Observational Constraints on Black Hole Spin

Abstract

The spin of a black hole is an important quantity to study, providing a window into the processes by which a black hole was born and grew. Further, spin can be a potent energy source for powering relativistic jets and energetic particle acceleration. In this review, I describe the techniques currently used to detect and measure the spins of black holes. It is shown that:• Two well understood techniques, X-ray reflection spectroscopy and thermal continuum fitting, can be used to measure the spins of black holes that are accreting at moderate rates. There is a rich set of other electromagnetic techniques allowing us to extend spin measurements to lower accretion rates. • Many accreting supermassive black holes are found to be rapidlyspinning, although a population of more slowly spinning black holes emerges at masses above M > 3 × 10 7 M as expected from recent structure formation models. • Many accreting stellar-mass black holes in X-ray binary systems are rapidly spinning and must have been born in this state. • The advent of gravitational wave astronomy has enabled the detection of spin effects in merging binary black holes. Most of the pre-merger black holes are found to be slowly spinning, a notable exception being an object that may itself be a merger product. • The stark difference in spins between the black hole X-ray binary and the binary black hole populations shows that there is a diversity of formation mechanisms.Given the array of new electromagnetic and gravitational wave capabilities currently being planned, the future of black hole spin studies is bright.

Hypercritical Accretion for Black Hole High Spin in Cygnus X-1

“…The recently updated measurements (Miller-Jones et al 2021) of the BH mass, as well as its spin a * and companion mass, are marked in blue (at 68% credibility). The currently reported BH spin is very extreme, i.e., a * > 0.983, and it is still consistent with previous measurements (Cantiello et al 2014;Orosz et al 2011;Reynolds 2021). Our finding shows that such high BH spin can be explained by the hypercritical accretion investigated here.…”

“…To date the BH mass of Cygnus X-1 has exceeded the previously highest measured one in the X-ray binary for the extragalactic system M33 X-7. We note that the BH dimensionless spin recently reported is extremely high, a * > 0.9985 (Miller-Jones et al 2021), which is consistent with the previous measurement, namely a * > 0.983 (Reynolds 2021;Gou et al 2014;Cantiello et al 2014;Orosz et al 2011;Zhao et al 2021). In addition, it has been found that the surface helium-to-hydrogen ratio is about more than a factor of two relative to the solar composition (Shimanskii et al 2012).…”

“…It has been suggested that the two types of BH binaries (BBHs and BH-HMXBs) likely have distinct formation paths (Qin et al 2019a;Reynolds 2021;Fishbach & Kalogera 2021). This work is motivated by the inconsistent finding for the BH spin measurements in the two types of BH binaries.…”

Mass, Spin, and Ultralight Boson Constraints from the Intermediate Mass Black Hole in the Tidal Disruption Event 3XMM J215022.4-055108

“…Far from the horizon, a • is a higher order correction to the gravitational potential, challenging to observe even for resolved S-star orbits in the Milky Way (Merritt et al 2010), and impossible for unresolved stellar orbits around extragalactic SMBHs. Iron K-α spectroscopy allows spin measurements in bright AGN (Reynolds 2020), which, however, may not be representative of the SMBH population (Berti & Volonteri 2008).…”

A timing-based estimate of the spin of the black hole in MAXI J1820+070

“…The measurement of the spin in a black hole (BH) is one of the ★ E-mail: yash@iucaa.in important challenges in X-ray astronomy. Estimates of the spin can be made using spectroscopic methods in which either the relativistic broadening of the Fe K𝛼 line is measured or the continuum X-ray emission is modelled with thermal components (Miller et al 2009;Reynolds 2020). In both cases the innermost radius of the disk is measured and is assumed identical to the innermost stable circular orbit (ISCO).…”