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IAUC 8190: Poss. N IN Sct; CXOU J180951.1-194351 = RXTE J1810-197

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                                                  Circular No. 8190
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)
CBAT@CFA.HARVARD.EDU (science)
URL http://cfa-www.harvard.edu/iau/cbat.html  ISSN 0081-0304
Phone 617-495-7440/7244/7444 (for emergency use only)


POSSIBLE NOVA IN SCUTUM
     S. Nakano, Sumoto, Japan; and H. Sato, National Astronomical
Observatory of Japan (NAOJ), report the discovery of a possible
nova by Hideo Nishimura (Kakegawa, Shizuoka-ken, Japan) on T-Max
films taken on Aug. 28.58 (mag 8.5) and 29.436 UT (mag about 8.4)
with a Pentax 200-mm f/4.0 lens, the position given as R.A. =
18h49m38s, Decl. = -9o33'45" (equinox 2000.0).  T. Nakamura, NAOJ,
reports the following mean precise position end figures by S. Wakuda
(Hamamatsu, Japan) from six CCD images (0.25-m f/5.1 reflector, Aug.
29.354, V = 9.5): 37s.62, 50".3.  H. Yamaoka, Kyushu University,
reports that ASAS-3 images show an object at position end figures
37s.7, 53" with the following V magnitudes:  Aug. 24.105, [14.0;
26.096, 10.15; 28.096, 8.94.  Yamaoka adds that a USNO-A2.0 star of
red mag 16.4 and blue mag 17.2 has position end figures 37s.684,
53".76; this star is also present in the GSC-2.2 and USNO-B1.0
catalogues, and a 2MASS star (J = 14.32, K = 13.82) has position end
figures 37s.70, 54".1.  Visual magnitude estimate by A. Pearce,
Nedlands, W. Australia:  Aug. 30.461, 9.3.


CXOU J180951.1-194351 = RXTE J1810-197
     E. V. Gotthelf and J. P. Halpern, Columbia University; C.
Markwardt, Goddard Space Flight Center (GSFC); A. I. Ibrahim,
George Washington University and GSFC; M. Roberts and S. M. Ransom,
McGill University; and P. Woods, USRA and National Space Science
and Technology Center, on behalf of a larger collaboration, report
a highly significant detection of a pulsed source with a
barycentric period of 5.5391(3) s, from a 2833-s Chandra HRC-I
observation starting Aug. 27.95166 UT.  This source shows a
strongly modulated (52-percent pulsed fraction) sinusoidal pulse
signal with a countrate of 0.917 counts/s in the HRC-I energy band
0.08-10.0 keV.  The pulsar is located at R.A. = 18h09m51s.13, Decl.
= -19o43'51".7 (equinox 2000.0; uncertainty radius of 2".1 at the
off-axis HRC-I position).  This position is offset by 14' from the
centroid reported for the 5.5-s pulsar RXTE J1810-197 (IAUC 8168);
the large offset (6 sigma) is likely due to systematic errors in
the derived RXTE position which are based on scans of the complex
region.  The location of CXOU J180951.1-194351 is consistent with a
source seen in archival ROSAT and ASCA images.  The lack of an
optical counterpart within the 1-sigma Chandra error circle
suggests that CXOU J180951.1-194351 is an isolated neutron star,
one most similar to the transient anomalous x-ray pulsar AX
J1844.8-0256 (cf. Vasisht et al. 2000, Ap.J. 43, L49).

                      (C) Copyright 2003 CBAT
2003 August 30                 (8190)            Daniel W. E. Green

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