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IAUC 7274: XTE J1859+226; C/1999 S3

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                                                  Circular No. 7274
Central Bureau for Astronomical Telegrams
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
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XTE J1859+226
     A. Wood and D. A. Smith, Massachusetts Institute of Technology
(MIT); and F. E. Marshall and J. Swank, Goddard Space Flight Center
(GSFC), on behalf of the RXTE All-Sky Monitor (ASM) team at MIT and
GSFC, report the emergence of a new x-ray transient:  "The source's
intensity was measured to rise slowly during approximately a 12-hr
interval to 160 +/- 15 mCrab (2-12 keV) on Oct 9.658 UT and 250
mCrab by Oct. 10.523, confirmed by a PCA observation (with source
position given by Markwardt et al., below).  The rise was
consistent with a linear increase at a rate of about 6 mCrab/hr.
Extrapolation indicates that the outburst began around Oct. 8.55 UT.
The ASM spectrum is roughly Crab-like and shows no indication of
evolution.  Neither the ASM catalogue nor SIMBAD lists any known
x-ray sources in or near the positional error box.  Observations at
other wavelengths would be timely."
     C. B. Markwardt, University of Maryland and GSFC; and F. E.
Marshall and J. H. Swank, GSFC, write:  "The RXTE PCA observed the
transient source XTE J1859+226 on Oct. 10.56-10.60 UT for about
2000 s.  The intensity of the source was 1.7 x 10**-8 erg s**-1
cm**-2 in the band 2-60 keV (= 250 mCrab in the band 2-10 keV),
with fractional r.m.s. variability of about 32 percent.  PCA scans
during the observation determined a source position of R.A. =
18h58m35s, Decl. = +22o39'.3 (equinox 2000.0), with a 95-percent
uncertainty radius of 1', which also accounts for the intrinsic
variability of the source.  A power spectrum shows a quasiperiodic
oscillation and two harmonics, the fundamental being centered at
0.45 Hz (FWHM 0.11 Hz), with a fractional r.m.s. amplitude of about
17 percent.  No high-frequency pulsations or variability were seen.
The energy spectrum can be modeled as a power law with photon index
1.7 and neutral hydrogen absorption of about 3 x 10**21 cm**-2.
These timing and spectral characteristics together are reminiscent
of a galactic blackhole candidate.  Additional PCA observations are

     Total visual magnitude estimates:  Sept. 27.88 UT, 12.6 (K.
Hornoch, Lelekovice, Czech Republic, 0.35-m reflector; moonlight);
29.82, 12.5 (R. J. Bouma, Groningen, The Netherlands, 0.25-m
reflector); Oct. 1.79, 12.7 (W. Hasubick, Buchloe, Germany, 0.44-m
reflector); 2.82, 12.5 (M. Lehky, Hradec Kralove, Czech Republic,
0.42-m reflector); 2.92, 12.3 (G. Comello, Roden, The Netherlands,
0.31-m reflector); 6.10, 12.6 (Bouma).

                      (C) Copyright 1999 CBAT
1999 October 11                (7274)            Daniel W. E. Green

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