Nuclear Resonances: The Quest for Large Column Densities and a New Tool

Nuclear physics offers us a powerful tool: using nuclear resonance absorption lines to infer the physical conditions in astrophysical settings which are otherwise difficult to deduce. Present-day technology provides an increase in sensitivity over previous gamma-ray missions large enough to utilize this tool for the first time. The most exciting promise is to measure gamma-ray bursts from the first star(s) at redshifts 20–60, but also active galactic nuclei are promising targets. Nucleonic Absorption-Line Spectroscopy We propose to add the utilization of gamma-ray absorption line spectroscopy to the astronomical toolbox. Nuclear level transitions carry their own specific information, which can complement studies in particular of violent and embedded objects such as GRBs and nuclei of active galaxies. I. The physical effect I.1 Nucleonic cross sections The detection of (resonant) absorption lines is the most frequently used tool for studying matter towards an astrophysical source at low and high redshifts as illuminated by distant background sources such as quasars. The depth and shape of these absorption lines tell us about the physical conditions of gas located between the source and the observer. These are used to derive densities, velocities and metallicities, in order to constrain and unravel cosmological structure and evolution. Similar to X-ray and optical absorption lines which are due to transitions between electronic levels, resonant absorption processes in atomic nuclei exist which leave characteristic absorption lines in the γ-ray range. The most prominent and astrophysically relevant are the nuclear excitation and Pygmy resonances (element-specific narrow lines between 5–9 MeV), the Giant Dipole resonance (GDR; proton versus neutron fluid oscillations; ∼ 25 MeV; two nucleons and more) and the Deltaresonance (individual-nucleon excitations, 325 MeV; all nucleons, including H!). The following is a short description of each of these astrophysically relevant resonances a more detailed description can be found in Iyudin et al. (2005, AA the width is somewhat larger for nuclei as compared to that of protons. Giant Dipole Resonance: First observed in 1947 in photonuclear reactions, the GDR is a collective oscillation of all protons against all neutrons in a nucleus, and as such does not occur for hydrogen. All other elements contribute, and for A>4 the maximum of the cross section is in the 20-30 MeV range. For a solar abundance medium, Helium provides the largest contribution. Pygmy Resonances: Resonance-like absorption below the photoproduction threshold can be produced either via photon absorption to the excitation level of the nuclei or via the photon capture into the so-called ”Pygmy” dipole resonance. The majority of the abundant isotopes in the interstellar matter have the ground state with a zero spin value and a positive parity; e.g., He, C, O, Mg, Si, and Fe, producing a cross-section maximum around 7 MeV. Single resonances are narrow, but in any realistic observing condition many elements with cross-section maxima at slightly different energies overlap so this is the most challenging resonance for observational astronomy. Nuclear Level Transitions: These are the more conventional analogue to atomic line transitions, if the nuclear shell model is adopted. They cover the energy range between about 0.5 and 8 MeV, one prominent example being the 4.430 MeV line of C excitation. 200keV 50 MeV Figure 1: A flat νFν spectrum with NH = 10 28 cm resonance absorption lines for two different redshifts (black: z=25; color: z=0) and different metallicities: Z=0.1 (green) and Z=1 (red) solar metallicity. Note the obvious difference in the relative strengths of the absorptions. The solid vertical lines bracket the energy band from 200 keV to 50 MeV which would be ideal to measure resonances in the high-redshift Universe. Some galactic foreground absorption has been been included (curvature of the green line at the very left).