THE ASTRONOMER Electronic Circular No 729       1993 Apr 10 13.20UT
Ed:Guy M Hurst, 16,Westminster Close, Kempshott Rise,  Basingstoke,
Hants, RG22 4PP,England. Telephone/FAX(0256)471074 Int:+44256471074
TELEX: 9312111261 Answerback: TA G      TELECOM GOLD: 10074:MIK2885
GMH at UK.AC.RUTHERFORD.STARLINK.ASTROPHYSICS  STARLINK: RLSAC::GMH
GMH at UK.AC.CAM.ASTRONOMY.STARLINK            STARLINK: CAVAD::GMH
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SUPERNOVA IN NGC 3031
Editor:
It appears that the outburst of this object may have occurred
between 1993 Mar 27.91UT when J.C.-Merlin, Le Creusot, France
advises us that it was not visible to a limit of magnitude 16 on
an Ektachrome 400 exposure and Mar. 28.30UT when A.Neely, Silver
City, USA recorded it at 13.8 on an unfiltered CCD image.
If anyone has photographs around these dates, even if the object is
absent, please advise the Editor immediately as it is important
that the onset of outburst is determined as accurately as possible.

As observers will have noted we have so far not published on these
e-circulars the estimates reported to us, which by last night,
had reached 304 observations and which already suggests this will
be the most comprehensively-observed supernova in the history of
BAA/TA.

The reason for this is that there is undoubtedly a major problem
with bias where observers liaising with each other appear to be
anticipated the brightness only to find difficulty in reconciling
subsequent estimates and wondering if they should be revised. It
should be stressed that both novae and supernovae are notoriously
unpredictable and that it does not necessarily mean that a maximum
is always followed by a smooth decline. Indeed a double maximum can
occur and in some supernovae a plateau of steady brightness may
also be a feature.

Similarly there can be rapid fluctuations during the course of the
same night and we already have cases where experienced observers
have differed by 1.0 magnitudes or more on the same night in the
case of SN 1993J. It does not follow automatically that any one
observer has made a substantial error.

Please report what you see and do not concern yourself with other
observers' results of which you may become aware.

Dr.Gerard de Vaucouleurs, Texas, USA e-mails some notes for us:

SN1993J Comparison Stars
Except for neglected color terms in the transformation to the
Johnson V system, the main source of scatter in the various
provisional magnitudes is in the adopted standards, often
star B = GSC 0928.  The transformation from GSC V magnitudes
to Johnson V magnitudes published by Harold Corwin in IAU
Circular 5742 is
       HGC V = 12.87 + 0.969(+/- 0.052)(GSC V - 12.43)
from an impartial l.s. line fit to GSC #0434,0928,1023,1031,1123.
These stars have all nearly the same color, <(B-V)> = + 0.54,
range 0.46-0.63. Stars #0224 (red) and #0308 (aberrant) are
excluded.
Neglecting the barely significant scale coefficient, the mean
systematic difference between the two scales is HGCV-GSCV =
12.87 - 12.43 = + 0.44. This is close to the difference for
star B (11.90 - 11.42 = + 0.48).
A preliminary reduction to 50 available estimates to the Johnson V
system (leaving still a large dispersion ~ 0.2 mag) suggests that
the V maximum was reached on JD9077.7 at V = 10.5, B-V = + 0.3,
with onset near JD9072.0.