Search

Context. X-ray flashes (XRFs) are a class of gamma-ray bursts (GRBs) with a peak energy of the time-integrated $\nu\,F_\nu$spectrum, Ep, typically below 30 keV, whereas classical GRBs have Epof a few hundreds of keV. Apart from Epand the systematically lower luminosity, the properties of XRFs, such as their duration or spectral indices, are typical of the classical GRBs. Yet, the nature of XRFs and their differences from GRBs are not understood. In addition, there is no consensus on the interpretation of the shallow decay phase observed in most X-ray afterglows of both XRFs and GRBs.

Context. Previously detected in only a few gamma-ray bursts (GRBs), X-ray flares are now observed in ∼50% of SwiftGRBs, though their origins remain unclear. Most flares are seen early on in the afterglow decay, while some bursts exhibit flares at late times of 104to 105s, which may have implications for flare models.

We report on the temporal and spectral characteristics of the early X-ray emission from the GRB 050822 as observed by Swift. This burst is likely to be an XRF showing major X-ray flares in its XRT light-curve. The quality of the data allows a detailed spectral analysis of the early afterglow in the X-ray band. During the X-ray flares, a positive correlation between the count rate and the spectral hardness (i.e. the higher the count rate, the harder the spectrum) is clearly seen for the X-ray flares. This behaviour, similar to that seen for Gamma-ray pulses, indicates that the energy peak of the spectrum is in the XRT energy band and it moves towards lower energies with time. We show evidence for the possible detection of the emergence of the forward-shock emission, produced at a radius larger than 4 $\times$1016cm in the case of a CBM afterglow model (a formation region clearly different from that producing the prompt emission). Finally, we show that the null detection of a jet break up to T0+4$\times$106s in the X-ray light curve of this XRF can be understood: i) if the jet seen on-axis is uniform with a large opening angle ($\theta > 20^\circ$); or ii) if the jet is a structured Gaussian-like jet with the line-of-sight outside the bright Gaussian core.

Context.Swiftdata are revolutionising our understanding of Gamma Ray Bursts. Since bursts fade rapidly, it is desirable to create and disseminate accurate light curves rapidly.

We report on the temporal and spectral characteristics of the early X-ray emission from the Gamma Ray Burst 051117A as observed by Swift . The superb quality of the early X-ray light-curve and spectra of this source, one of the brightest seen by the X-ray Telescope at such early times, allows an unprecedented look at the spectral and temporal evolution of the prompt and early afterglow emission for this GRB and allows us to place stringent limits on the detection of lines. GRB 051117A displays a highly complex light-curve, with an apparent initial slow decline of slope $\alpha=0.77\pm0.07$($f(t)\propto t^{-\alpha}$) dominated by numerous superposed flares of varying amplitude and duration. Between orbits 2 and 3, the X-ray light-curve drops abruptly, highlighting the dominance of flaring activity at early times, and indicating that the central engine for this burst remains active for several kiloseconds after the initial explosion. The late time slope ($t>10^{4}$s) also decays relatively slowly with a powerlaw index of $\alpha=0.66$, breaking to a steeper slope of 1.1, 170 ks after the BAT trigger. The X-ray light-curve at early times is characteristic of a noise process, consisting of random shots superposed on an underlying powerlaw decay, with individual shots well-modelled by a fast-rise and exponential decay spanning a broad range in rise-times and decay rates. A temporal spectral analysis of the early light-curve shows that the photon index and source intensity are highly correlated with the spectrum being significantly harder when brighter, consistent with the movement of the peak of the Band function to lower energies following individual flares. The high quality spectrum obtained from the first orbit of WT mode data, enables us to place a $3\sigma$upper limit on the strength of any emission line features of ${\it EW}< 15$eV, assuming a narrow emission-line of 100 eV at the peak of the effective area.

Aims.We present our analysis of the multiwavelength photometric & spectroscopic observations of GRB 060210and discuss the results in the overall context of current GRB models.

