A hard X-ray flare of SAX J1712.6-3739: a superburst event ?

ATel #11623; Jie Lin and Wenfei Yu (Shanghai Astronomical Observatory)
on 9 May 2018; 08:42 UT
Credential Certification: Wenfei Yu (wenfei@shao.ac.cn)

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SAX J1712.6-3739 was discovered by BeppoSAX/WFC in 1999 and a type I X-ray burst was detected with an unabsorbed bolometric flux of (5.1+/-0.5) x 10^-8 ergs/cm^2/s, which led to its identification of a neutron star LMXB and the estimate of its distance of ~7 kpc, by assuming a neutron star with 1.4 solar mass ( Cocchi et al. 2001). It was later classified as a persistent source by Fiocchi et al. (2008) and it is likely an ultracompact X-ray binary candidate (in't Zand et al., 2007; Fiocchi et al., 2008).

By investigating the orbital monitoring data of Swift/BAT, we found the 15-50 keV X-ray intensity of SAX J1712.6-3739 rose to 845.7+/-63.0 mCrab on May 8th, 2018 at 09:18:12 UT, then declined to 148.0+/-31.0 mCrab at 09:24:44 UT and to 87.8+/-23.8 mCrab at 09:32:36 UT on the same day. The peak bolometric luminosity (0.1-100 keV), if estimated as a thermonuclear burst by assuming a black body temperature of 3 keV and a distance of 7 kpc, is about 2.3x10^38 erg/s, roughly corresponding to the Eddington luminosity of a neutron star. This favors the identification of the X-ray flare as a thermonuclear burst.

The hard X-ray burst last more than 860 seconds based on BAT monitoring in the 15-50 keV band. This is much longer than those of typical type I bursts. The hard X-ray flux declined with an e-folding decay time around 260+/-78 seconds, which is also much shorter than the typical e-folding decay time of superbursts based the measurements in the soft X-ray energy band (typically 3-20 keV) (see, for example, Atel #8253; Atel #9882; Atel #11422). Ballantyne & Strohmayer (2004) and Keek et al. (2014) showed that the blackbody temperature of the X-ray spectrum during superbursts in 4U 1820-30 and 4U 1636-536 decreased from ~3 keV to below 2 keV in the first few thousand seconds, corresponding to the decrease in the ratio between the X-ray flux in the 15-50 keV band to that in the 3-20 keV band by a factor of 5, from about 0.30 to 0.06. The e-folding decay time of the X-ray flare as seen in the soft X-ray band would be at least 5 times longer. Therefore the hard X-ray flare detected by Swift/BAT would correspond to a superburst event in the direction of the ultracompact LMXB candidate SAX J1712.6-3739. X-ray observations of the source are highly encouraged.