[M96]

Supernova 1998bu in M96

[SN 1998bu image] Supernova 1998bu occurred in M96 on May 9, 1998 and was discovered by Italian amateur astronomer Mirko Villi, one of the maintainers of the International Supernova Network, on CCD image when it was at 13th magnitude, and rapidly brightening to 11.8 mag. This supernova is situated about 1 arc minute north of the center of M96, and is the first one ever discovered in this galaxy.

This supernova was the first one the present author (hf) observed with his own eyes, when it appeared almost as bright as the nucleus of M96.

Our image was obtained by Norbert Stapper on May 14, 1998 at 21:53 UT with a 8-inch SC telescope, LPR filter and ST7 CCD camera at 1.86 arc seconds per pixel, under modest observing conditions. Norbert estimated the brightness of the supernova as 12.0 +/- 0.1 at that time.

Spectral investigations indicated that this supernova is of type Ia, as there is no hydrogen, but helium present in the spectrum (see e.g. Mike Richmond's Supernova Taxonomy). In case of Supernova 1998bu, this may be of particular interest with respect to the cosmic distance scale determination, as supernovae of this type are often regarded as one of the more reliable "standard candles" for determining distances of more remote galaxies, and M96 has a Cepheid distance determined by the Hubble Space Telescope. Therefore, the brightness of SN type Ia might be better "gauged" by this supernova. These investigations have been actually carried out by Nick Suntzeff and co-workers (Suntzeff et.al. 1999).

Type Ia supernovae are commonly interpreted as explosions of a White Dwarf which acquires too much mass from a companion star, so that its mass exceeds the Chandrasekhar limit of stability (about 1.4 solar masses) and finally collapses, causing a thermonuclear explosion, or as the result of a collision of two White Dwarf stars. As this process is thought to occur under very similar conditions each time, it is assumed that all type Ia supernova explosions are similar, i.e. are of similar luminosity and show similar light curves. Observational results give supplementary evidence to this assumption.

Supernova 1998bu showed a rarely seen effect: A light echo was discovered after about 500 days, when the light curve suddenly flattened and the spectrum of the supernova chenged back to that seen in earlier phases (Cappellaro et.al. 2001). This indicates that light reflected by foreground dust had arrived at the detectors, a light echo.

References:

Hartmut Frommert
Christine Kronberg
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Last Modification: June 23, 2014