MAXI and RXTE-ASM detect superburst candidate for 4U 1820-30

ATel #3625; Jean in 't Zand (SRON), Motoko Serino (RIKEN), Nobuyuki Kawai (Tokyo Tech), Craig Heinke (U. Alberta)
on 2 Sep 2011; 13:03 UT
Credential Certification: Jean in' t Zand (jeanz@sron.nl)

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On March 17, 2010, MAXI/GSC and RXTE/ASM data detected a flare from the prototypical ultracompact X-ray binary 4U 1820-30 that has a duration of at least 57 min and a peak 4-10 keV flux of 1.9 Crab units. Given the duration and the fact that 4U 1820-30 for decades only exhibited fluxes below 0.5 Crab (e.g., Priedhorsky & Terrell, ApJ, 284, L17, 1984; Zdziarski et al., MNRAS 377, 1006, 2007) except during thermonuclear (type-I) X-ray bursts (e.g., Galloway et al., ApJS 179, 360, 2008), we tentatively identify this as a superburst. This would be the second superburst from 4U 1820-30, 10.5 yr after the first one (Strohmayer & Brown, ApJ, 566, 1045, 2002).

The MAXI/GSC flare data consist of a single 50-s long scan over the source centered on 17:22:03 UT (MJD 55272.72365) during which the triangular scan response is accurately followed so that the source must be constant during this time within statistical margins. Ordinary X-ray bursts from 4U 1820-30 are always shorter with e-folding decay times that are less than 8 s (e.g., Cornelisse et al., A&A, 405, 1033, 2003; Galloway et al. 2008). The fitted 4-10 keV flux is 1.9+/-0.1 Crab units. This is, within 10%, consistent with the peak flux of ordinary X-ray bursts from 4U 1820-30 (e.g., Galloway et al. 2008). The last MAXI scan over 4U 1820-30 prior to the high point is 1.5 hr before. It shows a flux of 0.30+/-0.05 Crab units. In the 24 hrs prior to the high point the average flux is 0.33 +/- 0.02 Crab units. The first data point after the high point is 1.5 hr later, at 0.50+/-0.06 Crab. The next one is 12 hr later at 0.43+/-0.06 Crab.

The ASM data nearest to the MAXI high point are 2 consecutive dwells 57 min afterward with an average flux of 0.50+/-0.03 Crab. The next ASM dwell is 9.0 hrs afterward with a flux of 0.30+/-0.03 Crab. The last ASM data prior to the high MAXI point is 15.9 hr before, at a flux of 0.33+/-0.02 Crab.

The flux histories from MAXI and ASM are roughly equivalent with an e-folding decay time of 0.5+/-0.1 hr. This is 2 times smaller than the first superburst, but the sampling is very sparse. In fact, the flux profile is consistent with the profile of the first superburst if it would have started roughly 0.5 hr before the high MAXI point.

The (10-20 keV)/(4-10 keV) flux hardness ratio of the high MAXI data point is 0.65+/-0.06 (after subtraction of pre-flare fluxes). The ratio is 0.25+/-0.04 for the 24-hr pre-flare data. This rough doubling of the hardness ratio is as expected between the persistent and thermonuclear emission.

The first SWIFT-BAT data point after the MAXI high point is 1.2 hr afterward. The 15-50 keV is 1.6+/-0.4 times higher than the daily average.

The 10.5 yr superburst recurrence time would be consistent with the ~13 yr prediction by Strohmayer & Brown (2002) for 4U 1820-30. Note that this applies to a hydrogen-deficient system which is different than for all other known superbursters except possibly 4U 0614+09 (Keek & in 't Zand, Proc. 7th INTEGRAL Workshop, 2008). However, the duty cycles of RXTE-ASM and MAXI for superbursts are not a perfect 100% but perhaps around 50% and, so, there is a non-negligible chance that a superburst was missed.