Evidence for chromatic X-ray light-curve breaks in Swift gamma-ray burst afterglows and their theoretical implications

The power-law decay of the X-ray emission of gamma-ray burst (GRB) afterglows 050319, 050401, 050607, 050713A, 050802 and 050922C exhibits a steepening at about 1–4 h after the burst which, surprisingly, is not accompanied by a break in the optical emission. If it is assumed that both the optical and X-ray afterglows arise from the same outflow then, in the framework of the standard forward shock model, the chromaticity of the X-ray light-curve breaks indicates that they do not arise solely from a mechanism related to the outflow dynamics (e.g. energy injection) or the angular distribution of the blast-wave kinetic energy (structured outflows or jets). The lack of a spectral evolution accompanying the X-ray light-curve break shows that these breaks do not arise from the passage of a spectral break (e.g. the cooling frequency) either. Under these circumstances, the decoupling of the X-ray and optical decays requires that the microphysical parameters for the electron and magnetic energies in the forward shock evolve in time, whether the X-ray afterglow is synchrotron or inverse-Compton emission. For a steady evolution of these parameters with the Lorentz factor of the forward shock and an X-ray light curve arising cessation of energy injection into the blast wave, the optical and X-ray properties of the above six Swift afterglows require a circumburst medium with a r−2 radial stratification, as expected for a massive star origin for long GRBs. Alternatively, the chromatic X-ray light-curve breaks may indicate that the optical and X-ray emissions arise from different outflows. Neither feature (evolution of microphysical parameters or the different origin of the optical and X-ray emissions) was clearly required by pre-Swift afterglows

Swift/X-ray Telescope monitoring of the candidate supergiant fast X-ray transient IGR J16418-4532

We report on the Swift monitoring of the candidate supergiant fast X-ray transient (SFXT) IGR J16418−4532, for which both orbital and spin periods are known (∼3.7 d and ∼1250 s, respectively). Our observations, for a total of ∼43 ks, span over three orbital periods and represent the most intense and complete sampling of the light curve of this source with a sensitive X-ray instrument. With this unique set of observations, we can address the nature of this transient. By applying the clumpy wind model for blue supergiants to the observed X-ray light curve, and assuming a circular orbit, the X-ray emission from this source can be explained in terms of the accretion from a spherically symmetric clumpy wind, composed of clumps with different masses, ranging from ∼5 × 10[superscript: 16] to 10[superscript: 21] g. Our data suggest, based on the X-ray behaviour, that this is an intermediate SFXT

Confirmation of the supergiant fast X-ray transient nature of AX J1841.0-0536 from Swift outburst observations

Swift observed an outburst from the supergiant fast X-ray transient (SFXT) AX J1841.0−0536 on 2010 June 5, and followed it with X-ray Telescope (XRT) for 11 d. The X-ray light curve shows an initial flare followed by a decay and subsequent increase, as often seen in other SFXTs, and a dynamical range of ∼1600. Our observations allow us to analyse the simultaneous broad-band (0.3–100 keV) spectrum of this source, for the first time down to 0.3 keV, which can be fitted well with models usually adopted to describe the emission from accreting neutron stars in high-mass X-ray binaries, and is characterized by a high absorption (NH∼ 2 × 1022 cm−2), a flat power law (Γ∼ 0.2) and a high-energy cut-off. All of these properties resemble those of the prototype of the class, IGR J17544−2619, which underwent an outburst on 2010 March 4, whose observations we also discuss. We show how well AX J1841.0−0536 fits in the SFXT class, based on its observed properties during the 2010 outburst, its large dynamical range in X-ray luminosity, the similarity of the light curve (length and shape) to those of the other SFXTs observed by Swift and the X-ray broad-band spectral properties

GRB 060607A: A gamma-ray burst with bright asynchronous early X-ray and optical emissions

