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


Greenhill, JG and Giles, AB and Coutures, C, A transient I-band excess in the optical spectrum of the accreting millisecond pulsar SAX J1808.4-3658, Monthly Notices of the Royal Astronomical Society, 370, (3) pp. 1303-1308. ISSN 0035-8711 (2006) [Refereed Article]

DOI: doi:10.1111/j.1365-2966.2006.10544.x


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 broadband 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 -1 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. © 2006 RAS.

Item Details

Item Type:Refereed Article
Research Division:Physical Sciences
Research Group:Astronomical sciences
Research Field:Galactic astronomy
Objective Division:Expanding Knowledge
Objective Group:Expanding knowledge
Objective Field:Expanding knowledge in the physical sciences
UTAS Author:Greenhill, JG (Dr John Greenhill)
UTAS Author:Giles, AB (Dr Barry Giles)
ID Code:41061
Year Published:2006
Web of Science® Times Cited:12
Deposited By:Physics
Deposited On:2006-08-01
Last Modified:2007-03-14

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