Abstracts of Interest
Selected by:
Gavin Rowell
Abstract: 1806.03167
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title: A spatial likelihood analysis for MAGIC telescope data
(Submitted on 8 Jun 2018)
Abstract: Context. The increase in sensitivity of Imaging Atmospheric Cherenkov Telescopes (IACTs) has lead to numerous detections of extended $\gamma$-ray sources at TeV energies, sometimes of sizes comparable to the instrument's field of view (FoV). This creates a demand for advanced and flexible data analysis methods, able to extract source information by utilising the photon counts in the entire FoV.
Aims. We present a new software package, "SkyPrism", aimed at performing 2D (3D if energy is considered) fits of IACT data, possibly containing multiple and extended sources, based on sky images binned in energy. Though the development of this package was focused on the analysis of data collected with the MAGIC telescopes, it can further be adapted to other instruments, such as the future Cherenkov Telescope Array (CTA).
Methods. We have developed a set of tools that, apart from sky images (count maps), compute the instrument response functions (IRFs) of MAGIC (effective exposure throughout the FoV, point spread function (PSF), energy resolution and background shape), based on the input data, Monte-Carlo simulations and the pointing track of the telescopes. With this information, the presented package can perform a simultaneous maximum likelihood fit of source models of arbitrary morphology to the sky images providing energy spectra, detection significances, and upper limits.
Results. We demonstrate that the SkyPrism tool accurately reconstructs the MAGIC PSF, on and off-axis performance as well as the underlying background. We further show that for a point source analysis with MAGIC's default observational settings, SkyPrism gives results compatible with those of the standard tools while being more flexible and widely applicable.
Abstract: 1806.02833
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title: Testing the equation of state of neutron stars with electromagnetic observations
(Submitted on 7 Jun 2018)
Abstract: Neutron stars are the densest, directly observable stellar objects in the universe and serve as unique astrophysical laboratories to study the behavior of matter under extreme physical conditions. This book chapter is devoted to describing how electromagnetic observations, particularly at X-ray, optical and radio wavelengths, can be used to measure the mass and radius of neutron stars and how this leads to constraints on the equation of state of ultra-dense matter. Having accurate theoretical models to describe the astrophysical data is essential in this effort. We will review different methods to constrain neutron star masses and radii, discuss the main observational results and theoretical developments achieved over the past decade, and provide an outlook of how further progress can be made with new and upcoming ground-based and space-based observatories.
Abstract: 1806.02806
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title: Mapping the Interstellar Magnetic Field Around the Heliosphere with Polarized Starlight
(Submitted on 7 Jun 2018)
Abstract: Starlight that becomes linearly polarized by magnetically aligned dust grains provides a viable diagnostic of the interstellar magnetic field (ISMF). A survey is underway to map the local ISMF using data collected at eight observatories in both hemispheres. Two approaches are used to obtain the magnetic structure: statistically evaluating magnetic field directions traced by multiple polarization position angles, and least-squares fits that provide the dipole component of the magnetic field. We find that the magnetic field in the circumheliospheric interstellar medium (CHM), which drives winds of interstellar gas and dust through the heliosphere, drapes over the heliopause and influences polarization measurements. We discover a polarization band that can be described with a great circle that traverses the heliosphere nose and ecliptic poles. A gap in the band appears in a region coinciding both with the highest heliosheath pressure, found by IBEX, and the center of the Loop I superbubble. The least-squares analysis finds a magnetic dipole component of the polarization band with the axis oriented toward the ecliptic poles. The filament of dust around the heliosphere and the warm helium breeze flowing through the heliosphere trace the same magnetic field directions. Regions along the polarization band near the heliosphere nose have magnetic field orientations within 15 degrees of sightlines. Regions in the IBEX ribbon have field directions within 40 degrees of the plane of the sky. Several spatially coherent magnetic filaments are within 15 pc. Most of the low frequency radio emissions detected by the two Voyager spacecraft follow the polarization band. The geometry of the polarization band is compared to the Local Interstellar Cloud, the Cetus Ripple, the BICEP2 low opacity region, Ice Cube IC59 galactic cosmic ray data, and Cassini results.
Abstract: 1806.02040
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title: Minimum Spanning Tree cluster analysis of the LMC region above 10 GeV: detection of the SNRs N 49B and N 63A
(Submitted on 6 Jun 2018)
Abstract: We present the results of a cluster search in the gamma-ray sky images of the Large Magellanic Cloud region by means of the Minimum Spanning Tree algorithm at energies higher than 10 GeV, using 9 years of Fermi-LAT data. Several significant clusters were found, the majority of which associated with previously known gamma-ray sources. New significant clusters associated with the supernova remnants N 49B and N 63A are also found, and confirmed with a Maximum Likelihood analysis of the Fermi-LAT data.
