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Swift sees a return to quiescence in XMMSL1 J162940.2-112024: A possible massive stellar flare?

ATEL # 1718; A M Read (Leicester), R D Saxton (ESAC), J Osborne (Leicester), J Pye (Leicester), P A Evans (Leicester)
on 15 Sep 2008; 19:01 UT
Password Certification: Dr. Andy Read (amr30@star.le.ac.uk)

Subjects: X-ray, Request for Observations, Transients, Stars

XMMSL1 J162940.2-112024, a bright new X-ray transient discovered on Aug 20, 2008 by XMM-Newton in its Slew Survey mode [ATel #1689] has been observed by Swift for ~1.3 ksec on Sep 5, 2008. No significant emission is seen at the slew source position (RA:16 29 40.2, DEC:-11 20 24; err. 8''). A 2 sigma upper limit to the Swift-XRT (0.2-10 keV) count rate was calculated at <3.2e-3 ct/s.

The optical and IR magnitudes of the brighter, and closer, of the two catalogued optical sources within the slew error circle, USNO-B1.0 0786-0301344, are consistent with a star of effective temperature 3600K and bolometric luminosity 6e-5 L(solar). That is, it appears to be a late-type, main sequence M2/M3 star, a likely flaring star. For an M2 dwarf (Mv=+10, B-V=1.5) with Av=0, the distance to USNO-B1.0*0786-0301344 (B=19.5) is approximately 400pc.

At this distance, the slew source (0.2-10 keV) luminosity was 5.5e32 erg/s (assuming a best fit power-law model fit to the Slew spectrum; note a thermal model fits the slew spectrum well also, though the parameters are largely unconstrained). This is extremely bright for a stellar flare (the superflare from EV Lac for example [ATel #1499], is seen at >8e31 erg/s), though within the upper range of previously-observed large stellar flares (see e.g. compilation in Guedel 2004, A&AR 12, 71). The Swift-XRT (0.2-10 keV) upper limit (<3.6e30 erg/s) is consistent with typical quiescent X-ray luminosity of a dMe star (e.g. the quiescent value for EV Lac is ~2e30 erg/s).

We encourage further observations to ascertain the true nature of USNO-B1.0*0786-0301344 and its neighbour.


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