Balmer filaments in Tycho's supernova remnant: an interplay between cosmic-ray and broad-neutral precursors

We present H$\alpha$ spectroscopic observations and detailed modelling of the Balmer filaments in the supernova remnant Tycho. We used Galaxy H$\alpha$ Fabry-P'erot Spectrometer on the William Herschel Telescope with a 3.4’$\times$3.4’ field-of-view, 0.2" pixel scale and ${\sigma }_{\mathrm{i}\mathrm{n}\mathrm{s}\mathrm{t}\mathrm{r}}=8.1$ km/s resolution at 1" seeing for $\sim 10$ hours, resulting in 82 spatial-spectral bins that resolve the narrow H$\alpha$ line in the entire Tycho’s northeastern rim. For the first time, we can mitigate artificial line broadening from unresolved differential motion, and probe H$\alpha$ emission parameters in varying shock and ambient medium conditions. Broad H$\alpha$ line remains unresolved within spectral coverage of 392 km/s. We employed Bayesian inference to obtain reliable parameter confidence intervals, and quantify the evidence for models with multiple line components. The median H$\alpha$ narrow-line full-width at half-maximum of all bins and models is $W_{\mathrm{N}\mathrm{L}}=(54.8\pm 1.8)$ km/s at the $95\mathrm{\%}$ confidence level, varying within [35, 72] km/s between bins and clearly broadened compared to the intrinsic (thermal) $\approx 20$ km/s. Possible line splits are accounted for, significant in $\approx 18\mathrm{\%}$ of the filament, and presumably due to remaining projection effects. We also find wide-spread evidence for intermediate-line emission of a broad-neutral precursor, with median $W_{\mathrm{I}\mathrm{L}}=(180\pm 14)$ km/s ($95\mathrm{\%}$ confidence). Finally, we present a measurement of the remnant’s systemic velocity, $V_{\mathrm{L}\mathrm{S}\mathrm{R}}=-34$ km/s, and map differential line-of-sight motions. Our results confirm the existence and interplay of shock precursors in Tycho’s remnant. In particular, we show that suprathermal narrow-line emission is near-universal in Tycho and that, in absence of an alternative explanation, collisionless supernova remnant shocks constitute a viable acceleration source for Galactic TeV Cosmic-Ray protons.