We present observations of the X-ray afterglow of GRB 050408, a gamma-ray burst discovered by HETE-II. Swiftbegan observing the field 42 min after the burst, performing follow-up over a period of 38 d (thus spanning three decades in time). The X-ray light curve showed a steepening with time, similar to many other afterglows. However, the steepening was unusually smooth, over the duration of the XRT observation, with no clear break time. The early decay was too flat to be described in terms of standard models. We therefore explore alternative explanations, such as the presence of a structured afterglow or of long-lasting energy injection into the fireball from the central GRB engine. The lack of a sharp break puts constraints on these two models. In the former case, it may indicate that the angular energy profile of the jet was not a simple power law, while in the second model it implies that injection did not stop abruptly. The late decay may be due either to a standard afterglow (that is with no energy injection), or to a jetted outflow still being refreshed. A significant amount of absorption was present in the X-ray spectrum, corresponding to a rest-frame hydrogen column density $N_{\rm H} = 1.2_{-0.3}^{+0.4} \times 10^{22}$cm-2, indicative of a dense environment.

Aims.We report a multi-wavelength analysis of the prompt emission and early afterglow of GRB 051111 and discuss its properties in the context of current fireball models.

Context.GRB 050721 was detected by Swiftand promptly followed-up, in the X-ray by Swiftitself and, in the optical band, by the VLT operated, for the first time, in rapid response mode starting observations about 25 m after the burst. A multiwavelength monitoring campaign was performed in order to study its afterglow's behavior.

We present observations of GRB 060124, the first event for which both the prompt and the afterglow emission could be observed simultaneously and in their entirety by the three Swift instruments. Indeed, Swift-BAT triggered on a precursor ~570 s before the main burst peak, and this allowed Swift to repoint the narrow field instruments to the burst position ~350 s beforethe main burst occurred. GRB 060124also triggered Konus-Wind, which observed the prompt emission in a harder gamma-ray band (up to 2 MeV). Thanks to these exceptional circumstances, the temporal and spectral properties of the prompt emission can be studied in the optical, X-ray and gamma-ray ranges. While the X-ray emission (0.2–10 keV) clearly tracks the gamma-ray burst, the optical component follows a different pattern, likely indicating a different origin, possibly the onset of external shocks. The prompt GRB spectrum shows significant spectral evolution, with both the peak energy and the spectral index varying. As observed in several long GRBs, significant lags are measured between the hard- and low-energy components, showing that this behaviour extends over 3 decades in energy. The GRB peaks are also much broader at soft energies. This is related to the temporal evolution of the spectrum, and can be accounted for by assuming that the electron spectral index softened with time. The burst energy ($E_{\rm iso} \sim 5$$\times$1053erg) and average peak energy ($E_{\rm p} \sim 300$keV) make GRB 060124consistent with the Amati relation. The X-ray afterglow is characterized by a decay which presents a break at $t_{\rm b} \sim 10^5$s.

Aims.The short/hard GRB 051210 was detected and located by the Swift-BAT instrument and rapidly pointed towards by the narrow field instruments. The XRT was able to observe a bright, rapidly fading X-ray emission. We present the analysis of the prompt and afterglow emission of this event.

Short duration ($\la $2 s) Gamma-ray bursts (GRBs) have been a mystery since their discovery. Until May 2005 very little was known about short GRBs, but this situation has changed rapidly in the last few months since the Swift and HETE-2 satellites have made it possible to discover X-ray and optical counterparts to these sources. Positional associations indicate that short GRBs arise in close-by galaxies ($z<0.7$). Here we report on a detailed study of the short GRB 050724 X-ray afterglow. This burst shows strong flaring variability in the X-ray band. It clearly confirms early suggestions of X-ray activity in the 50–100 s time interval following the GRB onset seen with BATSE. Late flare activity is also observed. These observations support the idea that flares are related to the inner engine for short GRBs, as well as long GRBs.

Aims.The aim of this work is to identify and study spectral features in the quasar 3C 273 usually blended by its strong jet emission.

Gamma-ray burst (GRB) progenitors are observationally linked to the death of massive stars. X-ray studies of the GRB afterglows can deepen our knowledge of the ionization status and metal abundances of the matter in the GRB environment. Moreover, the presence of local matter can be inferred through its fingerprints in the X-ray spectrum, i.e. the presence of absorption higher than the Galactic value. A few studies based on BeppoSAX and XMM-Newton found evidence of higher than Galactic values for the column density in a number of GRB afterglows. Here we report on a systematic analysis of 17 GRBs observed by Swift up to April 15, 2005. We observed a large number of GRBs with an excess of column density. Our sample, together with previous determinations of the intrinsic column densities for GRBs with known redshift, provides evidence for a distribution of absorption consistent with that predicted for randomly occurring GRB within molecular clouds.

