The spins of the Galactic black holes in MAXI J1535-571 and 4U 1630-472 from Insight-HXMT

TL;DR

This study uses Insight-HXMT X-ray observations to measure the spins of black holes in two Galactic X-ray binaries, confirming a near-maximal spin for MAXI J1535-571 and a high but not extremal spin for 4U 1630-472.

Contribution

First application of Insight-HXMT data to determine black hole spins via reflection features in these sources, providing measurements consistent with previous NuSTAR results.

Findings

MAXI J1535-571 has a spin parameter close to 1.

4U 1630-472 has a high but not extremal spin.

Results are consistent with previous NuSTAR measurements.

Abstract

Insight-HXMT is the first Chinese X-ray astronomical mission. With a 1-250 keV bandpass, an energy resolution of 150 eV at 6 keV, and without being subject to pile-up distortions, it is suitable to study bright X-ray sources like Galactic black holes. In the present paper, we study Insight-HXMT observations of the X-ray binaries MAXI J1535-571 and 4U 1630-472 during their outbursts in 2017 and 2020, respectively. From the analysis of the reflection features, we are able to infer the black hole spin parameter in the two sources. For MAXI J1535-571, we find a spin parameter very close to 1, $a_* = 0.9916 \pm 0.0012$ (90% C.L., statistical error), which is consistent with the previous NuSTAR measurement. For 4U 1630-472, we find a moderately high value of the black hole spin parameter, $a_* =0.817 \pm 0.014$ (90% C.L., statistical error), which is lower than the almost extremal value found with NuSTAR data.