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Title:Supernovae as gamma ray emitters from the MeV to the TeV
Author(s):Wang, Xilu
Director of Research:Fields, Brian D.
Doctoral Committee Chair(s):Fields, Brian D.
Doctoral Committee Member(s):Ricker, Paul M.; Wong, Tony; Gammie, Charles F.
Department / Program:Astronomy
Degree Granting Institution:University of Illinois at Urbana-Champaign
gamma ray
cosmic ray
starburst galaxy
Type Ia supernova
gamma lines
Abstract:Gamma rays offer unique probes of supernovae as cosmic-ray accelerators and as nucleosynthesis sites. My thesis work focuses on how to explore these two aspects of supernovae. (1) Both starburst galaxies and ultra luminous infrared galaxies are new gamma-ray source classes discovered by Fermi and TeV telescopes. These extreme star-forming galaxies have high supernova rates, and thus accelerating cosmic rays that collide with dense interstellar gas to produce gamma rays. Indeed these galaxies are expected to be "thick" to cosmic-ray protons and thus act as "calorimeters", where a substantial fraction of cosmic-ray energy input is emitted in gamma rays. Here we build a one-zone, "thick-target" model implementing calorimetry and placing a firm upper bound on gamma-ray emission from cosmic-ray interactions. The model assumes that cosmic rays are accelerated by supernovae, and all suffer nuclear interactions rather than escape. Our model has only two free parameters: the cosmic-ray proton acceleration energy per supernova e_cr, and the proton injection spectral index s. We calculate the pionic gamma-ray emission from 10 MeV to 10 TeV, and derive the thick-target parameters for six galaxies with Fermi, H.E.S.S., and/or VERITAS data. Our model provides good fi ts for the M82 and NGC 253, and yields e_cr and s values suggesting that supernova cosmic-ray acceleration is similar in starbursts and in our Galaxy. We find that these starbursts are indeed nearly if not fully proton calorimeters. For NGC 4945 and NGC 1068, the models are consistent with calorimetry but are less well-constrained due to the lack of TeV data. However, the Circinus galaxy and the ultraluminous infrared galaxy Arp 220 exceed our pionic upper-limit; possible explanations are discussed. (2) In our own Galaxy, any supernova explosion would be a spectacular "once in lifetime" event. Tragically, a Galactic Type Ia supernova (SNIa) could go entirely unnoticed due to the large optical and near-IR extinction in the Milky Way plane, low radio and X-ray luminosities, and a weak neutrino signal. But fortunately SNIa emit nuclear gamma-ray lines from 56Ni -> 56Co -> 56Fe radioactive decays. These lines fall within the Fermi/GBM energy range, and the 56Ni 158 keV line is detectable by Swift/BAT. Both instruments frequently monitor the Galactic plane, which is transparent to gamma rays. Thus GBM and BAT are ideal Galactic SNIa early warning systems. We simulate SNIa MeV light curves and spectra to show that GBM and BAT could con rm a Galactic SNIa explosion, followed by Swift localization and observation in X-rays and UVOIR band. The time needed to sound the alarm depends on the 56Ni distribution, and can be as early as a few days if > 10% of the 56Ni is in an exterior shell as suggested by SN2014J gamma data.
Issue Date:2018-08-31
Rights Information:Copyright 2018 Xilu Wang
Date Available in IDEALS:2019-02-08
Date Deposited:2018-12

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