Recent optical variability in LSI +61 303

ATel #1765; F. Lewis (Las Cumbres Observatory Global Telescope Network (LCOGTN)/Open Univ.), D. M. Russell (Univ. of Amsterdam), N. Rea (Univ. of Amsterdam), P. Roche (LCOGTN/Cardiff Univ.), S. Clark (Open Univ.)
on 6 Oct 2008; 13:27 UT
Credential Certification: David M. Russell (D.M.Russell@uva.nl)

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We have been monitoring the optical counterpart of the TeV emitting binary LSI +61 303 (V615 Cas) since November 2005 with the Faulkes Telescope North (on Haleakala, Maui), as part of a program to monitor long-term variability in X-ray binaries. Prompted by the recent SGR-like burst observed from a source in the region of LSI +61 303 (GCN 8209; ATel #1715; see also ATel #1731, #1740), we searched for unusual optical activity during the last two months (3rd August - 1st October).

Using differential photometry with respect to three nearby stars, we find long-term variations of amplitude ~ 0.1 magnitudes in u' B V R i'-bands; similar to that found over an 8-year period by Paredes et al. (1994). Our light curves are here. There is a dip in the light curves on 9th September 14:23 UT - the date closest to the SGR-like burst (10th September at 12:52:21 UT; ATel #1715). The dip, of ~ 0.054 mag (compared to the observations either side of this date: 5th and 13th September) is sharp compared to the long-term variations and exists in all five filters. However, there is also a slightly less prominent dip on 30th August, and it is difficult to assess how significant the 9th September dip is. We cannot therefore solidly associate the SGR-like burst with unusual optical behaviour in LSI +61 303. In addition, we do not find any unusual optical activity around the time of the X-ray flaring from a source in the region of LSI +61 303 on 21st August (ATel #1730).

Note that if the burst was emitted by a magnetar hosted in the LSI +61 303 system, in principle one would not expect a strong variability in the optical band, as this is dominated by the emission of the massive companion star. However, no magnetar in an X-ray binary has ever been observed so far, hence no comparison can be made.

We will continue to monitor the source for the next few months while it remains visible to Faulkes Telescope North. The Faulkes Telescope Project is an educational and research arm of the Las Cumbres Observatory Global Telescope Network (LCOGTN). FL acknowledges support from the Dill Faulkes Educational Trust.

Lightcurve of LS I +61 303