SWIFT-BAT observations of the recently discovered magnetar SGR 0501+4516

arXiv:1004.4036v1 [astro-ph.HE] 23 Apr 2010 Accepted for publication in the Astrophysical Journal: April 20, 2010 Preprint typeset using LATEX style emulateapj v. 11/10/09 SWIFT-BAT OBSERVATIONS OF THE RECENTLY DISCOVERED MAGNETAR SGR 0501+4516 Harsha S. Kumar1, Alaa. I. Ibrahim2,3 & Samar Safi-Harb1,4 Accepted for publication in the Astrophysical Journal: April 20, 2010 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 8 models, among which the cut-offpowerlaw and the two- blackbody models provided the best fit in the 15–150 keV energy range. The cut-offpowerlaw model fit yields a mean photon index ΓCP L = 0.54±0.11 and a cut-offenergy EC = 19.1±1.8 keV for the bursts. The mean hard and soft blackbody temperatures are found to be kTBBh = 12.8±0.7 keV and kTBBs = 4.6±0.5 keV, respectively, and are anti-correlated with the square of the radii of the hard and soft emitting regions (RBBh and RBBs) as R2 BBh ∝kT −5.8 and R2 BBs ∝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 ∼4.4×10−9 ergs cm−2 to ∼2.7×10−6 ergs cm−2, consistent with those observed for typical short SGR bursts. Subject headings: gamma rays: bursts — stars: individual (SGR 0501+4516) — stars: neutron — X-rays: bursts 1. INTRODUCTION The Swift mission (Gehrels et al. 2004), launched in November 2004 to explore the gamma-ray bursts (GRBs), has provided an excellent opportunity to de- tect and study the γ-ray activities from known Soft Gamma Repeaters (SGRs) as well as to discover new SGRs. SGRs, belonging to the class of ‘magnetars’ (Dun- can & Thompson 1992), are highly magnetized (B ∼ 1014−15 G) and slowly rotating (P ∼2−12 s) neutron stars characterized by short, bright bursts of hard X- rays and soft γ-rays (see Mereghetti 2008 for a recent review). During its active state, an SGR can go through periods of intense bursting activity lasting from a few days to months, however, it can also remain dormant for many years. The bursts, often varying in duration, are classified into three main categories: short, intermediate and giant flares. The short bursts are the typical kind observed during an outburst, marked by timescales ∼ 0.1–0.5 s and luminosities ∼1038–1041 ergs s−1, whereas the intermediate bursts have timescales ∼1–60 s and lu- minosities ∼1041–1043 ergs s−1 (Mereghetti 2008). Gi- ant flares, the rare and unique events, are distinguished by their extreme energies (1044–1047 ergs s−1), long du- rations (∼hundreds of seconds), and the presence of a coherent pulsating decaying tail, consistent with the spin period of the neutron star. Persistent X-ray emission, in- 1 Department of Physics & Astronomy, University of Manitoba, Winnipeg, MB R3T 2N2, Canada; har- sha@physics.umanitoba.ca 2 Department of Physics, American University in Cairo; Faculty of Science, Cairo University, Cairo, Egypt 3 Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA; ai@space.mit.edu 4 Canada Research Chair; samar@physics.umanitoba.ca terpreted as originating due to the magnetospheric cur- rents driven by twists in the evolving ultra-high magnetic field (Thompson & Duncan 1995), has also been observed from SGRs in the 0.1–10 keV band with a typical X-ray luminosity of ∼1035 ergs s−1, and the spectrum is gen- erally described by an absorbed powerlaw (PL) plus a blackbody component (Mereghetti 2008). According to the magnetar model (Thompson & Dun- can 1995), the dominant form of energy powering an SGR is its decaying ultra-strong magnetic field. The surface of the neutron star is heated and fractured by instabil- ities generating Alfv´en waves that accelerate electrons, and in turn give away their energy in short bursts. The model also suggests that the high-energy dissipated re- mains trapped in the magnetosphere as the ‘trapped fire- ball’, and it shrinks in size with time. Alternatively, the bursts can arise from heating of the corona by magnetic reconnection in the stellar magnetosphere resulting in intermediate-type flares (Lyubarsky 2002). The giant flares are caused by the sudden rearrangement of the star’s magnetic field producing global crustal fractures (Thompson & Duncan 19

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