The optical counterpart of XTE J0929-314: the third transient millisecond X-ray pulsar

A blue and variable optical counterpart of the X-ray transient XTE J0929-314 was identified on 2002 May 1. We conducted frequent BVRI broad-band photometry on this object using the Mt Canopus 1-m telescope during May and June until it had faded to below 21st magnitude. Nearly continuous I-band CCD photometry on 2002 May 2–4 revealed a ∼10 per cent sinusoidal modulation at the binary period lasting ∼6 cycles during the latter half of May 2. The phase indicates that the modulation may be due to a combination of emission by a hotspot on the disc and X-ray heating of the secondary. The emission generally trended bluer with B−I decreasing by 0.6 mag during the observations, but there were anomalous changes in colour during the first few days after optical identification when the I-band flux decreased slightly while fluxes in other bands increased. Spectral analysis of the BVRI broad-band photometry shows evidence of a variable excess in the R and I bands. We suggest that this may be due to synchrotron emission in matter flowing out of the system, and note that similar processes may have been responsible for anomalous V- and I-band measurements in 1998 of the persistent millisecond X-ray pulsar SAX J1808.4-3658.