Supergiant fast X-ray transients (SFXTs) are high mass X-ray binaries (HMXBs) displaying X-ray outbursts that can reach peak luminosities up to 1038 erg s-1 and spend most of their lives in more ...quiescent states with luminosities as low as 1032-1033 erg s-1. During the quiescent states, less luminous flares are also frequently observed with luminosities of 1034-1035 erg s-1. The main goal of the comprehensive and uniform analysis of the SFXT Swift triggers presented in this paper is to provide tools to predict whether a transient that has no known X-ray counterpart may be an SFXT candidate. These tools can be exploited for the development of future missions exploring the variable X-ray sky through large field-of-view instruments. We examined all available data on outbursts of SFXTs that triggered the Swift/Burst Alert Telescope (BAT) collected between 2005 August 30 and 2014 December 31, in particular those for which broad-band data, including the Swift/X-ray Telescope (XRT) data, are also available. This work complements and extends our previous catalogue of SFXT flares detected by BAT from 2005 February 12 to 2013 May 31, since we now include the additional BAT triggers recorded until the end of 2014 (i.e. beyond the formal first 100 months of the Swift mission). Due to a change in the mission’s observational strategy, virtually no SFXT triggers obtained a broad-band response after 2014. We processed all BAT and XRT data uniformly by using the Swift Burst Analyser to produce spectral evolution dependent flux light curves for each outburst in the sample. The BAT data allowed us to infer useful diagnostics to set SFXT triggers apart from the general γ-ray burst population, showing that SFXTs uniquely give rise to image triggers and are simultaneously very long, faint, and ‘soft’ hard-X-ray transients. We find that the BAT data alone can discriminate very well the SFXTs from other classes of fast transients, such as anomalous X-ray pulsars and soft gamma repeaters. On the contrary, the XRT data collected around the time of the BAT triggers are shown to be decisive for distinguishing SFXTs from, for instance, accreting millisecond X-ray pulsars and jetted tidal disruption events. The XRT observations of 35 (out of 52 in total) SFXT BAT triggers show that in the soft X-ray energy band, SFXTs display a decay in flux from the peak of the outburst of at least three orders of magnitude within a day and rarely undergo large re-brightening episodes, favouring in most cases a rapid decay down to the quiescent level within three to five days (at most).
We report on follow-up observations of the gamma-ray burst GRB 060927 using the robotic ROTSE-IIIa telescope and a suite of larger aperture ground-based telescopes. An optical afterglow was detected ...20 s after the burst, the earliest rest-frame detection of optical emission from any GRB. Spectroscopy performed with the VLT about 13 hr after the trigger shows a continuum break at lambda approximately 8070 AA, produced by neutral hydrogen absorption at unk approximately 5.6. We also detect an absorption line at 8158 AA, which we interpret as Si II lambda 1260 at unk = 5.467. Hence, GRB 060927 is the second most distant GRB with a spectroscopically measured redshift. The shape of the red wing of the spectral break can be fitted by a damped Lya profile with a column density with log(N sub(H)/cm super(-2)) = 22.50 plus or minus 0.15. We discuss the implications of this work for the use of GRBs as probes of the end of the dark ages and draw three main conclusions: (1) GRB afterglows originating from unk unk unk should be relatively easy to detect from the ground, but rapid near-infrared monitoring is necessary to ensure that they are found; (2) the presence of large H I column densities in some GRB host galaxies at unk > 5 makes the use of GRBs to probe the reionization epoch via spectroscopy of the red damping wing challenging; and (3) GRBs appear crucial to locate typical star-forming galaxies at unk > 5, and therefore the type of galaxies responsible for the reionization of the universe.
Swift/BAT and MAXI/GSC broadband transient monitor Sakamoto, Takanori; Oda, Ryoma; Mihara, Tatehiro ...
Publications of the Astronomical Society of Japan,
06/2016, Letnik:
68, Številka:
SP1
Journal Article
Recenzirano
Odprti dostop
We present a newly developed broadband transient monitor using the Swift Burst Alert Telescope (BAT) and the MAXI Gas Slit Camera (GSC) data. Our broadband transient monitor keeps vigil for ...high-energy transient sources from 2 keV to 200 keV in seven energy bands by combining the BAT (15–200 keV) and the GSC (2–20 keV) data. Currently, daily and 90-minute (one orbit) averaged light curves are available for 106 high-energy transient sources. This transient monitor is available to the public through our web server, http://yoshidalab.mydns.jp/bat_gsc_trans_mon/, for wider use by the community. We discuss a daily sensitivity of our monitor and possible future improvements on our pipeline.
We present the first combined study of the recently discovered source IGR J16283-4838 with Swift, INTEGRAL, and RXTE. The source, discovered by INTEGRAL on 2005 April 7, shows a highly absorbed ...variable N sub(H) = (0.4-1.7) x 10 super(23) cm super(-2) and flat ( 6 1) spectrum in the Swift XRT and RXTE PCA data. No optical counterpart is detectable (V > 20 mag), but a possible infrared counterpart within the Swift XRT error radius is detected in the 2MASS and Spitzer GLIMPSE. The observations suggest that IGR J16283-4838 is a high-mass X-ray binary (HMXB) containing a neutron star embedded in Compton thick material. This makes IGR J16283-4838 a member of the class of highly absorbed HMXBs, discovered by INTEGRAL.
