2012MNRAS.422..981S

We present results of our analysis of the optical emission of the unique source GRB 070610/SWIFT J195509+261406 during its 2007 outburst and discuss the implications of the non-detection of the very faint optical counterpart in quiescence. We analyse the statistical properties of the optical brightness and X-ray intensity during the outburst and determined an approximate value of the mean colour index R-I. The optical emission has the form of spikes whose duration decreases with the outburst's progress, and the spectral energy distribution from the X-ray to the optical bands undergoes variations during the spikes. We show that the optical emission can be explained by a synchrotron mechanism while any thermal component is below the detection limit. We discuss the similarities between synchrotron emission of J1955 and the optical afterglows (OAs) of the extragalactic long gamma-ray bursts. We conclude that while the emission mechanisms in J1955 and OAs are quite similar, the configurations of their light-emitting regions largely differ. The synchrotron emission of the spike must be primarily generated near (several times 10¹⁰ cm) the central object, and cannot be explained by density enhancements in the interstellar medium. We argue that the activity of a central engine plays a great role in generating the afterglow emission of J1955. We discuss the possibility that J1955 is an ultracompact binary with a very low time-averaged mass transfer rate on to the neutron star. This can fulfil the conditions required for J1955, with the 2007 outburst as the extreme case seen from the very close X-ray binary SAX J1808.4-3658 in 2005.