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
Credential Certification: Dr. Andy Read (amr30@star.le.ac.uk)
Subjects: X-ray, Request for Observations, Star, Transient
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.
