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IAUC 6877: 1998ar; SAX J1808.4-3658 = XTE J1808-369; 1998an

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                                                 Circular No. 6877
Central Bureau for Astronomical Telegrams
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions)
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SUPERNOVA 1998ar IN NGC 2916
     Y. L. Qiu, Q.Y. Qaio, W. D. Li, and J. Y. Hu, Beijing
Astronomical Observatory (BAO) report the discovery of an apparent
supernova on unfiltered CCD frames taken in the course of the BAO
Supernova Survey on Apr. 14 and 15 UT, when the estimated
magnitudes of the new star were 18.4 and 18.2, respectively.  SN
1998ar is located at R.A. = 9h34m58s.86, Decl. = +21o42'57".5
(equinox 2000.0), which is 16".9 east and 38".5 north of the center
of NGC 2916.  CCD frames taken on Mar. 14 (limiting mag about 18.6)
show no star at the position of SN 1998ar.

SAX J1808.4-3658 = XTE J1808-369
     D. Chakrabarty and E. H. Morgan, Massachusetts Institute of
Technology, report for the Rossi XTE team:  "The pulsations from
XTE J1808-369 (= SAX J1808.4-3658) recently discovered in RXTE
observations (IAUC 6876) show clear evidence for a binary orbit.
Using RXTE/PCA data from Apr. 11.81-11.82, 11.86-11.89, and 13.07-
13.09 and assuming a constant pulsar-spin frequency for all these
data, we find the following provisional orbital parameters for the
neutron star:  an orbital period of 7251(1) s, a projected
semimajor axis of 0.0632(3) lt-s, and an orbital epoch (time of 90
degrees mean longitude) of JD 2450915.3995(1).  The corresponding
barycentric spin frequency is 400.9753(1) Hz.  The orbital
parameters give a pulsar mass function of 3.9 x 10E-5 solar mass,
one of the smallest mass functions for any x-ray binary.  Since
x-ray heating of the mass donor and/or the accretion disk in such a
low-mass x-ray binary is likely to be very strong, observations at
other wavelengths are urgently required to identify the counterpart
while the x-ray source is still active."

SUPERNOVA 1998an IN UGC 3683
     D. Hanzl, N. Copernicus Observatory and Planetarium, Brno,
reports the following position for SN 1998an (cf. IAUC 6871)
obtained with a 0.4-m reflector (+ CCD + V filter) on Apr. 11.8 UT:
R.A. = 7h08m17s.13 +/- 0s.02, Decl. = +46o06'54".3 +/- 0".8
(equinox 2000.0).  Hanzl also provides the following CCD V
magnitudes (based on GSC comparison stars):  Apr. 8.792, 15.6;
11.819, 15.6; 13.815, 15.7; 14.856, 15.6.

                      (C) Copyright 1998 CBAT
1998 April 15                  (6877)            Daniel W. E. Green

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