A burst and simultaneous short-term pulsed flux enhancement from the magnetar candidate 1E 1048.1-5937

We report on the 2004 June 29 X-ray burst detected from the direction of the AXP 1E 1048.1-5937 using the RXTE. We find a simultaneous increase of similar to 3.5 times the quiescent value in the 2-10 keV pulsed flux of 1E 1048.1-5937 during the tail of the burst, which identifies the AXP as the burst's origin. The burst was overall very similar to the two others reported from the direction of this source in 2001. The unambiguous identification of 1E 1048.1-5937 as the burster here confirms that it was the origin of the 2001 bursts as well. The epoch of the burst peak was very close to the arrival time of 1E 1048.1-5937's pulse peak. The burst exhibited significant spectral evolution, with the trend going from hard to soft. Although the average spectrum of the burst was comparable in hardness (Gamma similar to 1.6) to those of the 2001 bursts, the peak of this burst was much harder (Gamma similar to 0.3). During the 11 days following the burst, the AXP was observed further with RXTE, XMM-Newton, and Chandra. Pre- and postburst observations revealed no change in the total flux or spectrum of the quiescent emission. Comparing all three bursts detected thus far from this source, we find that this event was the most fluent (> 3.3; 10(-8) ergs cm(-2) in the 2-20 keV band), had the highest peak flux (59 +/- 9; 10(-10) ergs s(-1) cm(-2) in the 2-20 keV band), and had the longest duration (> 699 s). The long duration of the burst differentiates it from SGR bursts, which have typical durations of similar to 0.1 s. Bursts that occur preferentially at pulse maximum, have fast rises, and long X-tails containing the majority of the total burst energy have been seen uniquely from AXPs. The marked differences between AXP and SGRs bursts may provide new clues to help understand the physical differences between these objects.