NGC 4051: Black hole mass and photon index-mass accretion rate correlation

TL;DR

This paper discovers a correlation between the X-ray photon index and mass accretion rate in AGN NGC 4051, enabling a new method to estimate black hole mass using spectral data and scaling techniques.

Contribution

It introduces a novel analytical model and applies a scaling technique to determine the black hole mass in NGC 4051 from spectral correlations.

Findings

Black hole mass in NGC 4051 estimated to be over 6x10^5 solar masses.

Established a correlation between photon index and mass accretion rate in AGN.

Validated the scaling method using reference sources Cygnus X-1 and GRO J1550-564.

Abstract

We present a discovery of the correlation between the X-ray spectral (photon) index and mass accretion rate observed in AGN NGC 4051. We analyzed spectral transition episodes observed in NGC 4051 using XMM/Newton, Suzaku and RXTE. We applied a scaling technique for a black hole (BH) mass evaluation which uses a correlation between the photon index and normalization of the seed (disk) component, which is proportional to a mass accretion rate. We developed an analytical model that shows the spectral (photon) index of the BH emergent spectrum undergoes an evolution from lower to higher values depending on a mass accretion rate in the accretion disk. We considered Cygnus X-1 and GRO~J1550-564 as reference sources for which distances, inclination angles and the BH masses are evaluated by dynamical measurements. Application of the scaling technique for the photon index-mass accretion rate correlation provides an estimate of the black hole mass in NGC 4051 to be more than 6x10^5 solar masses.