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J.P. Magué & B. Ménard

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[1]  arXiv:1708.06348 [pdf, other]
Title: Stellar multiplicity in the Milky Way Galaxy
Comments: 2 pages, 1 figure, to appear in the proceedings of the IAU Symposium 334 "Rediscovering our Galaxy", Potsdam, 10-14 July 2017, eds. C. Chiappini, I. Minchev, E. Starkenburg, M. Valentini
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Astrophysics of Galaxies (astro-ph.GA)

We present our models of the effect of binaries on high-resolution spectroscopic surveys. We want to determine how many binary stars will be observed, whether unresolved binaries will contaminate measurements of chemical abundances, and how we can use spectroscopic surveys to better constrain the population of binary stars in the Galaxy. Using a rapid binary-evolution algorithm that enables modelling of the most complex binary systems we generate a series of large binary populations in the Galactic disc and evaluate the results. As a first application we use our model to study the binary fraction in APOGEE giants. We find tentative evidence for a change in binary fraction with metallicity.

[2]  arXiv:1708.06351 [pdf, other]
Title: Spectroscopic confirmation of the low-latitude object FSR 1716 as an old globular cluster
Comments: 4 pages, 4 figures, submitted to Astronomy & Astrophysics
Subjects: Astrophysics of Galaxies (astro-ph.GA)

Star clusters are invaluable tracers of the Galactic components and the discovery and characterization of low-mass stellar systems can be used to appraise their prevailing disruption mechanisms and time scales. However, owing to the significant foreground contamination, high extinction, and still uncharted interfaces of the underlying Milky Way components, objects at low Galactic latitudes are notoriously difficult to characterize. Here, we present the first spectroscopic campaign to identify the chemodynamical properties of the low-latitude star cluster FSR 1716. While its photometric age and distance are far from settled, the presence of RR Lyrae variables indicates a rather old cluster variety. Using medium-resolution (R$\sim$10600) calcium triplet (CaT) spectroscopy obtained with the wide-field multi-fibre AAOmega instrument, we identified six member candidates with a mean velocity of $-30$ km s$^{-1}$ and a velocity dispersion of 2.5$\pm$0.9 km s$^{-1}$. The latter value implies a dynamic mass of $\sim$1.3$\times$10$^4$ M$_{\odot}$, typical of a low-mass globular cluster. Combined with our derived CaT metallicity of $-1.38\pm0.20$ dex, this object is finally confirmed as an old, metal-poor globular cluster.

[3]  arXiv:1708.06352 [pdf, other]
Title: The curvature radiation absorption cross section
Authors: Nicola Locatelli (1,2), Gabriele Ghibellini (1) ((1) INAF - Osservatorio Astronomico di Brera, (2) Univ. Milano-Bicocca)
Comments: 6 pages, 4 figures, submitted to A&A
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

When treating the absorption of light, one focuses on the absorption coefficient, related to the probability of photons to survive while traversing a layer of material. From the point of view of particles doing the absorption, however, the elementary interaction of the particle with the photon is best described by the corresponding cross section. We revisit curvature radiation in order to find the absorption cross section for this process, making use of the Einstein coefficients and their relations with spontaneous and stimulated emission and true absorption. We derive the cross section as a function of the emission angle psi (i.e. the angle between the instantaneous velocity vector and the direction of the photon), and the cross section integrated over angles. Both are positive, contrary to the synchrotron case for which the cross section can be negative for large psi. Therefore, it is impossible to have curvature radiation masers. This has important consequences on sources of very large brightness temperatures that require a coherent emission process, such as pulsars and Fast Radio Bursts.

[4]  arXiv:1708.06353 [pdf, other]
Title: Twelve years of spectroscopic monitoring in the Galactic Center: the closest look at S-stars near the black hole
Comments: Accepted for publication in ApJ
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Astrophysics of Galaxies (astro-ph.GA)

We study the young S-stars within a distance of 0.04 pc from the supermassive black hole in the center of our Galaxy. Given how inhospitable the region is for star formation, their presence is more puzzling the younger we estimate their ages. In this study, we analyse the result of 12 years of high resolution spectroscopy within the central arcsecond of the Galactic Center (GC). By co-adding between 55 and 105 hours of spectra we have obtained high signal to noise H- and K-band spectra of eight stars orbiting the central supermassive black hole. Using deep H-band spectra, we show that these stars must be high surface gravity (dwarf) stars. We compare these deep spectra to detailed model atmospheres and stellar evolution models to infer the stellar parameters. Our analysis reveals an effective temperature of 21000-28500 K, a rotational velocity of 60-170 km/s, and a surface gravity of 4.1-4.2. These parameters imply a spectral type of B0-B3V for these stars. The inferred masses lie within 8-14 Msun. We derive an age of 6.6^{+3.4}{-4.7} Myr for the star S2, which is compatible with the age of the clockwise rotating young stellar disk in the GC. We estimate the age of all other studied S-stars to be less than 15 Myr, which are compatible with the age of S2 within the uncertainties. The relatively low ages for these S-stars favor a scenario in which the stars formed in a local disk rather than the field-binary-disruption scenario throughout a longer period of time.

[5]  arXiv:1708.06356 [pdf, other]
Title: The Canada-France Imaging Survey: First results from the u-band component
Comments: 12 pages, 11 figures, accepted for publication in ApJ
Subjects: Astrophysics of Galaxies (astro-ph.GA)

The Canada-France Imaging Survey (CFIS) will map the northern high Galactic latitude sky in the $u$-band ("CFIS-u", 10,000$\, {\rm deg^2}$) and in the $r$-band ("CFIS-r", 5,000$\, {\rm deg^2}$), enabling a host of stand-alone science investigations, and providing some of the ground-based data necessary for photometric redshift determination for the Euclid mission. In this first contribution we present the $u$-band component of the survey, describe the observational strategy, and discuss some first highlight results, based on approximately one third of the final area. We show that the Galactic anticenter structure is distributed continuously along the line of sight, out to beyond 20 kpc, and possesses a metallicity distribution that is essentially identical to that of the outer disk sampled by APOGEE. This suggests that it is probably a buckled disk of old metal-rich stars, rather than a stream or a flare. We also discuss the future potential for CFIS-u in discovering star-forming dwarf galaxies around the Local Group, the characterization of the white dwarf and blue straggler population of the Milky Way, as well as its sensitivity to low-surface brightness structures in external galaxies.

