8.6 Hour Period in XMMU J115113.3-623730

ATel #2777; J. Patterson, J. P. Halpern (Columbia U.), C. Stockdale (CBA-Hazelwood), G. Bolt (CBA-Perth), S. Lowther (CBA-Pukekohe), and B. Monard (Bronberg Observatory and CBA-Pretoria)
on 6 Aug 2010; 20:16 UT
Credential Certification: Jules Halpern (jules@astro.columbia.edu)

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We have obtained time-series differential photometry of XMMU J115113.3-623730, the eruptive star reported by Greiner et al. (ATel #2746) and Hughes et al. (ATel #2771). Coverage consisted of 17 separate time series in V and unfiltered light, from 19 July to 6 August 2010, using several telescopes in the Center for Backyard Astrophysics (CBA) network. We used overlaps of V-light and unfiltered-light time series to convert the latter to a V scale; this produced an overall calibration uncertainty of 0.10-0.15 mag. Despite the unfavorable sky position (Centaurus in August), aliasing problems were overcome by the global distribution of telescopes (New Zealand-Australia-South Africa). The power spectrum of the 19-night light curve has a strong peak at 0.3604(5) days, along with a host of smaller peaks that are all aliases of the true frequency. An ephemeris for maximum light of the mean light curve is

HJD 2,455,397.224(5) + 0.3604(5) E.

Mean light was sensibly constant near V=13.9 during this interval. Apart from the 8.6 hour signal of full amplitude 0.44 mag, the only other variability was ~0.1 mag flickering seen on essentially every night. Figures illustrating these effects are linked below.

This 8.6 hour period is very likely the orbital signature of the underlying binary. The origin of the signal is unknown, although reprocessing of soft X-rays in the atmosphere of the orbiting "donor star" is a good working hypothesis. Still less known is the place of the star in the zoo of eruptive binaries. Probably it belongs to the V Sge class, for the reasons discussed by Hughes et al. (ATel #2771) (as well as the orbital period reported here). The 2008 ASAS-3 light curve seems oddly faint for a true classical nova, although the peak of the outburst may have been missed due to the seasonal gap. Archival study of the star's long-term light curve would be very helpful - as would a search for other signatures of a true nova event (radio, infrared emission, expanding shell).

Light Curves and Power Spectrum