http://arxiv.org/abs/1612.07290
Several starburst galaxies have been observed in the GeV and TeV bands; in this regime, gamma rays are mainly produced through the pionic process from cosmic-ray proton interactions with the interstellar medium. Furthermore, the dense environments of starbursts have been proposed to act as proton “calorimeters” in which collisions dominate proton losses, so that a substantial fraction of cosmic-ray proton energy input is emitted in gamma rays. Here we build a one-zone, “thick-target” model that implements calorimetry and thus places a firm upper bound to the gamma-ray emission due to cosmic-ray interactions. The model assumes that cosmic rays are accelerated by supernovae, and escape is neglected. Our model has only two free parameters: the cosmic-ray proton acceleration energy per supernova, and the proton injection spectral index. We find that in our thick-target limit, the emergent gamma-ray spectral index is the same as the injection index. We calculate the pionic gamma-ray emission from 10 MeV to 10 TeV for five starburst galaxies (M82, NGC 253, NGC 4945, NGC 1068 and Circinus) and the ultraluminous infrared galaxy Arp 220. Using Fermi, H.E.S.S., and VERITAS data, we perform chi-squared fits to derive best-fit parameters for each galaxy. We find that for the best-observed galaxies M82 and NGC 253, the cosmic-ray acceleration energetics and spectral indices are consistent with Fermi observations of Galactic supernova remnants; this suggests that cosmic-ray acceleration by supernovae is similar in starbursts and in our Galaxy. For NGC 4945 and NGC 1068, the models are not well-constrained due to the lack of TeV data, but the GeV data also are consistent with near or full proton calorimetry. Consequently, we conclude that these starbursts, and perhaps most starbursts, are consistent with near or total proton calorimetry. (Abridged)
X. Wang and B. Fields
Thu, 22 Dec 16
22/65
Comments: 34 pages, comments welcome, submitted to ApJ