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 Γevolving 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.

We present observations of XRF 050406, the first burst detected by Swift showing a flare in its X-ray light curve. During this flare, which peaks at $t_{\rm peak} \sim 210$s after the BAT trigger, a flux variation of $\delta F / F \sim 6$in a very short time $\delta t / t_{\rm peak} \ll 1$was observed. Its measured fluence in the 0.2-10 keV band was ~$1.4 \times 10^{-8}$erg cm-2, which corresponds to 1-15% of the prompt fluence. We present indications of spectral variations during the flare. We argue that the producing mechanism is late internal shocks, which implies that the central engine is still active at 210 s, though with a reduced power with respect to the prompt emission. The X-ray light curve flattens to a very shallow slope with decay index of ~0.5 after ~4400 s, which also supports continued central engine activity at late times. This burst is classified as an X-ray flash, with a relatively low fluence (~10-7erg cm-2in the 15-350 keV band, $E_{\rm iso} \sim 10^{51}$erg), a soft spectrum (photon index 2.65), no significant flux above ~50 keV and a peak energy $E_{\rm p} < 15$keV. XRF 050406is one of the first examples of a well-studied X-ray light curve of an XRF. We show that the main afterglow characteristics are qualitatively similar to those of normal GRBs. In particular, X-ray flares superimposed on a power-law light curve have now been seen in both XRFs and GRBs. This indicates that a similar mechanism may be at work for both kinds of events.

We present optical observations of the short/hard gamma-ray burst GRB 050709, the first such event with an identified optical counterpart. The object is coincident with a weak X-ray source and is located inside a galaxy at redshift $z = 0.1606 \pm 0.0002$. Multiband photometry allowed us to study the broad-band spectral energy distribution.
Late-time monitoring places strong limits on any supernova simultaneous with the GRB. The host galaxy is not of early type. Spectra show that the dominant stellar population is relatively young (∼1 Gyr), and that ongoing star formation is present at a level of $2\mbox{--}3\,L/L_*~M_\odot$yr-1. This is at least 2 orders of magnitude larger than that observed in the elliptical hosts of the short GRB 050509B and GRB 050724. This shows that at least some short GRBs originate in a young population. Short/hard GRB models based on the merger of a binary degenerate system are compatible with the host galaxy characteristics, although there is still the possibility of a connection between young stars and at least a fraction of such events.

We present low-resolution VLT spectroscopy of the afterglow of the gamma-ray bursts (GRBs) 991216, 011211 and 021211. Our spectrum of GRB 991216 is the only optical spectrum for this afterglow. It shows two probable absorption systems at $z=0.80$and $z=1.02$, where the highest redshift most likely reflects the distance to the host galaxy. A third system may be detected at $z=0.77$. HST imaging of the field, obtained 4 months after the burst, has resulted in the detection of two amorphous regions of emission, one at the projected afterglow position, and the other 0$.\!\!^{\prime\prime}$6 away. The spectrum shows a depression in flux in between 4000 Å and 5500 Å. This could be the result of a 2175 Å-type extinction feature in the host of GRB 991216, but at a rather red wavelength of 2360 Å. If this interpretation is correct, it is the first time the extinction feature is seen in a GRB afterglow spectrum. It is centered at a wavelength similar to that of the ultra-violet (UV) bumps inferred from observations of a few UV-strong, hydrogen-poor stars in the Galaxy. All significant absorption lines (except for one) detected in the spectrum of GRB 011211 are identified with lines originating in a single absorption system at $z=2.142\pm0.002$, the redshift of the GRB 011211 host galaxy. We also detect the Lyαabsorption line in the host, to which we fit a neutral hydrogen column density of log N($\ion{H}{i}$) = $20.4\pm0.2$, which indicates that it is a damped Lyαsystem. Using a curve-of-growth analysis, we estimate the Si, Fe and Al metallicity at the GRB 011211 redshift to be [Si/H] = $-0.9^{+0.6}_{-0.4}$, $\rm [Fe/H]=-1.3\pm0.3$, and [Al/H] = $-1.0^{+0.5}_{-0.3}$. For GRB 021211, we detect a single emission line in a spectrum obtained tens of days after the burst, which we identify as [$\ion{O}{ii}$] λ3727 at $z=1.006$. The corresponding unobscured [$\ion{O}{ii}$] star-formation rate is 1.4 M$_{\odot}$yr-1.

