Central Bureau for Astronomical Telegrams
IAUC 7395: C/1996 B2; XTE J1118+480
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Circular No. 7395
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
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COMET C/1996 B2 (HYAKUTAKE)
G. H. Jones and A. Balogh, Imperial College, London; and T. S.
Horbury, Queen Mary and Westfield College, London, write: "Analysis
of magnetometer data returned from the ESA/NASA Ulysses spacecraft
for several hours centered on 1996 May 1.375 UT reveals solar-wind
magnetic-field signatures consistent with a cometary ion-tail
crossing. Our search for a source comet revealed that C/1996 B2
passed across the sun-Ulysses line during 1996 Apr. 23. This 8-day
time difference equals the travel time for cometary ions to be
carried anti-sunward to Ulysses at up to 750 km/s (the exact travel
time depending on the acceleration rate within the tail).
Magnetic-field vectors were consistent with a tail parallel to the
comet's orbital plane, as expected, and the tail diameter of > 7 x
10**6 km (inferred from the magnetometer data) is consistent with
that expected for gas-production-rate estimates for the comet on
Apr. 23. Consideration of the relative positions of Ulysses and
the comet's nucleus implies a tail length > 3.8 AU. Details are
given by Jones et al. (2000, Nature 404, 574). We also note that
the earth-comet-sun phase angle during the Ulysses tail crossing on
May 1 was 32 deg, and if the tail (out to the position of Ulysses)
had been then visible from the earth (with the comet's head at
solar elongation about 7 deg), its apparent length would have been
> 80 deg. This is due to the tail's curvature, which is clear in
the magnetometer data. This curvature may provide an explanation
for the reported apparent tail lengths in late Mar. 1996 that were
larger than the phase angle (cf. IAUC 6360)."
XTE J1118+480
V. Dhawan, National Radio Astronomy Observatory; G. G. Pooley,
Mullard Radio Astronomy Observatory, University of Cambridge; and
R. N. Ogley and I. F. Mirabel, Service d'Astrophysique, Laboratoire
du Commissariat a l'Energie Atomique, Saclay, report on additional
radio photometry of XTE J1118+480 (cf. IAUC 7390) using the Ryle
and VLA telescopes: "Observations at 15 GHz with the Ryle
telescope on Mar. 31.77-31.89 and Apr. 2.74-02.88 UT measure flux
densities of 7.8 +/- 0.35 and 7.5 +/- 0.30 mJy. Observations with
the VLA on Apr. 2.98-3.13 produce flux densities at 1.4, 8.3, and
23 GHz of 2.1, 6.0 +/- 0.1, and 8.8 +/- 0.3 mJy. The position of
the source measured by the VLA at 23 GHz is R.A. = 11h18m10s.79,
Decl. = +48o02'12".3 (equinox 2000.0; uncertainty 0".3). The
inverted radio spectrum is consistent with emission from an
optically thick ejection."
(C) Copyright 2000 CBAT
2000 April 5 (7395) Daniel W. E. Green
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