Positron annihilation as a cosmic ray probe

Abstract

Recently, the gamma-ray telescopes AGILE and Fermi observed several middle-aged supernova remnants (SNRs) interacting with molecular clouds. A plausible emission mechanism of the gamma rays is the decay of neutral pions produced by cosmic ray (CR) nuclei (hadronic processes). However, observations do not rule out contributions from bremsstrahlung emission due to CR electrons. TeV gamma-ray telescopes also observed many SNRs and discovered many unidentified sources. It is still unclear whether the TeV gamma-ray emission is produced via leptonic processes or hadronic processes. In this Letter, we propose that annihilation emission of secondary positrons produced by CR nuclei is a diagnostic tool of the hadronic processes. We investigate MeV emissions from secondary positrons and electrons produced by CR protons in molecular clouds. The annihilation emission of the secondary positrons from SNRs can be robustly estimated from the observed gamma-ray flux. The expected flux of the annihilation line from SNRs observed by AGILE and Fermi is sufficient for the future Advanced Compton Telescope to detect. Moreover, synchrotron emission from secondary positrons and electrons and bremsstrahlung emission from CR protons can be also observed by the future X-ray telescope NuSTAR and ASTRO-H.

Synchrotron X-ray diagnostics of cutoff shape of nonthermal electron spectrum at young supernova remnants

“…1 but for p = 2.3 and 3.3. Vink & Laming 2003;Vink 2008;Ohira et al 2012a), and so on. Similar discussion may be done with Fig.…”

“…Figure 3 shows that all the curves are degenerate with each other, along which the observed data points should lie if the X-ray emission arises from synchrotron radiation. If there are outliers, we can discuss the existence of extra components, such as a bump in the electron spectrum near E max,e due to effects of pile-up via synchrotron cooling (Longair 1994;Drury et al 1999;Zirakashvili & Aharonian 2007) and/or nonlinear acceleration (e.g., Malkov & Drury 2001), jitter or diffusive synchrotron emission (Toptygin & Fleishman 1987;Medvedev 2000;Reville & Kirk 2010;Teraki & Takahara 2011), secondary synchrotron radiation that originates from accelerated protons generating charged pions (e.g., Yamazaki et al 2006), nonthermal bremsstrahlung emission (Laming 2001a(Laming , 2001bVink & Laming 2003;Vink 2008;Ohira et al 2012a), and so on. Similar discussion may be done with Figure 4.…”

Escape of cosmic-ray electrons from supernova remnants

“…The escape of CR nuclei from SNRs has been investigated by several authors (e.g. Ptuskin & Zirakashvili ; Ohira, Murase & Yamazaki ; Caprioli, Amato & Blasi ; Drury ; Ohira & Ioka ), and the emission from runaway CR nuclei has also been investigated (Aharonian & Atoyan ; Gabici, Aharonian & Casanova ; Ellison & Bykov ; Ohira et al ; Ohira, Kohri & Kawanaka ). Runaway CR nuclei can emit gamma‐rays from the exterior of accelerators, and their radiation spectrum is softer than that from the interior because of the energy‐dependent diffusion of CRs (Aharonian & Atoyan ).…”