We present hydrodynamical calculations of radiative shocks with low Mach numbers and find that the well-known global overstability can occur if the temperature exponent (α) of the cooling is sufficiently negative. We find that the stability of radiative shocks increases with decreasing Mach number, with the result that $M=2$shocks require $\alpha \la -1.2$in order to be overstable. Such values occur within a limited temperature range of many cooling curves. We observe that Mach numbers of order 100 are needed before the strong shock limit of $\alpha_{\rm cr} \approx 0.4$is reached, and we discover that the frequency of oscillation of the fundamental mode also has a strong Mach number dependence. We find that feedback between the cooling region and the cold dense layer (CDL) further downstream is a function of Mach number, with stronger feedback and oscillation of the boundary between the CDL and the cooling region occuring at lower Mach numbers. This feedback can be quantified in terms of the reflection coefficient of sound waves, and in those cases where the cooling layer completely disappears at the end of each oscillation cycle, the initial velocity of the waves driven into the upstream pre-shock flow and into the downstream CDL, and the velocity of the the boundary between the CDL and the cooling layer, can be understood in terms of the solution to the Riemann problem. An interesting finding is that the stability properties of low Mach number shocks can be dramatically altered if the shocked gas is able to cool to temperatures less than the pre-shock value (i.e. when $\chi < 1$, where χis the ratio of the temperature of the cold dense layer to the pre-shock temperature). In such circumstances, low Mach number shocks have values of $\alpha_{\rm cr}$which are comparable to values obtained for higher Mach number shocks when $\chi = 1$. For instance, $\alpha_{\rm cr}=-0.1$when $M=2$and $\chi=0.1$, comparable to that when $M=10$and $\chi=1$. Thus, it is probable that low Mach number astrophysical shocks will be overstable in a variety of situations. We also explore the effect of different assumptions for the initial hydrodynamic set up and the type of boundary condition imposed downstream, and find that the properties of low Mach number shocks are relatively insensitive to these issues. The results of this work are relevant to astrophysical shocks with low Mach numbers, such as supernova remnants (SNRs) immersed in a hot interstellar medium (e.g., within a starburst region), and shocks in molecular clouds, where time-dependent chemistry can lead to overstability.

We present the results from an XMM-Newtonobservation (January 24, 2001) of the bright Seyfert 1 galaxy ESO 198-G24($z=0.045$). We found that this Seyfert has an intrinsic 2–10 keV luminosity of about 1044erg s-1. This source shows no intrinsic absorption in addition to the Galactic absorption (${\cal N}_{\rm H}\sim3\times10^{20}$cm-2). We found both with EPIC and RGS that this source possesses significantly steeper spectra below ~1.5–2 keV than observed at higher X-ray energies, the so-called soft X-ray excess. The RGS spectra reveal no significant narrow absorption lines suggesting that if there is a warm absorber, it either has a relatively low column density, or a very high ionization parameter. The RGS data are well described by the combination of a power-law, a modified black body continuum, and weak relativistic lines of $\ion{O}{viii}$, and $\ion{C}{vi}$Lyα. However other interpretations are not definitely excluded. The 2–10 keV energy band is well fitted by an absorbed power-law with a photon spectral index of $\Gamma=1.77\pm0.04$(consistent with the typical $\Gamma \sim1.7$found in Seyfert 1 galaxies). We found the presence of a narrow Gaussian emission line at 6.41 keV (i.e. <$\ion{Fe}{xvii}$) with a moderate equivalent width of about 60–70 eV, and we found an upper limit for a broad component, if any, of 75 eV. We also found a weak absorption edge associated with cold iron with an optical depth of about 0.2.

