The first outburst of the new magnetar candidate SGR 0501+4516

Abstract

We report here on the outburst onset and evolution of the new Soft Gamma\nRepeater SGR 0501+4516. We monitored the new SGR with XMM-Newton starting on\n2008 August 23, one day after the source became burst-active, and continuing\nwith 4 more observations, with the last one on 2008 September 30. Combining the\ndata with the Swift-XRT and Suzaku data, we modelled the outburst decay over\n160 days, and we found that the source flux decreased exponentially with a\ntimescale of t_c=23.8 days. In the first XMM-Newton observation a large number\nof short X-ray bursts were observed, the rate of which decayed drastically in\nthe following observations. We found large changes in the spectral and timing\nbehavior of the source during the outburst, with softening emission as the flux\ndecayed, and the non-thermal soft X-ray spectral component fading faster than\nthe thermal one. Almost simultaneously to our XMM-Newton observations (on 2008\nAugust 29 and September 2), we observed the source in the hard X-ray range with\nINTEGRAL, which clearly detected the source up to ~100keV in the first\npointing, while giving only upper limits during the second pointing,\ndiscovering a variable hard X-ray component fading in less than 10 days after\nthe bursting activation. We performed a phase-coherent X-ray timing analysis\nover about 160 days starting with the burst activation and found evidence of a\nstrong second derivative period component (\\ddot{P} = -1.6(4)x10^{-19}\ns/s^{-2}). Thanks to the phase-connection, we were able to study the the\nphase-resolved spectral evolution of SGR 0501+4516 in great detail. We also\nreport on the ROSAT quiescent source data, taken back in 1992 when the source\nexhibits a flux ~80 times lower than that measured during the outburst, and a\nrather soft, thermal spectrum.\n