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 ------------------------------------------------------------------- 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.