Unusual Strong Hard Flare or Burst in 1A 1246-588

ATel #830; A. M. Levine (MIT), R. A. Remillard (MIT), and D. Galloway (U. of Melbourne) for the ASM team at MIT and NASA/GSFC
on 8 Jun 2006; 20:07 UT
Credential Certification: Alan M. Levine (aml@space.mit.edu)

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Observations with the RXTE All-Sky Monitor revealed a fast flare or unusual burst in the X-ray source 1A 1246-588 (RA 192.414, Dec -59.088; J2000) in four consecutive dwells on 2006 May 26. Each dwell was 90 s long with a 6 s interval between dwells to rotate the camera assembly. The X-ray flare began at 2006 May 26 18:57:34 (UTC; MJD 53881.78998) and lasted at least 340 s. The average source strength in each of the four dwells was 103 +- 4, 144 +- 4, 73 +- 3, and 39 +- 2 ASM counts/s in the 1.5-12 keV band (corresponding to 1.37 +- 0.05, 1.91 +- 0.06, 0.97 +- 0.03, and 0.52 +- 0.02 Crab). The source was not detected (0.1 Crab limit per dwell at 5 sigma) in preceding or succeeding observations even though it was observed 50 minutes before the beginning of the first of the four dwells and 35 minutes after the end of the last.

The spectrum of the emission during the flare was significantly harder than the spectrum of either the Crab Nebula or typical X-ray bursts with average rates relative to those of the Crab in the second of the four dwells of 0.56 (1.5-3 keV), 1.08 (3-5 keV), and 3.43 (5-12 keV), with an HR2 value (5-12 keV rate/3-5 keV rate) of 3.47. HR2 values this high are typical of spectra of accreting X-ray pulsars in HMXBs; they are not typical of the persistent emission from LMXBs.

In a review of the RXTE/ASM 10-year light curve of 1A 1246-588, we found no other flares or bursts from this source in the ten year record of ASM observations which are comparable in duration and peak intensity to the event reported here. Bassa et al. (A&A, 446, L17, 2006) recently reported an optical identification of 1A 1246-588 which suggests that the system is an ultracompact binary and they note that type I bursts have been seen from this system in BeppoSAX and RXTE/ASM data. This suggests that the present event may be an unusually hard and long Type I burst.