Circular No. 6985
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)
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Phone 617-495-7244/7440/7444 (for emergency use only)


SAX J1810.8-2609 = RX J1810.7-2609
     J. Greiner, Astrophysical Institute, Potsdam; A. J. Castro-
Tirado, Instituto de Astrofisica de Andalucia, Granada; and T.
Boller, Max Planck Insitute for Extraterrestrial Physics, Garching,
report:  "A 1150-s-exposure ROSAT HRI observation was performed on
Mar. 24 of the error box of the transient x-ray burster SAX
J1810.8-2609 (IAUC 6838).  One strong x-ray source was detected at
a mean countrate of 0.5 counts/s, corresponding to roughly 1.5
mCrab level in the band 0.1-2.4 keV (absorbed).  The best-fit x-ray
position is R.A. = 18h10m44s.5, Decl. = -26o09'01" (equinox 2000.0),
with an error radius of +/- 10".  The x-ray intensity does not vary
by more than a factor of 3, and no x-ray burst is detected.  The
above position was serendipituously covered by a ROSAT PSPC
pointing on 1993 Sept. 10, when SAX J1810.8-2609 was not detected
(3-sigma upper limit of 0.027 PSPC counts/s, corresponding roughly
to 0.01 HRI counts/s).  It was also not detected during the ROSAT
all-sky survey in Sept. 1990.  This proves the transient nature of
our detected ROSAT HRI source.  Assuming a power law of photon
index 2.2 and full galactic absorption of 3.7 x 10E21 cmE-2, the
unabsorbed flux is 1.0 x 10E-10 erg cmE-2 sE-1 in the band 0.1-2.4
keV (and 3.5 x 10E-11 erg cmE-2 sE-1 in the band 2-10 keV) -- i.e.,
a factor of 4 lower than during the BeppoSAX observations on Mar.
10-12 (IAUC 6843).  The HRI position of SAX J1810.8-2609 agrees
with the BeppoSAX WFC position (with 3' error radius) to within
30" (IAUC 6838), but it is more than 3' off the BeppoSAX NFI
position (with 1' error radius; IAUC 6843).  Thorough analysis of
the ROSAT attitude solution does not reveal any anomaly (courtesy
W. Grimm, German Space Operation Center, Oberpfaffenhofen) for our
ROSAT HRI observation.  Images in the Bessel B and the Gunn r and
z bands taken on 1998 Mar. 13 with the Danish 1.5-m telescope at La
Silla show several optical objects within the HRI error circle.  A
comparison image in the z band taken on July 25 by G. Israel
(Osservatorio di Roma) and S. Covino (Osservatorio di Brera) does
not reveal any variability of these objects down to z = 20.  Images
are posted at http://www.obs.aip.de/~jcg/xrt/sax1810m2609.html."


COMET C/1998 K2 (LINEAR)
     Visual m_1 estimates:  June 20.25 UT, 12.1 (R. Keen, Mt.
Thorodin, CO, 0.32-m reflector); 28.25, 12.4 (A. Hale, Cloudcroft,
NM, 0.41-m reflector); July 1.54, 12.1 (D. A. J. Seargent, Cowra,
N.S.W., 0.25-m reflector); 21.45, 12.7 (M. Mattiazzo, Wallaroo,
South Australia, 0.20-m reflector).

                      (C) Copyright 1998 CBAT
1998 August 6                  (6985)            Daniel W. E. Green