[6]  arXiv:1708.06357 [pdf, ps, other]
Title: Strongly lensed repeating Fast Radio Bursts precisely probe the universe
Comments: 5 pages, 2 figures. Comments are welcome
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); General Relativity and Quantum Cosmology (gr-qc)

Fast Radio bursts (FRBs) are bright transients with millisecond duration at $\sim$ GHz frequencies, whose physical origin is subject to intense debate. Most FRBs are located at high galactic latitudes and have anomalously large dispersion measures (DMs). Attributing DM to an intergalactic medium origin, the corresponding redshifts z are around $0.5-1$. In this case, FRBs have great chance to be gravitationally lensed by intervening galaxies. Since in a lensed FRB system, the time delay between images can be measured to extremely high precision because of the large ratio $\sim10^9$ between the typical galaxy-lensing delay time $\sim\mathcal{O}$(10 days) and the narrow width of the bursts $\sim\mathcal{O}$(ms), we propose accurate measurements of time delays between images of lensed FRBs as a powerful probe for precision cosmology. Here we show that, within the flat $\Lambda$CDM model, the Hubble constant $H_0$ can be constrained with an uncertainty of $0.48\%$ from accurate measurements of time delays of 10 such systems. More importantly, on the basis of the distance sum rule, the cosmic curvature will be constrained to a precision of $\sim0.056$ in a model-independent way. Such a direct and model-independent constraint on the cosmic curvature will provide a stringent direct test for the validity of the Friedmann-Lema\^{i}tre-Robertson-Walker (FLRW) metric and break the intractable degeneracy between the cosmic curvature and dark energy, offering the opportunity in investigating the nature of dark sectors of the universe.

[7]  arXiv:1708.06358 [pdf, ps, other]
Title: Morphological evidence for a past minor merger in the Seyfert galaxy NGC 1068
Comments: 15 pages, 9 figures. Accepted for publication in the Publications of the Astronomical Society of Japan (PASJ)
Subjects: Astrophysics of Galaxies (astro-ph.GA)

Deep optical imaging with both Hyper Suprime-Cam and Suprime-Cam on the 8.2 m Subaru Telescope reveals a number of outer faint structures around the archetypical Seyfert galaxy NGC 1068 (M 77). We find three ultra diffuse objects (UDOs) around NGC 1068. Since these UDOs are located within the projected distance of 45 kpc from the center of NGC 1068, they appear to be associated with NGC 1068. Hereafter, we call them UDO-SW, UDO-NE, and UDO-SE where UDO = Ultra Diffuse Object, SW = south west, NE = north west, and SE = south east; note that UDO-SE was already found in the SDSS Stripe 82 data. Among them, both UDO-NE and UDO-SW appear to show a loop or stream structure around the main body of NGC 1068, providing evidence for the physical connection to NGC 1068. We consider that UDO-SE may be a tidal dwarf galaxy. We also find another UDO-like object that is 2 magnitudes fainter and smaller by a factor of 3 to 5 than those of the three UDOs. This object may belong to a class of low surface brightness galaxy. Since this object is located along the line connecting UDO-NE and UDO-SW, it is suggested that this object is related to the past interaction event that formed the loop by UDO-NE and UDO-SW, thus implying the physical connection to NGC 1068. Another newly-discovered feature is an asymmetric outer one-arm structure emanated from the western edge of the outermost disk of NGC 1068 together with a ripple-like structure at the opposite side. These structures are expected to arise in a late phase of a minor merger according to published numerical simulations of minor mergers. All these lines of evidence show that NGC 1068 experienced a minor merger several billions years ago. We then discuss the minor-merger driven triggering of nuclear activity in the case of NGC 1068.

[8]  arXiv:1708.06359 [pdf, other]
Title: Chemical Mapping of the Milky Way With The Canada-France Imaging Survey: A Non-parametric Metallicity-Distance Decomposition of the Galaxy
Comments: 21 pages, 17 figures, accepted for publication in ApJ
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We present the chemical distribution of the Milky Way, based on 2,900$\, {\rm deg^2}$ of $u$-band photometry taken as part of the Canada-France Imaging Survey. When complete, this survey will cover 10,000$\, {\rm deg^2}$ of the Northern sky. By combing the CFHT $u$-band photometry together with SDSS and Pan-STARRS $g,r,$ and $i$, we demonstrate that we are able to measure reliably the metallicities of individual stars to $\sim 0.2$ dex, and hence additionally obtain good photometric distance estimates. This survey thus permits the measurement of metallicities and distances of the dominant main-sequence population out to approximately 30 kpc, and provides much higher number of stars at large extraplanar distances than have been available from previous surveys. We develop a non-parametric distance-metallicity decomposition algorithm and apply it to the sky at $30\deg < |b| < 70\deg$ and to the North Galactic Cap. We find that the metallicity-distance distribution is well-represented by three populations whose metallicity distributions do not vary significantly with vertical height above the disk. As traced in main-sequence stars, the stellar halo component shows a vertical density profile that is close to exponential, with a scale height of around 3 kpc. This may indicate that the inner halo was formed partly from disk stars ejected in an ancient minor merger.

[9]  arXiv:1708.06360 [pdf, other]
Title: Feeling the pull, a study of natural Galactic accelerometers. II: kinematics and mass of the delicate stellar stream of the Palomar 5 globular cluster
Comments: 13 pages, 13 figures, ApJ in print
Journal-ref: The Astrophysical Journal, Volume 842, Issue 2, article id. 120, 12 pp. (2017)
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We present two spectroscopic surveys of the tidal stellar stream of the Palomar 5 globular cluster, undertaken with the VLT/FLAMES and AAT/AAOmega instruments. We use these data in conjunction with photometric data presented in the previous contribution in this series to classify the survey stars in terms of their probability of belonging to the Palomar 5 stellar stream. We find that high-probability candidates are only found in a very narrow spatial interval surrounding the locus of the stream on the sky. PanSTARRS RRLyrae stars in this region of sky are also distributed in a similar manner. The absence of significant "fanning" of this stellar stream confirms that Palomar 5 does not follow a chaotic orbit. Previous studies have found that Palomar 5 is largely devoid of low-mass stars, and we show that this is true also of the stellar populations along the trailing arm out to $6\deg$. Within this region, which contains 73\% of the detected stars, the population is statistically identical to the core, implying that the ejection of the low-mass stars occurred before the formation of the stream. We also present an updated structural model fit to the bound remnant, which yields a total mass of $4297\pm98 {\rm\,M_\odot}$ and a tidal radius $0.145\pm0.009$ kpc. We estimate the mass of the observed system including the stream to be $12200\pm400 {\rm\,M_\odot}$, and the initial mass to have been $\sim47000\pm1500 {\rm\,M_\odot}$. These observational constraints will be employed in our next study to model the dynamics of the system in detail.