The early optical emission of the moderately high redshift (2 = 3.08) GRB 060607A shows a remarkable broad and strong peak with a rapid rise and a relatively slow power-law decay. It is not coincident with the strong early-time flares seen in the X-ray and gamma-ray energy bands. There is weak evidence for variability superposed on this dominant component in several optical bands that can be related to flares in high-energy bands. While for a small number of gamma-ray bursts (GRBs), well-sampled optical flares have been observed simultaneously with X-ray and gamma-ray pulses, GRB 060607A is one of the few cases where the early optical emission shows no significant evidence for correlation with the prompt emission. In this work we first report in detail the broad-band observations of this burst by Swift. Then by applying a simple model for the dynamics and the synchrotron radiation of a relativistic shock, we show that the dominant component of the early emissions in optical wavelengths has the same origin as the tail emission produced after the main gamma-ray activity. The most plausible explanation for the peak in the optical light curve seems to be the cooling of the prompt after the main collisions, shifting the characteristic synchrotron frequency to the optical bands. The fact that the early emission in X-ray does not show a steep decay, like what is observed in many other GRBs, is further evidence for slow cooling of the prompt shell within this GRB. It seems that the cooling process requires a steepening of the electron energy distribution and/or a break in this distribution at high energies. From simultaneous gamma-ray emission during the first flare, the behaviour of hardness ratio, and the lack of spectral features, we conclude that the X-ray flares are due to the collision of late shells rather than late reprocessing of the central engine activities. The sharp break in the X-ray light curve at few thousands of seconds after the trigger, is not observed in the infrared/optical/ultraviolet bands, and therefore cannot be a jet break. Either the X-ray break is due to a change in the spectrum of the accelerated electrons or the lack of an optical break is due to the presence of a related delayed response component

The nature of the outflow in gamma-ray bursts

The Swift satellite has enabled us to follow the evolution of gamma-ray burst (GRB) fireballs from the prompt γ-ray emission to the afterglow phase. The early-time X-ray and optical data for GRBs obtained by telescopes aboard the Swift satellite show that the source for prompt γ-ray emission, the emission that heralds these bursts, is short lived, and is distinct from the source for the long-lived afterglow emission that follows the initial burst. Using these data we determine the distance of the γ-ray source from the centre of the explosion. We find this distance to be 1015–1016 cm for most bursts, and show that this is within a factor of about 10 of the radius of the shock heated circumstellar medium (CSM) producing the X-ray photons. Furthermore, using the early γ-ray, X-ray and optical data we show that the prompt gamma-ray emission cannot be produced in internal shocks nor can it be produced in the external shock; in a more general sense γ-ray generation mechanisms based on shock physics have problems explaining the GRB data for ten Swift bursts analyzed in this work. A magnetic field dominated outflow model for GRBs has a number of attractive features, although evidence in its favour is inconclusive. Finally, the X-ray and optical data allow us to provide an upper limit on the density of the CSM of about 10 protons cm−3 at a distance of ∼5 × 1016 cm from the centre of explosion

Investigating the nature of the INTEGRAL gamma-ray bursts and sub-threshold triggers with Swift follow-up

We explore the potential of the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) to improve our understanding of the low-fluence regime for explosive transients, such as Gamma-ray Bursts (GRBs). We probe the nature of the so-called 'WEAK' INTEGRAL triggers, when the gamma-ray instruments record intensity spikes that are below the usual STRONG significance thresholds. In a targeted Swift follow-up campaign, we observed 15 WEAK triggers.We find six of these can be classified as GRBs. This includes GRB 150305A, a GRB discovered from our campaign alone. We also identified a source coincident with one trigger, IGRW151019, as a candidate active galactic nucleus.We show that real events such as GRBs exist within the INTEGRAL Burst Alert System (IBAS) WEAK trigger population. A comparison of the fluence distributions of the full INTEGRAL IBAS and Swift-BAT GRB samples showed that the two are similar.We also find correlations between the prompt gamma-ray and X-ray properties of the two samples, supporting previous investigations.We find that both satellites reach similar, low fluence levels regularly, although Swift is more sensitive to short, low-fluence GRBs

Multiple flaring activity in the supergiant fast X-ray transient IGR J08408-4503 observed with Swift

