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INTEGRAL study of MAXI J1535-571, MAXI J1820+070 and MAXI J1348-630 outbursts -- I. Detection and polarization properties of the high-energy emission

In black hole X-ray binaries, a non-thermal high-energy component is sometimes detected at energies above 200 keV. The origin of this component is debated and distinct spectral modelizations can lead to different interpretations. High-energy polarimetry measurements with INTEGRAL allow new diagnostics on the physics responsible for the MeV spectral component. In this work, we aim to investigate the high-energy behavior of three bright sources discovered by the MAXI: MAXI J1535-571, MAXI J1820+070 and MAXI J1348-630. We take advantage of their brightness to investigate their soft gamma-ray (0.1-2 MeV) properties with INTEGRAL. We use both spectral and polarimetric approaches to probe their high-energy emission with the aim to bring new constraints on the ~ MeV emission. We first study the spectral characteristics of the sources in the 3-2000 keV using JEM-X, IBIS and SPI with a semi-phenomenological description of the data. We then use IBIS as a Compton telescope in order to evaluate the polarization properties of the sources above 300 keV. A high-energy component is detected during the HIMS and SIMS of MAXI J1535-571, the LHS of MAXI J1820+070 and the LHS of MAXI J1348-630. The components detected in MAXI J1820+070 and MAXI J1348-630 are polarized with a polarization fraction of 26 +/- 9° and > 56 % in the 300-1000 keV, 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 non-thermal distribution of electrons from a hybrid corona. For MAXI J1348-630, the high fraction of polarization points towards a jets origin, however, we cannot formally conclude without any infrared data giving information on the optically thin part of the synchrotron spectrum.