Toward Understanding the Cosmic-Ray Acceleration at Young Supernova Remnants Interacting With Interstellar Clouds: Possible Applications to Rx J1713.7–3946

Abstract

Using three-dimensional magnetohydrodynamics simulations, we investigate general properties of a blast wave shock interacting with interstellar clouds. The pre-shock cloudy medium is generated as a natural consequence of the thermal instability that simulates realistic clumpy interstellar clouds and their diffuse surrounding. The shock wave that sweeps the cloudy medium generates a turbulent shell through the vorticity generations that are induced by shock-cloud interactions. In the turbulent shell, the magnetic field is amplified as a result of turbulent dynamo action. The energy density of the amplified magnetic field can locally grow comparable to the thermal energy density, particularly at the transition layers between clouds and the diffuse surrounding. In the case of a young supernova remnant (SNR) with a shock velocity 10 3 km s −1 , the corresponding strength of the magnetic field is approximately 1 mG. The propagation speed of the shock wave is significantly stalled in the clouds because of the high density, while the shock maintains a high velocity in the diffuse surrounding. In addition, when the shock wave hits the clouds, reflection shock waves are generated that propagate back into the shocked shell. From these simulation results, many observational characteristics of a young SNR RX J1713.7−3946 that is suggested to be interacting with molecular clouds, can be explained as follows: The reflection shocks can accelerate particles in the turbulent downstream region where the magnetic field strength reaches 1mG, which causes short-time variability of synchrotron X-rays. Since the shock velocity is stalled locally in the clouds, the temperature in the shocked cloud is suppressed far below 1 keV. Thus, thermal Xray line emission would be faint even if the SNR is interacting with molecular clouds. We also find that the photon index of the π 0 -decay gamma rays generated by cosmic-ray protons can be 1.5 (corresponding energy flux is νF ν ∝ ν 0.5 ), because the penetration depth of high-energy particles into the clumpy clouds depends on their energy. This suggests that, if we rely only on the spectral study, the hadronic gamma-ray emission is indistinguishable from the leptonic inverse Compton emission. We propose that the spatial correlation of the gamma-ray, X-ray, and CO line emission regions can be conclusively used to understand the origin of gamma rays from RX J1713.7−3946.

Prospects for Cherenkov Telescope Array Observations of the Young Supernova Remnant RX J1713.7−3946

“…Depending on the total mass fraction and the energy dependence of the CR penetration depth in such dense cores in a clumpy medium interacting with the shock, an overall hard gamma-ray spectrum consistent with the Fermi/ LAT and H.E.S.S. data can be realized (Zirakashvili & Aharonian 2010;Inoue et al 2012;Gabici & Aharonian 2014). On the other hand, it has been argued that since a hadronic model requires a high ambient gas density in order for the gamma-ray emission to be dominated by the π 0 -decay component, bright thermal X-ray emission should be expected from the shocked gas, which is nevertheless not detected so far (e.g., Ellison et al 2012).…”

“…(1) Hadronic model based on MHD simulations: The shock interaction with dense clouds excites turbulence, which amplifies the magnetic field up to the mG order (Inoue et al 2012;Sano et al 2013Sano et al , 2015. These results may be crucial for our better understanding of the gamma-ray and X-ray images of SNRs.…”

“…In a shock−cloud interaction scenario, however, it is quite possible that the dense clumps of gas swept up by the blast wave can remain "intact" and stay at a low average ionization state and temperature. If these dense clumps bear a significant mass fraction behind the shock, faint thermal X-rays can be naturally explained (Inoue et al 2012).…”

Synthetic Observation of Turbulent Supernova Remnants

“…Shock propagation in an inhomogeneous ISM leads to make velocity shear behind shock wave and drives anisotropic turbulence biased toward the shock propagation direction. The roll of RMI in the SNR has been studied using magnetohydrodynamics (MHD) simulations [6][7][8][9][10][11]15]. The turbulence driven by RMI induces small-scale dynamo effect that amplifies the downstream magnetic field.…”

Collisionless Shocks in Partly Ionized Plasma with Cosmic Rays: Microphysics of Non-thermal Components

“…Raymond et al (1988b) analyzed a shock in the Cygnus Loop SNR and found that the nonthermal pressure exceeds the thermal pressure by an order of magnitude in the zones where the [S II] lines are formed, though they were unable to distinguish between magnetic and cosmic ray contributions. Neutral particles could affect the processes of particle acceleration (e.g., Draine and McKee 1993;O'C Drury et al 1996;Malkov et al 2005;Blasi et al 2012;Inoue et al 2012;Ohira 2012;Helder et al 2012;Morlino et al 2012a), magnetic field amplification and plasma heating in the upstream region (e.g., Bykov and Toptygin 2005;Reville et al 2007).…”