Black hole Spin Measurement Based on Time-domain VLBI Observations of Infalling Gas Cloud

TL;DR

This paper proposes a novel method to measure black hole spin by analyzing relativistic flux variations in time-domain VLBI observations of infalling gas clouds, demonstrated through simulations of EHT data for SgrA*.

Contribution

The paper introduces a new technique for black hole spin measurement based on time-domain flux analysis of infalling gas clouds using VLBI data, applicable to real EHT observations.

Findings

Spin dependence detectable in synthetic EHT data.

Method effective even with multiple gas clouds and realistic conditions.

Applicable to existing EHT observations of SgrA*.

Abstract

The black hole spacetime is described by general relativity and characterized by two quantities: the black hole mass and spin. Black hole spin measurement requires information from the vicinity of the event horizon, which is spatially resolved for the Galactic center SagittariusA* (SgrA*) and nearby radio galaxy M87 by means of very long baseline interferometry (VLBI) observations with the Event Horizon Telescope (EHT). In this paper, we simulate EHT observations for a gas cloud intermittently falling onto a black hole, and construct a method for spin measurement based on its relativistic flux variation. The light curve of the infalling gas cloud is composed of peaks formed by photons which directly reach a distant observer and by secondary ones reaching the observer after more than one rotation around the black hole. The time interval between the peaks is determined by a period of photon rotation near the photon circular orbit which uniquely depends on the spin. We perform synthetic EHT observations for SgrA* under a more realistic situation that a number of gas clouds intermittently fall towards the black hole with various initial parameters. Even for this case, the black hole spin dependence is detectable in correlated flux densities which are accurately calibrated by baselines between sites with redundant stations. The synthetic observations indicate that our methodology can be applied to EHT observations of Sgr A* since April 2017.