We report the Swift discovery of the nearby long, soft gamma-ray burst GRB100316D, and the subsequent unveiling of its low-redshift host galaxy and associated supernova. We derive the redshift of the ...event to be z= 0.0591 ± 0.0001 and provide accurate astrometry for the gamma-ray burst (GRB) supernova (SN). We study the extremely unusual prompt emission with time-resolved γ-ray to X-ray spectroscopy and find that the spectrum is best modelled with a thermal component in addition to a synchrotron emission component with a low peak energy. The X-ray light curve has a remarkably shallow decay out to at least 800 s. The host is a bright, blue galaxy with a highly disturbed morphology and we use Gemini-South, Very Large Telescope and Hubble Space Telescope observations to measure some of the basic host galaxy properties. We compare and contrast the X-ray emission and host galaxy of GRB100316D to a subsample of GRB-SNe. GRB100316D is unlike the majority of GRB-SNe in its X-ray evolution, but resembles rather GRB060218, and we find that these two events have remarkably similar high energy prompt emission properties. Comparison of the host galaxies of GRB-SNe demonstrates, however, that there is a great diversity in the environments in which GRB-SNe can be found. GRB100316D is an important addition to the currently sparse sample of spectroscopically confirmed GRB-SNe, from which a better understanding of long GRB progenitors and the GRB-SN connection can be gleaned. PUBLICATION ABSTRACT
We report on the temporal and spectral characteristics of the early X-ray emission from the Gamma Ray Bursts GRB050126 and GRB050219A as observed by Swift . The X-ray light-curves of these 2 bursts ...both show remarkably steep early decays ($F(t)\propto t^{-3}$), breaking to flatter slopes on timescales of a few hundred seconds. For GRB050126 the burst shows no evidence of spectral evolution in the 20–150 keV band, and the spectral index of the γ-ray and X-ray afterglows are significantly different suggesting a separate origin. By contrast the BAT spectrum of GRB050219A displays significant spectral evolution, becoming softer at later times, with Γ evolving toward the XRT photon index seen in the early X-ray afterglow phase. For both bursts, the 0.2–10 keV spectral index pre- and post-break in the X-ray decay light-curve are consistent with no spectral evolution. We suggest that the steep early decline in the X-ray decay light-curve is either the curvature tail of the prompt emission; X-ray flaring activity; or external forward shock emission from a jet with high density regions of small angular size (${>}\Gamma^{-1}$). The late slope we associate with the forward external shock.
Context. Supergiant fast X-ray transients (SFXTs) are high mass X-ray binaries (HMXBs) that are defined by their hard X-ray flaring behaviour. During these flares they reach peak luminosities of ...1036–1037 erg s-1 for a few hours (in the hard X-ray), which are much shorter timescales than those characterizing Be/X-ray binaries. Aims. We investigate the characteristics of bright flares (detections in excess of 5σ) for a sample of SFXTs and their relation to the orbital phase. Methods. We have retrieved all Swift/BAT Transient Monitor light curves and collected all detections in excess of 5σ from both daily- and orbital-averaged light curves in the time range of 2005 February 12 to 2013 May 31 (MJD 53 413–56 443). We also considered all on-board detections as recorded in the same time span and selected those in excess of 5σ and within 4 arcmin of each source in our sample. Results. We present a catalogue of over a thousand BAT flares from 11 SFXTs, down to 15–150 keV fluxes of ~6 × 10-10 erg cm-2 s-1 (daily timescale) and ~1.5 × 10-9 erg cm-2 s-1 (orbital timescale, averaging ~800 s); the great majority of these flares are unpublished. The catalogue spans 100 months. This population is characterized by short (a few hundred seconds) and relatively bright (in excess of 100 mCrab, 15–50 keV) events. In the hard X-ray, these flares last generally much less than a day. Clustering of hard X-ray flares can be used to indirectly measure the length of an outburst, even when the low-level emission is not detected. We construct the distributions of flares, of their significance (in terms of σ), and of their flux as a function of orbital phase to infer the properties of these binary systems. In particular, we observe a trend of clustering of flares at some phases as Porb increases, which is consistent with a progression from tight circular or mildly eccentric orbits at short periods to wider and more eccentric orbits at longer orbital periods. Finally, we estimate the expected number of flares for a given source for our limiting flux and provide the recipe for calculating them for the limiting flux of future hard X-ray observatories.
Context. Swift discovered the high redshift ( z =6.29) GRB 050904 with the Burst Alert Telescope (BAT) and began observing with its narrow field instruments 161 s after the burst onset. This ...gamma-ray burst is the most distant cosmic explosion ever observed. Because of its high redshift, the X-ray Telescope (XRT) and BAT simultaneous observations provide 4 orders of magnitude of spectral coverage (0.2-150 keV; 1.4-1090 keV in the source rest frame) at a very early source-frame time (22 s). The X-ray emission was monitored by the XRT up to 10 days after the burst. Aims. We present the analysis of BAT and XRT observations of GRB 050904 and a complete description of its high energy phenomenology. Methods. We performed time resolved spectral analysis and light curve modeling. Results. GRB 050904 was a long, multi-peaked, bright GRB with strong variability during its entire evolution. The light curve observed by the XRT is characterized by the presence of a long flaring activity lasting up to 1-2 h after the burst onset in the burst rest frame, with no evidence of a smooth power-law decay following the prompt emission as seen in other GRBs. However, the BAT tail extrapolated to the XRT band joins the XRT early light curve and the overall behavior resembles that of a very long GRB prompt. The spectral energy distribution softens with time, with the photon index decreasing from-1.2 during the BAT observation to-1.9 at the end of the XRT observation. The dips of the late X-ray flares may be consistent with an underlying X-ray emission arising from the forward shock and with the properties of the optical afterglow reported by Tagliaferri et al. (2005b, A&A, 443, L1). Conclusions. We interpret the BAT and XRT data as a single continuous observation of the prompt emission from a very long GRB. The peculiarities observed in GRB 050904 could be due to its origin within one of the first star-forming regions in the Universe; very low metallicities of the progenitor at these epochs may provide an explanation.
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