Circular No. 7098
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions)
BMARSDEN@CFA.HARVARD.EDU or DGREEN@CFA.HARVARD.EDU (science)
URL http://cfa-www.harvard.edu/iau/cbat.html  ISSN 0081-0304
Phone 617-495-7244/7440/7444 (for emergency use only)


XTE J1550-564
     B. A. Harmon, M. H. Finger, M. L. McCollough, S. N. Zhang, W.
S. Paciesas, and C. A. Wilson, Marshall Space Flight Center, NASA,
report, for the Compton Gamma Ray Observatory/BATSE Team:  "The
x-ray transient XTE J1550-564 (IAUC 7008) was detected in BATSE
data beginning on Jan. 21.  As of Jan. 23, the one-day averaged
flux was 300 mCrab (+/- 20 percent, 20-100 keV), with a hard
spectrum (power-law photon-number index -2.3 +/- 0.2).  This is a
reflare in hard x-rays, following the primary outburst in 1998
Sept.-Oct. (IAUC 7010).  The source has been active in soft x-rays
(2-12 keV) since about Dec. 18, according to observations by the
Rossi X-ray Timing Explorer ASM."


GRB 990123
     W. Offutt, Cloudcroft, NM, obtained two 600-s and two 900-s
exposures of the optical candidate for GRB 990123, using a 0.60-m
f/7 Ritchey-Chretien reflector (+ CCD + Bessell R-band filter).
The mean position from the four exposures is R.A. = 15h25m30s.35,
Decl. = +44o45'59".2 (equinox 2000.0; twelve USNO SA2.0 stars as
reference).  The following photometry (mean residual 0.25 mag) is
based on the reference star located about 11".4 west of the target
object (R = 19.1):  Jan. 24.4400 UT, 19.8; 24.4567, 19.8; 24.4666,
20.0; 24.4781, 20.3.
     B. A. Skiff, Lowell Observatory, writes, in reference to IAUC
7096:  "In the absence of a proper calibration sequence, optical
observers of GRB 990123 have been making photometric zero-point
adjustments using USNO A2.0 magnitudes.  It is perhaps worth noting
that these data do not in general correspond to either the Johnson
B or Cousins R systems, and that they often possess substantial
systematic errors in scale, in the zero-point, and as a function of
star color.  For the GSPC sequence near the center of the relevant
plate, plus a few other stars from the Minnesota APS group
(http://isis.spa.umn.edu/Docs/get_photom.html), it turns out that
the A2.0 'mb' values are close to standard B; 'mr' is offset about
+0.3 mag from Cousins R (A2.0 being too faint).  More typically,
both 'mb' and 'mr' are about 0.6 mag too bright compared to B and R.
My large file of faint photometric reference stars can be used to
make local corrections to A2.0 or other catalogues of faint stars:
ftp://ftp.lowell.edu/pub/bas/starcats/loneos.stds."

                      (C) Copyright 1999 CBAT
1999 January 25                (7098)            Daniel W. E. Green