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THE ASTRONOMER Electronic Circular No 2916      2013 May 27 15.18UT
Ed:Guy M Hurst, 16,Westminster Close, Kempshott Rise,  Basingstoke,
Hants, RG22 4PP,England.Telephone/FAX(01256)471074Int:+441256471074
INTERNET: GUY@TAHQ.DEMON.CO.UK      Backup: gmh@wdcc1.bnsc.rl.ac.uk
WORLD WIDE WEB                         http://www.theastronomer.org
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COMET ASTROMETRY
We have received a number of queries since the BAA meeting in Northampton
recently regarding reporting procedures for comets.

One theme concerns astrometry and to whom results should be sent. Jonathan
Shanklin, Director of the BAA Comet Section confirms that he does not need
these reports and so they should, as previously, be sent to the TA magazine
with a copy to the Minor Planet Centre using the latter organisation's
format.

The magnitudes, although usually approximate and somewhere between m1 and m2
allow the editor to monitor any substantial deviation from predicted values
and issue an alert on these circulars if brighter than expected. In
addition, NASA ADS and our own search facilities of PDF files allow
investigation of field coordinates and dates for potential discoveries and
pre-discoveries of other objects which may have been recorded in the same
field. There have already been a number of such instances where an enquiry
from professionals has been received so sending these reports in promptly is
helpful.

Could we also ask that where any unusual general activity has been recorded
descriptive comments should be added to your report for possible inclusion
in the magazine.

GRB 130427A = SN 2013cq
A.J. Levan (University of Warwick) et. al., report on GCN 14686 they have
observed the optical/NIR counterpart of GRB 130427A with the Hubble Space
Telescope beginning at 02:23 UT on 2013 May 20. The afterglow is well
detected in multi-band observations in the UV (F336W), optical (F606W) and
NIR (F160W) and is offset approximately 0.8" from the optical centroid of
its host. The host itself also contains additional star forming complexes
including a bright UV source approximately 0.25" from the GRB position.
In the three bands we measure preliminary magnitudes of the afterglow +
supernova of
F336W=23.10 +/- 0.02; F606W=21.85 +/- 0.02; F160W=21.34 +/- 0.03

These magnitudes show significant curvature in the optical likely due to the
underlying supernova SN 2013cq (de Ugarte Postigo CBET 3529; Xu et al. GCN
14597).    If the optical light were entirely dominated by supernova
emission the absolute magnitude at z=0.34 would be M_B~ -19.1 at 17
rest-frame days post burst. However, SNe are weaker UV and IR emitters and
so under the naive assumption that the UV and IR bands are dominated by
power-law afterglow emission with minimal supernova contribution the
inferred magnitude of the supernova in the V-band (rest frame B-band) is
V~23. This corresponds to an absolute magnitude of M_B ~ -17.9,
approximately a magnitude fainter than the B-band peak of SN 1998bw (which
occurred at a comparable epoch of 15 days post burst).     However, the SN
could contribute as much as one half of the flux we are seeing in the NIR
and UV and there may be substantial host emission underneath the object in
the optical and UV.   Thus the SN magnitude should be considered very
approximate.


Images of the field are posted at
http://www.star.le.ac.uk/~anl/GRB130427A

Editor: Although the evolving supernova is very faint we are publishing
these notes to provide feedback to a number of recent enquiries.

Guy M Hurst