[vsnet-alert 1717] alert notice

[Ron Remillard <rr@space.mit.edu>]

Likely optical counterpart for X-ray binary (2 hr period)
that contains msec pulsar:

P. Roche, D. Chakrabarty, L. Morales-Rueda, R. Hynes,  S. M. Slivan, 
C. Simpson, and P. Hewett have reported a possible optical counterpart for the millisecond
X-ray pulsar SAX J1808.4-3658 (IAUC 6876, 6877, 6878).  Observations with the
1-m Jacobus Kapteyn Telescope in La Palma and the 3.8-m UK Infrared Telescope 
in Hawaii on UT April 18 detected a star which is not present to a limiting
magnitude of V>19 in the STScI Digitized Sky Survey (DSS) image of the field 
(from a 1987 UK Schimdt plate).   Its position is R.A.18h08m27s.5, Decl. 
-36o58'42" (equinox 2000.0), 19 arcsec from the center of the 0.6 arcmin (68 
percent confidence) X-ray error circle from SAX (in't Zand et al. 1998, A&A, 
331, L25).  Its position is also consistent with the smaller 25 arcsec 
uncertainty from the X-ray timing data (Chakrabarty and Morgan 1998, 
astro-ph/9804248).  No other stars in our V band images of the SAX error circle
showed significant variability with respect to the DSS image, to a limiting 
magnitude of about 17.5.  The magnitudes of the candidate are V=16.6, R=16.1, 
I=15.6, J=15.0, H=14.4, K=13.8, with an uncertainty of 0.2 mag in VRI and less
than 0.1 mag in JHK.  We note that a fainter star (about V=18) 5 arcsec SSE of 
the candidate position is visible on both the DSS image and our JHK images 
and so is not our candidate.  Spectroscopy and further photometry are urgently
required to check whether the candidate is an unrelated background variable, 
and to look for fading as the X-ray source declines as well as a possible 
2-hour orbital modulation in the intensity. Observations with the Rossi X-Ray 
Timing Explorer indicate that the X-ray source is declining on a timescale of 
2 weeks. 
------------

cheers,
Ron Remillard
rr@space.mit.edu

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