Verification of the transient in the M31 globular cluster Bol 194 as super-soft X-ray source

ATel #1306; F. Haberl (Max-Planck-Institut fuer extraterrestrische Physik, MPE), W. Pietsch (MPE), M. Henze (MPE)
on 27 Nov 2007; 17:24 UT
Credential Certification: Frank Haberl (fwh@mpe.mpg.de)

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Following the discovery of the transient X-ray source in the M31 globular cluster Bol 194 with Chandra HRC-I (Haberl et al. 2007, ATel #1296) we asked for a Swift follow-up target of opportunity observation. During the Swift 7.26 ks exposure (observation with ID 00031027001 starting on 2007-11-24.34) the transient was detected with 40+/-6 cts with the XRT at energies below 750 eV. The temperature of a blackbody fit to the very soft X-ray spectrum can be constrained to 50 eV < kT < 100 eV (90% confidence) with the formal best fit value at 84 eV. The best fit absorbing column density is 5.9e20 cm-2, consistent with the Galactic foreground NH, with an upper limit of 2.3e21 cm-2. The observed flux in the 0.2-2.4 keV band is 1.8e-13 erg/cm2/s which corresponds to a source luminosity (for a distance of 780 kpc) of 4e37 erg/s to 3e38 erg/s, depending on the actual column density. Assuming the blackbody model with the best fit parameters, the estimated Chandra HRC-I count rate is 0.021 cts/s, fully compatible with the measured count rate (ATel #1296).

The spectrum is consistent with that of a super-soft X-ray source while a black hole system in soft/high state seems to be excluded due to the non-detection of a hard emission component. The source could either be a highly variable (transient) super-soft X-ray source like e.g. RX J0513.9-6951 in the LMC or an optical nova in the globular cluster Bol 194 (similar to M31N 2007-06b; Pietsch et al. 2007, ATel #1294). However, in the latter case no nova outburst was reported from optical observations which may have been missed due to the location of the source in the globular cluster.

We would like to thank the Swift team for the scheduling of the observations.