[10]  arXiv:1708.06362 [pdf, other]
Title: General-Relativistic Simulations of Four States of Accretion onto Millisecond Pulsars
Comments: 9 pages, 5 figures, 1 movie. Submitted to ApJ Letters. 7 movies available at this https URL
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); General Relativity and Quantum Cosmology (gr-qc)

Accreting neutron stars can power a wide range of astrophysical phenomena including short- and long-duration gamma-ray bursts, ultra-luminous X-ray sources, X-ray binaries, and transitional millisecond pulsars. Detailed modeling of these systems can reveal the complex accretion-disk-magnetosphere interaction and help to interpret their observations. However, force-free methods, which can robustly handle the magnetically dominated regions near the star, do not include the accreting gas, while magnetohydrodynamic (MHD) methods become numerically unstable at high magnetization. We present a new approach that combines essentially force-free and MHD regions in the same simulation, and describe the first time-dependent relativistic simulations of magnetized accretion onto millisecond pulsars. Our axisymmetric general-relativistic (GR) MHD simulations follow a turbulent magnetized accretion flow as it penetrates the pulsar wind, crosses the light cylinder into the closed, approximately dipolar zone of the pulsar magnetosphere, and reaches the stellar surface. These are the first simulations to model (i) the transition of a millisecond pulsar from the isolated to the accreting state and (ii) the formation of powerful, Poynting-flux-dominated relativistic jets by accreting neutron stars. The simulations produce four accretion regimes, in order of increasing stellar magnetic field strength (equivalently, decreasing mass accretion rate): (a) crushed magnetosphere and direct accretion; (b) magnetically channeled accretion onto the stellar poles; (c) the propeller state, where material enters through the light cylinder but is prevented from accreting by the centrifugal barrier; (d) almost perfect exclusion of the accretion flow from the light cylinder by the pulsar wind.

[11]  arXiv:1708.06364 [pdf]
Title: Realising the full potential of X-ray astronomy in the UK
Authors: P. Gandhi (Southampton), N. Degenaar (API, Amsterdam), C. Done (Durham), M.G. Watson (Leicester)
Comments: Summary of community meeting on UK X-ray astronomy to be submitted to A&G (2017 Feb 10 RAS specialist discussion meeting; this https URL). Comments welcome until end of August
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM); High Energy Astrophysical Phenomena (astro-ph.HE); Space Physics (

X-ray astronomy is our gateway to the hot universe. More than half of the baryons in the cosmos are too hot to be visible at shorter wavelengths. Studying the extreme environments of black hole and neutron star vicinities also requires X-ray observations. With the successful launch of ISRO's AstroSat in 2015, and a few transformative results that emerged from JAXA's Hitomi mission in 2016, a new window has been opened into high sensitivity fast timing and high X-ray spectral resolution. Together with upcoming all-sky survey missions expected very soon, X-ray astronomy is entering a new era of parameter space exploration. The UK has been at the forefront of this field since the 1970s. But flat cash science budgets, compounded with the rising costs of cutting-edge space missions, imply inevitably diminishing roles for the UK in terms of both payload development and science exploitation in the future. To review the novel science possibilities enabled by recent and upcoming missions, and to discuss how to pave the way forward for X-ray astronomy in the UK, a specialist RAS discussion meeting was held in London on Feb 10 2017, summarised herein. A consolidated effort by the community to come together and work cohesively is a suggested natural first step in the current climate.

[12]  arXiv:1708.06365 [pdf, other]
Title: Optimizing measurements of cluster velocities and temperatures for CCAT-prime and future surveys
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); Instrumentation and Methods for Astrophysics (astro-ph.IM)

Galaxy cluster velocity correlations and mass distributions are sensitive probes of cosmology and the growth of structure. Upcoming microwave surveys will enable extraction of velocities and temperatures from many individual clusters for the first time. We forecast constraints on peculiar velocities, electron temperatures, and optical depths of galaxy clusters obtainable with upcoming multi-frequency measurements of the kinematic, thermal, and relativistic Sunyaev-Zeldovich effects. The forecasted constraints are compared for different measurement configurations with frequency bands between 90 GHz and 1 THz, and for different survey strategies for the 6-meter CCAT-prime telescope. We study methods for improving cluster constraints by removing emission from dusty star forming galaxies, and by using X-ray temperature priors from eROSITA. Cluster constraints are forecast for several model cluster masses. A sensitivity optimization for seven frequency bands is presented for a CCAT-prime first light instrument and a next generation instrument that takes advantage of the large optical throughput of CCAT-prime. We find that CCAT-prime observations are expected to enable measurement and separation of the SZ effects to characterize the velocity, temperature, and optical depth of individual massive clusters ($\sim10^{15}\,M_\odot$). Submillimeter measurements are shown to play an important role in separating these components from dusty galaxy contamination. Using a modular instrument configuration with similar optical throughput for each detector array, we develop a rule of thumb for the number of detector arrays desired at each frequency to optimize extraction of these signals. Our results are relevant for a future "Stage IV" cosmic microwave background survey, which could enable galaxy cluster measurements over a larger range of masses and redshifts than will be accessible by other experiments.

[13]  arXiv:1708.06366 [pdf, other]
Title: Using planetary transits to estimate magnetic cycles lengths in Kepler stars
Comments: 8 pages, 4 figures, Proceedings of the IAU Symposium No. 328: "Living Around Active Stars"
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Earth and Planetary Astrophysics (astro-ph.EP)

Observations of various solar-type stars along decades revealed that they can have magnetic cycles, just like our Sun. An investigation of the relation between their cycle length and rotation period can shed light on the dynamo mechanisms operating in these stars. Previous works on this relation suggested that the stars could be separated into active and inactive branches, with the Sun falling between them. In this work, we determined short magnetic activity cycles for 6 active solar-type stars observed by the Kepler telescope. The method adopted here estimates the activity from the excess in the residuals of the transitlight curves. This excess is obtained by subtracting a spotless model transit from the light curve, and then integrating over all the residuals during the transit. The presence of long term periodicity is estimated from the analysis of a Lomb-Scargle periodogram of the complete time series. Finally, we investigate the rotation-cycle period relation for the stars analysed here and find that some active stars do not follow the behaviour proposed earlier, falling in the inactive branch. In addition, we also notice a considerable spread from other stars in the literature in the active/inactive branches.

[14]  arXiv:1708.06368 [pdf, other]
Title: Gravitationally Lensed HI with MeerKAT
Comments: 15 pages, 3 figures, accepted for publication, Proceedings of Science, workshop on "MeerKAT Science: On the Pathway to the SKA", held in Stellenbosch 25-27 May 2016. Comments welcome
Subjects: Astrophysics of Galaxies (astro-ph.GA); Cosmology and Nongalactic Astrophysics (astro-ph.CO)

The SKA era is set to revolutionize our understanding of neutral hydrogen (HI) in individual galaxies out to redshifts of z~0.8; and in the z > 6 intergalactic medium through the detection and imaging of cosmic reionization. Direct HI number density constraints will, nonetheless, remain relatively weak out to cosmic noon (z~2) - the epoch of peak star formation and black hole accretion - and beyond. However, as was demonstrated from the 1990s with molecular line observations, this can be overcome by utilising the natural amplification afforded by strong gravitational lensing, which results in an effective increase in integration time by the square of the total magnification (\mu^2) for an unresolved source. Here we outline how a dedicated lensed HI survey will leverage MeerKAT's high sensitivity, frequency coverage, large instantaneous bandwidth, and high dynamic range imaging to enable a lasting legacy of high-redshift HI emission detections well into the SKA era. This survey will not only provide high-impact, rapid-turnaround MeerKAT science commissioning results, but also unveil Milky Way-like systems towards cosmic noon which is not possible with any other SKA precursors/pathfinders. An ambitious lensed HI survey will therefore make a significant impact from MeerKAT commissioning all the way through to the full SKA era, and provide a more complete picture of the HI history of the Universe.

