NLS1 galaxies and estimation of their central black hole masses from the X-ray excess variance method

TL;DR

This paper extends the X-ray excess variance method for black hole mass estimation from broad line Seyfert 1 galaxies to Narrow Line Seyfert 1 galaxies, revealing the need for different scaling factors and spectral considerations.

Contribution

It introduces a modified mass estimation approach for NLS1 galaxies and highlights the importance of soft X-ray spectral properties in classification.

Findings

A single scaling coefficient works for most NLS1s, increasing mass estimates by a factor of 20.

Some NLS1 galaxies with intermediate variability and harder X-ray spectra challenge the simple classification.

Soft X-ray slope may be a better classification criterion than Hbeta line width.

Abstract

Black hole mass determination in active galaxies is a key issue in understanding various luminosity states. In the present paper we try to generalise the mass determination method based on the X-ray excess variance, successfully used for typical broad line Seyfert 1 galaxies (BLS1) to Narrow Line Seyfert 1 (NLS1) galaxies. NLS1 galaxies differ from BLS1 with respect to several properties. They are generally more variable in 2-10 keV energy band so the natural expectation is the need to use a different scaling coefficient between the mass and the variance in these two types of sources. However, we find that such a simple approach is not enough. Although for majority of the 21 NLS1 galaxies in our sample a single scaling coefficient (larger by a factor 20) provided us with a satisfactory method of mass determination, in a small subset of NLS1 galaxies this approach failed. Variability of those objects appeared to be at the intermediate level between NLS1 and BLS1 galaxies. These exceptional NLS1 galaxies have much harder soft X-ray spectra than majority of NLS1 galaxies. We thus postulate that the division of Seyfert 1 galaxies into BLS1 and NLS1 according to the widths of the Hbeta line is less generic than according to the soft X-ray slope.