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Swift Observations of the Supersoft X-ray Transient in the M31 Globular Cluster Bol 111

ATEL # 1333; A.K.H. Kong (NTHU), R. Di Stefano (CfA)
on 16 Dec 2007; 9:36 UT
Password Certification: Albert Kong (

Subjects: X-ray, Nova, Transients
Referred to by ATEL #: 1335

We report follow-up Swift observations of the supersoft X-ray transient in the M31 globular cluster Bol 111 (ATel #1294). Bol 111 was observed with Swift on 2007 Dec 13, 14, and 15. The source is still active and appears to rebrighten. In the two Swift observations taken on 2007 Nov 18, the source has a count rate of 0.0085 c/s with all counts below 0.6 keV (ATel #1294). The count rate of the supersoft source drops to 0.0045+/-0.0011 c/s on 2007 Dec 13 while on 2007 Dec 14 and 15, the source brightens to 0.0086+/-0.0017 c/s and 0.0092+/-0.0017 c/s, respectively with all photons below 1 keV. The combined X-ray spectrum from all 5 Swift observations can be fitted with an absorbed blackbody model with N_H=1.21e21 cm^-2 and kT=56 eV. This yields an average unabsorbed 0.3-1 keV luminosity of 1.57e38 erg/s (d=780 kpc).

In addition to the transient in Bol 111, we also discovered another supersoft X-ray transient in the field. The source is located at R.A.=00:42:09.9, Decl.=+40:57:48 (J2000) with a 90% error radius of 5.6 arcsec. The source has a count rate of 0.0026+/-0.0007 c/s with all photons below 0.7 keV on 2007 Nov 18. Assuming an absorbed blackbody model with N_H=1e21 cm^-2 and kT=50 eV, the corresponding unabsorbed 0.3-1 keV luminosity is 2.2e37 erg/s. On 2007 Dec 13, the source becomes brighter with a count rate of 0.0061+/-0.0007 c/s but it fades significantly on 2007 Dec 14 and 15 with a count rate of 0.0011+/-0.0006 c/s.

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

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R. E. Rutledge , Editor
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