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IAUC 7265: C/1999 S3; GM Sgr, SAX J1819.3-2525 = XTE J1819-254

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                                                  Circular No. 7265
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions)
BMARSDEN@CFA.HARVARD.EDU or DGREEN@CFA.HARVARD.EDU (science)
URL http://cfa-www.harvard.edu/iau/cbat.html  ISSN 0081-0304
Phone 617-495-7244/7440/7444 (for emergency use only)


COMET C/1999 S3 (LINEAR)
     Additional astrometry is published on MPEC 1999-S18, from
whence the following preliminary parabolic orbital elements are
taken:

     T = 1999 Nov.  6.073 TT          Peri. =  42.810
                                      Node  =  12.476   2000.0
     q = 1.92488 AU                   Incl. =  71.679

1999 TT     R. A. (2000) Decl.   Delta     r    Elong. Phase    m1
Sept.19     1 41.16   +42 23.8   1.229   2.014  128.6   23.0   13.5
     24     1 23.34   +46 19.7   1.194   1.997  130.5   22.4   13.4
     29     1 01.17   +50 01.1   1.172   1.982  131.3   22.3   13.3
Oct.  4     0 34.49   +53 15.2   1.162   1.968  130.8   22.6   13.3
      9     0 03.81   +55 50.1   1.166   1.956  129.1   23.4   13.2


GM SAGITTARII AND SAX J1819.3-2525 = XTE J1819-254
     Further to their report on IAUC 7254, R. M. Hjellming et al.
write:  "Continued VLA observations of GM Sgr show that, since it
became optically thin on Sept. 17.0 UT, the radio fluxes have
decayed with a power law and spectrum approximated by
0.33[(JD-2451437)**-2.05](f**-0.66), where f is the frequency in
GHz, reaching 3.9, 1.8, and 1.1 mJy at 1.49, 4.9, and 8.5 GHz on
Sept. 22.0.  VLA observations at 21 cm on Sept. 17.0 show strong H
I absorption, identical within the noise to that seen against a
nearby extragalactic calibrator (J1820-2528), suggesting that GM
Sgr is at least a few hundred parsecs away; these data give no
constraints on the maximum distance.  The corresponding total H I
column density is 1.8 x 10**21 (T_s/100 K) cm**-2, where T_s is the
spin temperature.  On Sept. 16.02 the 0.41-Jy source at 4.9 GHz was
resolved into a 0.34-Jy source (at the position reported on IAUC
7254) and extensions roughly 0".25 north and south.  On Sept.
17.93, a 14.9-GHz image obtained with 'fast-phase' referencing to
J1820-2528 showed a about 4-mJy source at the IAUC 7254 radio
position and a about 9-mJy source located about 0".25 south, at the
same location as the southern extension on Sept. 16.02.
Observations on Sept. 22.00 and 24.1 show only this southern source.
Accurate optical astrometry is needed to show where GM Sgr lies
with respect to the radio emission.  This would determine whether
the Sept. 22-24 radio source should be interpreted as unusually
long-lived ejecta, perhaps interacting with some surrounding
material (cf. XTE J1748-288), or simply residual emission from the
central object."

                      (C) Copyright 1999 CBAT
1999 September 26              (7265)            Daniel W. E. Green

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