INTEGRAL study of MAXI J1535−571, MAXI J1820+070, and MAXI J1348 − 630 outbursts

In black hole X-ray binaries, a nonthermal high-energy component is sometimes detected at energies above 200 keV. The origin of this high-energy component is debated and distinct spectral modelizations can lead to different interpretations. High-energy polarimetry measurements with the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) enable new diagnostics on the physics responsible for the MeV spectral component in black hole X-ray binaries. In this work, we aim to investigate the high-energy behavior of three bright sources discovered by the Monitor of All-sky X-ray Image: MAXI J1535−571, MAXI J1820+070, and MAXI J1348−630. We took advantage of their brightness to investigate their soft γ-ray (0.1–2 MeV) properties with INTEGRAL. We used both spectral and polarimetric approaches to probe their high-energy emission with the aim of bringing new constraints on the MeV emission in black hole X-ray binaries. We first studied the spectral characteristics of the sources in the 3–2000 keV range using JEM-X, IBIS, and SPI, with a semi-phenomenological description of the data. We then used IBIS as a Compton telescope in order to evaluate the polarization properties of the sources above 300 keV. A high-energy component was detected during the hard-intermediate state and soft-intermediate state of MAXI J1535−571, the low-hard state of MAXI J1820+070, and the low-hard state of MAXI J1348−630. The components detected in MAXI J1820+070 and MAXI J1348 − 630 were polarized with a polarization fraction of 26 ± 9° and > 56% in the 300–1000 keV range, respectively. With no polarization information for MAXI J1535−571, the component detected could either come from the jets or the corona. In the case of MAXI J1820+070, the extrapolation of the synchrotron spectrum measured in the infrared indicates that the component is likely due to a nonthermal distribution of electrons from a hybrid corona. For MAXI J1348−630, the high fraction of polarization points toward a jets origin; however, we cannot formally conclude this without any infrared data giving information on the optically thin part of the synchrotron spectrum.