Transient Black Hole Candidate XTE J1550-564 Brightens to 6.8 Crab
ATel #37; R. Rutledge (Caltech), D. Fox, and D. A. Smith (MIT)
on 19 Sep 1998; 22:10 UT
Credential Certification: R. E. Rutledge (rutledge@astron.berkeley.edu)
Subjects: X-ray, Binary, Transient
Referred to by ATel #: 58
The black hole candidate (BHC) X-ray transient XTE J1550-564 (IAUC
# 7008 ) continues to brighten in X-rays, according to RXTE
All-Sky Monitor (ASM; 2-12 keV) observations, and at the time of
this posting has reached 6.8 Crab (513 +/- 10 cts/sec) in its
brightest 90-second ASM dwell.
A total of 113 ASM dwells on the source between 7.0 Sep and 13.0 Sep
1998 (UT) show the X-ray intensity increased at an average rate of
0.23 +/-0.002 Crab/day (1 Crab=75.5 ASM c/s), up to 1.45+/-0.14 Crab,
with little (<5%) variability about this linear rise. Between 13.0
and 18.8 Sep, the X-ray intensity increased at the (slower) average
rate of 0.15 +/- 0.01 Crab/day, with considerable variability about
this trend (approximately 10-30%, increasing with time). Between Sep
18.4-18.8, 20 ASM dwells showed a variable intensity between 170-240
ASM c/s (about 2.3-3.2 Crab), far in excess of the 2-4 ASM c/s
uncertainty on each measurement.
After a 24-hr period with no ASM coverage, realtime analysis of 3
90-sec ASM observations between Sep 19.758-19.759 found the object
with countrates of 472, 476, and 513 (all +/- 10) ASM c/s,
corresponding to an average flux of 6.5+/-0.1 Crab.
X-ray colors, determined from the ASM data, show that the source
spectrum softened significantly over the first five days of the
outburst, from relative (1.5-3 keV):(3-5 keV):(5-12 keV) count rates
of 10:12.3(1.5):17.2(2.0) at the start of the outburst to
10:9.0(0.2):7.9(0.2) by 13 Sep. Since then, the X-ray colors have
remained roughly constant, hardening again only within the last day
(coincident with the jump in X-ray intensity) to
10:11.4(0.1):11.4(0.2).
The source is currently the brightest BHC yet observed by RXTE. As
such, observations at all wavelengths in continuation of the reported
optical (ATEL #34, #35; IAUC # 7009 , # 7013 ) and radio (IAUC # 7010 )
observations are encouraged.
We are grateful to the RXTE/ASM team for access to their quicklook
data and associated software products.
