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IAUC 7856: XTE J1908+094; gamma-RAY TRANSIENT

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                                                  Circular No. 7856
Central Bureau for Astronomical Telegrams
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
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XTE J1908+094
     P. M. Woods, C. Kouveliotou, M. H. Finger, and E. Gogus,
National Space Science and Technology Center; and J. Swank, C.
Markwardt, and T. Strohmayer, Goddard Space Flight Center, report
the serendipitous discovery of a new x-ray transient, XTE J1908+094,
in RXTE PCA observations of the soft-gamma-ray repeater SGR 1900+14,
triggered following the burst activity on Feb. 17-18 (GCN 1253).
These observations failed to detect the 5.2-s SGR pulsations,
pointing towards a possible new source as the origin of the high
x-ray flux. An RXTE PCA scan of the region around SGR 1900+14 on
Feb. 21 was consistent with emission only from known sources (and
no new sources).  However, the scans required SGR 1900+14 to be 20
times brighter than its quiescent flux level (GCN 1256).  A
Director's Discretionary Time Chandra observation on Mar. 11 showed
that the SGR was quiescent and did not reveal any new source within
the Chandra ACIS field-of-view.  A subsequent RXTE PCA scan on Mar.
17, taken in combination with the first scan, required that a new
source be included in the fit.  The best-fit position is R.A. =
19h08m50s, Decl. = +9 22'.5 (equinox J2000.0; estimated 2'
systematic error radius), or approximately 24' away from the SGR
source.  The source spectrum (2-30 keV) can be best fit with a
power-law function including photoelectric absorption (column
density N_h = 2.3 x 10**22, photon index = 1.55).  Iron line
emission is present, but may be due to the Galactic ridge.  Between
Feb. 19 and Mar. 17, the source flux (2-10 keV) has risen from 26
to 64 mCrab.  The power spectrum is flat between 1 mHz and 0.1 Hz,
falling approximately as 1/f**0.5 up to 1 Hz.  At 1 Hz is seen a
broad quasiperiodic oscillation peak and a break to a 1/f**2 power
law, which continues to 4 Hz.  The fractional rms amplitude from 1
mHz to 4 Hz is 43 percent.  No coherent pulsations are seen between
0.001 and 1024 Hz.  The authors conclude that XTE J1908+094 is a
new blackhole candidate.

     M. Schmidt, California Institute of Technology, writes:  "The
two gamma-ray transients observed by Konus and Ulysses on 1995 Jan.
10 and Mar. 25 (IAUC 7840) are clearly present in BATSE DISCLA data
from GRO Compton.  In a systematic search for gamma-ray bursts in
DISCLA data completed in 2001, it was found that three software
triggers occurred during the Jan. 1995 event and two during the
Mar. event.  The localizations of the triggers are all within 17
deg of Cyg X-1, consistent with the conclusion on IAUC 7840 that
these two outbursts originated from Cyg X-1."

                      (C) Copyright 2002 CBAT
2002 March 21                  (7856)            Daniel W. E. Green

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