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GALEX pre-outburst detection of OT_J213806.6+261957

ATEL # 2605; Boris Gaensicke
on 9 May 2010; 12:08 UT
Password Certification: Boris Gaensicke (Boris.Gaensicke@warwick.ac.uk)

Subjects: Optical, Ultra-Violet

The optical transient in Pegasus, recently reported in CBET 2273 and CBET 2275 (currently designated VSX J213806.5+261957 or OT_J213806.6+261957) has been detected in the GALEX All-Sky Survey in quiescence (MJD=53955.8) at m(fuv)=16.15+-0.03, F(fuv)=1257.58 +/- 33.16 microJy and m(NUV)=16.15+-0.02, F(nuv)=1260.16 +/- 23.79 microJy. The behaviour of this transient resembles an outburst of a WZ Sge star. The ultraviolet spectrum of WZ Sge stars is dominated by emission from the white dwarf photosphere. The flat ultraviolet spectrum of OT_J213806.6+261957 observed by GALEX is compatible with a white dwarf temperature of ~15000K, similar to that of WZ Sge itself. Assuming a 0.6Msun white dwarf mass, the GALEX fluxes imply a distance of ~70pc. Such a short distance is consistent with the proper motion of the object discussed by Henden in CBET 2275. It appears now most likely that OT_J213806.6+261957 is indeed a WZ Sge star, i.e. a short-period cataclysmic variable with a low accretion rate and rare but large-amplitude outbursts, and it might be among the closest known members of this class.


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