IGR J08408−4503 is a supergiant fast X–ray transient discovered in 2006 with a confirmed association with a O8.5Ib(f) supergiant star, HD 74194. We report on the analysis of two outbursts caught by Swift/Burst Alert Telescope (BAT) on 2006 October 4 and 2008 July 5, and followed up at softer energies with Swift/X-ray Telescope (XRT). The 2008 XRT light curve shows a multiple-peaked structure with an initial bright flare that reached a flux of ∼10[superscript: −9] erg cm[superscript: -2] s[superscript: −1] (2–10 keV), followed by two equally bright flares within 75 ks. The spectral characteristics of the flares differ dramatically, with most of the difference, as derived via time-resolved spectroscopy, being due to absorbing column variations. We observe a gradual decrease in the N[subscript: H], derived with a fit using absorbed power-law model, as time passes. We interpret these N[subscript: H] variations as due to an ionization effect produced by the first flare, resulting in a significant decrease in the measured column density towards the source. The durations of the flares as well as the times of the outbursts suggest that the orbital period is ∼35 d, if the flaring activity is interpreted within the framework of the Sidoli et al. model with the outbursts triggered by the neutron star passage inside an equatorial wind inclined with respect to the orbital plane

The Swift Burst Analyser I. BAT and XRT spectral and flux evolution of gamma ray bursts

Context: Gamma ray burst models predict the broadband spectral evolution and the temporal evolution of the energy flux. In contrast, standard data analysis tools and data repositories provide count-rate data, or use single flux conversion factors for all of the data, neglecting spectral evolution. Aims: We produce Swift BAT and XRT light curves in flux units, where the spectral evolution is accounted for. Methods: We have developed software to use the hardness ratio information to track spectral evolution of GRBs, and thus to convert the count-rate light curves from the BAT and XRT instruments on Swift into accurate, evolution-aware flux light curves. Results: The Swift Burst Analyser website (http://www.swift.ac.uk/burst_analyser) contains BAT, XRT and combined BAT-XRT flux light curves in three energy regimes for all GRBs observed by the Swift satellite. These light curves are automatically built and updated when data become available, are presented in graphical and plain-text format, and are available for download and use in research

X-RAY FLASHES IN RECURRENT NOVAE: M31N 2008-12a AND THE IMPLICATIONS OF THE SWIFT NONDETECTION

Models of nova outbursts suggest that an X-ray flash should occur just after hydrogen ignition. However, this X-ray flash has never been observationally confirmed. We present four theoretical light curves of the X-ray flash for two very massive white dwarfs (WDs) of 1.380 and 1.385 ${M}_{\odot }$ and for two recurrence periods of 0.5 and 1 yr. The duration of the X-ray flash is shorter for a more massive WD and for a longer recurrence period. The shortest duration of 14 hr (0.6 days) among the four cases is obtained for the $1.385\,{M}_{\odot }$ WD with a 1 yr recurrence period. In general, a nova explosion is relatively weak for a very short recurrence period, which results in a rather slow evolution toward the optical peak. This slow timescale and the predictability of very short recurrence period novae give us a chance to observe X-ray flashes of recurrent novae. In this context, we report the first attempt, using the Swift observatory, to detect an X-ray flash of the recurrent nova M31N 2008-12a (0.5 or 1 yr recurrence period), which resulted in the nondetection of X-ray emission during the period of 8 days before the optical detection. We discuss the impact of these observations on nova outburst theory. The X-ray flash is one of the last frontiers of nova studies, and its detection is essential for understanding the pre-optical-maximum phase. We encourage further observations

An online repository of Swift/XRT light curves of Γ-ray bursts

Context.Swift data are revolutionising our understanding of Gamma Ray Bursts. Since bursts fade rapidly, it is desirable to create and disseminate accurate light curves rapidly. Aims.To provide the community with an online repository of X-ray light curves obtained with Swift. The light curves should be of the quality expected of published data, but automatically created and updated so as to be self-consistent and rapidly available. Methods.We have produced a suite of programs which automatically generates Swift/XRT light curves of GRBs. Effects of the damage to the CCD, automatic readout-mode switching and pile-up are appropriately handled, and the data are binned with variable bin durations, as necessary for a fading source. Results.The light curve repository website (http://www.swift.ac.uk/xrt_curves) contains light curves, hardness ratios and deep images for every GRB which Swift's XRT has observed. When new GRBs are detected, light curves are created and updated within minutes of the data arriving at the UK Swift Science Data Centre