by Steve H. Lucas


The bright supernova (hereafter SN) in the galaxy NGC 4527 was first sighted by S. Knight (USA) on April 13.17UT at magnitude ~14.0v. The report of the sighting was subsequently forwarded by phone to the AAVSO. On the 14th. of April, the SN had brightened by almost one-half magnitude....this was also reported to the AAVSO by Mr. Knight.

On April 15.5UT, the suspect was observed by R. Evans (Australia) at magnitude ~13.0v. By direct measurement from a photographic plate, and with a spectrum (with the assistance of Rob McNaught of Siding Spring Observatory), the SN was verified and forwarded to the Central Bureau of Astronomical Telegrams.

A few hours later, reports began pouring in from all over the world regarding the discovery. M. Villi and G. Cortini (Italy) spotted the suspect, now a bonifide SN, and reported their findings to Asiago Observatory. They too forwarded their observation to CBAT. In the meantime, W. Johnson (USA) also provided a discovery possibility to Mr. Gene Shoemaker at Mt. Palomar Observatory, who mentioned that he had just received a call from CBAT requesting a further confirmation of a SN in NGC 4527....the SN, now had an identity...SN 1991T.

The galaxy NGC 4527 is an Sb(s)II type galaxy with an apparent Blue magnitude of 11.32 (Shapley-Ames, 1981). NGC 4527 has a distance Modulus of 32.49 and is probably a member of the "X" component of the Virgo II Cloud (Buta, Turner, PASP., _95_, 1983). This is just a "tad" closer (on an astronomical scale) to us than the Virgo Cluster proper.

An analysis of this event utilizing visual observations by amateurs, plus a light curve is presented.....



The following exercise utilizes a reproduction of the mean light curve of the type Ia supernova, as presented by Doggett and Branch, AJ.,_,90_,2303, 1985, and is used to reference amateur observations of this event. There were many magnitude estimates which appeared in the IAU Circulars, these have been included as well as many unpublished estimates that were received by this author. This article is an attempt to display that a meaningful light curve can be weaned from observations of experienced amateur astronomers. (No attempt will be made to justify or explain the mechanics concerning the behaviour of this event. This information can be weaned from:(1.) Filippenko, ApJ., _384_:L15-L18 (1992), (2.) Phillips,, AJ., _103_, (1992), (3.)Yamaoka and Nomoto, ApJ., _393_:L55-L58 (1992), and (4.) LaPuente, ApJ., _387_,L33-L36 (1992) (SHL).


The early light curve (pre-maximum) of the SN 1991T had some peculiarties as compared to the mean light curve. This is indicated by a sharp rise to maximum light which began almost at its discovery, which began ~14 days before on 13 April, 1991. Below is a comparison with the pre and post maximum points as determined by eye and applied to the mean light curve.

Mean Light Curve

Days		Magnitude	Per day

-15.7d		-1.0m		0.063m
-9.2		-0.5		0.054
+11.84		+0.5		0.042
+21.05		+1.0		0.047
+31.57		+1.5		0.047
+43.57		+2.0		0.046
+61.84		+2.5		0.040
+81.57		+3.0		0.038
Days		Magnitude	Per Day

-14.0d		-2.7m		0.192m
-12.0		-1.7		0.141
-09.0		-0.7		0.077
+14.4		+0.5		0.034
+22.0		+1.0		0.045
+30.0		+1.5		0.050
+42.0		+2.0		0.047
+63.8		+2.5		0.039
+78.0		+3.0		0.038

It can be seen that the maximum in the visual occured on ~4/27/91 [see (1) above], and the V-Band maximum occured three days later [see (2) above]. This event was a bit brighter than theorized (11.3v versus 12.4v)

The FWFM (Full Width Full Maximum[at 0.5m]) of the light curve determined from the mean versus the observed value was 21.2d and 21d respectively. The FWFM at magnitude 1.0 was 35.4d compared to 30d.

From maximum light to the inflection point (the rapid decline yielding to a more linear posture), the visual observations were in close agreement to the mean, with some spurrious observations displaying a slightly brighter estimate than theorized for the mean average.

The point of inflection [visual] was estimated to have occured at ~day 47 (the authors opinion). This value has some uncertainty attached, due to the fact that there is a gap in visual estimates from this point to ~day 63 (probably due to the presence of a full moon).

It might also be noted that the visual observations at this point in the event displayed a slightly fainter posture when compared to the mean light curve. This fact and a possible relation to the color index of the event versus what the human eye can decipher has been brought up by many astronomers. Many more observations are needed to form a difinitive conclusion to this phenomena. However, in the paper "UBVRI Photometry of SN 1993J in M81: The First 120 Days" (Richmond et. al, AJ_107_, [1994], pg.1026-1027), a color-dependant relationship has been noted. In this particular event (1993J) the author cites a linear-fit formula (to fit the data):

m*vis=visual - 0.266 + 0.53(B-V)

It might be worthwhile to continue detailed magnitude estimate studies on bright supernova by the amateur community in the future to help determine such a color dependant correlation.

From this point onward (to day ~77) the few scattered data points followed the mean curve in fairly good agreement. The event was then lost to visual observation after this point.


The response by the amateur community in monitoring this bright event was noteworthy. Many estimates were duplicates, thus they were indicated by just one data point. The total number of estimates exceeded over 100. In only three occasions were scattered data points not included....they were estimates that were sent in by observers who contibuted just once. Any correlation between their experience and the data provided is therefore not available and suspect.

Generally speaking, the visual monitoring of this event was in good agreement with the mean light curve decay. It is actually quite remarkable that there wasn't any significant amount of scatter throughout the entire event even though this event was considered to have displayed some peculiarities in its explosive mechanism (see 1, 2, 3, and 4 above).

I would like to take this time to thank the individuals who contributed to the evolutionary process of this supernova. Especially the Spanish observers (M-1 Supernova Group) who contributed over 40% of the data point estimates, the SUNSEARCH team for their role in this event, and the amateur community as a whole for their contributions. If only this kind of response could be applied to other supernova events then a viable database catagorizing the behaviour of many events could be realized.

Steve H. Lucas

Parts of this article were originally written in August, 1991(Starburst #27) (were lost, then found again on my old Apple IIc computer) and updated on September 28, 1996. The original title was: "Fitting the Supernova in NGC a "T"?)

Other SN1991T References:

  • ISN SN 1991T web page
  • 1996 HST/FOC image (deconvolved) showing the resolved light echo is cresent shaped. Diameter is about 0.2".

    Back to Supernovae in NGC and IC galaxies.

    David Bishop