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IAUC 6424: 1996aj; Sco X-1

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                                                  Circular No. 6424
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)

     The Mount Stromlo Abell Cluster Supernova Search Team (cf.
IAUC 6418) reports the discovery of an apparent supernova on CCD
images taken by S. Chan on the Mount Stromlo Observatory 1.27-m
telescope (+ Macho Camera).  SN 1996aj is located at R.A. =
13h29m06s.82, Decl. = -29o14'02".0, which is 4".45 west and 0".4
north of a galaxy situated near Abell 3559.  A nearby star has
position end figures 07s.99, 20".9.  SN 1996aj is visible on V and
R CCD images taken on June 15 at mag about 18.5 and was not
detected on a poor frame obtained on May 28.
     P. Garnavich, P. Challis, and R. Kirshner, Harvard-Smithsonian
Center for Astrophysics, write:  "We obtained a spectrogram of SN
1996aj on June 19.3 UT with the Multiple Mirror Telescope.  The
spectrum shows that this is a type-Ia supernova between 10 and 20
days past maximum.  The redshift of the supernova is between 0.10
and 0.11, making it background to the nearby Abell cluster 3559."

     M. van der Klis and R. Wijnands, Astronomical Institute "Anton
Pannekoek", University of Amsterdam; W. Chen, University of
Maryland at College Park and Goddard Space Flight Center, NASA; F.
K. Lamb and D. Psaltis, University of Illinois at Urbana-Champaign;
E. Kuulkers, Astrophysics Division, European Space Agency and ESTEC;
W. H. G. Lewin, Massachusetts Institute of Technology; B. Vaughan,
California Institute of Technology; J. van Paradijs and S. Dieters,
University of Alabama at Huntsville and University of Amsterdam;
and K. Horne, University of St. Andrews, report:  "Rossi X-ray
Timing Explorer observations of Sco X-1 performed on May 25.0-25.2
UT show two kHz quasiperiodic oscillations (QPOs) similar to those
observed in Feb. (IAUC 6319; van der Klis et al., Ap.J. Lett.,
submitted).  Sco X-1 is at a lower accretion rate (as evidenced by
the absence of 6-Hz QPO) than in the Feb. observations and the QPOs
are at lower frequency (634 +/- 2 and 926 +/- 1 Hz).  Their
frequency difference is 292 +/- 2 Hz, significantly larger than
that in February (247 +/- 3 Hz).  This implies that the beat-
frequency interpretation proposed for the two kHz QPOs in 4U
1728-34 (IAUC 6320, 6387; Strohmayer et al., Ap.J. Lett., submitted),
where one QPO is due to the Keplerian rotation of the inner edge of
the accretion disk and the other to the beat of this same Keplerian
frequency with the neutron-star spin, can, for Sco X-1, be excluded."

                      (C) Copyright 1996 CBAT
1996 June 21                   (6424)            Daniel W. E. Green

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