Anomalous X-ray emission in GRB 060904B: a nickel line?

In 2006 March the soft gamma-ray repeater SGR 1900+14 resumed its bursting activity after ~2 yr of quiescence. The Swift mission observed the source several times. We report on the intense burst ``forest'' recorded on March 29, which lasted for ~30 s, when Swift was pointing at the source with the narrow field of view instruments. More than 40 bursts were detected by BAT and XRT, 7 of which were rare intermediate flares (IFs). The BAT data were used to carry out time-resolved spectroscopy in the 14-100 keV range down to 8 ms timescales. BAT and XRT simultaneous data were used to characterize the broadband energy spectra of IFs and verify the results obtained from the BAT-only spectral fits. This unique data set allowed us to test the magnetar model predictions, such as the magnetically trapped fireball and twisted magnetosphere, over an unprecedented range of fluxes and with large statistics. We confirmed that a two-blackbody component adequately fits the time-resolved and integrated spectra of IFs. However, Comptonization models give comparably good reduced χ2. Moreover, we found a change of behavior, around ~1041 erg s-1, above which the softer blackbody shows a sort of saturation, while the harder one still grows to a few times 1041 erg s-1, and a rather sharp correlation between temperature and radii of the blackbodies (R2~kT-3), which holds for the most luminous parts of the flares (~Ltot>=1041 erg s-1). Within the magnetar model, the majority of these findings are accounted for in terms of thermalized emission from the E-mode and O-mode photospheres. Interestingly, the maximum observed luminosity coming from a region of ~15 km matches the magnetic Eddington luminosity at the same radius, for a surface dipole field of ~8×1014 G (virtually equal to that deduced from the spin-down of SGR 1900+14)