有哪些信誉好的足球投注网站A transient I band excess in the optical spectrum of the accreting millisecond pulsar SAX J.pdf
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A transient I band excess in the optical spectrum of the accreting millisecond pulsar SAX J
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Mon. Not. R. Astron. Soc. 000, 1–7 (2005) Printed 5 February 2008 (MN LATEX style file v2.2)
A transient I band excess in the optical spectrum of the
accreting millisecond pulsar SAX J1808.4–3658
J. G. Greenhill1, A. B. Giles1,2 and C Coutures3
1 School of Mathematics and Physics, University of Tasmania, Private bag 37, Hobart, Tasmania 7001, Australia
2 Spurion Technology Pty. Ltd., 200 Mt. Rumney Road, Mt. Rumney, Tasmania 7170, Australia
3 CEA/Saclay, 91191 Gif-sur-Yvette cedex, France
Accepted 10 May 2006
ABSTRACT
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 out-
burst. 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 sug-
gest 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 BV R 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 occurr
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