[madbadgalaxyman (Robert)]

Peter asked how bright this supernova is going to get.

This is a tough question in the absence of a reliable distance measurement for IC 4901. There have been a couple of individual estimates of the distance of IC 4901 using the Tully-Fisher method, which unfortunately is no more accurate than using the redshift of a galaxy to estimate its distance. In fact, the two Tully-Fisher distance determinations are highly discrepant.

Sadly, IC 4901 has had very little study by professional astronomers: it gets just a brief mention in a handful of papers. This situation is exactly as I would expect from the poor results of my previous searches for info about various southern IC galaxies!

In the circumstances, it might be best to estimate the distance of this galaxy from its redshift, which is 2038 km/s in the Local Group reference frame (according to the NED database). Assuming a Hubble Constant of 73 km/s/Mpc one can then use this redshift to estimate that the distance is 28 Megaparsecs ( = 91 million light years) .

The other parameter that we need in order to estimate the probable peak magnitude of this supernova is its luminosity (absolute magnitude), which introduces further uncertainty. As I am unfamiliar with the literature on supernovae, my most recent information about the absolute magnitudes of Type II-P supernovae comes from the year 2002 (D. Richardson et al., (2002), AJ, 123, 745) and it is an average of the luminosities of a small number of these supernovae. Richardson et al. find a mean Blue absolute magnitude of -17 for Supernovae of type II-P, but they also find that the luminosities of the individual II-P supernovae events occupy a wide scatter between
B absolute magnitudes -16 and -18

Assuming a peak absolute B-band magnitude of -17 for this supernova, and a distance of 27.9 Megaparsecs (a distance modulus of
m-M = 32.23), and a total extinction (the internal extinction within IC 4901, plus the foreground extinction from the Milky Way) of 0.6 magnitudes, we can then use the standard equation for distance modulus and absolute magnitude to estimate a peak apparent blue (B-band) magnitude for this supernova of 15.8, which is quite faint.
If the supernova was absolute blue magnitude -18 in luminosity, it would probably end up being closer to Blue (B-band) magnitude 14.8

Given the uncertainties as to the distance of this galaxy and the luminosity of this supernova, not to mention uncertainty as to the precise amount of dust extinction, a peak apparent Blue magnitude of 14.6 to 16.3 can be considered a ballpark estimate.

cheers,
mad galaxy man

P.S. the amount of extinction from dust within the host galaxy and within our own Milky Way galaxy can make a considerable difference to the apparent magnitude of a supernova. I assumed only 0.2 mags of foreground extinction, together with a further 0.4 magnitudes of dimming of the SN's light coming from within IC 4901 itself. The foreground extinction from our own Galaxy can be calculated with reasonable accuracy, but the extinction of a supernova's light caused by the dust within its host galaxy will be unique for every observed supernova.

I can't put a figure on the distance uncertainty regarding IC 4901, but I assume that it could make at least a 0.3 magnitude difference to the observed magnitude of the supernova.

If, for instance, I assume that the SN is more luminous than the average for its type (in this example, assuming that the SN is -18 abs. magnitude), minimal dimming of the SN's light by dust (say, 0.4 blue magnitudes of all-sources extinction), and a 15 percent closer distance for the host galaxy (say 77 million light years), then the apparent B magnitude of the SN at its peak will be B = 14.3 magnitude.