[15]  arXiv:1708.06369 [pdf, other]
Title: Mahalo Deep Cluster Survey I. Accelerated and enhanced galaxy formation in the densest regions of a protocluster at z=2.5
Comments: Submitted to MNRAS, 23 pages, 17 figures, 5 tables
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We carried out deep H$\alpha$ narrowband imaging with 10 hours net integrations towards the young protocluster, USS1558$-$003 at $z=2.53$ with the Subaru Telescope. This system is composed of four dense groups with massive local overdensities, traced by 107 H$\alpha$ emitters (HAEs) with stellar masses and star formation rates down to $1\times10^8$ M$_\odot$ and 3 M$_\odot$yr$^{-1}$, respectively. We have investigated the environmental dependence of various physical properties within the protocluster by comparing distributions of HAEs in higher and lower densities with a standard Kolmogorov--Smirnov test. At 97\% confidence level, we find enhanced star formation across the star-forming main sequence of HAEs living in the most extreme `supergroup', corresponding to the top quartile of overdensities. Furthermore, we derive distribution functions of H$\alpha$ luminosity and stellar mass in group and intergroup regions, approximately corresponding to 30 times and 8 times higher densities than the general field. As a consequence, we identify by 0.7 and 0.9 dex higher cutoffs in H$\alpha$ luminosity and stellar mass functions in the dense groups, respectively. On the other hand, HAEs in the intergroup environment of the protocluster show similar distribution functions to those of field galaxies despite residing in significant overdensities. In the early phase of cluster formation, as inferred from our results, the densest parts in the protocluster have had an accelerated formation of massive galaxies. We expect that these eventually grow and transform into early-type galaxies at the bright end of the red sequence as seen in present-day rich clusters of galaxies.

[16]  arXiv:1708.06372 [pdf, ps, other]
Title: Vigorous atmospheric motion in the red supergiant star Antares
Comments: 27 pages, 8 figures, published in Nature
Journal-ref: Nature, 2017, 548, 310
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

Red supergiant stars represent a late stage of the evolution of stars more massive than about nine solar masses, in which they develop complex, multi-component atmospheres. Bright spots have been detected in the atmosphere of red supergiants using interferometric imaging. Above the photosphere of a red supergiant, the molecular outer atmosphere extends up to about two stellar radii. Furthermore, the hot chromosphere (5,000 to 8,000 kelvin) and cool gas (less than 3,500 kelvin) of a red supergiant coexist at about three stellar radii. The dynamics of such complex atmospheres has been probed by ultraviolet and optical spectroscopy. The most direct approach, however, is to measure the velocity of gas at each position over the image of stars as in observations of the Sun. Here we report the mapping of the velocity field over the surface and atmosphere of the nearby red supergiant Antares. The two-dimensional velocity field map obtained from our near-infrared spectro-interferometric imaging reveals vigorous upwelling and downdrafting motions of several huge gas clumps at velocities ranging from about -20 to +20 kilometres per second in the atmosphere, which extends out to about 1.7 stellar radii. Convection alone cannot explain the observed turbulent motions and atmospheric extension, suggesting that an unidentified process is operating in the extended atmosphere.

[17]  arXiv:1708.06386 [pdf, other]
Title: Multiwavelength observations of the blazar BL Lacertae: a new fast TeV $γ$-ray flare
Comments: 8 pages, 5 figures, in Proceedings of the 35th International Cosmic Ray Conference (ICRC 2017), Busan (South Korea)
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

Observations of fast TeV $\gamma$-ray flares from blazars reveal the extreme compactness of emitting regions in blazar jets. Combined with very-long-baseline radio interferometry measurements, they probe the structure and emission mechanism of the jet. We report on a fast TeV $\gamma$-ray flare from BL Lacertae observed by VERITAS, with a rise time of about 2.3 hours and a decay time of about 36 minutes. The peak flux at $>$200 GeV measured with the 4-minute binned light curve is $(4.2 \pm 0.6) \times 10^{-6} \;\text{photons} \;\text{m}^{-2}\, \text{s}^{-1}$, or $\sim$180% the Crab Nebula flux. Variability in GeV $\gamma$-ray, X-ray, and optical flux, as well as in optical and radio polarization was observed around the time of the TeV $\gamma$-ray flare. A possible superluminal knot was identified in the VLBA observations at 43 GHz. The flare constrains the size of the emitting region, and is consistent with several theoretical models with stationary shocks.

[18]  arXiv:1708.06393 [pdf, other]
Title: A citizen-science approach to muon events in imaging atmospheric Cherenkov telescope data: the Muon Hunter
Comments: 8 pages, 3 figures, in Proceedings of the 35th International Cosmic Ray Conference (ICRC 2017), Busan, South Korea
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM)

Event classification is a common task in gamma-ray astrophysics. It can be treated with rapidly-advancing machine learning algorithms, which have the potential to outperform traditional analysis methods. However, a major challenge for machine learning models is extracting reliably labelled training examples from real data. Citizen science offers a promising approach to tackle this challenge.
We present "Muon Hunter", a citizen science project hosted on the Zooniverse platform, where VERITAS data are classified multiple times by individual users in order to select and parameterize muon events, a product from cosmic ray induced showers. We use this dataset to train and validate a convolutional neural-network model to identify muon events for use in monitoring and calibration. The results of this work and our experience of using the Zooniverse are presented.

[19]  arXiv:1708.06406 [pdf, ps, other]
Title: On production of gamma rays and Relativistic Runaway Electron Avalanches from Martian dust storms
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

Production of runaway electron avalanches and gamma rays originating inside Martian dust storms are studied using Monte Carlo simulations. In the absence of in situ measurements, we use theoretical predictions of electric fields inside dust storms. Electrons are produced through the relativistic runaway electron avalanches process, and energetic photons are results of the bremsstrahlung scattering of the electrons with the air. Characteristic lengths of the runaway electron avalanche for different electric fields and the energy spectrum of electrons are derived and compared to their terrestrial counterparts. It is found that it is possible for Martian dust storms to develop energetic electron avalanches and produce large fluxes of gamma ray photons similar to terrestrial gamma ray flashes from Earth's thunderstorms. The phenomenon could be called Martian gamma ray flash, and due to the very thin atmosphere on Mars, it can be observed by both ground-based instruments or satellites orbiting the planet.

[20]  arXiv:1708.06408 [pdf]
Title: Solar Line Asymmetries: Modelling the Effect of Granulation on the Solar Spectrum
Authors: Timo A. Nieminen
Comments: 331 pages, 98 figures, PhD thesis, The University of Queensland, Brisbane, Australia, 1995
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

A parametric model of granulation employing a small number of parameters was developed. Synthetic spectra calculated using this model closely match observed spectra and, in particular, reproduce the asymmetries observed in spectral lines. Both the microturbulent motions and the large-scale flow velocity decrease exponentially with a scale height of 368 km as the height within the photosphere increases. The model agrees with observations of the solar granulation (from which it was derived).
The horizontal motions associated with granulation were found and used to calculate spectra emergent away from disk centre. These calculated spectra were compared to observed spectra, with the agreement supporting the accuracy of the granular model.
Also in the course of this work, the Brueckner-O'Mara damping theory was found to predict damping constants accurately. The photospheric abundances of a number of elements were determined. The abundance obtained for iron agrees with the meteoric iron abundance. Astrophysical f-values for some lines were also determined.

