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Discovery of Year-scale Time Variability from Thermal X-Ray Emission in Tycho’s Supernova Remnant

Presentation #102.29 in the session ISM/Galaxies.

Published onJul 01, 2023
Discovery of Year-scale Time Variability from Thermal X-Ray Emission in Tycho’s Supernova Remnant

Electron heating at collisionless shocks is closely related to a number of astrophysical problems, such as generation processes of the shocks and particle injections to the diffusive shock acceleration in supernova remnants (SNRs) (R. A. Treumann, 2009, A&ARv, 17, 409). However, the heating mechanism of shocked plasma is still open due to a lack of observational information. In our study, we discovered a year-scale time variability of thermal X-ray emissions of Tycho’s SNR in Chandra data (M. Matsuda et al., 2022, ApJ, 940, 105). Our spectral analysis shows an increase in the electron temperature (kTe) from ~0.30 keV to ~0.69 keV between 2000 and 2015. Energy transfer via Coulomb collisions can well explain the time scale of the kTe change, assuming that the plasma density is about ten times higher than the surroundings. The high density of the plasma is also suggested by a spatial correlation between the time variable X-ray and bright Hα filaments in an optical image taken by the Hubble Space Telescope (J.-J. Lee et al., 2010, ApJL, 715, L146) as well as the best-fit values of the emission measures. Besides, we constrain the efficiency of collisionless electron heating by comparing the observation to calculations of kTe change under different assumptions of electron-to-proton temperature ratios (β=Te/Tp) immediately downstream of the shock. The ratio is constrained to be me/mp≤β≤0.15, in agreement with an efficiency estimated by previous Hα studies of Tycho’s SNR (e.g., M. van Adelsberg et al., 2008, ApJ, 689, 1089). Our discovery helps us understand the evolution of the shock-heated plasma, which can provide clues to the heating mechanism. Our poster will present the details of the above and future observations aiming at measuring ion temperatures with the microcalorimeter onboard XRISM (M. Tashiro et al., 2018, SPIE, 10699, 1069922).

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