Swift-bat observations of the recently discovered magnetar SGR 0501+4516

Abstract

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 15-150keV energy range. The cut-off PL model fit yields a mean photon index ΓCPL = 0.54 ± 0.11 and a cut-off energy EC = 19.1 ± 1.8keV for the bursts. The mean hard and soft blackbody temperatures are found to be = 12.8 0.7keV and = 4.6 ± 0.5keV, respectively, and are anti-correlated with the square of the radii of the hard and soft emitting regions (RBBh and RBBs) as R2BBh ∝ kT -5.8 and R2 ∝ 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 ∼ 4.4 × 10-9ergs cm-2 to ∼ 2.7 × 10-6ergs cm-2, consistent with those observed for typical short SGR bursts. © 2010. The American Astronomical Society. All rights reserved.