[21]  arXiv:1708.06473 [pdf, other]
Title: Scale-Dependent Galaxy Bias from Massive Particles with Spin during Inflation
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); High Energy Physics - Phenomenology (hep-ph); High Energy Physics - Theory (hep-th)

The presence of additional particles during inflation leads to non-Gaussianity in late-time correlators of primordial curvature perturbations. The shape and amplitude of this signal depend on the mass and spin of the extra particles. Constraints on this distinct form of primordial non-Gaussianity, therefore, provide a wealth of information on the particle content during inflation. We investigate the potential of upcoming galaxy surveys in constraining such a signature through its impact on the observed galaxy power spectrum. Primordial non-Gaussianity of various shapes induces a scale-dependent bias on tracers of large-scale structure, such as galaxies. Using this signature we obtain constraints on massive particles during inflation, which can have non-zero spins. In particular, we show that the prospects for constraining particles with spins 0 and 1 are promising, while constraining particles with spin 2 from power spectrum alone seems challenging. We show that the multi-tracer technique can significantly improve the constraints from the power spectrum by at least an order of magnitude. Furthermore, we analyze the effect of non-linearities due to gravitational evolution on the forecasted constraints on the masses of the extra particles and the amplitudes of the imprinted non-Gaussian signal. We find that gravitational evolution affects the constraints by less than a factor of 2.

[22]  arXiv:1708.06493 [pdf, ps, other]
Title: Using Chebyshev polynomials interpolation to improve the computation efficiency of gravity near an irregular-shaped asteroid
Comments: 17 pages, 10 figures, 3 tables, accepted to Research in Astronomy and Astrophysics
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

In asteroid rendezvous missions, the dynamical environment near the asteroid's surface should be made clear prior to the mission launch. However, most of the asteroids have irregular shapes, which lower the efficiency of calculating their gravitational field by adopting the traditional polyhedral method. In this work, we propose a method to partition the space near the asteroid adaptively along three spherical coordinates and use Chebyshev polynomials interpolation to represent the gravitational acceleration in each cell. Moreover, we compare four different interpolation schemes to obtain the best precision with the identical initial parameters. An error-adaptive octree division is combined to improve the interpolation precision near the surface. As an example, we take the typical irregular-shaped near-Earth asteroid 4179 Toutatis to show the advantage of this method, as a result, we show that the efficiency can be increased by hundreds to thousands times with our method. In a word, this method can be applicable to other irregular-shaped asteroids and can greatly improve the evaluation efficiency.

[23]  arXiv:1708.06501 [pdf, other]
Title: The Evolution of Bulge-Dominated Field Galaxies from z~1 to the Present
Comments: Accepted for publication in The Astrophysical Journal 08/20/2017
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We analyze the stellar populations and evolutionary history of bulge-dominated field galaxies at redshifts 0.3<z<1.2 as part of the Gemini/HST Galaxy Cluster Project (GCP). High signal-to-noise optical spectroscopy from the Gemini Observatory and imaging from the Hubble Space Telescope are used to analyze a total of 43 galaxies, focusing on the 30 passive galaxies in the sample. Using the size-mass and velocity dispersion-mass relations for the passive field galaxies we find no significant evolution of sizes or velocity dispersions at a given dynamical mass between z~1 and the present. We establish the Fundamental Plane and study mass-to-light (M/L) ratios. The M/L vs. dynamical mass relation shows that the passive field galaxies follow a relation with a steeper slope than the local comparison sample, consistent with cluster galaxies in the GCP at z=0.86. This steeper slope indicates that the formation redshift is mass dependent, in agreement with "downsizing," meaning that the low mass galaxies formed their stars more recently while the high mass galaxies formed their stars at higher redshift. The zero point differences of the scaling relations for the M/L ratios imply a formation redshift of z_form=1.35(+0.10)(-0.07) for the passive field galaxies. This is consistent with the (Hdelta_A + Hgamma_A)' line index which implies a formation redshift of z_form=1.40(+0.60)(-0.18).

[24]  arXiv:1708.06502 [pdf, other]
Title: Limit on graviton mass from galaxy cluster Abell 1689
Authors: Shantanu Desai
Comments: 6 pages, 2 figures
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO)

To date, the only limit on graviton mass using galaxy clusters was obtained by Goldhaber and Nieto in 1974, using the fact that the orbits of galaxy clusters are bound and closed, and extend up to 580 kpc. From positing that only a Newtonian potential gives rise to such stable bound orbits, a limit on the graviton mass $m_g<10^{-29}$ eV was obtained (PRD 9,1119, 1974). Recently, it has been shown that one can get closed bound orbits for a whole class of non-Newtonian potentials (arXiv:1707.04937 and arXiv:1705.02444), thus invalidating the main \emph{ansatz} used in Goldhaber and Nieto to obtain the graviton mass bound. In order to obtain a revised estimate using galaxy clusters, we use dynamical mass models of the Abell 1689 (A1689) galaxy cluster to check their compatibility with a Yukawa gravitational potential. We assume mass models for the gas, dark matter, and galaxies for A1689 from arXiv:1703.10219 and arXiv:1610.01543, who used this cluster to test various alternate gravity theories, which dispense with the need for dark matter. We quantify the deviations in the acceleration profile using these mass models, assuming a Yukawa potential and that obtained assuming a Newtonian potential, by calculating the $\chi^2$ residuals between the two profiles. The 90\% c.l. upper limit on the graviton mass corresponds to the minimum mass for which $\Delta \chi^2>2.71$. Our estimated 90\% c.l. bound on the graviton mass ($m_g$) is thereby given by, $m_g < 1.64 \times 10^{-29}$ eV or in terms of the graviton Compton wavelength, $\lambda_g>7.6 \times 10^{19}$ km.

[25]  arXiv:1708.06505 [pdf, ps, other]
Title: Imaging Spectroscopy of Solar Radio Burst Fine Structures
Comments: 20 pages, 9 figures
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

Solar radio observations provide a unique diagnostic of the outer solar atmosphere. However, the inhomogeneous turbulent corona strongly affects the propagation of the emitted radio waves, so decoupling the intrinsic properties of the emitting source from the effects of radio-wave propagation has long been a major challenge in solar physics. Here we report quantitative spatial and frequency characterization of solar radio burst fine structures observed with the LOw Frequency Array (LOFAR), an instrument with high time resolution that also permits imaging at scales much shorter than those corresponding to radio-wave propagation in the corona. The observations demonstrate that radio-wave propagation effects, and not the properties of the intrinsic emission source, dominate the observed spatial characteristics of radio burst images. These results permit more accurate estimates of source brightness temperatures, and open opportunities for quantitative study of the mechanisms that create the turbulent coronal medium through which the emitted radiation propagates.

