2010ApJ...716...97K

We present results on the soft gamma repeater (SGR) 0501+4516, discovered by the Swift Burst Alert Telescope (BAT) on 2008 August 22. More than 50 bursts were identified from this source, out of which 18 bursts had enough counts to carry out spectral analysis. We performed time-averaged spectral analysis on these 18 bursts using eight models, among which the cut-off power-law (PL) and the two-blackbody models provided the best fit in the 15-150 keV energy range. The cut-off PL model fit yields a mean photon index Γ_CPL = 0.54±0.11 and a cut-off energy E_C= 19.1±1.8 keV for the bursts. The mean hard and soft blackbody temperatures are found to be {kT_{BB_h}} = 12.8±0.7 keV and {kT_{BB_s}} = 4.6±0.5 keV, respectively, and are anti-correlated with the square of the radii of the hard and soft emitting regions ({R_{BB_h}} and {R_{BB_s}}) as {R_{BB_h}^2} {vprop} kT ^–5.8 and {R_{BB_s}^2} {vprop} kT ^–2.7, respectively. The soft and hard component temperatures with different indices support the idea of two distinct emitting regions with the hard component corresponding to a smaller radius and the soft component corresponding to a larger radius, which further corroborate the idea of the propagation of extraordinary (E) and ordinary (O) mode photons across the photosphere, as predicted in the magnetar model. We notice strong burst fluence-duration correlation as well as hardness ratio-duration and hardness ratio-fluence anti-correlations for the SGR 0501+4516 bursts. The burst fluences range from ∼4.4x10^–9 ergs/cm2 to ∼2.7x10^–6 ergs/cm2, consistent with those observed for typical short SGR bursts.