Scaling of X-ray spectral properties of a black hole in the Seyfert 1 galaxy NGC~7469

Abstract

We present our analysis of X-ray spectral properties observed from the Seyfrert 1 galactic nucleus NGC~7469 using the RXTE and ASCA observations. We demonstrate strong observational evidence that NGC~7469 undergoes spectral transitions from the low hard state (LHS) to the intermediate state (IS) during these observations. The RXTE observations (1996--2009) show that the source was in the IS ~ 75 % of the time only, ~ 25 % of the time in the LHS. The spectra of NGC~7469 are well fitted by the so-called bulk motion Comptonization (BMC) model for all spectral states. We have established the photon index saturation level, Gamma_{sat}+2.1+/-0.1, in the Gamma versus mass accretion rate, Mdot correlation. This Gamma- Mdot correlation allows us to estimate the black hole (BH) mass in NGC~7469 to be M__BH> 3 x 10^6 solar masses assuming the distance to NGC~7469 of 70 Mpc. For this BH mass estimate, we use the scaling method taking Galactic BHs, GRO~J1655--40, Cyg~ X--1 and an extragalactic BH source, NGC~4051 as reference sources. The Gamma versus Mdot correlation revealed in NGC~7469 is similar to those in a number of Galactic and extragalactic BHs and it clearly shows the correlation along with the strong Gamma saturation at ~2.1. This is robust observational evidence for the presence of a BH in NGC~7469. We also find that the seed photon temperatures are quite low, of the order of 140-200 eV, which are consistent with a high BH mass in NGC~7469 that is more than 3x10^6 solar masses.