Balmer Filaments in Tycho’s Supernova Remnant: An Interplay between Cosmic-ray and Broad-neutral Precursors

Abstract

We present H α spectroscopic observations and detailed modeling of the Balmer filaments in the supernova remnant (SNR) Tycho (SN 1572). We used GH α FaS (Galaxy H α Fabry–Pérot Spectrometer) on the William Herschel Telescope with a 3.′4 × 3.′4 field of view, 0.″2 pixel scale, and σ instr = 8.1 km s −1 resolution at 1″ seeing for ∼10 hr, resulting in 82 spatial−spectral bins that resolve the narrow H α line in the entire SN 1572 northeastern rim. For the first time, we can therefore mitigate artificial line broadening from unresolved differential motion and probe H α emission parameters in varying shock and ambient medium conditions. Broad H α line remains unresolved within spectral coverage of 392 km s −1 . We employed Bayesian inference to obtain reliable parameter confidence intervals and to quantify the evidence for models with multiple line components. The median H α narrow-line (NL) FWHM of all bins and models is W NL = ( 54.8 ± 1.8 ) km s −1 at the 95% confidence level, varying within [35, 72] km s −1 between bins and clearly broadened compared to the intrinsic (thermal) ≈20 km s −1 . Possible line splits are accounted for, significant in ≈ 18 % of the filament, and presumably due to remaining projection effects. We also find widespread evidence for intermediate-line emission of a broad-neutral precursor, with a median W IL = ( 180 ± 14 ) km s −1 (95% confidence). Finally, we present a measurement of the remnant’s systemic velocity, V LSR = − 34 km s −1 , 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 NL emission is near-universal in SN 1572, and that, in the absence of an alternative explanation, collisionless SNR shocks constitute a viable acceleration source for Galactic TeV cosmic-ray protons.