A transient I-band excess in the optical spectrum of the accreting millisecond pulsar SAX J1808.4-3658

The optical counterpart of the transient, millisecond X-ray pulsar SAX J1808.4-3658 was observed in four colours (BVRI) for five weeks during the 2005 June-July outburst. The optical fluxes declined by similar to 2 mag during the first 16d and then commenced quasi-periodic secondary outbursts, with time-scales of several days, similar to those seen in 2000 and 2002. The broad-band spectra derived from these measurements were generally consistent with emission from an X-ray heated accretion disc. During the first 16d decline in intensity the spectrum became redder. We suggest that the primary outburst was initiated by a viscosity change driven instability in the inner disc and note the contrast with another accreting millisecond pulsar, XTE J0929-314, for which the spectrum becomes bluer during the decline. On the night of 2005 June 5 (HJD 245 3527) the I-band flux was similar to 0.45-mag brighter than on the preceding or following nights whereas the BV and R bands showed no obvious enhancement. A type I X-ray burst was detected by the Rossi X-ray Timing Explorer spacecraft during this I-band integration. It seems unlikely that reprocessed radiation from the burst was sufficient to explain the observed increase. We suggest that a major part of the I-band excess was due to synchrotron emission triggered by the X-ray burst. Several other significant short duration changes in V - I were detected. One occurred at about HJD 245 3546 in the early phase of the first secondary outburst and may be due to mass-transfer instability or to another synchrotron emission event.