Abstract: 1806.01893
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title: Expanding CO Shells in the Orion A Molecular Cloud
(Submitted on 5 Jun 2018)
Abstract: We present the discovery of expanding spherical shells around low to intermediate-mass young stars in the Orion A giant molecular cloud using observations of $^{12}$CO (1-0) and $^{13}$CO (1-0) from the Nobeyama Radio Observatory 45-meter telescope. The shells have radii from 0.05 to 0.85 pc and expand outward at 0.8 to 5 km/s. The total energy in the expanding shells is comparable to protostellar outflows in the region. Together, shells and outflows inject enough energy and momentum to maintain the cloud turbulence. The mass-loss rates required to power the observed shells are two to three orders of magnitude higher than predicted for line-driven stellar winds from intermediate-mass stars. This discrepancy may be resolved by invoking accretion-driven wind variability. We describe in detail several shells in this paper and present the full sample in the online journal.
Abstract: 1806.01839
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title: Observing small-scale gamma-ray anisotropies with the Cherenkov Telescope Array
(Submitted on 5 Jun 2018)
Abstract: Disentangling the composition of the diffuse gamma-ray background (DGRB) is a major challenge in gamma-ray astronomy. It is presumed that at the highest energies, the DGRB is dominated by relatively few, still unresolved point sources. This conjecture has recently been supported by the measurement of small-scale anisotropies in the DGRB by the Fermi Large Area Telescope (LAT) up to energies of 500 GeV. We show how such anisotropies can be searched for with the forthcoming Earth-bound Cherenkov Telescope Array (CTA) up to the TeV range. We investigate different observation modes to analyse CTA data for small-scale anisotropies and propose the projected extragalactic large-area sky survey as the most promising data set. Relying on an up-to-date model of the performance of the southern CTA, we find that CTA will be able to probe anisotropies in the DGRB from unresolved point sources at a relative amplitude of $C_{\rm P}^I/I^2_{\rm DGRB}\gtrsim 4\times 10^{-3}\,{\rm sr}$ at energies above 30 GeV and angular scales $\lesssim 1.5^{\circ}$. Such DGRB anisotropies have not yet been ruled out by the Fermi-LAT. The proposed analysis would primarily clarify the contribution from blazars and misaligned active galactic nuclei to the very-high-energy regime of the DGRB, as well as provide insight on dark matter annihilation in Galactic and extragalactic density structures. Finally, it constitutes a measurement with complementary systematic uncertainties compared to the Fermi-LAT.
Abstract: 1806.01624
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title: Binary Neutron Star Merger Remnants as Sources of Cosmic Rays Below the "Ankle"
(Submitted on 5 Jun 2018)
Abstract: We investigate non-thermal electron and nuclei energy losses within the binary neutron star merger remnant produced by the event GW170817. The lack of a cooling feature within the detected synchrotron emission from the source is used to constrain the magnetic field at the mG level, assuming that this emission is electron synchrotron in origin, and that the accelerated spectrum in the electrons follows the form $dN/dE_e \propto E_e^{-2}$. The level of subsequent gamma-ray emission from the source is demonstrated to provide a further constraint on the source magnetic field strength. We also put forward alternative strong ($\sim$10~G) magnetic field scenarios able to support this emission. For such stronger fields, the photodisintegration of non-thermal nuclei within the source is considered, and a bottleneck period of $\sim$5-30 days is found when this process peaks. We find that this class of source is in principle able to support the population of cosmic rays detected at Earth below the "ankle".
