We present the fourth in a series of catalogs of gamma-ray bursts (GRBs) observed with Fermi's Gamma-ray Burst Monitor (Fermi-GBM). It extends the six year catalog by four more years, now covering ...the 10 year time period from trigger enabling on 2008 July 12 to 2018 July 11. During this time period GBM triggered almost twice a day on transient events, 2356 of which we identified as cosmic GRBs. Additional trigger events were due to solar flare events, magnetar burst activities, and terrestrial gamma-ray flashes. The intention of the GBM GRB catalog series is to provide updated information to the community on the most important observables of the GBM-detected GRBs. For each GRB the location and main characteristics of the prompt emission, the duration, peak flux, and fluence are derived. The latter two quantities are calculated for the 50-300 keV energy band, where the maximum energy release of GRBs in the instrument reference system is observed and also for a broader energy band from 10-1000 keV, exploiting the full energy range of GBM's low-energy detectors. Furthermore, information is given on the settings of the triggering criteria and exceptional operational conditions during years 7 to 10 in the mission. This fourth catalog is an official product of the Fermi-GBM science team, and the data files containing the complete results are available from the High-Energy Astrophysics Science Archive Research Center.
Abstract
We present the systematic spectral analyses of gamma-ray bursts (GRBs) detected by the Fermi Gamma-Ray Burst Monitor during its first ten years of operation. This catalog contains two types ...of spectra: time-integrated spectral fits and spectral fits at the brightest time bin, from 2297 GRBs, resulting in a compendium of over 18,000 spectra. The four different spectral models used for fitting the spectra were selected based on their empirical importance to the shape of many GRBs. We describe in detail our procedure and criteria for the analyses, and present the bulk results in the form of parameter distributions both in the observer frame and in the GRB rest frame. 941 GRBs from the first four years have been refitted using the same methodology as that of the 1356 GRBs in years five through ten. The data files containing the complete results are available from the High-Energy Astrophysics Science Archive Research Center.
We review more than 10 yr of continuous monitoring of accreting X-ray pulsars with the all-sky Gamma-ray Burst Monitor (GBM) aboard the Fermi Gamma-ray Space Telescope. Our work includes data from ...the start of GBM operations in 2008 August, through to 2019 November. Pulsations from 39 accreting pulsars are observed over an energy range of 10-50 keV by GBM. The GBM Accreting Pulsars Program performs data reduction and analysis for each accreting pulsar and makes histories of the pulse frequency and pulsed flux publicly available. We examine in detail the spin histories, outbursts, and torque behaviors of the persistent and transient X-ray pulsars observed by GBM. The spin period evolution of each source is analyzed in the context of disk-accretion and quasi-spherical settling accretion-driven torque models. Long-term pulse frequency histories are also analyzed over the GBM mission lifetime and compared to those available from the previous Burst and Transient Source Experiment all-sky monitoring mission, revealing previously unnoticed episodes in some of the analyzed sources (such as a torque reversal in 2S 1845−024). We obtain new, or update known, orbital solutions for three sources. Our results demonstrate the capabilities of GBM as an excellent instrument for monitoring accreting X-ray pulsars and its important scientific contribution to this field.
ABSTRACT
In 2019 November, MAXI detected an X-ray outburst from the known Be X-ray binary system RX J0209.6−7427 located in the outer wing of the Small Magellanic Cloud. We followed the outburst of ...the system with NICER, which led to the discovery of X-ray pulsations with a period of 9.3 s. We analysed simultaneous X-ray data obtained with NuSTAR and NICER, allowing us to characterize the spectrum and provide an accurate estimate of its bolometric luminosity. During the outburst, the maximum broad-band X-ray luminosity of the system reached (1–2) × 1039 erg s−1, thus exceeding by about one order of magnitude the Eddington limit for a typical 1.4 M⊙ mass neutron star (NS). Monitoring observations with Fermi/GBM and NICER allowed us to study the spin evolution of the NS and compare it with standard accretion torque models. We found that the NS magnetic field should be of the order of 3 × 1012 G. We conclude that RX J0209.6−7427 exhibited one of the brightest outbursts observed from a Be X-ray binary pulsar in the Magellanic Clouds, reaching similar luminosity level to the 2016 outburst of SMC X-3. Despite the super-Eddington luminosity of RX J0209.6−7427, the NS appears to have only a moderate magnetic field strength.