We report on an XMM-Newtonobservation of the X-ray afterglow of the Gamma Ray Burst GRB 011211, originally detected by Beppo-SAXon 11th December 2001. The early afterglow spectrum obtained by XMM-Newton, observed 11 hours after the initial burst, appeared to reveal decaying H-like Kaemission lines of Mg, Si, S, Ar and Ca, arising in enriched material with an outflow velocity of order 0.1c (Reeves et al. [CITE]). This was attributed to matter ejected from a massive stellar progenitor occurring shortly before the burst itself. Here, we present a detailed re-analysis of the XMM-NewtonEPIC observations of GRB 011211. In particular, we show that the detection of the soft X-ray line emission appears robust, regardless of detector background, calibration, spectral binning, or the spectral model that is assumed. We demonstrate that thermal emission, from an optically thin plasma, is the most plausible model that can account for the soft X-ray emission, which appears to be the case for at least two burst afterglow spectra observed by XMM-Newton. The X-ray spectrum of GRB 011211 appears to evolve with time after the first 10 ks of the XMM-Newtonobservation as the Si and S emission lines are only detected during the first 10 ks of observation. The observations suggest that thermal emission is present during the early afterglow spectrum, whilst a power-law component dominates the latter stages. Finally we estimate the mass of the ejected material in GRB 011211 to be of the order 4–20 solar masses.

The spectrum of the afterglow of GRB 020322 is the highest-quality X-ray spectrum of a GRB afterglow available to date. It was detected by XMM-Newtonin an observation starting fifteen hours after the GRB with a mean 0.2–10.0 keV observed flux of $3.5\pm0.2\times10^{-13}$erg cm-2s-1, making it the brightest X-ray afterglow observed so far with XMM-Newton. The source faded; its lightcurve was well fit by a power-law with a decay index of $1.26\pm0.23$. The spectrum is adequately fit with a power-law absorbed with neutral or ionised gas significantly in excess of the foreground Galactic column, at redshift $1.8_{-1.1}^{+1.0}$or with low metal abundances. No spectral line or edge features are detected at high significance, in particular, a thermal emission model fits the data poorly, the upper limit on its contribution to the spectrum is $3.7\times10^{-14}$erg cm-2s-1, or ~10% of the total flux. No spectral variability is observed.

The X-ray afterglows of GRB 001025A and GRB 010220 were detected by XMM-Newtonwith an average 0.2–10.0 keV flux of 4.4 and $3.3\times10^{-14}$erg cm-2s-1respectively; the afterglow of GRB 001025A is observed to decay. Afterglows at other wavelengths were not detected for either burst. A set of broadened soft X-ray emission lines are detected in the afterglow of GRB 001025A, at $5.0\,\sigma$significance above a Galactic-absorbed power-law continuum. The spectra of both afterglows are significantly better fit by a variable abundance thermal plasma model than by an absorbed power-law and are consistent with the observations of GRB 011211, indicating that thermal emission from light elements may be common in the early X-ray afterglows of GRBs.

Mid-infrared images of the Seyfert 1 galaxy Mrk 279obtained with the ISO satellite are presented together with the results of a one-year monitoring campaign of the 2.5-11.7 μm spectrum. Contemporaneous optical photometric and spectrophotometric observations are also presented. The galaxy appears as a point-like source at the resolution of the ISOCAM instrument (4-5″). The 2.5-11.7 μm average spectrum of the nucleus in Mrk 279shows a strong power law continuum with $\alpha = -0.80\pm0.05$(${ F_{\nu} \propto \nu^{\alpha}}$) and weak PAH emission features. The Mrk 279spectral energy distribution shows a mid-IR bump, which extends from 2 to 15-20 μm. The mid-IR bump is consistent with thermal emission from dust grains at a distance of $\ga 100$lt-d. No significant variations of the mid-IR flux have been detected during our observing campaign, consistent with the relatively low amplitude (~10% rms) of the optical variability during the campaign. The time delay for Hβline emission in response to the optical continuum variations is $\tau = 16.7^{+5.3}_{-5.6}$days, consistent with previous measurements.

We present the temporal analysis of X-ray observations of the radio-loud Narrow-Line Seyfert 1 galaxy (NLS1) PKS 0558-504obtained during the XMM-NewtonCalibration and Performance Verification (Cal/PV) phase. The long term light curve is characterized by persistent variability with a clear tendency for the X-ray continuum to harden when the count rate increases. Another strong correlation on long time scales has been found between the variability in the hard band and the total flux. On shorter time scales the most relevant result is the presence of smooth modulations, with characteristic time of ~2 hours observed in each individual observation. The short term spectral variability turns out to be rather complex but can be described by a well defined pattern in the hardness ratio-count rate plane.

