The Peak Brightness of SN 1981B in NGC 4536 and the Hubble Constant

Abstract

The light curve of the Type Ia supernova SN 1981B (in NGC 4536) is important because Sandage et al. plan to measure the distance to the host galaxy by means of Cepheid variables and will thus derive the Hubble constant. Important parameters in this derivation include the peak magnitude as well as the decline rate of the supernova. In this Letter, I report on the results of my quantitative light-curve template fitting to all available data. I find observed peak magnitudes of 11.74 ± 0.06, 12.04 ± 0.04, and 11.98 ± 0.04 in the U, B, and V bands. The infrared brightness parameter, H20, is 12.94 ± 0.02. For E(B - V) of 0.04 ± 0.07, the dereddened magnitudes are 11.54 ± 0.35, 11.88 ± 0.29, 11.86 ± 0.22, and 12.92 ± 0.04. The decline rate of SN 1981B (Δm15 = 1.07 ± 0.09) is closely matched with the average light curve of Type Ia events, so that both the standard candle and decline rate calibrations will yield similar absolute magnitudes. These magnitudes can then be used to derive the Hubble constant as soon as the distance modulus to NGC 4536 (μ) is measured. The combined results from the U, B, V, and H bands are that the Hubble constant will equal 50 km s-1 Mpc-1 100.2[(31.67±0.17) − μ]. The case of SN 1981B is particularly clean, so that there are not any grounds for challenging the resulting Ho.