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 magnitudes during the first 16 days and then commenced quasi-periodic secondary outbursts, with time-scales of several days, similar to those seen in 2000 and 2002. The broadband spectra derived from these measurements were generally consistent with emission from an X-ray heated accretion disc. During the first 16 days 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 2453527) the I band flux was ~0.45 magnitudes brighter than on the preceding or following nights whereas the BVR bands showed no obvious enhancement. A Type I X-ray burst was detected by the RXTE 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 2453546 in the early phase of the first secondary outburst and may be due to a mass transfer instability or to another synchrotron emission event. 
J. G. Greenhill (1), A. B. Giles (1 and 2), C. Coutures (3) ((1)   University of Tasmania (2) Spurion Technology (3) CEA / S...