Electronic Telegram No. 3888
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
CBAT Director:  Daniel W. E. Green; Hoffman Lab 209; Harvard University;
 20 Oxford St.; Cambridge, MA  02138; U.S.A.
e-mail:  cbatiau@eps.harvard.edu (alternate cbat@iau.org)
URL http://www.cbat.eps.harvard.edu/index.html
Prepared using the Tamkin Foundation Computer Network


COMET C/2013 A1 (SIDING SPRING)
     D. Bodewits, T. Farnham, and M. F. A'Hearn, University of Maryland,
College Park, report results from observations of comet C/2013 A1 using the
UltraViolet-Optical Telescope (UVOT) onboard the Swift space telescope.  They
obtained photometry using broadband V (central wavelength 546.8 nm; FWHM =
75.0 nm) and UVW1 (central wavelength 260.0 nm; FWHM = 70.0 nm) filters.
Fluxes were derived using the UVOT photometric system (Breeveld et al. 2011,
AIP Conf. Proc. 1358, 373).  Comet C/2013 A1 was observed between May 28 and
30 at r = 2.46 AU.  Within apertures of radius 5".7 (140000 km at the comet),
they measured a visual magnitude of 15.20 +/- 0.05.  This V-band magnitude
corresponds to values of Af(rho) of 860 cm (+/- 5 percent), normalized to a
phase of 0 degrees using the phase function by D. Schleicher (see website URL
http://asteroid.lowell.edu/comet/dustphase.html).  The present authors used
the UVW1 filter to detect OH emission and derive a water-production rate of
QH_2O = 1.7 x 10^27 (+/- 1 x 10^27) molecules/s.  Assuming a rapid-rotator
model (Cowan and A'Hearn 1979, Moon and the Planets 21, 155), this production
rate requires a minimum active area of 5.7 +/- 4 square km or a minimum radius
of 0.34 km.


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                         (C) Copyright 2014 CBAT
2014 June 6                      (CBET 3888)              Daniel W. E. Green