Activity from Magnetar Candidate 4U 0142+61: Bursts and Emission Lines

Abstract

After 6 years of quiescence, Anomalous X-ray Pulsar (AXP) 4U 0142+61 entered an active phase in 2006 March that lasted several months. During the active phase, several bursts were detected, and many aspects of the X-ray emission changed. We report on the discovery of six X-ray bursts, the first ever seen from this AXP in ~10 years of Rossi X-ray Timing Explorer (RXTE) monitoring. All the bursts occurred in the interval between 2006 April 6 and 2007 February 7. The bursts had the canonical fast rise slow decay profiles characteristic of SGR/AXP bursts. The burst durations ranged from 8-3x10^3 s as characterized by T90,these are very long durations even when compared to the broad T90 distributions of other bursts from SGRs and AXPs. The first five burst spectra are well modeled by simple blackbodies, with temperature kT ~2-6 keV. However, the sixth burst had a complicated spectrum consisting of at least three emission lines with possible additional emission and absorption lines. The most significant feature was at ~14 keV. Similar 14-keV spectral features were seen in bursts from AXPs 1E 1048.1-5937 and XTE J1810-197. If this feature is interpreted as a proton cyclotron line, then it supports the existence of a magnetar-strength field for these AXPs. Several of the bursts were accompanied by a short-term pulsed flux enhancement. We discuss these events in the context of the magnetar model.