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IAUC 7019: XTE J1550-564; 1998eb

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                                                 Circular No. 7019
Central Bureau for Astronomical Telegrams
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
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XTE J1550-564
     R. Remillard and E. Morgan, Massachusetts Institute of
Technology; and J. McClintock and G. Sobczak, Harvard-Smithsonian
Center for Astrophysics, write:  "The x-ray nova and blackhole
candidate XTE J1550-564 has brightened dramatically since its
discovery at 0.07 Crab (2-12 keV) on Sept. 7 (IAUC 7008).
Observations with the RXTE All Sky Monitor show a steady rise to
1.7 Crab on Sept. 15.  Thereafter, there is increased variability,
with the intensity reaching 3.2 Crab on Sept. 18.7 UT.  On Sept. 19
and 20, a large flare peaked at 6.8 Crab, and the intensity fell
back to the range of 2.7-3.6 Crab on Sept. 20 and 21.  PCA
observations on Sept. 22.15 indicate a flux of 2.0 Crab at 2-30
keV.  The long-term trend shows a steeper x-ray spectrum with
increased brightness, while a local hardening of the spectrum was
observed during the Sept. 19-20 flare.  This x-ray nova is the
brightest yet observed by RXTE, and observations at other
wavelengths are strongly encouraged."
     J. D. Hawley, S. M. Petrinec, and D. L. Chenette, Lockheed
Martin Missiles and Space ATC Space Physics, communicate:  "XTE
J1550-564 (IAUC 7008) has been observed by the PIXIE instrument
onboard the NASA/POLAR spacecraft since it first became visible on
Sept. 7.  The first PIXIE observation with adequate counting
statistics was at Sept. 7.7 UT.  The object is visible for about
5.5 hr every orbit (18-hr period, apogee over north pole at 9 earth
radii).  Although the primary mission of PIXIE is to study the
bremsstrahlung x-rays of the earth's auroral regions, we also have
the opportunity to observe celestial x-ray sources that appear off
the limb of the earth.  Averaged x-ray flux measurements (E = 2-12-
keV, 64 logarithmically spaced energy channels) for each apogee
pass show a nearly-linear slope of +0.8 photon sE-1 cmE-2 dayE-1
for Sept. 7-16.  On Sept. 16, the x-ray flux dipped but then rose
rapidly (flux of 5.8 photons sE-1 cmE-2 on Sept. 16.0, versus 24
photons sE-1 cmE-2 on Sept. 19.5), with the flux peaking on Sept.
20.  PIXIE also measures Cir X-1, and on Sept. 12 the intensity of
XTE J1550-564 surpassed the typical level of Cir X-1.  Preliminary
analysis shows similar spectral shapes for Cir X-1 and XTE
J1550-564.  Spectra along with more detailed information and plots
can be found on the PIXIE home page, http://pixie.spasci.com."

SUPERNOVA 1998eb IN NGC 1961
     S. Pesci, Milan, Italy, reports that a visual search of NGC
1961 with a 0.5-m telescope on Aug. 26.9 UT showed no new star
(limiting mag 15.8).

                      (C) Copyright 1998 CBAT
1998 September 23              (7019)            Daniel W. E. Green

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