The acceleration of non-thermal particles in collisionless shocks, especially in supernova remnant shocks, is a long standing problem for revealing the origin of Galactic cosmic-rays. In the most relied scenario, the Galactic cosmic-rays originate in the diffusive shock acceleration mechanism, and the pressure of accelerated cosmic-rays should be large comparable to the ram pressure of upstream background protons to realize the maximum energy of Galactic cosmic-rays, 3 PeV. If the case, the upstream plasma is decelerated "before" entering the shock front (measured in the shock rest frame). This is called the cosmic-ray modified shock and one of the essential predictions for collisionless shocks efficiently accelerating cosmic-rays. Nevertheless, the modification has unfortunately never been observed yet. In numerical simulation, an inferred degree of velocity modification is just 10 percent level. Thus, we must examine this 10 percent modification of plasma located at a distance of kpc scale. We have found that the polarimetry of Hα emission at supernova remnants is a potentially powerful tool to quantify the velocity modification: the polarization direction of Hα is parallel to the shock normal vector in the modified shock case, while the direction is perpendicular without modification. Note that although observations of Hα in supernova remnants has been done in past decades, linearly polarized Hα is recently discovered by Sparks et al. (2015). We have also found that among observable values we calculated, only the polarization direction responds sensitively to the velocity modification (5 percent in our model). Therefore, the polarimetry of Hα will be a unique diagnostics of cosmic-ray modified shock and will bring new insights to particle acceleration physics in collisionless shocks.
Supernova Remnants; Particle acceleration