Anomalous X-Ray Pulsar 1E 1048.1-5937: Pulsed Flux Flares and Large Torque Variations

We report on continued monitoring of the anomalous X-ray pulsar (AXP) 1E 1048.1-5937 using the Rossi X-R ay Timing Explorer . We confirm that this pulsar has exhibited significant pulsed flux variability. The principal features of this variability are two pulsed X-ray flares. Both flares lasted several months and had well-resolved few-week-long rises. The long rise times of the flares are a phenomenon not previously reported for this class of object. The epochs of the flare peaks were MJD 52,218.8±4.5 and 52,444.4±7.0. Both flares had shorter rise than fall times. The flares had peak fluxes of 2.21 ± 0.16 and 3.00 ± 0.13 times the quiescent value. We estimate a total 2-10 keV energy release of ~2.7 × 10 40 and ~2.8 × 10 41 ergs for the flares, assuming a distance of 5 kpc. We also report large (factor of ~12) changes to the pulsar's spin-down rate on timescales of weeks to months, shorter than has been reported previously. We find marginal evidence for correlation between the flux and spin-down rate variability, with probability of nonrandom correlation 6%. We discuss the implications of our findings for AXP models.