Disk-Outflow Symbiosis in GRMHD Simulations: Explaining Hard-State ULXs

TL;DR

This paper uses GRMHD simulations to show that highly magnetized accretion flows around stellar-mass black holes can produce the super-Eddington luminosities observed in ultraluminous X-ray sources, explaining their spectral features.

Contribution

It demonstrates through simulations that magnetized advective accretion flows can account for ULX luminosities and magnetic field strengths, advancing understanding of ULX physics.

Findings

Simulations produce ULX-like luminosities exceeding Eddington limit.

Magnetic field strengths around $10^7$ Gauss are sufficient for high luminosity.

Highly magnetized accretion flows can explain ULX spectral characteristics.

Abstract

Ultraluminous X-ray sources (ULXs) have captivated researchers for decades due to their exceptionally high luminosities and unique spectral characteristics. Some of these sources defy expectations by exhibiting super-Eddington luminosities with respect to stellar mass sources even in their low-hard state. Numerical steady-state calculations suggest that ULXs in this state can be explained as highly magnetized advective accretion sources around stellar-mass black holes. To explore this further, we employ GRMHD simulations using the publicly available code, BHAC (Black Hole Accretion Code), to model the behavior of highly magnetized advective accretion flows around a black hole. Our simulations demonstrate that such systems can indeed produce the intense luminosities observed in ULXs. Additionally, we validate that the magnetic fields required for these high emissions are of the order of $10^7$ Gauss, consistent with previous numerical steady-state findings.