[26]  arXiv:1708.06515 [pdf, other]
Title: The baryonic halos of isolated elliptical galaxies
Comments: 5 pages. Published in the special issue of Galaxies, "On the Origin (and Evolution) of Baryonic Galaxy Halos"
Journal-ref: Galaxies, 5, 42 (2017)
Subjects: Astrophysics of Galaxies (astro-ph.GA)

Without the interference of a number of events, galaxies may suffer in crowded environments (e.g., stripping, harassment, strangulation); isolated elliptical galaxies provide a control sample for the study of galaxy formation. We present the study of a sample of isolated ellipticals using imaging from a variety of telescopes, focusing on their globular cluster systems as tracers of their stellar halos. Our main findings are: (a) GC color bimodality is common even in the most isolated systems; (b) the specific frequency of GCs is fairly constant with galaxy mass, without showing an increase towards high-mass systems like in the case of cluster ellipticals; (c) on the other hand, the red fraction of GCs follows the same inverted V shape trend with mass as seen in cluster ellipticals; and (d) the stellar halos show low S\'ersic indices which are consistent with a major merger origin.

[27]  arXiv:1708.06526 [pdf, other]
Title: The shapes and alignments of the satellites of the Milky Way and Andromeda
Comments: Accepted for publication in MNRAS
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We measure the intrinsic shapes and alignments of the dwarf spheroidal (dSph) galaxies of the Local Group. We find the dSphs of the Milky Way are intrinsically flatter (mean intrinsic ellipticity $\mu_E\sim0.6$) than those of M31 ($\mu_E\sim0.5$) and that the classical Milky Way dSphs ($M_V<-8.5\,\mathrm{mag}$) are rounder ($\mu_E\sim0.5$) than the ultrafaints ($\mu_E\sim0.65$) whilst in M31 the shapes of the classical and ultrafaint dSphs are very similar. The M31 dSphs are preferentially radially aligned with a dispersion of $\sim45\deg$. This signal is driven by the ultrafaint population whilst the classical M31 dSphs are consistent with a random orientation. We compare our results to the Aquarius mock stellar catalogues of Lowing et al. and find the subhalo radial alignment distribution matches the Local Group dSphs results, whilst the Aquarius intrinsic ellipticities are significantly smaller than the data ($\Delta\langle E\rangle\approx0.4$). We provide evidence that the major axes of the Milky Way satellites lie within a preferential plane with normal vector pointing towards $(\ell,b)=(127,5)\deg$. We associate this preferred direction with the Vast Polar Orbital structure although their respective great circles are offset by $\sim30\deg$. No signal in the alignments of the major axes is found in M31, suggesting that the Great Plane of Satellites is formed from recent accretion or chance alignment. Finally, we provide predictions for the discrepancy between the velocity dispersion versus scale radius distributions for the Milky Way and M31 populations and demonstrate that the projection effect from viewing similar populations from two different locations does not account for the discrepancy which is probably caused by increased tidal disruption in M31.

[28]  arXiv:1708.06528 [pdf, other]
Title: Polarized radiative transfer modeling of warped and clumpy dusty tori
Comments: 15 pages, 13 Figures, accepted for publication in A&A
Subjects: Astrophysics of Galaxies (astro-ph.GA); High Energy Astrophysical Phenomena (astro-ph.HE)

Context: Active galactic nuclei (AGN) are anisotropic objects surrounded by an optically thick equatorial medium whose true geometry still defy observers. Aims: In this paper, we aim to explore the optical, scattering-induced, polarization emerging from clumpy and warped dusty tori to check whether they can fit with the unified model predictions. Methods: We run polarized radiative transfer simulations in a set of warped and non warped clumpy tori to explore the differences induced by distorted dust distributions. We then include warped tori in a more complex model representative of an AGN to check, using polarimetry and imaging methods, if warps can reproduce the expected polarization dichotomy between Seyfert-1 and Seyfert-2 AGN. Results: The main results from our simulations highlight that isolated warped structures imprint the polarization degree and angle with distinctive signatures at Seyfert-1 orientations. Included in an AGN model, the signatures of warps are easily (but not always) washed out by multiple scattering in a clumpy environment. Imaging polarimetry may help to detect warped tori but we prove that warps can exist in AGN circumnuclear regions without contradicting observations. Conclusions: Two warped tori with a non significant difference in geometry in terms of photometry or spectroscopy can have totally different signatures in polarimetry. Testing the geometry of any alternative model to the usual dusty torus using polarized radiative transfer is a necessary approach to verify or reject a hypothesis.

[29]  arXiv:1708.06560 [pdf, other]
Title: Statistical analysis of 2D patterns and its application to astrometry
Comments: 11 pages, 13 figures, 1 table
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM); High Energy Physics - Phenomenology (hep-ph)

A general statistical procedure for analysis of finite 2D patterns, inspired by analysis of heavy-ion data, is developed. The method is verified in the study of publicly available data obtained by the Gaia-ESA mission. We prove that the procedure can be sensitive to the limits of accuracy of measurement, but it can also clearly identify the real physical effects on the large background of random distributions. As an example, the method confirms presence of binary and ternary star systems in the studied data. At the same time the possibility of statistical detection of gravitational microlensing effect is discussed.

[30]  arXiv:1708.06591 [pdf, ps, other]
Title: Study of the molecular and ionized gas in a possible precursor of an ultra-compact HII region
Comments: Accepted in A&A. August 22, 2017
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We study the molecular and the ionized gas in a possible precursor of an UC HII region to contribute to the understanding of how high-mass stars build-up their masses once they have reached the zero-age main secuence. We carried out molecular observations toward the position of the Red MSX source G052.9221-00.4892, using ASTE in the 12CO, 13CO, and C18O J=3-2, and HCO+ J=4-3 lines. We also present radio continuum observations at 6 GHz carried out with the JVLA interferometer. Combining these observations with public infrared data allowed us to inquire about the nature of the source. The analysis of the molecular observations reveals the presence of a kinetic temperature and H2 column density gradients across the molecular clump in which the source is embedded, with the hotter and less dense gas in the inner region. The 12CO J=3-2 emission shows evidence of misaligned massive molecular outflows, with the blue lobe in positional coincidence with a jet-like feature seen at 8 um. The radio continuum emission shows a slightly elongated compact radio source in positional coincidence with the Red MSX source. The polar-like morphology of this compact radio source perfectly matches the hourglass-like morphology exhibited by the source in the Ks-band. The axes of symmetry of the radio source and the near-IR nebula are perfectly aligned. Based on the multiwavelength analysis, we suggest that the analyzed source could be transiting a HC HII region phase, in which the young central star emits winds and ionizing radiation through the poles. By the other hand, according to a comparison between the Br-gamma and the radio flux density, the source would be in a more evolved evolutionary stage of an optically thin UC HII region in photoionization equilibrium. If this is the case, from the radio continuum emission, we can conjecture upon the spectral type of its exciting star which would be a B0.5V.

