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 ∼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 ∼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.