2006/02/28 by J. G. Greenhill, A. B. Giles, C. Coutures
Physics and Astronomy · #Accretion (finance) #Astronomy and Astrophysical Research #Astrophysical Phenomena and Observations #Flux (metallurgy) #Instability #Millisecond #Millisecond pulsar #Pulsar #Pulsars and Gravitational Waves Research #Synchrotron #Synchrotron radiation #astro-ph
paper · pdf · doi:10.1111/j.1365-2966.2006.10544.x
published as Mon.Not.Roy.Astron.Soc.370:1303-1308,2006 · 7 pages, 5 figures, accepted by MNRAS, 10 May 2006. Ver. 2 - larger Table 2 + various modest text changes
arxiv created 2006/05/25 · openalex publication_date 2006/06/15 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/05
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.