Supernova 1993J in M81,
02 April - 19 May 1993
F Garcia of Lugo, Spain discovered a supernova in the galaxy M81 (in Ursa Major) on 27 March 1993. In spring, M81 is situated near the zenith and is thus in an excellent position for observing. The star chart below (figure 1) shows the position of the galaxy: the easiest way to find it is by star-hopping from the magnitude 4.6 star 24 UMa. M81 is an almost face-on, regular spiral galaxy with a bright and condensed central nucleus. Some 38' to the north of it lies the irregular galaxy M82. Both galaxies are at a distance of about 8.5 million light years from the Earth. It is possible to fit both galaxies in the same field of view if using a low power, forming a splendid pair even in a small telescope.
Figure 1. M81 finder chart.
SN1993J is located approximately 5' south of the centre of the nucleus of M81. On 01 April 1993, the BAA published Circular C725, showing finder and comparison stars; it is reproduced in figure 2. For comparison, figure 3 shows a long-exposure photograph by OASI member Ric Pecce (150 minute exposure on hypersensitised Technical Pan film using 250 mm SCT at f10) taken in 1994 from Walton-on-the-Naze; it is rotated approximately to match the orientation of the BAA finder chart, and the finder/comparison stars are marked. (Click on the image for the full-scale, un-anotated version.) SN1993J is easily identified as the right hand member of a small triangle of 11-12 magnitude stars, of which the brightest member is star "F". It lies within a spiral arm surrounding the nucleus of the galaxy; although the arm is too faint to be seen in a small telescope, the photograph reveals it clearly. The magnitudes of stars marked in figures 2 and 3 are as follows: A 8.9, E 11.8, F 11.91, G 12.45, H 12.89, J 13.00, K 13.78, L 14.10, N 14.99.
Figure 2. BAA finder chart.
Figure 3. Long-exposure photograph.
When F Garcia discovered the supernova, its magnitude was estimated to be 14. David Payne, Gary Marriott and Pete Richards made the first
observation of the object from Orwell Park, using the 26 cm refractor, at 22:00 UT on 02 April 1993, estimating its magnitude at 11.8. Altogether, members of OASI obtained six magnitude estimates using the 26 cm refractor as follows.
|22:00 UT, 02 April
|21:30 UT, 14 April
|22:00 UT, 21 April
Table 1. Magnitude estimates at Orwell Park.
In fact, a report by Ron Arbour in Astronomy Now showed the light curve of the supernova to be rather peculiar, exhibiting a double peak in magnitude. The supernova rose from its discovery magnitude of 14.0 on 27 March to a peak of 10.5 on 31 March, then faded to a minimum of around magnitude 12 on 05 April, after which it rose to a second peak around magnitude 10.5 in early April.
The observations at Orwell Park were rather sparse and missed the initial rapid increase in brightness and the subsequent initial minimum on 05 April. The first observation from Orwell Park took place during the initial fall in magnitude, and subsequent observations captured the second increase in magnitude and the subsequent long period of fading (with some evidence that the rate of decline occurred more rapidly in the immediate aftermath of the second peak. The following graph shows the initial portion of the "official" light curve and the estimates made at Orwell Park.
Figure 4. Light curves.