[Paper Review] The gamma-ray visibility of supernova remnants: a test of cosmic ray origin

L. O’C. Drury, F. Aharonian+1 more|arXiv (Cornell University)|May 28, 1993

Astrophysics and Cosmic PhenomenaPhysics and Astronomy130 citations

TL;DR

This paper uses nonlinear diffusive shock acceleration models in supernova remnants (SNRs) to predict gamma-ray luminosities from hadronic interactions, showing that SNRs could be detectable in the 1–10 TeV band with imaging atmospheric Cherenkov telescopes if the interstellar medium density exceeds 0.1 cm⁻³. The study predicts a gamma-ray luminosity of ~10³⁶ erg s⁻¹ for nearby SNRs, providing a crucial test for the SNR origin of Galactic cosmic rays.

ABSTRACT

Gamma ray production in supernova remnants is discussed on the basis of current ideas about cosmic ray acceleration.

Motivation & Objective

• To assess the detectability of gamma-ray emission from supernova remnants (SNRs) as a direct test of their role as the primary source of Galactic cosmic rays.
• To evaluate the expected gamma-ray luminosity from SNRs based on modern particle acceleration models, particularly diffusive shock acceleration.
• To determine the feasibility of detecting SNRs in the 100 MeV to TeV energy ranges using existing and planned instruments such as EGRET, Cherenkov telescopes, and air-shower arrays.
• To explore how spectral measurements and upper limits across multiple energy bands (GeV, TeV, PeV) can constrain the maximum energy and efficiency of particle acceleration in SNRs.
• To assess the potential for detecting high-energy neutrinos from SNRs as a complementary probe of cosmic ray acceleration.

Proposed method

• Uses simplified nonlinear diffusive shock acceleration models (e.g., DMV model) to estimate cosmic ray energy density and particle injection rates in SNRs.
• Calculates gamma-ray emissivity per unit volume as $ q_{ ext{\gamma}} = \mathcal{E}_{\text{\gamma}} / E_{\text{C}} $, where $ \mathcal{E}_{\text{\gamma}} $ is the gamma-ray production rate and $ E_{\text{C}} $ is the cosmic ray energy density.
• Applies the gamma-ray emissivity to three regions of the SNR: interior, post-shock, and shock precursor, using gas number density $ n $ and energy density $ E_{\text{C}} $.
• Estimates total gamma-ray luminosity by integrating over the SNR volume, assuming a power-law spectrum in momentum with index $ \alpha \approx 4.1 $–$ 4.3 $.
• Evaluates instrumental sensitivities of EGRET (100 MeV–10 GeV), Cherenkov telescopes (1–10 TeV), and air-shower arrays (≥10 TeV) for detecting SNR gamma-ray emission.
• Considers background levels and source confusion to assess detectability thresholds, especially for EGRET and future instruments.

Experimental results

Research questions

• RQ1 Can supernova remnants produce sufficient gamma-ray emission at energies >100 MeV to be detectable by EGRET?
• RQ2 What is the expected gamma-ray luminosity of SNRs if they are the primary source of Galactic cosmic rays below 10¹⁴ eV?
• RQ3 How does the gamma-ray visibility of SNRs depend on interstellar medium density and SNR evolutionary stage?
• RQ4 Can modern Cherenkov telescopes detect SNRs in the 1–10 TeV band, and under what conditions?
• RQ5 Can air-shower arrays provide meaningful upper limits on cosmic ray spectra above 100 TeV to constrain the maximum acceleration energy in SNRs?

Key findings

• The gamma-ray production efficiency for SNRs with power-law spectra is about 2–3 times lower than the conventional value used for the diffuse interstellar medium, but this depends weakly on spectral index.
• If SNRs are the main source of Galactic cosmic rays, their gamma-ray luminosity in the >100 MeV band is almost model-independent and predicted to be ~10³⁶ erg s⁻¹ for a typical SNR.
• Detection of SNRs in the 100 MeV band with EGRET is unlikely due to instrumental sensitivity and background limitations, but not impossible.
• Detection prospects improve significantly in the 1–10 TeV band, where modern imaging atmospheric Cherenkov telescopes could detect SNRs up to 10 kpc if the ISM density is >0.1 cm⁻³.
• Air-shower arrays can place firm upper limits on cosmic ray spectra extending beyond 100 TeV, providing a crucial test for acceleration models.
• Neutrino fluxes above 1 TeV from SNRs could reach ~10⁻¹⁰ cm⁻² s⁻¹ for nearby remnants, potentially detectable by DUMAND in all-sky surveys.