RXTE Detection of 1E 1740.7-2942
ATel #258; C. B. Markwardt (U. Maryland & NASA/GSFC), D. M. Smith and N. Bezayiff (U. C. Santa Cruz), J. H. Swank (NASA/GSFC), W. A. Heindl (U. C. San Diego)
on 25 Mar 2004; 04:35 UT
Credential Certification: Craig B. Markwardt (craigm@lheamail.gsfc.nasa.gov)
Subjects: X-ray, A Comment, Black Hole, Transient
Grebenev, Revnivtsev and Sunyaev discuss the recent non-detection by
INTEGRAL of 1E 1740.7-2942 (ATEL #257). This source is regularly
monitored by RXTE PCA scanning observations and direct pointings,
which each occur twice weekly.
In the 2-10 keV band the source has a typical flux of about 25 mCrab.
However, in the past half a year, the flux has declined to about 9
mCrab, but is still strongly detectable.
The current spectrum is consistent with the soft state, and not the
hard state, of black-hole binaries. In 2002, 1E 1740.7-2942 had
another soft X-ray low state, where the X-ray flux dipped by a similar
amount (ATEL #94). Thus, such transitions may be more common than
previously thought. An even more dramatic transition was observed in
2001 from the very similar system GRS 1758-258 (ATEL #66).
