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IAUC 7880: XTE J1901+014; XTE J1751-305

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                                                  Circular No. 7880
Central Bureau for Astronomical Telegrams
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
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XTE J1901+014
     R. A. Remillard, Massachusetts Institute of Technology (MIT);
and D. A. Smith, University of Michigan, for the RXTE ASM Team at
MIT and NASA Goddard Space Flight Center, report the discovery of
a fast x-ray transient.  The detection was initially thought to be
a gamma-ray burst (cf. http://gcn.gsfc.nasa.gov/gcn/gcn3/1332.gcn3),
with a peak flux of 0.9 Crab (2-12 keV).  The event began on Apr.
6.76268 UT, lasting > 2 min and < 3.15 hr (see
http://xte.mit.edu/xtej1901+104).  The ASM data show rapid
evolution to a very hard spectrum during this outburst, but the IPN
instruments failed to detect the source above 15 keV (K. Hurley,
private communication).  Furthermore, archival ASM data show a
previous outburst at the same position on 1997 June 21.215, which
was longer than 6 min and < 8 hr, with a peak flux of 0.4 Crab.
The combined ASM observations yield a best position R.A. =
19h01m36s, Decl. = +1 26'.4 (equinox J2000.0; estimated uncertainty
3' in R.A. and 2' in Decl.).  The error box contains one very faint,
unidentified x-ray source from the Einstein Galactic Plane Survey:
2E 1859.1+0122 (R.A. = 19h01m40s.2, Decl. = +1 27'13", equinox
J2000.0; error radius 48"; Hertz and Grindlay 1988, A.J. 96, 233).
The time scales for these eruptions are reminiscent of the
microquasar and blackhole binary V4641 Sgr, so optical and radio
observations of XTE J1901+014 are strongly encouraged.

XTE J1751-305
     Z. Wang and D. Chakrabarty, MIT, report that the candidate
infrared counterpart that they proposed for XTE J1751-305 (IAUC
7874) is now likely excluded.  Further analysis of their J-band
images of the field from Apr. 9 UT have led to a revised
astrometric solution (0".5 rms) that places the candidate outside
the 0".6-radius Chandra x-ray error circle (IAUC 7876), lying 0".8
west and 0".4 north of its center.  No stars are detected within
the error circle (limiting mag J = 20.5).  The two detected stars
closest to the Chandra position are the former candidate (0".9
distant) and one (J = 18.0 +/- 0.5; blended with a neighbor to the
southwest having J = 13.5) that is 1".2 distant.
     C. B. Markwardt, University of Maryland and Goddard Space
Flight Center, reports that XTE J1751-305 (IAUC 7867) has become
undetectable by the RXTE PCA instrument.  Since discovery, RXTE
monitoring has shown a decrease in the x-ray flux, and as of Apr.
17.5 UT the 2-10-keV upper limit was approximately 1 mCrab (due
primarily to other nearby sources).

                      (C) Copyright 2002 CBAT
2002 April 18                  (7880)            Daniel W. E. Green

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