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IAUC 6897: 1989af; SAX J1808.4-3658 = XTE J1808-369

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                                                 Circular No. 6897
Central Bureau for Astronomical Telegrams
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
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SUPERNOVA 1989af IN UGC 6433
     Jean Mueller reports her discovery of an apparent supernova
(red mag about 16.5) on a plate taken on 1989 Jan. 15 by P. Moniot
with the 1.2-m Oschin Schmidt Telescope in the course of the second
Palomar Sky Survey.  The star is located at R.A. = 11h25m31s.46,
Decl. = +38o03'43".6 (equinox 2000.0), which is 6".3 west and 6".1
north of the center of UGC 6433 (measures by G. V. Williams, SAO).
SN 1989af is also present (mag 17.5-18) on films taken on 1989 Mar.
30 with the 0.46-m Palomar Schmidt telescope by H. Holt and N. G.
Thomas.  There is no object at this position on original Palomar
Sky Survey prints, on the Digital Sky Survey, or on a second Survey
IIIa-J plate taken on 1990 Apr. 22 by C. Brewer and J. D. Mendenhall.

SAX J1808.4-3658 = XTE J1808-369
     A. V. Filippenko and D. C. Leonard, University of California
at Berkeley, report:  "Keck spectra (range 554-685 nm) of the
suspected optical counterpart (cf. IAUC 6885, 6886) of SAX
J1808.4-3658 reveal a possible very weak, double-peaked H-alpha
emission line of velocity (full width) 1000 km/s.  Absorption lines
characteristic of mid- to late-type stars are present throughout
the spectrum."
     M. Gilfanov, M. Revnivtsev, and R. Sunyaev, Max-Planck-
Institut fur Astrophysik and Russian Space Research Institute,
report:  "The analysis of the public-domain data of RXTE target-of-
opportunity observations of this 2.5-ms binary x-ray pulsar (IAUC
6876-6878, 6885, 6886) during Apr. 11-May 2 has shown that an
abrupt decline of the source luminosity occurred between Apr. 25
and 29.  Whereas the luminosity was decreasing at an approximately
constant rate corresponding to an e-folding time of about 10 days
between Apr. 11 and 25, it has dropped by a factor of about 4
between Apr. 25 and 29, and by another factor of about 10 by May 2.
The RXTE observations have revealed surprising stability of the
source spectrum during this luminosity decrease by more than two
orders of magnitude.  The source spectrum (energy range 3-100 keV
for Apr. 11-29 data; 3-25 keV for May 2 data) had a remarkable
Crab-like power-law shape with a photon index of about 2, while the
luminosity dropped from about 8 x 10E36 to about 6 x 10E34 erg/s
(assuming a distance of 4 kpc).  Such behavior is surprising within
a framework of conventional models of the accretion onto a weakly
magnetized neutron star.  The shape of the x-ray light curve
suggests that the magnetospheric radius has reached the corotation
radius at the luminosity level of a few x 10E35 erg/s."

                      (C) Copyright 1998 CBAT
1998 May 7                     (6897)            Daniel W. E. Green

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