All-sky Medium Energy Gamma-ray Observatory Research Articles

Prospects of direct detection of 48V gamma-rays from thermonuclear supernovae

ABSTRACT Detection of gamma-rays emitted by radioactive isotopes synthesized in stellar explosions can give important insights into the processes that power transients such as supernovae, as well as providing a detailed census of the abundance of different isotope species relevant to the chemical evolution of the Universe. Observations of nearby supernovae have yielded observational proof that 57Co powered the late-time evolution of SN1987A’s light curve, and conclusive evidence that 56Ni and its daughter nuclei power the light curves of Type Ia supernovae. In this paper, we describe the prospects for detecting nuclear decay lines associated with the decay of 48V, the daughter nucleus of 48Cr, which is expected to be synthesized in large quantities – $M_{\mathrm{Cr}}\sim 1.9\times 10^{-2}\, \mathrm{M_\odot }$ – in transients initiated by explosive helium burning (α-capture) of a thick helium shell. We calculate emergent gamma-ray line fluxes for a simulated explosion model of a thermonuclear explosion of carbon–oxygen white dwarf core of mass $0.45\, \mathrm{ M}_{\odot }$ surrounded by a thick helium layer of mass $0.21\, \mathrm{ M}_{\odot }$. We present observational limits on the presence of 48V in nearby SNe Ia 2014J using the INTEGRAL space telescope, excluding a 48Cr production on the surface of more than $0.1\, \mathrm{M_{\odot }}$. We find that the future gamma-ray mission the All-Sky Medium Energy Gamma-Ray Observatory (AMEGO) will have an approximately 5 per cent chance of observing 48V gamma-rays from such events during the currently planned operational lifetime, based on our birthrate predictions of faint thermonuclear transients. We describe the conditions for a 3σ detection by the gamma-ray telescopes INTEGRAL/SPI, Compton Spectrometer and Imager (COSI) , and AMEGO.

Gamma Rays from Kilonova: A Potential Probe of r-process Nucleosynthesis

Abstract The mergers of compact binaries with at least one neutron star component are the potential leading sites of the production and ejection of r-process elements. Discoveries of galactic binary pulsars, short gamma-ray bursts, and gravitational-wave detections have all been constraining the rate of these events, while the gravitational wave plus broadband electromagnetic coverage of binary neutron star merger (GW170817) has also placed constraints on the properties (mass and composition) of the merger ejecta. But uncertainties and ambiguities in modeling the optical and infrared emission make it difficult to definitively measure the distribution of heavy isotopes in these mergers. In contrast, gamma rays emitted in the decay of these neutron-rich ejecta may provide a more direct measurement of the yields. We calculate the gamma production in remnants of neutron star mergers, considering two epochs: a kilonova epoch, lasting about two weeks, and a much later epoch of tens and hundreds of thousands of years after the merger. For the kilonova epoch, when the expanding ejecta is still only partially transparent to gamma radiation, we use 3D radiative transport simulations to produce the spectra. We show that the gamma-ray spectra associated with beta- and alpha-decay provide a fingerprint of the ejecta properties and, for a sufficiently nearby remnant, may be detectable, even for old remnants. We compare our gamma spectra with the potential detection limits of next generation detectors, including the Lunar Occultation Explorer (LOX), the All-sky Medium Energy Gamma-ray Observatory (AMEGO), and the Compton Spectrometer and Imager (COSI). We show that fission models can be discriminated via the presence of short-lived fission fragments in the remnant spectra.

The all-sky medium energy gamma-ray observatory

The All-sky Medium Energy Gamma-ray Observatory (AMEGO) is an Astrophysics Probe mission concept designed to explore the MeV sky. It has a unique capability to cover with high sensitivity both the Compton and pair conversion regimes from ∼ 200 keV to > 10 GeV . To date the MeV regime was explored with significantly less sensitivity with respect to the nearby energy bands, and promises a rich return similar to what Fermi-LAT achieved in the GeV band. The AMEGO instrument features an anti-coincidence detector, a double-sided Silicon microstrip tracker with analog readout, a segmented CZT calorimeter optimized for the Compton regime, and a segmented CsI calorimeter optimized for the pair regime. This design choice provides substantial performance improvements relative to CGRO-COMPTEL and Fermi-LAT in the overlapping energy bands. In particular, the detector is tailored to the challenging task of imaging Compton events with unprecedented resolution.