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-Ray 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 and times the quiescent value. We 2.21 +/- 0.16 and 3.00 +/- 0.13 times the quiescent value. We estimate a total 2 - 10 keV energy release of similar to2.7 x 10(40) and similar to2.8 x 10(41) ergs for the flares, assuming a distance of 5 kpc. We also report large ( factor of similar to 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.