Evidence for an anticorrelation between the duration of the shallow decay phase of GRB X-ray afterglows and redshift

One of the most intriguing features discovered by Swift is a plateau phase in the X-ray flux decay of about 70% of the afterglows of gamma-ray bursts (GRBs). The physical origin of this feature is still being debated. We constrain the proposed interpretations, based on the intrinsic temporal properties of the plateau phase. We selected and analyzed all the Swift/XRT GRB afterglows at known redshift observed between March 2005 and June 2008 featuring a shallow decay phase in their X-ray lightcurves. For our sample of 21 GRBs we find an anticorrelation of the logarithm of the duration of the shallow phase with re dshift, with a Spearman rank-order correlation coefficient of r=-0.4 and a null hypothesis probability of 5%. When we correct the durations for cosmological dilation, the anticorrelation strenghtens, with r=-0.6 and a null hypothesis probability of 0.4%. Considering only those GRBs in our sample that have a well-measured burst peak energy (8 out of 21), we find an anticorrelation between the energy of the burst and the shallow phase duration, with r=-0.80 and a null hypothesis probability of 1.8%. If the burst energy anticorrelation with the shallow phase duration is real, then the dependence of the shallow phase on redshift could be the result of a selection effect, since on average high-redshift bursts with lower energies and longer plateaus would be missed. A burst energy anticorrelation with the shallow phase duration would be expected if the end of the plateau arises from a collimated outflow. Alternative scenarios are briefly discussed involving a possible cosmological evolution of the mechanism responsible for the X-ray shallow decay.

Where are the missing gamma ray burst redshifts?

In the redshift range z = 0-1, the gamma ray burst (GRB) redshift distribution should increase rapidly because of increasing differential volume sizes and strong evolution in the star formation rate. This feature is not observed in the Swift redshift distribution and to account for this discrepancy, a dominant bias, independent of the Swift sensitivity, is required. Furthermore, despite rapid localization, about 40-50% of Swift and pre-Swift GRBs do not have a measured redshift. We employ a heuristic technique to extract this redshift bias using 66 GRBs localized by Swift with redshifts determined from absorption or emission spectroscopy. For the Swift and HETE+BeppoSAX redshift distributions, the best model fit to the bias in z < 1 implies that if GRB rate evolution follows the SFR, the bias cancels this rate increase. We find that the same bias is affecting both Swift and HETE+BeppoSAX measurements similarly in z < 1. Using a bias model constrained at a 98% KS probability, we find that 72% of GRBs in z < 2 will not have measurable redshifts and about 55% in z > 2. To achieve this high KS probability requires increasing the GRB rate density in small z compared to the high-z rate. This provides further evidence for a low-luminosity population of GRBs that are observed in only a small volume because of their faintness.

The exceptionally extended flaring activity in the X-ray afterglow of GRB 050730 observed with Swift and XMM-Newton

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.