Balmer-dominated shocks in Tycho's SNR: omnipresence of CRs

TL;DR

This study uses high-resolution spectroscopy of Tycho's supernova remnant to demonstrate that cosmic-ray acceleration is widespread, affecting Balmer line profiles and indicating variable shock conditions across the remnant.

Contribution

It provides the first evidence that broadening of the narrow Hα line is intrinsic and not due to observational artifacts, confirming ubiquitous cosmic-ray acceleration in Tycho's SNR.

Findings

Narrow Hα line width exceeds 20 km/s across the entire NE rim.

CR acceleration influences both narrow and broad Hα components.

Presence of an intermediate component with ~180 km/s width in some spectra.

Abstract

We present wide-field, spatially and highly resolved spectroscopic observations of Balmer filaments in the northeastern rim of Tycho's supernova remnant in order to investigate the signal of cosmic-ray (CR) acceleration. The spectra of Balmer-dominated shocks (BDSs) have characteristic narrow (FWHM $\sim$ 10 kms$^{-1}$) and broad (FWHM $\sim$ 1000 kms$^{-1}$) H$\alpha$ components. CRs affect the H$\alpha$-line parameters: heating the cold neutrals in the interstellar medium results in broadening of the narrow H$\alpha$-line width beyond 20 kms$^{-1}$, but also in reduction of the broad H$\alpha$-line width due to energy being removed from the protons in the post-shock region. For the first time we show that the width of the narrow H$\alpha$ line, much larger than 20 kms$^{-1}$, is not a resolution or geometric effect nor a spurious result of a neglected intermediate (FWHM $\sim$ 100 kms$^{-1}$) component resulting from hydrogen atoms undergoing charge exchange with warm protons in the broad-neutral precursor. Moreover, we show that a narrow line width $\gg$ 20 kms$^{-1}$ extends across the entire NE rim, implying CR acceleration is ubiquitous, and making it possible to relate its strength to locally varying shock conditions. Finally, we find several locations along the rim, where spectra are significantly better explained (based on Bayesian evidence) by inclusion of the intermediate component, with a width of 180 kms$^{-1}$ on average.