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MAXI/GSC detections of new superbursts from Ser X-1 and 4U 1705-44

ATel #9882; Wataru Iwakiri (RIKEN), Jean in 't Zand (SRON), Motoko Serino (RIKEN)
on 20 Dec 2016; 01:09 UT
Credential Certification: Wataru Iwakiri (wataru.iwakiri@riken.jp)

Subjects: X-ray, Binary, Neutron Star

We report the MAXI/GSC detection of new candidate superbursts from Ser X-1 and 4U 1705-44.

The event from Ser X-1 was found in MAXI/GSC observations on 2011 Dec 06 (MJD 55901). Three superbursts were reported previously from this source (Cornelisse et al. 2002, A&A, 382, 174; Kuulkers 2009, ATel #2140). The new MAXI event was detected at 08:10 UT and lasted for at least 4 hours with a fast-rise and exponential-decay light curve. The spectrum of each of the three scan transits is well fitted by an absorbed blackbody model with temperatures kT = 2.1 (+0.3, -0.2), 1.5 (+0.3, -0.2) and 1.1 (+0.6, -0.4) keV at 08:10, 09:42 and 11:14 UT, respectively. The hydrogen column density was fixed at 0.5 x1022 cm-2 (Bhattacharyya & Strohmayer 2007, ApJL, 664, L103). The results prove cooling along the decay. The bolometric flux declined from 6.7 (+/- 1.1) x10-9 to 1.9 (+/- 0.7) x10-9 erg/s/cm2 over the course of 3 hours. The e-folding decay time is 2.3 +/- 0.7 hours. On the basis of these results, we conclude that this is the 4th superburst event from Ser X-1. The recurrence time between the 3rd and the 4th superburst is 1178 days although there is a possibility of missing events during data gaps.

The new candidate superburst from 4U 1705-44 was found in observations on 2016 Oct 22 (MJD 57683). Although it was predicted that 4U 1705-44 is a prospective superburster by in 't Zand et al. (2003, A&A, 411, L487), no superburst was observed from this source yet. The observed new event detected at 20:19 UT lasted for at least 2 hours with a fast-rise and exponential-decay light curve. The energy-resolved lightcurves show that the X-ray emission softens throughout the decay. The spectrum obtained during the first and second scan transits (20:19 and 21:52 UT) are well fitted by an absorbed blackbody model with temperatures of kT = 1.8 (+/- 0.1) and 1.8 (+/- 0.2) keV. The hydrogen column density was fixed at 1.9 x1022 cm-2 (Piraino et al. 2007, A&A, 471, L17). This result shows that the blackbody temperature did not decrease significantly but is typical of superbursts. The bolometric flux declined from 1.4 (+/- 0.1) x10-8 to 0.7 (+/- 0.1) x10-8 erg/s/cm2 over the course of 1.5 hours. The e-folding decay time is 2.2 +/- 0.6 hours. From these results we conclude that this is the first superburst observed from 4U 1705-44.

All the quoted errors are statistical 1 sigma errors.