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Keywords:

  • radiation mechanisms: non-thermal;
  • gamma-ray burst: general;
  • stars: neutron;
  • ISM: jets and outflows;
  • X-rays: binaries

ABSTRACT

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 RI. 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.