[31]  arXiv:1708.06592 [pdf, other]
Title: The Pierre Auger Observatory: Contributions to the 35th International Cosmic Ray Conference (ICRC 2017)
Comments: compendium of 26 proceedings for the ICRC 2017
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); Cosmology and Nongalactic Astrophysics (astro-ph.CO); Instrumentation and Methods for Astrophysics (astro-ph.IM)

Contributions of the Pierre Auger Collaboration to the 35th International Cosmic Ray Conference (ICRC 2017), 12-20 July 2017, Bexco, Busan, Korea.

[32]  arXiv:1708.06662 [pdf, other]
Title: Spontaneous generation of bending waves in isolated Milky Way-like discs
Comments: Accepted for publication in MNRAS, 15 pages, 10 figures, 1 table
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We study the spontaneous generation and evolution of bending waves in $N$-body simulations of two isolated Milky Way-like galaxy models. The models differ by their disc-to-halo mass ratios, and hence by their susceptibility to the formation of a bar and spiral structure. Seeded from shot noise in the particle distribution, bending waves rapidly form in both models and persist for many billions of years. Waves at intermediate radii manifest as corrugated structures in vertical position and velocity that are tightly wound, morphologically leading, and dominated by the $m=1$ azimuthal Fourier component. A spectral analysis of the waves suggests they are a superposition of modes from two continuous branches in the Galactocentric radius-rotational frequency plane. The lower-frequency branch is dominant and is responsible for the corrugated, leading, and warped structure. Over time, power in this branch migrates outward, lending credence to an inside-out formation scenario for the warp. Our power spectra qualitatively agree with results from linear perturbation theory and a WKB analysis, both of which include self-gravity. Thus, we conclude that the waves in our simulations are self-gravitating and not purely kinematic. These waves are reminiscent of the wave-like pattern recently found in Galactic star counts from the Sloan Digital Sky Survey and smoothly transition to a warp near the disc's edge. Velocity measurements from \textit{Gaia} data will be instrumental in testing the true wave nature of the corrugations. We also compile a list of "minimum requirements" needed to observe bending waves in external galaxies.

[33]  arXiv:1708.06681 [pdf, other]
Title: Comparison between two scalar field models using rotation curves of spiral galaxies
Comments: 7 pages, 1 figure, three tables
Subjects: Astrophysics of Galaxies (astro-ph.GA)

Scalar fields have been used as candidates for dark matter in the universe, from axions with masses $\sim10^{-5}$eV until ultra-light scalar fields with masses $\sim10^{-22}$eV. Axions behave as cold dark matter while the ultra-light scalar fields galaxies are Bose-Einstein condensate drops. The ultra-light scalar fields are also called scalar field dark matter model. In this work we study rotation curves for low surface brightness spiral galaxies using two scalar field models: the Gross-Pitaevskii Bose-Einstein condensate in the Thomas-Fermi approximation and a scalar field solution of the Klein-Gordon equation. We also used the zero disk approximation galaxy model where photometric data is not considered, only the scalar field dark matter model contribution to rotation curve is taken into account. From the best-fitting analysis of the galaxy catalog we use, we found the range of values of the fitting parameters: the length scale and the central density. The worst fitting results (values of $\chi^2_{red}$ much greater than 1, on the average) were for the Thomas-Fermi models, i.e., the scalar field dark matter is better than the Thomas-Fermi approximation model to fit the rotation curves of the analysed galaxies. To complete our analysis we compute from the fitting parameters the mass of the scalar field models and two astrophysical quantities of interest, the dynamical dark matter mass within 300 pc and the characteristic central surface density of the dark matter models. We found that the value of the central mass within 300 pc is in agreement with previous reported results, that this mass is $\approx 10^{7}$ $M_\odot/$pc$^2$, independent of the dark matter model. And, on the contrary, the value of the characteristic central surface density do depend on the dark matter model.

[34]  arXiv:1708.06692 [pdf, other]
Title: Lifetime measurements and oscillator strengths in singly ionised scandium and the solar abundance of scandium
Comments: Accepted for publication in MNRAS on 18 August 2017, this https URL
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

The lifetimes of 17 even-parity levels (3d5s, 3d4d, 3d6s, and 4p$^2$) in the region 57743-77837 cm$^{-1}$ of singly ionised scandium (\ion{Sc}{ii}) were measured by two-step time-resolved laser induced fluorescence spectroscopy. Oscillator strengths of 57 lines from these highly excited upper levels were derived using a hollow cathode discharge lamp and a Fourier transform spectrometer. In addition, Hartree--Fock calculations where both the main relativistic and core-polarisation effects were taken into account were carried out for both low- and high-excitation levels. There is a good agreement for most of the lines between our calculated branching fractions and the measurements of Lawler and Dakin (1989) in the region 9000-45000 cm$^{-1}$ for low excitation levels and with our measurements for high excitation levels in the region 23500-63100 cm$^{-1}$. This, in turn, allowed us to combine the calculated branching fractions with the available experimental lifetimes to determine semi-empirical oscillator strengths for a set of 380 E1 transitions in \ion{Sc}{ii}. These oscillator strengths include the weak lines that were used previously to derive the solar abundance of scandium. The solar abundance of scandium is now estimated to $\log~\epsilon_\odot = 3.04\pm0.13$ using these semi-empirical oscillator strengths to shift the values determined by Scott et al. (2015). The new estimated abundance value is in agreement with the meteoritic value ($\log~\epsilon_{\text{met}}=3.05\pm0.02$) of Lodders et al. (2009).

[35]  arXiv:1708.06701 [pdf, ps, other]
Title: Materia oscura escalar compleja (parte I): la versión hidrodinámica
Comments: 9 pages, in Spanish
Journal-ref: Revista Mexicana de Fisica E 58, 53-60 (2012)
Subjects: Astrophysics of Galaxies (astro-ph.GA)

In this work we use the Euler hydrodynamic equations of fluids to study a model of galactic halos minimally coupled to a complex scalar field, which in the Newtonian limit they become the Schr\"odinger-Poisson system. Applying a Madelung transformation, this system of equations takes the form of hydrodynamics equations, where there are a self-interacting potential and a kind of quantum potential that depends non-linearly on the density of the fluid. In this theoretical framework we analyze the Jeans' instability, which is useful for finding the scale length of perturbations of the scalar field that will form structures. In other words, perturbations of the scalar field with lengths less than this threshold length, can not lead to the formation of galactic structures. We also show that this scalar field hydrodynamic system has vorticity.

[36]  arXiv:1708.06706 [pdf, other]
Title: Analyzing γ-rays of the Galactic Center with Deep Learning
Comments: 24 pages, 11 figures
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); High Energy Physics - Phenomenology (hep-ph)

We present a new method to interpret the $\gamma$-ray data of our inner Galaxy as measured by the Fermi Large Area Telescope (Fermi LAT). We train and test convolutional neural networks with simulated Fermi-LAT images based on models tuned to real data. We use this method to investigate the origin of an excess emission of GeV $\gamma$-rays seen in previous studies. Interpretations of this excess include $\gamma$ rays created by the annihilation of dark matter particles and $\gamma$ rays originating from a collection of unresolved point sources, such as millisecond pulsars. Our new method allows precise measurements of the contribution and properties of an unresolved population of $\gamma$-ray point sources in the interstellar diffuse emission model.

