The supernova remnant SN 1006 as a Galactic particle accelerator

Abstract

The origin of cosmic rays is a pivotal open issue of high-energy astrophysics. Supernova remnants are strong candidates to be the Galactic factory of cosmic rays, their blast waves being powerful particle accelerators. However, supernova remnants can power the observed flux of cosmic rays only if they transfer a significant fraction of their kinetic energy to the accelerated particles, but conclusive evidence for such efficient acceleration is still lacking. In this scenario, the shock energy channeled to cosmic rays should induce a higher post-shock density than that predicted by standard shock conditions. Here we show this effect, and probe its dependence on the orientation of the ambient magnetic field, by analyzing deep X-ray observations of the Galactic remnant of SN 1006. By comparing our results with state-of-the-art models, we conclude that SN 1006 is an efficient source of cosmic rays and obtain an observational support for the quasi-parallel acceleration mechanism.

Hadronic versus Leptonic Origin of Gamma-Ray Emission from Supernova Remnants

“…In particular, we include the effects of the postcursor, i.e., the region immediately downstream of the shock where there is net drift of magnetic fluctuations and CRs with respect to the thermal plasma. These effects, which were seen for the first time in kinetic simulations (Caprioli et al 2020;, lead to an enhanced shock compression and a steepening of the CR spectrum, both in excellent agreement with observations-see Diesing & Caprioli 2021 for a discussion on SNR spectra and Giuffrida et al (2022) for the specific case of SN1006. This steepening of the particle spectra is most relevant to the results of our study because this will impact the behavior of the resulting photon spectra.…”

Plasma heating and particle acceleration in collisionless shocks through astrophysical observations

“…The presence of shock modification has been recently revealed in SN 1006 [7]. In this remnant, thanks to the the almost uniform magnetic field, approximately aligned in the southwest-northeast direction, it is possible to observe, in the same object, regions with efficient particle acceleration (i.e.…”

“…1) and regions where we do not expect shock modification ( [6]). A careful spatially resolved spectral analysis of the X-ray spectra stemming from the shocked interstellar medium at different positions of the shell reveals a regular azimuthal modulation of r s , with a minimum r s = 4 in quasi-perpendicular conditions and a maximum which reaches values as high as r s = 7 in quasiparallel conditions ( [7]). The accurate comparison of the azimuthal profile of the shock compression ratio with state of the art theoretical models of modified shocks including the effect of the postcursor shows a nice agreement between model and observations and provide important constraints on the acceleration efficiency and on its dependence on the angle between the ambient magnetic field and the shock velocity.…”

“…In particular, in quasi-parallel conditions, the cosmic rays pressure is ∼ 12% of the total, while the normalized magnetic pressure is ∼ 5%. Moreover, the r s azimuthal profile shows a sharp minimum, which is strongly indicative of efficient (normalized pressure ∼ 6%) reacceleration of pre-existing Galactic cosmic rays ( [7]).…”

“…Soft X-rays (0.5 − 1 keV, in red) show a complex pattern, with ripples of emission revealing knots of shocked ejecta ( [3,4]), hard X-rays (2.5 − 7 keV, in blue) are associated with synchrotron radiation in the northeastern and southwestern limbs ( [5]), while optical H α emission (in green) mark the northwestern shock front in great detail. By considering characteristic values for the post-shock temperature and density (T = 10 7 K [3], and n = 0.1 cm −3 [6,7], respectively), the Coulomb collision mean free path is of the order of 10 19 cm, i. e. comparable with the radius of the remnant and much larger than the width of shock front. Since the viscous dissipation at the shock front is localized in an extremely narrow region, it cannot be provided by collisions and collective effects, as electromagnetic fluctuations and plasma waves, instead, are thought to be responsible of the plasma heating.…”