Polarized H-alpha Emission from Supernova Remnant Shock Waves Efficiently Accelerating Cosmic Rays

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• John M. Laming (NRL, Washington DC)
• Monday 13 May 2019, 14:00-15:00
• MR14, Centre for Mathematical Sciences, Wilberforce Road, Cambridge.

If you have a question about this talk, please contact Dr William Béthune.

It is frequently stated that over 99.999% of the visible universe is in the plasma “state”. It is much less frequently commented that the overwhelming majority of this plasma is collisionless, in that particle mean free paths against Coulomb collisions are much longer than typical plasma dimensions. The theory of shock waves propagating in such plasmas poses special problems, relying on plasma turbulence to compress and heat the postshock gas, and also to scatter and accelerate cosmic rays as a necessary part of the shock dissipation.

We develop spectropolarimetry of H alpha as a diagnostic of such phenomena. Neutral hydrogen in the interstellar medium impacted by a collisionless shock “sees” an anisotropic distribution of scattering electrons and ions. In such circumstances line emission excited by these scattering particles will in general be polarized, and such polarization can be used to make inferences about collisionless plasma processes at the shock. Following an initial prediction that such polarization should exist (Laming 1990), and recent observational validation in the NW limb of SN 1006 (Sparks et al. 2015), we revisit the calculations with updated atomic data as a diagnostic of shock energy loss to cosmic rays (Shimoda et al. 2018). Such energy loss has the effect of increasing the mild “collimation” of the shocked flow, increasing the degree of polarization expected. We make comparisons with existing observations of SN 1006 and for “knot g” in Tycho’s supernova remnant, attempting to infer the shock energy losses to cosmic rays in each case.

This talk is part of the DAMTP Astrophysics Seminars series.

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