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IAUC 6405: 1996ab; Cyg X-1

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                                                  Circular No. 6405
Central Bureau for Astronomical Telegrams
Postal Address: Central Bureau for Astronomical Telegrams
Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions)
Phone 617-495-7244/7440/7444 (for emergency use only)

     J. Mueller reports her discovery of a supernova (mag about
18.5) located at R.A. = 15h21m09s.0, Decl. = +27o55'23" (equinox
2000.0), which is 10" west and 15" north of the center of a galaxy
evidently identified as Abell 2065-356 by Postman et al. (1988,
A.J. 95, 267), but which apparently is not the host galaxy (see
report by Garnavich et al., below), a conclusion supported by N.
Reid's finding that there is extended emission underlying SN 1996ab
that is apparently not connected with the larger system.  SN 1996ab
was found on a IIIa-J plate taken by K. M. Rykoski and J. D.
Mendenhall on May 12 UT with the 1.2-m Oschin Schmidt telescope in
the course of the second Palomar Sky Survey.  No object appears at
the position of SN 1996ab on original Sky Survey prints; Reid
further reports that there is no object at this position on Sky
Survey plates exposed in 1986 May and June.
     P. Garnavich, A. Riess, P. Challis, and R. Kirshner, Harvard-
Smithsonian Center for Astrophysics, write:  "Spectra obtained by
P. Berlind with the 1.5-m Tillinghast telescope on May 22.4 UT show
that this is a type-Ia supernova near maximum light.  The Si II
635.5-nm absorption is observed at 692 nm, which implies a redshift
of z = 0.13 for the supernova (assuming an expansion velocity of
10 000 km/s).  SN 1996ab is probably not associated with the Abell
Cluster 2065 (z = 0.06), but rather with a fainter galaxy
background to the cluster."

     S. N. Zhang, W. S. Paciesas, B. A. Harmon, G. J. Fishman, and
D. Crary, Marshall Space Flight Center, NASA, report for the
Compton Gamma Ray Observatory BATSE team:  "Cygnus X-1 has been
decreasing rapidly in intensity in the band 20-200 keV, as observed
by BATSE, roughly coincident in time with the increased 2- to 12-
keV flux reported by ASM/RXTE (see IAUC 6404).  The flux changed
from 1.0 +/- 0.02 Crab units (20-100 keV) on May 11 to 0.5 +/- 0.02
on May 19-20.  The energy spectrum also becomes softer during this
time (the Optically Thin Thermal Bremsstrahlung model temperature
being from 80 to about 50 keV).  The r.m.s. fraction (0.01-0.488
Hz) also decreased from a typical value of 8 +/- 0.5 to 5 +/- 0.5
percent during this period."

                      (C) Copyright 1996 CBAT
1996 May 22                    (6405)            Daniel W. E. Green

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