Years of RXTE Monitoring of Anomalous X-ray Pulsar 4U 0142+61: Long-Term Variability

TL;DR

This 10-year RXTE study of AXP 4U 0142+61 reveals long-term stability, possible glitches, evolving pulse profiles, and flux increases, challenging existing magnetar models.

Contribution

The paper provides the first long-term, detailed timing and flux variability analysis of 4U 0142+61 over a decade, highlighting profile evolution and flux changes.

Findings

Stable rotation from 2000 to 2006 with 2.3% RMS residual

Possible glitch between 1998 and 2000, not confirmed

Pulse profile evolved, especially in 2-4 keV band

Abstract

We report on 10 years of monitoring of the 8.7-s Anomalous X-ray Pulsar 4U 0142+61 using the Rossi X-Ray Timing Explorer (RXTE). This pulsar exhibited stable rotation from 2000 March until 2006 February: the RMS phase residual for a spin-down model which includes nu, nudot, and nuddot is 2.3%. We report a possible phase-coherent timing solution valid over a 10-yr span extending back to March 1996. A glitch may have occured between 1998 and 2000, but is not required by the existing timing data. The pulse profile has been evolving since 2000. In particular, the dip of emission between its two peaks got shallower between 2002 and 2006, as if the profile were evolving back to its pre-2000 morphology, following an earlier event, which possibly also included the glitch suggested by the timing data. These profile variations are seen in the 2-4 keV band but not in 6-8 keV. We also detect a slow increase in the pulsed flux between 2002 May and 2004 December, such that it has risen by 36+/-3% over 2.6 years in the 2-10 keV band. The pulsed flux variability and the narrow-band pulse profile changes present interesting challenges to aspects of the magnetar model.