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IAUC 6129: SATURN; RX J0558+53

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                                                  Circular No. 6129
Central Bureau for Astronomical Telegrams
Postal Address: Central Bureau for Astronomical Telegrams
Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions)
Phone 617-495-7244/7440/7444     TWX 710-320-6842 ASTROGRAM CAM

     F. Colas, Bureau des Longitudes, Paris; E. Frappa, Planetarium
de Saint-Etienne; J. Gomez, G.E.A., Barcelona; P. Laques,
Observatoire du Pic du Midi; J. Lecacheux, Observatoire de Meudon,
DESPA; and M. Tagger, Service d'Astrophysique, Centre d'Etudes de
Saclay, report on recent observations at the Pic du Midi 1-m
telescope:  "During continuous CCD imaging of Saturn on 1994 Nov.
22.8 and Dec. 16.8-17.8 UT under very stable seeing conditions
(0".6), we convincingly detected several 'spokes' on the B ring
[see also Sheehan and O'Meara 1993, Sky Tel. 85(1), 20], the rings
being then inclined by 8 deg to our line-of-sight. Our best time
resolution was 4 images/min.  The reflectance contrast for the
features (spokes) in the rings was at maximum in the I band (855
nm), reaching 3 percent on our best images, but they were also
detected in R; from our 3.8-hr sequence on Dec. 17 (169 images),
their behavior was reminiscent of that detected by both Voyager
spacecraft, appearing on that part of the rings exiting the
saturnian shadow.  The spokes are observed close to the corotation
radius (where the keplerian frequency equals the rotation frequency
of Saturn and its magnetosphere), with some extension inwards and
outwards by +/- a few thousands of km.  The spokes vanished upon
arrival in front of the planet.  No significant detection was
obtained on the opposite half-ring.  The maximum activity was
observed when dawn occurred on the rings over 95-125 deg in
longitude (system III), consistent with the strong peak at 115 deg
observed from Voyager data (cf. Esposito et al. and Mendis et al.
1984, in Saturn, Univ. of Arizona Press).  Saturn is currently
nearing solar conjunction, and the current ring tilt is 5.7 deg; on
April 12, at a similar elongation after conjunction, the tilt will
be only 1.6 deg and the spokes difficult or impossible to image
through the ring-plane passages."

RX J0558+53
     K. Kasturirangan, Indian Space Research Organisation (ISRO),
Bangalore, reports:  "B. N. Ashoka, T. M. K. Marar, and S. Seetha
(ISRO Satellite Centre) conducted high-speed photometric
observations on Jan. 8 with the 1-m telescope at Vanna Bappu
Observatory, Kavalur, on the recently discovered intermediate polar
RX J0558+53.  Optical pulsations in white light were unambiguously
detected at the reported x-ray period of 272 s.  Typical intensity
modulation in the light curve was 15 percent.  Tentative evidence
of the beat period between  272 s and the orbital period of 4.15 hr
is also seen in the Fourier transform of the data."

1995 January 27                (6129)            Daniel W. E. Green

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