A Swift gaze into the 2006 March 29 burst forest of SGR 1900+14

In 2006 March the soft gamma-ray repeater SGR 1900+14 resumed its bursting activity after similar to 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 similar to 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 chi(2). Moreover, we found a change of behavior, around similar to 10(41) erg s(-1), above which the softer blackbody shows a sort of saturation, while the harder one still grows to a few times 10(41) erg s(-1), and a rather sharp correlation between temperature and radii of the blackbodies (R-2 proportional to kT(-3)), which holds for the most luminous parts of the flares (similar to L-tot >= 10(41) 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 similar to 15 km matches the magnetic Eddington luminosity at the same radius, for a surface dipole field of similar to 8 x 10(14) G (virtually equal to that deduced from the spin-down of SGR 1900+14).