[37]  arXiv:1708.06710 [pdf, other]
Title: Systematic errors in estimation of gravitational-wave candidate significance
Comments: A shortened version of arXiv:1601.00130
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM)

The statistical significance of a candidate gravitational-wave (GW) event is crucial to the prospects for a confirmed detection, or for its selection as a candidate for follow-up electromagnetic observation. To determine the significance of a GW candidate, a ranking statistic is evaluated and compared to an empirically-estimated background distribution, yielding a false alarm probability or p-value. The reliability of this background estimate is limited by the number of background samples and by the fact that GW detectors cannot be shielded from signals, making it impossible to identify a pure background data set. Different strategies have been proposed: in one method, all samples, including potential signals, are included in the background estimation, whereas in another method, coincidence removal is performed in order to exclude possible signals from the estimated background. Here we report on a mock data challenge, performed prior to the first detections of GW signals by Advanced LIGO, to compare these two methods. The all-samples method is found to be self-consistent in terms of the rate of false positive detection claims, but its p-value estimates are systematically conservative and subject to higher variance. Conversely, the coincidence-removal method yields a mean-unbiased estimate of the p-value but sacrifices self-consistency. We provide a simple formula for the uncertainty in estimate significance and compare it to mock data results. Finally, we discuss the use of different methods in claiming the detection of GW signals.

[38]  arXiv:1708.06723 [pdf, other]
Title: Hunting electromagnetic counterparts of gravitational-wave events using the Zwicky Transient Facility
Comments: 9 pages, 11 figures, Accepted for publication in Publications of the Astronomical Society of the Pacific
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM)

Detections of coalescing binary black holes by LIGO have opened a new window of transient astronomy. With increasing sensitivity of LIGO and participation of the Virgo detector in Cascina, Italy, we expect to soon detect coalescence of compact binary systems with one or more neutron stars. These are the prime targets for electromagnetic follow-up of gravitational wave triggers, which holds enormous promise of rich science. However, hunting for electromagnetic counterparts of gravitational wave events is a non-trivial task due to the sheer size of the error regions, which could span hundreds of square degrees. The Zwicky Transient facility (ZTF), scheduled to begin operation in 2017, is designed to cover such large sky-localization areas. In this work, we present the strategies of efficiently tiling the sky to facilitate the observation of the gravitational wave error regions using ZTF. To do this we used simulations consisting of 475 binary neutron star coalescences detected using a mix of two- and three-detector networks. Our studies reveal that, using two overlapping sets of ZTF tiles and a (modified) ranked-tiling algorithm, we can cover the gravitational-wave sky-localization regions with half as many pointings as a simple contour-covering algorithm. We then incorporated the ranked-tiling strategy to study our ability to observe the counterparts. This requires optimization of observation depth and localization area coverage. Our results show that observation in r-band with ~600 seconds of integration time per pointing seems to be optimum for typical assumed brightnesses of electromagnetic counterparts, if we plan to spend equal amount of time per pointing. However, our results also reveal that we can gain by as much as 50% in detection efficiency if we linearly scale our integration time per pointing based on the tile probability.

[39]  arXiv:1708.06726 [pdf, ps, other]
Title: Looking for the weak members of the C$_{60}^+$ family in the interstellar medium
Comments: this http URL
Journal-ref: Acta Astronomica, 2017, vol 67, no 2, p. 159-169
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We demonstrate, using the high resolution spectra from the ESPADONS spectrograph, fed with the 3.6m CFH telescope, that the strength ratios of the strong--to--weak spectral features, attributed to C$_{60}^+$, are variable. We found that in the range of expected 9366~\AA\ C$_{60}^+$ feature there are two diffuse bands centered at 9362.0$\pm$0.1 and 9365.3$\pm$0.1 \AA\ with variable intensity ratio. We confidently confirm the lack of 9428~\AA\ feature which, in the laboratory spectra of C$_{60}^+$, is stronger than 9366~\AA. The weakest laboratory feature, near 9348.4~\AA\, remains below the level of detection in all spectra. The intensity ratio 9577/9365 is variable. These facts contradict to their common origin and so -- the identification of some interstellar spectral features as being carried by the cation of the "soccer ball". We also refined the rest wavelength position of the strongest diffuse band in this range: it is 9576.8$\pm$0.1~\AA.

[40]  arXiv:1708.06728 [pdf, other]
Title: Multi-TeV Energy Resolution Studies with VERITAS
Authors: Rita Wells
Comments: 8 pages, 5 figures, Contribution to the Proceedings of the 35th International Cosmic Ray Conference (ICRC 2017), Busan, South Korea
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM)

This work aims to investigate the systematic uncertainty in gamma-ray spectra arising from saturation effects from bright images reconstructed by VERITAS. The goal of the work is to improve or validate the energy resolution used for deriving gamma-ray spectra in the sub-TeV to multi-TeV energy regime with VERITAS. Saturation from multi-TeV gamma-ray events affects the image brightness used to reconstruct the energy of the primary gamma ray, and potentially biases the energy estimate. We discuss a method for investigating the energy resolution bias in these multi-TeV gamma-ray events by looking at showers with large core distances that do not saturate the cameras.

[41]  arXiv:1708.06753 [pdf, other]
Title: Bayesian delensing of CMB temperature and polarization
Comments: 20 pages, 11 figures
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO)

We develop the first algorithm able to jointly compute the maximum {\it a posteriori} estimate of the Cosmic Microwave Background (CMB) temperature and polarization fields, the gravitational potential by which they are lensed, and cosmological parameters such as the tensor-to-scalar ratio, $r$. This is an important step towards sampling from the joint posterior probability function of these quantities, which, assuming Gaussianity of the CMB fields and lensing potential, contains all available cosmological information and would yield theoretically optimal constraints. Attaining such optimal constraints will be crucial for next-generation CMB surveys like CMB-S4, where limits on $r$ could be improved by factors of a few over currently used sub-optimal quadratic estimators. The maximization procedure described here depends on a newly developed lensing algorithm, which we term \textsc{LenseFlow}, and which lenses a map by solving a system of ordinary differential equations. This description has conceptual advantages, such as allowing us to give a simple non-perturbative proof that the lensing determinant is equal to unity in the weak-lensing regime. The algorithm itself maintains this property even on pixelized maps, which is crucial for our purposes and unique to \textsc{LenseFlow} as compared to other lensing algorithms we have tested. It also has other useful properties such as that it can be trivially inverted (i.e. delensing) for the same computational cost as the forward operation, and can be used to compute lensing adjoint, Jacobian, and Hessian operators. We test and validate the maximization procedure on flat-sky simulations covering up to 600\,deg$^2$ with non-uniform noise and masking.