Central Bureau for Astronomical Telegrams
IAUC 7390: XTE J1118+480
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Circular No. 7390
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
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XTE J1118+480
M. Uemura and T. Kato, Kyoto University (KU); and H. Yamaoka,
Kyushu University, on behalf of the VSNET collaboration team,
report the detection of the probable optical counterpart
(unfiltered CCD mag 12.92) of XTE J1118+480 on Mar. 30.726 UT,
using a KU 0.25-m telescope, at R.A. = 11h18m10s.79, Decl. =
+48o02'11".2 (equinox 2000.0; accuracy < 1"; measured by Yamaoka
with GSC-ACT stars), which is well within the error radius of the
ASM position (IAUC 7389). Yamaoka further notes that a star with
red mag 18.8 in the USNO A1.0 and A2.0 catalogues (A2.0 position
end figures 10s.851, 12".78) lies within 2" of this position, and
no other object can be seen on images of the second Digital Sky
Survey within 10". This suggests that this star is possibly the
quiescent counterpart of XTE J1118+480. D. Buczynski, Condor Brow
Observatory, confirmed the object at unfiltered CCD mag 12.91 on
Mar. 30.863. Kato forwards magnitudes by K. Takamizawa, Saku-machi,
Nagano (T-Max 400 film): fainter than 15.0-15.4 on 15 dates, 1994
Nov. 4-1999 Dec. 7.774; 2000 Jan. 8.743, 12.7; 27.595, 13.3; 28.604,
12.3; Feb. 27.617, 13.8; Mar. 24.481, 12.9. Visual magnitude
estimate by O. Trondal, Oslo, Norway: Mar. 30.867, 12.5.
G. G. Pooley and E. M. Waldram, Mullard Radio Astronomy
Observatory, University of Cambridge, write: "We obtained
observations at 15 GHz with the Ryle Telescope of a field of
diameter 15', centered on the position of XTE J1118+480 (IAUC 7389),
between Mar. 30.71 and 31.21 UT. Two sources were detected: NVSS
J111820+475659 with flux density 5 mJy, and a source (not detected
in NVSS) at R.A. = 11h18m10s.7, Decl. = +48o02'14" (equinox 2000.0;
uncertainty 3"; rms noise 0.5 mJy) with flux density 6.2 mJy. The
inferred variability or unusual spectrum of this source make it a
candidate for the counterpart of the x-ray source."
C. A. Wilson and M. L. McCollough, Marshall Space Flight
Center, report for the Compton Gamma Ray Observatory BATSE Team:
"The current outburst of XTE J1118+480 was first detected in 1-day
CGRO/BATSE earth-occultation measurements at a level of 61 +/- 16
mCrab (20-100 keV) on Mar. 26. We see evidence of a slow rise in
flux since about Mar. 5. The January outburst (IAUC 7389) is of a
much different character from the current outburst, having risen
abruptly from < 20 mCrab (20-100 keV) on Jan. 4 to 78 +/- 16 mCrab
on Jan. 5 and reaching a peak of 110 +/- 20 mCrab on Jan. 11; the
outburst slowly declined and the source was undetectable by Jan. 29.
The energy spectra from both the current outburst and the January
outburst are well characterized by a power law with a photon index
of 2.1 +/- 0.1, and the source is visible up to 120 keV."
(C) Copyright 2000 CBAT
2000 March 31 (7390) Daniel W. E. Green
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