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IAUC 6524: 1996cb; Sco X-1

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                                                  Circular No. 6524
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)

SUPERNOVA 1996cb IN NGC 3510
     S. Nakano, Sumoto, Japan, reports the discovery by Masakatsu
Aoki, Tsukioka Cho, Toyama, of a supernova (mag 16.5) on unfiltered
CCD frames taken on Dec. 15.707 UT with a 0.43-m f/6 reflector.  SN
1996ca is located at R.A. = 11h03m41s.98, Decl. = +28o54'13".7
(equinox 2000.0; measured using only three reference stars), which
is 20".9 west and 65".7 north of the center of NGC 3510.  There is
no image of the supernova on Aoki's patrol frames taken on Oct. 24
and Nov. 6, and nothing at this position on the Digital Sky Survey.
     P. Garnavich and R. Kirshner, Harvard-Smithsonian Center for
Astrophysics, report that P. Berlind obtained a spectrogram of SN
1996cb on Dec. 17.5 UT with the 1.5-m Tillinghast telescope; the
spectrum shows this object to be a type-II supernova near maximum.
Broad P-Cyg profiles of the Balmer lines dominate the spectrum.
The minimum of the H-alpha absorption trough indicates a supernova
expansion velocity of 21 000 km/s.  Narrow emission lines from the
host galaxy show a redshift of 770 km/s.

     A. G. Peele and N. E. White, Goddard Space Flight Center,
NASA, write:  "The public RXTE ASM lightcurve of Sco X-1 covering
the interval 1996 Feb. 22-Nov. 28 shows an intensity modulation
with a period of about 37 days.  The period is detected using a
Lomb-Scargle periodigram, and it has a false-alarm probablility of
< 10E-6; it is also detected using minimum entropy and chi**2-
folding techniques.  Detection of the period is complicated by the
well-known x-ray flaring from Sco X-1.  The modulation is most
evident in the low-energy (1.3-3.0-keV) channel, where the flaring
amplitude is a minimum, and it is a sinusoidal-like modulation of
the quiescent flux.  Fitting a sine wave to the data in this
channel gives an ephemeris of JD 2450160.9 (+/- 1.0) + (36.8 +/-
0.4)E, with phase zero defined as the modulation minimum and a full
amplitude of 13 +/- 2 percent.  The modulation amplitude for the
quiescent flux decreases by a factor of 4-5 between the soft- and
hard-energy (5.0-12.2-keV) channels.  The 37-day modulation does
not appear to be related to the flaring episodes.  The modulation
is not detected in archival Vela 5B and Ariel 5 ASM x-ray data (but
upper limits do not rule out its presence).  We encourage a search
for the 37-day period in the optical and radio emission from Sco
X-1 (= V818 Sco)."

                      (C) Copyright 1996 CBAT
1996 December 17               (6524)            Daniel W. E. Green

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