Glitches in Anomalous X-ray Pulsars

We report on 8.7 and 7.6 yr of Rossi X-ray Timing Explorer (RXTE) observations of the Anomalous X-ray Pulsars (AXPs) RXS J170849.0−400910 and 1E 1841−045, respectively. These observations, part of a larger RXTE AXP monitoring program, have allowed us to study the long-term timing, pulsed flux, and pulse profile evolution of these objects. We report on four new glitches, one from RXS J170849.0−400910 and three from 1E 1841−045. One of the glitches from 1E 1841−045 is among the largest ever seen in a neutron star, having fractional frequency jump ∆ν/ν = 1.6 × 10. With nearly all known persistent AXPs now seen to glitch, such behavior is clearly generic to this source class. We compare AXP glitches with those in radio pulsars. We show that in terms of fractional frequency change, AXPs are among the most actively glitching neutron stars, with glitch amplitudes in general larger than in radio pulsars. However, in terms of absolute glitch amplitude, AXP glitches are unremarkable. We show that the largest observed AXP glitches show recoveries that are unusual among those of radio pulsar glitches, with the combination of glitch recovery time scale and fraction yielding changes in spin-down rates in the days following the glitch of the order of, or larger than, the long-term average. We also observed a large fractional increase in the magnitude of the spin-down rate of 1E 1841−045, following its largest glitch, with ∆ν̇/ν̇ = 0.1. These observations are challenging to interpret in standard glitch models, as is the frequent occurence of large glitches given AXPs’ high measured temperatures. We speculate that the stellar core may be involved in the largest AXP glitches. Furthermore, we show that AXP glitches appear to fall in two classes: radiatively loud and radiatively quiet. The latter, of which the glitches of RXS J170849.0−400910 and 1E 1841−045 are examples, show little or no evidence for an accompanying radiative event such as a flux increase or pulse profile changes. We also show, however, that pulse profile changes and pulsed flux changes are very common among these AXPs, and are generally not correlated with any obvious timing behavior. Department of Physics, McGill University, Montreal, QC H3A 2T8 NASA Goddard Space Flight Center, Greenbelt, MD.