We have analyzed the timing properties of the Narrow-line Seyfert 1 galaxy Mrk 766 observed with XMM-Newtonduring the PV phase. The source intensity changes by a factor of 1.3 over the 29 000 s observation. If the soft excess is modeled by a black body component, as indicated by the EPIC pn data, the luminosity of the black body component scales with its temperature according to $L \sim T^4$. This requires a lower limit "black body size"of about $\rm 1.3 10^{25} cm^2$. In addition, we report the detection of a strong periodic signal with $\rm 2.4 10^{-4} Hz$. Simulations of light curves with the observed time sequence and phase randomized for a red noise spectrum clearly indicate that the periodicity peak is intrinsic to the distant AGN. Furthermore, its existence is confirmed by the EPIC MOS and RGS data. The spectral fitting results show that the black body temperature and the absorption by neutral hydrogen remain constant during the periodic oscillations. This observational fact tends to rule out models in which the intensity changes are due to hot spots orbiting the central black hole. Precession according to the Bardeen-Petterson effect or instabilities in the inner accretion disk may provide explanations for the periodic signal.

We present XMM-Newtonobservations of the high redshift ($z=3.104$), radio-loud quasar PKS 0537-286. The EPIC CCD cameras provide the highest signal-to-noise spectrum of a high-zquasar to date. The EPIC observations show that PKS 0537-286 is extremely X-ray luminous ($L_{\rm X}=2$1047erg s-1), with an unusually hard X-ray spectrum ($\Gamma=1.27\pm0.02$). The flat power-law emission extends over the whole observed energy range (0.4 to 40 keV in the quasar rest frame); there is no evidence of intrinsic absorption, which has been claimed in PKS 0537-286 and other high zquasars. However, there is evidence for weak Compton reflection. A redshiftediron K line, observed at 1.5 keV - corresponding to ~6.15 keV in the quasar rest frame - is detected at 95% confidence. If confirmed, this is the most distant iron K line known. The line equivalent width is small (33 eV), consistent with the "X-ray Baldwin effect" observed in other luminous quasars. The reflected continuum is also weak ($R \la$0.25). We find the overall spectral energy distribution of PKS 0537-286 is dominated by the X-ray emission, which, together with the flat power-law and weak reflection features, suggests that the X-radiation from PKS 0537-286 is dominated by inverse Compton emission associated with a face-on relativistic jet.

XMM-Newtonobservations of the low luminosity, radio-quiet quasar Markarian 205 have revealed a unique iron K emission line profile. In marked contrast to the broad and redshifted iron K line commonly seen in ASCAobservations of Seyfert 1 galaxies, we find that a substantial amount of the line flux in Mrk 205 occurs above the neutral line energy of 6.4 keV. Furthermore, we find that the iron line emission has two distinct components, a narrow, unresolved neutral line at 6.4 keV and a broadened line centred at 6.7 keV. We suggest that the most likely origin of the 6.7 keV line is from X-ray reflection off the surface of a highly ionised accretion disk, whilst the 6.4 keV component may arise from neutral matter distant from the black hole, quite possibly in the putative molecular torus. Overall this observation underlines the potential of XMM-Newtonfor using the iron K line as a diagnostic of matter in the innermost regions of AGN.

We present XMM-Newtonobservations of the bright quasar PKS 0558-504. The 0.2-10 keV spectrum is dominated by a large, variable soft X-ray excess. The fastest flux variations imply accretion onto a Kerr black hole. The XMM-Newtondata suggest the presence of a "big blue bump"in PKS 0558-504 extending from the optical band to ~3 keV. The soft X-ray spectrum shows no evidence for significant absorption or emission-line features. The most likely explanation for the hot big blue bump is Comptonization by the multi-temperature corona of a thermal accretion disc running at a high accretion rate.

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 SwiftBAT 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 Swiftinto accurate, evolution-aware flux light curves.Results. The SwiftBurst 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 Swiftsatellite. 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.