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IAUC 6885: SAX J1808.4-3658 = XTE J1808-369; 1998S

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                                                 Circular No. 6885
Central Bureau for Astronomical Telegrams
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
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SAX J1808.4-3658 = XTE J1808-369
     P. Roche, University of Sussex; D. Chakrabarty, Massachusetts
Institute of Technology (MIT); L. Morales-Rueda and R. Hynes,
University of Sussex; S. M. Slivan, MIT; C. Simpson, Jet Propulsion
Laboratory; and P. Hewett, Institute of Astronomy, Cambridge,
report:  "We have identified 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 at La Palma and
the 3.8-m U.K. Infrared Telescope at Mauna Kea on Apr. 18 UT
detected a star that is not present to a limiting magnitude of V >
19 in the Digitized Sky Survey (DSS) image of the field (from a
1987 U.K. Schimdt plate).  Its position is R.A. = 18h08m27s.5, Decl.
= -36o58'42" (equinox 2000.0), or 19" from the center of the 0'.6
(68-percent confidence) x-ray error circle from SAX (in't Zand et
al. 1998, A.Ap. 331, L25).  Its position is also consistent with
the smaller 25" uncertainty from the x-ray timing data (Chakrabarty
and Morgan 1998, http://xxx.lanl.gov/abs/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 < 0.1 mag in JHK.  We
note that a fainter star (V about 18) that is located 5" south-
southeast 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-hr 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."

     CCD photometry by D. Hanzl, Brno, Czech Republic, with a 0.4-m
reflector (G = comparison star GSC 3452.1093, V = 9.87 +/- 0.04,
B-V = +0.9 +/- 0.06; T = Tycho comparison stars):  Apr. 8.869 UT, V
= 12.77 (G); 11.892, V = 12.91 (G); 13.832, V = 13.02 +/- 0.02 (G);
20.895, V = 13.32 (T); 25.873, V = 13.49 +/- 0.01, B-V = +0.96 +/-
0.04 (T); 26.928, V = 13.54 +/- 0.03, B-V = +0.91 +/- 0.06 (G).

                      (C) Copyright 1998 CBAT
1998 April 27                  (6885)            Daniel W. E. Green

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