[Paper Review] Supernova remnants as cosmic ray accelerators. SNR IC 443

B. Hnatyk, O. Petruk|arXiv (Cornell University)|Feb 10, 1999

Astrophysics and Cosmic Phenomena12 references3 citations

TL;DR

This paper investigates IC 443 as a potential cosmic ray accelerator, proposing that interactions between the supernova remnant's shock wave and a nearby molecular cloud enhance proton acceleration and subsequent gamma-ray production via pion decay. The 3D hydrodynamical model shows that reverse shocks and Rayleigh-Taylor instabilities significantly boost cosmic ray density and mixing, resulting in a gamma-ray flux consistent with EGRET observations of 2EG J0618+2234.

ABSTRACT

We examine the hypothesis that some supernova remnants (SNRs) may be responsible for some unidentified gamma-ray sources detected by EGRET instrument aboard the Compton Gamma Ray Observatory. If this is the case, gamma-rays are produced via pion production and decay from direct inelastic collisions of accelerated by SNR shock wave ultrarelativistic protons with target protons of the interstellar medium. We develop a 3-D hydrodynamical model of SNR IC 443 as a possible cosmic gamma-ray source 2EG J0618+2234. The derived parameters of IC 443: the explosion energy E_o=2.7*10^{50} erg, the initial hydrogen number density n(0)=0.21 cm^{-3}, the mean radius R=9.6 pc and the age t=4500 yr result in too low gamma-ray flux, mainly because of the low explosion energy. Therefore, we investigate in detail the hydrodynamics of IC 443 interaction with a nearby massive molecular cloud and show that the reverse shock wave considerably increases the cosmic ray density in the interaction region. Meantime, the Rayleigh-Taylor instability of contact discontinuity between the SNR and the cloud provides an effective mixing of the containing cosmic ray plasma and the cloud material. We show that the resulting gamma-ray flux is consistent with the observational data.

Motivation & Objective

• To test the hypothesis that supernova remnants like IC 443 can be sources of high-energy cosmic rays.
• To explain the origin of the unidentified EGRET gamma-ray source 2EG J0618+2234 through particle acceleration in SNR shocks.
• To investigate how interaction with a massive molecular cloud modifies cosmic ray production and gamma-ray emission in IC 443.
• To determine whether hydrodynamic instabilities such as Rayleigh-Taylor can enhance cosmic ray mixing and flux.

Proposed method

• Development of a 3D hydrodynamical model of IC 443 to simulate shock propagation and interaction with a dense molecular cloud.
• Incorporation of reverse shock dynamics to model re-acceleration of cosmic rays in the post-shock region.
• Simulation of Rayleigh-Taylor instability at the contact discontinuity between the SNR ejecta and the molecular cloud to model turbulent mixing.
• Calculation of gamma-ray flux via inelastic p-p collisions (pion production and decay) using derived cosmic ray density and target proton density.
• Use of observed parameters—age, radius, explosion energy, and ambient density—to constrain the model and compare with EGRET data.
• Application of energy-dependent diffusion and acceleration efficiency in the shock front to estimate cosmic ray spectrum.

Experimental results

Research questions

• RQ1 Can the gamma-ray emission from IC 443, specifically the source 2EG J0618+2234, be explained by hadronic interactions of cosmic rays accelerated in the SNR shock?
• RQ2 How does the presence of a massive molecular cloud influence the efficiency of cosmic ray acceleration and gamma-ray production in IC 443?
• RQ3 To what extent do hydrodynamic instabilities such as Rayleigh-Taylor growth enhance cosmic ray mixing with the cloud material?
• RQ4 Why does the standard SNR model with low explosion energy fail to reproduce the observed gamma-ray flux, and how can this discrepancy be resolved?
• RQ5 What role does the reverse shock play in increasing the cosmic ray density in the interaction region?

Key findings

• The standard 3D hydrodynamical model of IC 443 with E₀ = 2.7 × 10⁵⁰ erg and n(0) = 0.21 cm⁻³ yields a gamma-ray flux too low to explain the EGRET observations.
• The reverse shock wave significantly increases the cosmic ray density in the interaction region between the SNR and the molecular cloud.
• Rayleigh-Taylor instability at the contact discontinuity between the SNR and the cloud leads to effective mixing of cosmic ray plasma with the cloud material.
• The combined effect of reverse shock acceleration and turbulent mixing results in a gamma-ray flux that matches the observed flux of 2EG J0618+2234.
• The model implies that the interaction with the molecular cloud is essential for explaining the high gamma-ray luminosity, resolving the discrepancy in the standard model.
• The derived parameters—age t = 4500 yr, radius R = 9.6 pc, and explosion energy E₀ = 2.7 × 10⁵⁰ erg—are consistent with observational constraints on IC 443.