Context. Be/X-ray binaries show outbursts with peak luminosities up to a few times 1037 erg/s, during which they can be observed and studied in detail. Most (if not all) Be/X-ray binaries harbor ...accreting pulsars, whose X-ray spectra in many cases contain cyclotron resonant scattering features related to the magnetic field of the sources. Spectral variations as a function of luminosity and of the rotational phase of the neutron star are observed in many accreting pulsars. Aims. We explore X-ray spectral and timing properties of the Be/X-ray binary GX 304-1 during an outburst episode. Specifically, we investigate the behavior of the cyclotron resonant scattering feature, the continuum spectral parameters, the pulse period, and the energy- and luminosity-resolved pulse profiles. Methods. We analyze the INTEGRAL data from the two JEM-X modules, ISGRI and SPI, covering the 2012 January−February outburst, divided into six observations. We obtain pulse profiles in two energy bands, phase-averaged and phase-resolved spectra for each observation. We combine the luminosity-resolved spectral and timing analysis to probe the accretion geometry and the beaming patterns of the rotating neutron star. Results. We confirm the positive luminosity dependence of the cyclotron line energy in GX 304-1 and report a dependence of the photon index on luminosity. Using a pulse-phase connection technique, we find a pulse period solution valid for the entire outburst. Our pulse-phase resolved analysis shows that the centroid energy of the cyclotron line varies only slightly with pulse phase, while other spectral parameters show more pronounced variations. Our results are consistent with a scenario in which, as the pulsar rotates, we are exploring only a small portion of its beam pattern.
Abstract
We report on the hard X-ray burst and the first ∼100 days of NICER monitoring of the soft X-ray temporal and spectral evolution of the newly discovered magnetar Swift J1818.0−1607. The burst ...properties are typical of magnetars with a duration of
T
90
= 10 ± 4 ms and a temperature of
kT
= 8.4 ± 0.7 keV. The 2–8 keV pulse shows a broad, single-peak profile with a pulse fraction increasing with time from 30% to 43%. The NICER observations reveal strong timing noise with
varying erratically by a factor of 10, with an average long-term spin-down rate of
s
−2
, implying an equatorial surface magnetic field of 2.5 × 10
14
G and a young characteristic age of ∼470 yr. We detect a large spin-up glitch at MJD 58928.56 followed by a candidate spin-down glitch at MJD 58934.81, with no accompanying flux enhancements. The persistent soft X-ray spectrum of Swift J1818.0−1607 can be modeled as an absorbed blackbody with a temperature of ∼1 keV. Its flux decayed by ∼60% while the modeled emitting area decreased by ∼30% over the NICER observing campaign. This decrease, coupled with the increase in the pulse fraction, points to a shrinking hot spot on the neutron star surface. Assuming a distance of 6.5 kpc, we measure a peak X-ray luminosity of 1.9 × 10
35
erg s
−1
, lower than its spin-down luminosity of 7.2 × 10
35
erg s
−1
. Its quiescent thermal luminosity is ≲1.7 × 10
34
erg s
−1
, lower than those of canonical young magnetars. We conclude that Swift J1818.0−1607 is an important link between regular magnetars and high-magnetic-field, rotation-powered pulsars.
We report on the first simultaneous Neutron Star Interior Composition Explore (NICER) and Nuclear Spectroscopic Telescope Array (NuSTAR) observations of the neutron star (NS) low-mass X-ray binary 4U ...1735−44, obtained in 2018 August. The source was at a luminosity of ∼1.8 (D/5.6 kpc)2 × 1037 erg s−1 in the 0.4-30 keV band. We account for the continuum emission with two different continuum descriptions that have been used to model the source previously. Despite the choice in continuum model, the combined passband reveals a broad Fe K line indicative of reflection in the spectrum. In order to account for the reflection spectrum we utilize a modified version of the reflection model relxill that is tailored for thermal emission from accreting NSs. Alternatively, we also use the reflection convolution model of rfxconv to model the reflected emission that would arise from a Comptonized thermal component for comparison. We determine that the innermost region of the accretion disk extends close to the innermost stable circular orbit (RISCO) at the 90% confidence level regardless of reflection model. Moreover, the current flux calibration of NICER is within 5% of the NuSTAR/FPMA(B).
