Hybrid Plasma Emission in the Black Hole X-ray Binary MAXI J1535-571

The corona consisting of high-energy electrons is responsible for the hard X-ray continuum emission from Seyfert active galactic nuclei and X-ray binaries. Previous studies show that the temperature of the hot, compact corona is regulated by electron-positron pair productions. However, a significant fraction of them is cooler than expected based on purely thermal pair productions. Therefore, a hybrid model consisting of lower-energy electrons in a quasi-Maxwellian distribution and higher-energy electrons in a non-thermal, power-law distribution has been proposed to explain the variety of observed coronal temperatures. I will present the NuSTAR and Insight-HXMT data of the X-ray binary MAXI J1535-571 in outburst, which is known to have a low coronal temperature of 19 keV in the hard state. The hard X-ray emission of MAXI J1535-571 significantly deviates from a purely thermal Comptonisation model, which was commonly assumed in previous spectral models. I will show that this hard X-ray excess emission can be explained by non-thermal distributed electrons. The best-fit spectral model suggests that the non-thermal component is responsible for 89% of the energy distribution in MAXI J1535-571. Our model also predicts a pair annihilation line at 511 keV. Future soft-Gamma-ray missions, e.g. COSI may be able to detect this feature in MAXI J1535-571.