Physics of Type Ia Supernovae

Type Ia supernovae (SNe Ia), the thermonuclear explosion of a white dwarf, were once considered standard candles. However, increased observations reveal inhomogeneities in chemical composition and luminosity behavior, roughly dividing SNe Ia into three luminosity classes; super-luminous, sub-luminous, and normally-luminous. After introducing the problem in the context of previous observations and modeling, this thesis explores the physical processes occurring in a SN Ia after explosion, and discusses observations of SN light curves. A simple model of the expanding ejecta calculates the energy deposition from the decay of radioactive Ni56 as well as photon diffusion. It produces light curves that match early bolometric observations of normal SNe Ia. Variable chemical composition of the ejecta allows for testing a number of explosion scenarios. It becomes apparent that the shape of the light curve is sensitive to the amount and location of synthesized Ni56. Monitoring gamma ray transport through Compton scattering indicates that gamma rays escape at late times. At this epoch an assumption of instantaneous deposition of energy is inaccurate. It is unclear whether positrons escape the ejecta or are trapped at even later times. The photometry of SN2007ax proved it to be the dimmest and reddest SN Ia observed. SN2008D was serendipitously observed in X-rays before it was even visible in optical light, revealing that an early x-ray outburst may accompany every core collapse SN. Subsequent observations resulted in a well-sampled, multi-band early light curve. Observations of SN2006D, another SN Ia, in B,V,R,I up to about 500 days after maximum light are also presented. The light curve may answer questions about the physics of SNe at late times, if more observations can be included. Future modifications of the simple model and strategies for useful observations are discussed

THE POWER OF THERMONUCLEAR SUPERNOVAE AFTER ONE YEAR

Type Ia supernovae (SNe Ia), the thermonuclear explosion of a white dwarf, shape our understanding of the expansion of the universe with the use of their uniformity in distance determinations. Powered by radioactivity synthesized in the explosion, they fade slowly over hundreds of days. Sometime after 200 days, the continually expanding ejecta allows γ-rays from 56 Ni and 56 Co decays to escape, and soon any radioactive power contributing to lighting up the SN comes from positrons formed in 19% of 56 Co decays. While at first it seemed that positrons escaped through the thinning ejecta, it has become apparent that conclusions can only be drawn from accounting for all the power from the near-infrared (NIR) as well as the optical. Only a handfull of SNe have been observed during epochs at a year after explosion in both the optical and NIR. These seem to make an argument for the complete trapping of positrons while also suggesting there is more power unobserved in other bands. This dissertation discusses observations of three nearby SNe; 2006E, 2006ce, and 2006mq, which were all discovered after maximum light, but bright enough to be observed to late times (the latest at ∼525 days after peak). The late multi-wavelength observations are converted to fluxes and luminosity and we assess the behvaior of different wavelength regimes. A simple positron deposition model is employed to estimate the feasibility of positron escape. We find that we cannot rule out positron escape, but that it seems likely that there is a color evolution that shifts power away from observed bands. This shifting of power seems to vary from SN to SN and is not uniform across all normal SNe Ia

The Rapidly Flaring Afterglow of the Very Bright and Energetic GRB 070125

We report on multi-wavelength observations, ranging from the X-ray to radio wave bands, of the IPN-localized gamma-ray burst GRB 070125. Spectroscopic observations reveal the presence of absorption lines due to O I, Si II, and C IV, implying a likely redshift of z = 1.547. The well-sampled light curves, in particular from 0.5 to 4 days after the burst, suggest a jet break at 3.7 days, corresponding to a jet opening angle of ~7.0 degrees, and implying an intrinsic GRB energy in the 1 - 10,000 keV band of around E = (6.3 - 6.9)x 10^(51) erg (based on the fluences measured by the gamma-ray detectors of the IPN network). GRB 070125 is among the brightest afterglows observed to date. The spectral energy distribution implies a host extinction of Av < 0.9 mag. Two rebrightening episodes are observed, one with excellent time coverage, showing an increase in flux of 56% in ~8000 seconds. The evolution of the afterglow light curve is achromatic at all times. Late-time observations of the afterglow do not show evidence for emission from an underlying host galaxy or supernova. Any host galaxy would be subluminous, consistent with current GRB host-galaxy samples. Evidence for strong Mg II absorption features is not found, which is perhaps surprising in view of the relatively high redshift of this burst and the high likelihood for such features along GRB-selected lines of sight.Comment: 50 pages, 9 figures, 5 tables Accepted to the Astrophysical Journa