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

Aims. We observed the high redshift ($z = 3.969$) GRB 050730 with Swift and XMM-Newton to study its prompt and afterglow emission. Methods. We carried out a detailed spectral and temporal analysis of Swift and XMM-Newton observations. Results. The X-ray afterglow of GRB 050730 was found to decline along 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 and 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 t1 = 237 ± 20 s and $t_2 = 10.1_{\rm -2.2}^{\rm +4.6}$ ks. The temporal decay slopes before, between and after these breaks were $\alpha_1$ = 2.1 ± 0.3, $\alpha_2 = 0.44_{\rm -0.08}^{\rm +0.14}$ and $\alpha_3 = 2.40_{\rm -0.07}^{\rm +0.09}$, respectively. The spectrum of the X-ray afterglow was well described by a photoelectrically absorbed power-law with an absorbing column density $N^{z}_{\rm H}=(1.28^{\rm +0.26}_{\rm -0.25})\times10^{22}$ cm-2 in the host galaxy. Evidence was found of flaring activity in the early UVOT optical afterglow, simultaneous with that observed in the X-ray band. Strong X-ray spectral evolution during the flaring activity was present. The rise and decay power-law slopes of the first three flares were in the range 0.8–1.8 using as zero times the beginning and the peak of the flares, respectively. In the majority of flares (6/7), the ratio $\Delta t/t_{\rm p}$ between the duration of the event and the time when the flare peaks was nearly constant and ~ 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, and 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 $E_{\rm{p}}$–$E_{\rm{iso}}$ Amati relation but is not consistent with the Ep vs. Ejet Ghirlanda relation.