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

• Published in: The Astrophysical journal Vol. 940; no. 2; pp. 105 - 114
• Main Authors: Matsuda, Masamune, Uchida, Hiroyuki, Tanaka, Takaaki, Yamaguchi, Hiroya, Tsuru, Takeshi Go
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01.12.2022; IOP Publishing
• Subjects: Astrophysics; Brightening; Electron energy; Electron heating; Emission analysis; Filaments; Heating; Hubble Space Telescope; Interstellar matter; Interstellar medium; Interstellar thermal emission; Plasma astrophysics; Space telescopes; Spectral analysis; Spectrum analysis; Supernova; Supernova remnants; Temperature ratio; X-ray emissions; X-ray sources; X-rays
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ac94cf

Abstract Mechanisms of particle heating are crucial to understanding the shock physics in supernova remnants (SNRs). However, there has been little information on time variabilities of thermalized particles so far. Here, we present a discovery of a gradually brightening thermal X-ray emission found in the Chandra data of Tycho’s SNR obtained during 2000–2015. The emission exhibits a knot-like feature (Knot1) with a diameter of ≃0.04 pc located in the northwestern limb, where we also find localized H α filaments in an optical image taken with the Hubble Space Telescope in 2008. The model with the solar abundance reproduces the spectra of Knot1, suggesting that Knot1 originates from the interstellar medium; this is the first detection of thermal X-ray emission from swept-up gas found in Tycho’s SNR. Our spectral analysis indicates that the electron temperature of Knot1 has increased from ∼0.30 to ∼0.69 keV within the period between 2000 and 2015. These results lead us to ascribe the time-variable emission to a small dense clump recently heated by the forward shock at the location of Knot1. The electron-to-proton temperature ratio immediately downstream of the shock ( β 0 ≡ T e / T p ) is constrained to be m e / m p ≤ β 0 ≤ 0.15 to reproduce the data, indicating the collisionless electron heating with efficiency is consistent with previous H α observations of Tycho and other SNRs with high shock velocities.