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Register a new userThe Swift X-ray flaring afterglow of GRB 050607
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Claudio Pagani
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The unique capability of the Swift satellite to perform a prompt and autonomous slew to a newly detected Gamma-Ray Burst (GRB) has yielded the discovery of interesting new properties of GRB X-ray afterglows, such as the steep early lightcurve decay and the frequent presence of flares detected up to a few hours after the GRB trigger. We present observations of GRB 050607, the fourth case of a GRB discovered by Swift with flares superimposed on the overall fading X-ray afterglow. The flares of GRB 050607 were not symmetric as in previously reported cases, showing a very steep rise and a shallower decay, similar to the Fast Rise, Exponential Decay that are frequently observed in the gamma-ray prompt emission. The brighter flare had a flux increase by a factor of approximately 25,peaking for 30 seconds at a count rate of approximately 30 counts s-1, and it presented hints of addition short time scale activity during the decay phase. There is evidence of spectral evolution during the flares. In particular, at the onset of the flares the observed emission was harder, with a gradual softening as each flare decayed. The very short time scale and the spectral variability during the flaring activity are indicators of possible extended periods of energy emission by the GRB central engine. The flares were followed by a phase of shallow decay, during which the forward shock was being refreshed by a long-lived central engine or by shells of lower Lorentz factors, and by a steepening after approximately 12 ks to a decay slope considered typical of X-ray afterglows.
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We present the results of a detailed spectral and temporal analysis of Swift and XMM-Newton observations of the high redshift (z=3.969) GRB 050730. The X-ray afterglow of GRB 050730 was found to decline with time with superimposed intense flaring activity that extended over more than two orders of magnitude in time. Seven distinct re-brightening events starting from 236 s up to 41.2 ks after the burst were observed. The underlying decay of the afterglow was well described by a double broken power-law model with breaks at t_1= 237 +/- 20 s and t_2 = 10.1 (-2.2) (+4.6) ks. The temporal decay slopes before, between and after these breaks were alpha_1 = 2.1 +/- 0.3, alpha_2 = 0.44 (-0.08) (+0.14) and alpha_3 = 2.40 (+0.07) (-0.09), respectively. The spectrum of the X-ray afterglow was well described by a photoelectrically absorbed power-law with an absorbing column density N_H=(1.28 +/- 0.26) 10^22 cm^-2 in the host galaxy. Strong X-ray spectral evolution during the flaring activity was present. In the majority of the flares (6/7) the ratio Delta_t/t_p between the duration of the event and the time when the flare peaks was nearly constant and about 0.6-0.7. We showed that the observed spectral and temporal properties of the first three flares are consistent with being due both to high-latitude emission, as expected if the flares were produced by late internal shocks, or to refreshed shocks, i.e. late time energy injections into the main afterglow shock by slow moving shells ejected from the central engine during the prompt phase. The event fully satisfies the E_p-E_iso Amati relation while is not consistent with the E_p-E_jet Ghirlanda relation.
We report the results of Swift X-Ray Telescope (XRT) observations of GRB 050318. This event triggered the Burst Alert Telescope (BAT) aboard Swift and was followed-up with XRT and UVOT for 11 consecutive orbits starting from 54 minutes after the trigger. A previously unknown fading X-ray source was detected and accurately monitored. The source was found to decrease in intensity with time and a clear temporal break occurring at ~18000 s after the trigger was observed. The X-ray light curve was found to be consistent with a broken power-law with decay indices -1.17 +/- 0.08 and -2.10 (+0.22) (-0.24) before and after the break. The spectrum of the X-ray afterglow was well described by a photoelectrically absorbed power-law with energy index of -1.09 +/-0.09. No evidence of spectral evolution was found. We compare these results with those obtained with UVOT and separately reported and refine the data analysis of BAT. We discuss our results in the framework of a collimated fireball model and a synchrotron radiation emission mechanism. Assuming the GRB redshift derived from the farthest optical absorption complex (z = 1.44), the event is fully consistent with the E_p-E_iso correlation.
Results are presented of early X-ray afterglow observations of GRB 060105 by Swift and Suzaku. The bright, long gamma-ray burst GRB 060105 triggered the Swift Burst Alert Telescope (BAT) at 06:49:28 on 5 January 2006. The Suzaku team commenced a pre-planned target of opportunity observation at 19 ks (5.3 hr) after the Swift trigger. Following the prompt emission and successive very steep decay, a shallow decay was observed from T_0+187 s to T_0+1287 s. After an observation gap during T_0 +(1.5-3) ks, an extremely early steep decay was observed in T_0+(4-30) ks. The lightcurve flattened again at T_0+30 ks, and another steep decay followed from T_0+50 ks to the end of observations. Both steep decays exhibited decay indices of 2.3 - 2.4. This very early break, if it is a jet break, is the earliest case among X-ray afterglow observations, suggesting a very narrow jet whose opening angle is well below 1 degree. The unique Suzaku/XIS data allow us to set very tight upper limits on line emission or absorption in this GRB. For the reported pseudo-redshift of z=4.0+/-1.3 the upper limit on the iron line equivalent width is 50 eV.
GRB 070724B is the first Gamma Ray Burst localized by SuperAGILE, the hard X-ray monitor aboard the AGILE satellite. The coordinates of the event were published $sim 19$ hours after the trigger. The Swift X-Ray Telescope pointed at the SuperAGILE location and detected the X-ray afterglow inside the SuperAGILE error circle. The AGILE gamma-ray Tracker and Minicalorimeter did not detect any significant gamma ray emission associated with GRB 070724B in the MeV and GeV range, neither prompt nor delayed. Searches of the optical afterglow were performed by the Swift UVOT and the Palomar automated 60-inch telescopes without any significant detection. Similarly the Very Large Array did not detect a radio afterglow. This is the first GRB event with a firm upper limit in the 100 MeV -- 30 GeV energy range, associated with an X-ray afterglow.
(Abridged) The Swift X-Ray Telescope (XRT) reveals some interesting features of early X-ray afterglows, including a distinct rapidly decaying component preceding the conventional afterglow component in many sources, a shallow decay component before the more ``normal decay component observed in a good fraction of GRBs (e.g. GRB 050128, GRB 050315, GRB 050319, and GRB 050401), and X-ray flares in nearly half of the afterglows (e.g. GRB 050406, GRB 050502B, GRB 050607, and GRB 050724). In this paper, we systematically analyze the possible physical processes that shape the properties of the early X-ray afterglow lightcurves, and use the data to constrain various models. We suggest that the steep decay component is consistent with the tail emission of the prompt gamma-ray bursts and/or of the X-ray flares. This provides clear evidence that the prompt emission and afterglow emission are two distinct components, supporting the internal origin of the GRB prompt emission. The shallow decay segment observed in a group of GRBs suggests that the forward shock keeps being refreshed for some time. This might be caused either by a long-lived central engine, or by a power law distribution of the shell Lorentz factors, or else by the deceleration of a Poynting flux dominated flow. X-ray flares suggest that the GRB central engine is still active after the prompt gamma-ray emission is over, but with a reduced activity at later times. In some cases, the central engine activity even extends days after the burst trigger. Analyses of early X-ray afterglow data reveal that GRBs are indeed highly relativistic events. Early afterglow data of many bursts, starting from the beginning of the XRT observations, are consistent with the afterglow emission from an interstellar medium (ISM) environment.
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