Central Bureau for Astronomical Telegrams
IAUC 7257: GM Sgr, SAX J1819.3-2525 = XTE J1819-254
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Circular No. 7257
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
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Phone 617-495-7244/7440/7444 (for emergency use only)
GM SAGITTARII AND SAX J1819.3-2525 = XTE J1819-254
M. L. McCollough and M. H. Finger, Universities Space Research
Association and Marshall Space Flight Center; and P. M. Woods,
University of Alabama, Huntsville, report for the BATSE team: "GM
Sgr appears as a strong and highly variable source in the BATSE
energy range. The recent x-ray event (IAUC 7253) was first
detected in the BATSE data on Sept. 14.88 UT with a 20-100-keV flux
of about 5 Crab (9.0 x 10**-8 erg cm**-2 s**-1 with a photon index
of -3.5). The occultation measurement prior to this showed no
excess flux above background. From Sept. 15.36 to 15.68, the
source was detected at fluxes reaching about 8 Crab (20-100 keV).
Power spectra made from 20-50-keV rates during this interval were
all consistent with a featureless 1/f continuum from 5 to 500 mHz.
The fractional rms amplitudes for the range 10-500 mHz varied from
17 to 38 percent. For the remainder of the day, the source was
undetected except for several short flares of about 10-500 s. The
peak flux values of these events reach 1.2 x 10**-7 erg cm**-2
s**-1 (20-100 keV), with the source being clearly detected above
100 keV."
C. B. Markwardt, University of Maryland and Goddard Space
Flight Center (GSFC); J. H. Swank, GSFC; and E. H. Morgan,
Massachusetts Institute of Technology, report that the RXTE PCA
observed the transient source XTE J1819-254 starting on Sept. 15.89
UT for about 3000 s. The intensity of the source during the
observation ranged from (1.8 to 860) x 10**-10 erg s**-1 cm**-2 in
the band 2-60 keV. After a flaring period, characterized by rapid
shots and lasting about 1000 s, it entered a quiescent phase at the
minimum flux level. The x-ray spectrum during the flare was
consistent with a power law having a photon index of 0.9, a rolloff
above 20 keV, a soft excess, and neutral hydrogen absorption < 2 x
10**21 cm**-2. During the quiescent phase, the spectrum steepened
to a photon index of 3. An iron emission line is present in both
spectra, with a best-fit centroid of 6.5 keV and equivalent width
of about 500 eV. No significant pulsations or quasiperiodic
variability were detected, although red noise was present below 30
Hz. Subsequent PCA observations on Sept. 16.06, 16.12, and 16.18
also showed quiescent behavior at similar intensities. We have
also undertaken regular monitoring observations of the galactic-
center region. These show that the source has varied irregularly
in the band 2-60 keV between (0.2 and 12) x 10**-10 erg s**-1
cm**-2 over the past 7 months, the most recent measurements being
(in the same units) Sept. 9.30, 2.1; 12.15, 3.1; 15.10, 4.1.
Additional public RXTE PCA observations are planned.
(C) Copyright 1999 CBAT
1999 September 17 (7257) Daniel W. E. Green
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