Abstract: 1806.01559
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title: Gamma-ray flaring activity of NGC 1275 in 2016-2017 measured by MAGIC
(Submitted on 5 Jun 2018)
Abstract: We report on the detection of flaring activity from the Fanaroff-Riley I radio galaxy NGC 1275 in very-high-energy (VHE, E $>$ 100 GeV) gamma rays with the MAGIC telescopes. Observations were performed between 2016 September and 2017 February as part of a monitoring program. The brightest outburst with $\sim1.5$ times the Crab Nebula flux above 100 GeV (C.U.) was observed during the night between 2016 December 31 and 2017 January 1 (fifty times higher than the mean previously measured in two observational campaigns between 2009 and 2011). Significant variability of the day-by-day light curve was measured, the shortest flux-doubling time-scales was found to be of $(611\pm101)$ min. The combined spectrum of the MAGIC data during the strongest flare state and simultaneous data from the Fermi-LAT around 2017 January 1 follows a power-law with an exponential cutoff at the energy $(492\pm35)$ GeV. Simultaneous optical flux density measurements in the R-band obtained with the KVA telescope are also presented and the correlation between the optical and gamma-ray emission is investigated. Due to possible internal pair-production, the fast flux variability constrains the Doppler factor to values which are inconsistent with a large viewing angle as observed in the radio band. We investigate different scenarios for the explanation of fast gamma-ray variability, namely emission from: magnetospheric gaps, relativistic blobs propagating in the jet (mini-jets) or external cloud (or star) entering the jet. We find that the only plausible model to account for the luminosities here observed would be the production of gamma rays in a magnetospheric gap around the central black hole only in the eventuality of an enhancement of the magnetic field threading the hole from its equipartition value with the gas pressure in the accretion flow.
Abstract: 1806.01542
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title: Towards Robotic Operation with the First G-APD Cherenkov Telescope
(Submitted on 5 Jun 2018)
Abstract: The First G-APD Cherenkov Telecope is an Imaging Air Cherenkov Tele- scope operating since 2011 at the Observatorio del Roque de los Muchachos. One of the major goals of the FACT collaboration is to achieve robotic operation of the telescope. Since 2011 FACT is operated remotely. To reduce the necessity of human interven- tion, several programs were developed, most notably the shifthelper together with the pycustos library. This software monitors the telescope system and environmental conditions and calls the shifters in case human intervention is required. This will lead to FACT being the first IACT with all shifters asleep during regular observations. The software presented here is open source and under MIT License.
Abstract: 1806.01300
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title: Optically thick HI does not dominate dark gas in the local ISM
(Submitted on 4 Jun 2018)
Abstract: The local interstellar medium (ISM) is suffused with "dark" gas, identified by excess infrared and gamma ray emission, yet undetected by standard ISM tracers such as neutral hydrogen (HI) or carbon monoxide emission. Based on observed dust properties from Planck, recent studies have argued that HI mixed with dust is strongly saturated and that dark gas is dominated by optically-thick HI. We test this hypothesis by reproducing this model using data from Planck and new 21 cm emission maps from GALFA-HI -- the first large-area 21cm emission survey with comparable angular resolution to Planck. We compare the results with those from a large sample of HI column densities based on direct observations of HI optical depth, and find that the inferred column density corrections are significantly lower than those inferred by the Planck-based model. Further, we rule out the hypothesis that the pencil-beam HI absorption sight lines preferentially miss opaque "blobs" with small covering fraction, as these structures require densities and pressures which are incompatible with ISM conditions. Our results support the picture that dark gas in the local ISM is not dominated by optically-thick HI, but is rather a combination of intrinsic changes in dust grain emissivities and H3 missed by CO observations.
Abstract: 1806.01231
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title: Diffuse X-ray emission in the Cygnus OB2 association
(Submitted on 4 Jun 2018)
Abstract: We present a large-scale study of diffuse X-ray emission in the nearby massive stellar association Cygnus OB2 as part of the Chandra Cygnus OB2 Legacy Program. We used 40 Chandra X-ray ACIS-I observations covering $\sim$1.0 deg$^2$. After removing 7924 point-like sources detected in our survey, background-corrected X-ray emission, the adaptive smoothing reveals large-scale diffuse X-ray emission. Diffuse emission was detected in the sub-bands Soft [0.5 : 1.2] and Medium [1.2 : 2.5], and marginally in the Hard [2.5 : 7.0] keV band. From X-ray spectral analysis of stacked spectra we compute a total [0.5 : 7.0 keV] diffuse X-ray luminosity of L$_{\rm x}^{\rm diff}\approx$4.2$\times$10$^{\rm 34}$ erg s$^{-1}$, characterized with plasma temperature components at kT$\approx$ 0.11, 0.40 and 1.18 keV, respectively. The HI absorption column density corresponding to these temperatures has a distribution consistent with N$_{\rm H}$ = 0.43, 0.80 and 1.39 $\times$10$^{22}$ cm$^{-2}$. The extended medium band energy emission likely arises from O-type stellar winds thermalized by wind-wind collisions in the most populated regions of the association, while the soft band emission probably arises from less energetic termination shocks against the surrounding Interstellar-Medium. Super-soft and Soft diffuse emission appears more widely dispersed and intense than the medium band emission. The diffuse X-ray emission is generally spatially coincident with low-extinction regions that we attribute to the ubiquitous influence of powerful stellar winds from massive stars and their interaction with the local Interstellar-Medium. Diffuse X-ray emission is volume-filling, rather than edge-brightened, oppositely to other star-forming regions. We reveal the first observational evidence of X-ray haloes around some evolved massive stars.
