Particle Acceleration in Young Supernova Remnants with Nonthermal X-ray and Gamma-ray Observations
Abstract
A supernova remnant (SNR) is a prominent candidate as an accelerator of galactic cosmic rays (CRs). In a widely accepted theory of diffusive shock acceleration (DSA), a particle is accelerated by diffusing back and forth across shock waves. The information on ongoing particle acceleration in shock waves of SNRs is accessible through observations of nonthermal radiation. The spectral cutoff shape of accelerated electrons is determined by diffusion and cooling and appears in the X-ray and TeV gamma-ray energy ranges, respectively, through synchrotron radiation and inverse Compton (IC) scattering. To measure the cutoff shape precisely and explore the corresponding diffusion coefficient and acceleration efficiency, we systematically analyze young SNRs that emit nonthermal X-rays and TeV gamma-rays to obtain a unified understanding of particle acceleration in SNRs. The nonthermal (synchrotron) X-ray spectrum provides us with a powerful means of studying the nature of particle (electron) acceleration in a shock wave formed at the surface of an SNR. If energy loss of synchrotron cooling is efficient, the electron spectrum becomes steeper (E^{‑3}) than predicted by the standard DSA (E^{‑2}), and exponentially drops off (exp(‑(E/E0)^2)) above the maximum energy of E0 in the case of Bohm diffusion, where the diffusion coefficient, D, is proportional to the particle energy (D ∝ Eα with α = 1). In this thesis, the relevant synchrotron model is applied to 11 young SNRs. We measure the cutoff energy parameter (ɛ0) in the synchrotron X-ray spectrum and estimate the Bohm factor (η) by a theoretically predicted relation of ɛ0 ∝ v_sh^2 η‑1, with v_sh being shock velocity. The η parameter is defined as a mean free path of an electron over v_sh its gyroradius and is indicative of acceleration efficiency (i.e., η = 1 indicates the most efficient acceleration derived from the minimum diffusion coefficient). We apply the method to the nonthermal X-ray spectra of 11 SNRs using broadband spectroscopy taken with Chandra and NuSTAR. The obtained ɛ0 values and shock speeds from previous studies show variations that depend on SNRs and even on regions within each SNR. Six of 11 SNRs can be spatially resolved to investigate the ɛ0–v_sh relation, resulting in different behaviors of acceleration as follows: one behavior is well-reproduced through the theoretical prediction with a constant η (Kepler's and Tycho's SNRs), another behavior is determined based on the surroundings, in particular, magnetic field orientation (SN 1006) and ambient density (Cassiopeia A), and our cooling-limited assumption might not be applicable in the inner regions of the northwest rim of RX J1713.7‑3946 and G1.9+0.3. With all 11 SNRs together, the η parameter tends to have a smaller value as the SNR evolves and becomes older. This evolution of acceleration efficiency could be related to turbulent production, which is expected to be self-generated by the accelerated particles. We also apply the IC scattering model to the TeV gamma-ray observations of five SNRs. We found that in the same way as we derived the Bohm factor using X-ray observations, the gamma-ray spectrum was also utilized for estimating η in the leptonic scenario. Compared to the η parameters estimated by using X-ray observations, slightly higher values of η are obtained due to the smaller cutoff energies of electrons. The difference can be attributed to the different sizes of regions for spectral extraction between X-ray and gamma-ray observations. In addition, we should address the issue of the application of the IC (leptonic) model, in which gamma-rays are emitted from high-energy electrons, to the gamma-ray spectra that might contain hadronic components radiated from accelerated protons. We construct a model for accelerated electrons in non-Bohm diffusion (α≠1). The difference of α shows an apparent distinction in the cutoff shape of the electron spectrum as exp(‑(E/E0)^{α+1}), particularly in the higher energies. We derive the corresponding synchrotron and IC models to apply to nonthermal observations in X-rays and TeV gamma-rays, respectively. The models with different α values are not significantly distinguished due to the limited statistics of the present spectra in the higher energies. With deeper observations and/or a combination of X-ray and gamma-ray spectra with better spatial resolution, the α parameter would be more constrained and provide feedback to theoretical studies of particle diffusion and turbulent generation.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- March 2020
- DOI:
- 10.14992/00019368
- Bibcode:
- 2020PhDT........36T
- Keywords:
-
- Cosmic rays;
- Particle acceleration;
- Supernova remnant;
- Nonthermal emission