The Suzaku Discovery of A Hard Power-Law Component in the Spectra of Short Bursts from SGR 0501+4516

Abstract: Using data with the Suzaku XIS and HXD, spectral studies of short bursts from the soft gamma repeater SGR 0501+4516 were performed. In total, 32 bursts were detected during the ~60 ks of observation conducted in the 2008 August activity. Excluding the strongest one, the remaining 31 bursts showed an average 2--40 keV fluence of 1.0(-0.5,+0.3)*10^-9 erg cm^-2. A 1--40 keV spectrum summed over them leaves significant positive residuals in the HXD-PIN band with chi^2/d.o.f. = 74/50, when fitted with a two-blackbody function. By adding a power law model, the fit became acceptable with chi^2/d.o.f. = 56/48, yielding a photon index of Gamma=1.0(-0.3,+0.4). This photon index is comparable to Gamma=1.33(-0.16,+0.23) (Enoto et al. 2010a) for the persistent emission of the same object obtained with Suzaku. The two-blackbody components showed very similar ratios, both in the temperature and the emission radii, to those comprising the persistent emission. However, the power-law to two-blackbody flux ratio was possibly higher than that of the persistent emission at 2.6 sigma level. Based on these measurements, average wide-band properties of these relatively weak bursts are compared with those of the persistent emission.