Abstract: 1806.00733
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title: Search for line signal candidates in the Fermi-LAT data
(Submitted on 3 Jun 2018)
Abstract: In this work, we aim to search for the line-like signals in the Fermi-LAT data. We have searched over 49000 regions of interest (ROIs) that cover the whole sky. No ROI displays a line signal with a test statistic (TS) value above 25, while for 50 ROIs weak line-like excesses with ${\rm TS}>16$ are present. These tentative line signals are most likely due to the statistical fluctuation, though the dark matter annihilation origin of a few of them can not be ruled out. We also use the number of the line signal candidates to constrain the cross section of dark matter annihilating to gamma-ray lines, $\left<\sigma{v}\right>_{\gamma\gamma}$.
Abstract: 1806.00703
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title: Novel Silicon Photomultipliers suitable for Dual-Mirror Small-Sized Telescopes of the Cherenkov Telescope Array
(Submitted on 2 Jun 2018)
Abstract: Many of the characteristics of Silicon Photomultipliers (SiPMs), such as high Photon Detection Efficiency (PDE), are well matched to the requirements of the cameras of the Small Sized Telescopes (SSTs) proposed for the Cherenkov Telescope Array in fact the double mirror Schwarzschild Couder configuration provides a much better Point Spread Function over large field of view compared to a single mirror. It allows better correction of aberrations at large off axis angles and facilitates the construction of compact telescopes. Moreover, the small plate scale of the dual mirror SSTs allows the use of SiPM detectors despite their small pixel sizes. However, one area in which SiPM performance has required improvement is Optical Cross Talk (OCT), where multiple avalanches are induced by a single impinging photon. OCT, coupled with the typical night sky background rate of 25 MCnts/s per pixel during Cherenkov observations, can place severe constraints on the triggering capability of the cameras. An OCT probability lower than 10 % meets the SST requirement. This paper describes the performance of novel Low Voltage Reverse (LVR) 2nd and 3rd generation Multi Pixel Photon Counters manufactured by Hamamatsu Photonics. These are designed to have both enhanced PDE and reduced OCT. Two 7 x 7 mm2 S14520 LVR2 MPPCs with 75 {\mu}m microcells are tested and compared with detectors of the same pixel size with 50 {\mu}m microcells. A comparative analysis of a 3 x 3 mm2 S14520 LVR2 and a S14520 LVR3 devices is also carried out, demonstrating that the LVR3 gives better photon detection in the 240 - 380 nm wavelength range. Finally, the effect on the OCT of an infrared filter is analysed.
Abstract: 1806.00614
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title: Possible Association of Two Stellar Bowshocks with Unidentified Fermi Sources
(Submitted on 2 Jun 2018)
Abstract: The bowshocks of runaway stars had been theoretically proposed as gamma-ray sources. However, this hypothesis has not been confirmed by observations up to date. In this paper, we present two runaway stars (Lambda Cephei and LS 2355) whose bowshocks are coincident with the unidentified Fermi gamma-ray sources 3FLG J2210.1+5925 and 3FGL J1128.7-6232, respectively. After performing a cross-correlation between different catalogues at distinct wavelengths, we found that these bowshocks are the most peculiar objects in the Fermi position ellipses. Then, we computed the inverse Compton emission and fitted the Fermi data in order to test the viability of both runaway stars as potential counterparts of the two high-energy sources. We obtained very reasonable values for the fitted parameters of both stars. We also evaluated the possibility for the source 3FGL J1128.7-6232, which is positionally coincident with an HII region, to be the result of background cosmic-rays protons interacting with the matter of the cloud, as well as the probability of a pure chance association. We conclude that the gamma rays from these Fermi sources might be produced in the bowshocks of the considered runaway stars. In such a case, these would be the first sources of this class ever detected at gamma rays.
This page created: Tue Jun 12 10:11:32 ACST 2018 by growell
For a printable title listing click here
For details on generating this page see the instructions. If there are problems with this page (and I expect there will be from time to time) contact Jose.
For previous lists of abstracts of interest click Previous abstracts of interest