2013MNRAS.429.3123E

We present a long-term study of the 2011 outburst of the magnetar Swift J1834.9-0846 carried out using new Chandra observations, as well as all the available Swift, RXTE and XMM-Newton data. The last observation was performed on 2011 November 12, about 100 d after the onset of the bursting activity that had led to the discovery of the source on 2011 August 7. This long time-span enabled us to refine the rotational ephemeris and observe a downturn in the decay of the X-ray flux. Assuming a broken power law for the long-term light curve, the break was at ∼ 46 d after the outburst onset, when the decay index changed from α ∼ 0.4 to ∼ 4.5. The flux decreased by a factor of ∼ 2 in the first ∼ 50 d and then by a factor of ∼ 40 until 2011 November (overall, by a factor of ∼ 70 in ∼ 100 d). At the same time, the spectrum, which was well described by an absorbed blackbody all along the outburst, softened, the temperature dropping from ∼ 1 to ∼ 0.6keV. Diffuse X-ray emission extending up to 20arcsec from the source was clearly detected in all Chandra observations. Its spatial and spectral properties, as well as its time evolution, are consistent with a dust-scattering halo due to a single cloud located at a distance of ~200 pc from Swift J1834.9-0846, which should be in turn located at a distance of ∼ 5kpc. Considering the time delay of the scattered photons, the same dust cloud might also be responsible for the more extended emission detected in XMM-Newton data taken in 2011 September. We searched for the radio signature of Swift J1834.9-0846 at radio frequencies using the Green Bank Radio Telescope and in archival data collected at Parkes from 1998 to 2003. No evidence for radio emission was found, down to a flux density of 0.05 mJy (at 2 GHz) during the outburst and ∼ 0.2-0.3 mJy (at 1.4 GHz) in the older data.