The type 1a supernova 1986G in NGC 5128 - Optical photometry and spectra

Abstract

Optical light curves and spectra of the Type la supernova 1986G in NGC 5128 (Centaurus A) are presented. SN 1986G was discovered approximately one week before maximum light. The initial rate of decline of the B light curve was remarkably fast and characteristic of the infrequently observed Pskovskii photometric class ß = 12. Although the spectral evolution closely resembled that of the more common "slower" photometric classes of Type la supernovae, subtle differences in the maximum-light spectra were detected. The expansion velocity of the photosphere of SN 1986G decreased rapidly at early phases, suggesting that the outer-envelope density gradient was less steep than in supernovae with smaller values of ß. SN 1986G appears to have been heavily obscured (E(B -V) = 0.90 ± 0.10) by the dust lane of NGC 5128. This circumstance accounts for the strong interstellar-absorption lines of Ca n H and K and Na ID observed in the spectra as well as for several weaker absorption features that we identify with the diffuse interstellar bands.SN 1986G provides graphic confirmation of the existence of intrinsic differences in the optical light curves and spectroscopic properties of Type la supernovae. Consequently, these objects must be used with considerable caution as cosmological standard candles. On the basis of the very close resemblance of SN 1986G to SN 19711 in NGC 5055, we derive a relative distance of D NGC5128 / D NG c5055 = 0.39 ± 0.04. Further distance estimates are hampered due to the lack of other well-observed Type la supernovae with ß = 12.

A reddening-free method to estimate the56Ni mass of Type Ia supernovae

“…This is consistent with the observed optical decline rate and lower B band luminosity (Phillips et al 1987). Using nebular spectra, Ruiz-Lapuente & Lucy (1992) calculate the M56 Ni for SN 1986G and find a value of 0.38 ± 0.03 M .…”

The Extremes of Thermonuclear Supernovae

“…Five years before SN 1991bg, the first peculiar SN Ia was discovered: SN 1986G in Centaurus A exhibited a low luminosity and unusually red colours (Phillips et al, 1987). However, since the SN was embedded in a dust lane, and showed prominent interstellar Na I D absorption lines in its spectra, it was not totally clear to which extent the observed peculiarities were intrinsic, or just caused by extinction and reddening along the line of sight.…”

Two transitional type Ia supernovae located in the Fornax cluster member NGC 1404: SN 2007on and SN 2011iv

“…The first clear indication of departure from homogeneity arrived with the study of the low-luminosity SN 1986G, located in Centaurus A (e.g., Branch 1987;Phillips et al 1987;Ashall et al 2016b). Five years later, with the discovery of both the high-luminosity SN 1991T (Filippenko et al 1992b;Ruiz-Lapuente et al 1992;Phillips et al 1992) and the low-luminosity SN 1991bg (Filippenko et al 1992a;Leibundgut et al 1993;Turatto et al 1996), it became obvious that SNe Ia exhibit significant diversity (see, e.g., Filippenko 1997).…”

“…In summary, the measured s BV values and the spectral characteristics discussed below, indicate SN 2007on and SN 2011iv are both similar to SN 2003gs (Krisciunas et al 2009), SN 2003hv (Leloudas et al 2009), and iPTF 13ebh (Hsiao et al 2015), all of which are transitional SNe Ia. Figure 7 displays the luminosity vs. decline-rate relation parametrised by ∆m 15 (left) and s BV (right) for an extended sample of SNe Ia observed by the CSP-I (e.g., Contreras et al 2010;Stritzinger et al 2011b) as well as the fast-declining SN 1986G (Phillips et al 1987) and SN 1991bg (Filippenko et al 1992a;Leibundgut et al 1993;Turatto et al 1996). Both SN 2007on and SN 2011iv are brighter than expected for typical fastdeclining SNe Ia and are therefore located closer to the faint end of the luminosity decline-rate relation of normal SNe Ia.…”