X-ray spectral states of microquasars

TL;DR

This paper investigates the origins of X-ray spectral differences in microquasars' states, using simulations to constrain magnetic fields and electron acceleration processes, suggesting similar coronae in different states with spectral variations driven by disc cooling.

Contribution

It introduces a new simulation code accounting for the synchrotron boiler effect and constrains magnetic fields and proton temperatures in microquasar coronae, challenging existing models.

Findings

Magnetic field in Cygnus X-1's hard state is below equipartition with radiation.

Proton temperatures are lower than in ADAF models.

Both spectral states may have similar coronae powered by non-thermal electron acceleration.

Abstract

We discuss the origin of the dramatically different X-ray spectral shapes observed in the Low Hard State (LHS: dominated by thermal comptonisation) and the High Soft State (HSS: dominated by the accretion disc thermal emission and non-thermal comptonisation in the corona). We present numerical simulations using a new code accounting for the so-called synchrotron boiler effect. These numerical simulations when compared to the data allow us to constrain the magnetic field and temperature of the hot protons in the corona. For the hard state of Cygnus X-1 we find a magnetic field below equipartition with radiation, suggesting that the corona is not powered through magnetic field dissipation (as assumed in most accretion disc corona models). On the other hand, our results also point toward proton temperatures that are substantially lower than typical temperatures of the ADAF models. Finally, we show that in both spectral states Comptonising plasma could be powered essentially through power-law acceleration of non-thermal electrons, which are then partly thermalised by the synchrotron and Coulomb boiler. This suggests that, contrary to current beliefs, the corona of the HSS and that of the LHS could be of very similar nature. The differences between the LHS and HSS coronal spectra would then be predominantly caused by the strong disc soft cooling emission which is present in the HSS and absent in the LHS.