Broadband X-ray spectra of short bursts from SGR 1900+14

We report on the X-ray spectral properties of 10 short bursts from SGR 1900+14 observed with the narrow-field instruments on board BeppoSAX in the hours following the intermediate flare of 2001 April 18. Burst durations are typically shorter than 1 s and often show significant temporal structure on timescales as short as similar to10 ms. Burst spectra from the MECS and PDS instruments were fitted across an energy range from 1.5 to above 100 keV. We fitted several spectral models and assumed NH values smaller than 5x10(22) cm(-2), as derived from observations in the persistent emission. Our results show that the widely used optically thin thermal bremsstrahlung law provides acceptable spectral fits for energies higher than 15 keV but severely overestimates the flux at lower energies. Several years ago similar behavior was observed in short bursts from SGR 1806-20, suggesting that the rollover of the spectrum at low energies is a universal property of this class of sources. Alternative spectral models, such as two blackbodies or a cutoff power law, provide significantly better fits to the broadband spectral data and show that all 10 bursts have spectra consistent with the same spectral shape.