Kink-driven magnetic reconnection in relativistic jets: consequences for X-ray polarimetry of BL Lacs

We investigate with relativisticMHD(magnetohydrodynamic) simulations the dissipation physics of BL Lac jets, by studying the synchrotron polarization signatures of particles accelerated by the kink instability in a magnetically dominated plasma column. The non-linear stage of the kink instability generates current sheets, where particles can be efficiently accelerated via magnetic reconnection. We identify current sheets as regions where s = Jd/B is above some predefined threshold (where B is the field strength, J is the current density, and delta is the grid scale), and assume that the particle injection efficiency scales as proportional to J(2). X-ray emitting particles have short cooling times, so they only probe the field geometry of their injection sites. In contrast, particles emitting in the optical band, which we follow self-consistently as they propagate away from their injection sites while cooling, sample a larger volume, and so they may be expected to produce different polarimetric signatures. We find that the degree of polarization is roughly the same between X-ray and optical bands, because even the optical emitting particles do not travel far from the current sheet where they were injected, due to lack of sufficient kink-generated turbulence. The polarization angle shows a different temporal evolution between the two bands, due to the different regions probed by X-ray and optical emitting particles. In view of the upcoming IXPE satellite, our results can help constrain whether kink-induced reconnection (as opposed to shocks) can be the source of multiwavelength emission from BL Lacs.

Automated summary

Using relativistic MHD simulations, we investigated the dissipation physics of BL Lac jets, focusing on the synchrotron polarization signatures of particles accelerated by the kink instability. Both X-ray and optical emitting particles show similar degree of polarization, but exhibit different temporal evolution in polarization angle due to probing different regions within the jet.