Plasma heating and particle acceleration in collisionless shocks through astrophysical observations

Abstract

Supernova remnants (SNRs), the products of stellar explosions, are powerful astrophysical laboratories, which allow us to study the physics of collisionless shocks, thanks to their bright electromagnetic emission. Blast wave shocks generated by supernovae (SNe) provide us with an observational window to study extreme conditions, characterized by high Mach (and Alfvenic Mach) numbers, together with powerful nonthermal processes. In collisionless shocks, temperature equilibration between different species may not be reached at the shock front. In this framework, different particle species might be heated at different temperatures (depending on their mass) in the post-shock medium of SNRs. SNRs are also characterized by a broadband nonthermal emission stemming at the shock front as a result of nonthermal populations of leptons and hadrons. These particles, known as cosmic rays, are accelerated up to ultrarelativistic energies via diffusive shock acceleration. If SNRs lose a significant fraction of their ram energy to accelerate cosmic rays, the shock dynamics should be altered with respect to the adiabatic case. This shock modification should result in an increase of the total shock compression ratio with respect to the Rankine-Hugoniot value of 4. Here I show that the combination of X-ray high resolution spectroscopy (to measure ion temperatures) and moderate resolution spectroscopy (for a detailed diagnostic of the post-shock density) can be exploited to study both the heating mechanism and the particle acceleration in collisionless shocks. I report on new results in the temperatures measured for different ion species in the remnant of the SN observed in 1987 in the Large Magellanic Cloud (SN 1987A). I also discuss evidence of shock modification recently obtained in the remnant of SN 1006 a. D., where the shock compression ratio increases significantly as the angle between the shock velocity and the ambient magnetic field is reduced.

Probing Shocked Ejecta in SN 1987A: A Novel Diagnostic Approach Using XRISM-Resolve

“…We then synthesize the XRISM-Resolve spectrum of SN 1987A, adding the contribution of the thermal broadening to that of the Doppler broadening. We here focus on two emission lines: the Si XIV Lyα line to have a comparison with previous studies (Miceli et al 2019;Miceli 2023) and the Fe XXV w, x, y, z lines (hereafter, Fe XXV lines complex), to explore the unprecedented capability of resolving the thermal broadening at high energy provided by this new instrument. We do not discuss here the line emission of H-like ion (Fe XXVI Lyα), which could have been a much more simple probe for the thermal broadening, because it does not emerge from the continuum (see Figure 2).…”

“…To self-consistently synthesize the thermal X-ray emission from the 3D MHD simulation, we employ an in-house developed tool in a similar fashion as Miceli et al (2019), Greco et al (2020), andMiceli (2023). In each cell of the computational domain, we derive the value of electron and proton temperature, electron density, and τ.…”

“…In addition to this effect, we also include the thermal broadening arising from the thermal motion of the ions in the plasma. To this end, we assume the ion temperature to be massproportional with respect to the proton temperature (Miceli et al 2019;Miceli 2023). Once we had obtained the complete spectral model by summing cell-by-cell the computed spectra, we applied a Poissonian randomization to synthesize the spectra.…”

“…They analyzed the Chandra High Energy Transmission Grating data from 2007 and 2011 comparing them with the 3D HD simulation presented by Orlando et al (2015). Since the simulation is able to reproduce self-consistently the whole broadening (due to the thermal motion of ions and to bulk motion along the line of sight) of the spectral lines, they were able to deduce the postshock temperature of protons and ions through the comparison of the model with observation (see also Ravi et al 2021;Miceli 2023).…”

Rankine–Hugoniot Relations and Magnetic Field Enhancement in Turbulent Shocks

“…They are encountered in all plasma environments, from a lab (Niemann et al 2014) to a galaxy cluster (Markevitch & Vikhlinin 2007), and at all spatial scales, from millimeters to kiloparsecs. Supernova remnant (SNR) shocks are one of the most efficient accelerators of charged particles in the Universe (Miceli 2023). SNR shocks can be studied only with remote measurements of the electromagnetic radiation coming from the accelerated and heated particles at the shock.…”