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

¹ LAPTh, Université Savoie-Mont-Blanc, CNRS, B.P. 110, 74941 Annecy-le-Vieux, France
² Lomonosov Moscow State University/Sternberg Astronomical Institute, Universitetsky Prospect 13, Moscow 119992, Russia
e-mail: seif@sai.msu.ru
³ George Mason University, College of Science, 4400 University Drive, Fairfax, VA 22030, USA
e-mail: achekhtm@gmu.edu
⁴ Dipartimento di Fisica, Università di Ferrara, Via Saragat 1, 44122 Ferrara, Italy
e-mail: titarchuk@fe.infn.it

Received: 4 November 2017
Accepted: 28 January 2018

Abstract

We present a discovery of the correlation between the X-ray spectral (photon) index and mass accretion rate observed in an active galactic nucleus, 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 6 × 10⁵ solar masses.