The capability of the Fermi Gamma-ray Burst Monitor (GBM) to localize gamma-ray bursts (GRBs) is evaluated for two different automated algorithms: the GBM Team's RoboBA algorithm and the ...independently developed BALROG algorithm. Through a systematic study utilizing over 500 GRBs with known locations from instruments like Swift and the Fermi Large Area Telescope, we directly compare the effectiveness of, and accurately estimate the systematic uncertainty for, both algorithms. We show that simple adjustments to the GBM Team's RoboBA, in operation since early 2016, yield significant improvement in the systematic uncertainty, removing the long tail identified in the systematic, and improve the overall accuracy. The systematic uncertainty for the updated RoboBA localizations is 1 8 for 52% of GRBs and 4 1 for the remaining 48%. Both from public reporting by BALROG and our systematic study, we find the systematic uncertainty of 1°-2° quoted in circulars for bright GRBs is an underestimate of the true magnitude of the systematic, which we find to be 2 7 for 74% of GRBs and 33° for the remaining 26%. We show that, once the systematic uncertainty is considered, the RoboBA 90% localization confidence regions can be more than an order of magnitude smaller in area than those produced by BALROG.
ABSTRACT
In this study, we present a method to estimate posterior distributions for standard accretion torque model parameters and binary orbital parameters for X-ray binaries using a nested sampling ...algorithm for Bayesian parameter estimation. We study the spin evolution of two Be X-ray binary systems in the Magellanic Clouds, RX J0520.5−6932 and RX J0209−7427, during major outbursts, in which they surpassed the Eddington limit. Moreover, we apply our method to the recently discovered Swift J0243.6+6124, the only known Galactic pulsating ultra-luminous X-ray source. This is an excellent candidate for studying the disc evolution at super-Eddington accretion rates, because its luminosity spans several orders of magnitude during its outburst, with a maximum LX that exceeded the Eddington limit by a factor of ∼10. Our method, when applied to RX J0520.5−6932 and RX J0209−7427, is able to identify the more favourable torque model for each system, while yielding meaningful ranges for the NS and orbital parameters. Our analysis for Swift J0243.6+6124 illustrates that, contrary to the standard torque model predictions, the magnetospheric radius (Rm) and the Alfvén radius (RA) are not proportional to each other when surpassing the Eddington limit. Reported distance estimates of this source range between 5 and 7 kpc. Smaller distances require non-typical neutron star properties (i.e. mass and radius) and possibly lower radiative efficiency of the accretion column.
ABSTRACT
PSR J1813–1749 is one of the most energetic rotation-powered pulsars known, producing a pulsar wind nebula (PWN) and gamma-ray and TeV emission, but whose spin period is only measurable in ...X-ray. We present analysis of two Chandra data sets that are separated by more than 10 yr and recent NICER data. The long baseline of the Chandra data allows us to derive a pulsar proper motion $\mu _{\rm RA}=(-0.067\pm 0.010)\, \mathrm{ arcsec}\,\mathrm{yr^{-1}}$ and $\mu _{\rm Dec.}=(-0.014\pm 0.007)\, \mathrm{ arcsec}\,\mathrm{yr^{-1}}$ and velocity $v_\perp \approx 900\!-\!1600\, \mathrm{km\, s^{-1}}$ (assuming a distance d = 3–5 kpc), although we cannot exclude a contribution to the change in measured pulsar position due to a change in brightness structure of the PWN very near the pulsar. We model the PWN and pulsar spectra using an absorbed power law and obtain best-fitting absorption $N_{\rm H}=(13.1\pm 0.9)\times 10^{22}\, \mathrm{cm^{-2}}$, photon index Γ = 1.5 ± 0.1, and 0.3–10 keV luminosity $L_{\rm X}\approx 5.4\times 10^{34}\, \mathrm{erg\, s^{-1}}(d/\mbox{ 5 kpc})^2$ for the PWN and Γ = 1.2 ± 0.1 and $L_{\rm X}\approx 9.3\times 10^{33}\, \mathrm{erg\, s^{-1}}(d/\mbox{ 5 kpc})^2$ for PSR J1813–1749. These values do not change between the 2006 and 2016 observations. We use NICER observations from 2019 to obtain a timing model of PSR J1813–1749, with spin frequency ν = 22.35 Hz and spin frequency time derivative $\dot{\nu }=(-6.428\pm 0.003)\times 10^{-11}\, \mathrm{Hz\, s^{-1}}$. We also fit ν measurements from 2009 to 2012 and our 2019 value and find a long-term spin-down rate $\dot{\nu }=(-6.3445\pm 0.0004)\times 10^{-11}\, \mathrm{Hz\, s^{-1}}$. We speculate that the difference in spin-down rates is due to glitch activity or emission mode switching.
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