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RXTE/ASM observations of new superbursts from Ser X-1 and 4U 1636-536

ATEL # 2140; Erik Kuulkers (ISOC, ESA/ESAC, Spain)
on 31 Jul 2009; 14:23 UT
Password Certification: Erik Kuulkers (

Subjects: X-ray, Binaries, Neutron Stars, Variables
Referred to by ATEL #: 2141

A systematic search for superbursts in the RXTE/ASM archive up to June 2009 of all type I X-ray bursters known to date revealed previously unnoticed long-duration events.

Two candidate superbursts were found in ASM data of Ser X-1. One superburst was previously known for this source (Cornelisse et al. 2002, A&A 382, 174). The new ones occurred on 1999 Aug 9 (MJD 51399) and 2008 Oct 14 (MJD 54753). The first event started between UT 01:41 and 03:16, during which it rose from a 1.5-12 keV persistent flux of ~0.18 Crab to an observed peak flux of 0.41+/-0.03 Crab. Subsequent measurements show a declining flux over a 3.2 hr time span, with a 1.5-12 keV count rate e-folding decay time of 3.6+/-0.3 hrs. At UT 16:03 near-to-persistent levels were seen. The X-ray emission softens throughout the decay, based on the ratio of the count rates in the 5-12 keV and 1.5-5 keV bands. Analysis of the 3-channel ASM data (see, e.g., Kuulkers 2002, A&A 383, L5) shows that the flare minus persistent emission is consistent with a black-body spectrum of temperatures (kT) between 1.8+/-0.2 and 2.3+/-0.3 keV, and marginal evidence for cooling during the decay.
The second event started between UT 05:07 and 06:41, during which it rose from a flux of about 0.16 Crab to an observed peak flux of 0.48+/-0.02 Crab. About 1.6 hrs later the flux declined to a flux of 0.30+/-0.01 Crab. It reached near-to-persistent flux on UT 14:31. The e-folding decay time is 1.4+/-0.15 hrs. The inferred temperatures clearly show cooling along the decay from 1.9+/-0.2 keV near the observed peak to 1.0+/-0.2 keV near the end.
Both events are similar to the previous superburst.

Cornelisse et al. noted an increase in the flux of Ser X-1 for several ASM dwells on on UT 2001 Jul 30 (MJD 52120) 03:39, being possibly related to another superburst. During the 9.5 min time span fluxes between ~0.36 and ~0.55 Crab are observed. 0.7 hrs before and 1.4 hrs after this time span the flux was at persistent values near 0.15 Crab. The e-folding decay time is 13+/-4 min. Only marginal evidence for spectral softening can be seen. The net-event emission shows typical type I X-ray burst black-body temperatures (1.8-1.9+/-0.2 keV). Based on the duration, I suggest it not to be a superburst, but a possible intermediate-duration burst (see Cumming et al. 2006, ApJ 646, 429).

A candidate superburst was found in the ASM data of 4U 1636-536 on 1997 Jul 13 (MJD 50642). Three superbursts were previously seen (Wijnands 2001, ApJ 554, L59; Strohmayer & Markwardt 2002, ApJ 577, 337; Kuulkers et al. 2004, AIPC, 714, 257). The new event started between UT 07:09 and 08:44, and reached an observed peak flux of 0.78+/-0.03 Crab. 1.7 hrs later it had declined to a flux of 0.47+/-0.01 Crab. The e-folding decay time is 1.8+/-0.2 hrs. 4.8 to 15.6 hrs after the observed peak the flux (~0.28 Crab) was still above the pre-event persistent level (~0.19 Crab). The inferred black-body temperatures clearly show cooling along the decay from 2.3+/-0.4 keV at the observed peak to 1.2+/-0.2 keV near the end. This 4th event is similar to the other 3 superbursts.

This gives for the first time constraints on the recurrence time scale of superbursts in Ser X-1 (892 days between the event seen by Cornelisse et al. on 1997 Feb 28 [MJD 50507] and the one on 1999 Aug 9), and more stringent constraints in the case of 4U 1636-536 (389 days between the event seen by Wijnands on 1996 Jun 19 [MJD 50253] and the one on 1997 Jul 13). The difference in the e-folding decay times in Ser X-1 may point to different initial conditions in the same source.

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