Narrow Line Seyfert 1 galaxies at hard X-rays

TL;DR

This study analyzes the X-ray spectral properties of 14 Narrow Line Seyfert 1 galaxies selected at hard X-rays, revealing their spectral characteristics, variability, and black hole mass distribution, and comparing them with broad-line Seyfert 1 galaxies.

Contribution

First comprehensive analysis of hard X-ray selected NLSy1 galaxies, combining broad-band spectra and spectral features, highlighting their spectral slopes, variability, and black hole mass distribution.

Findings

NLSy1 show steep spectral indices compared to BLSy1 in broad-band spectra.

Hard X-ray selected NLSy1 do not exhibit strong soft excess emission.

Approximately 15% of the hard X-ray sky NLSy1 are present, with small black hole masses around 10^7 solar masses.

Abstract

Narrow Line Seyfert 1 (NLSy1) galaxies are a peculiar class of type 1 AGN (BLSy1). The X-ray properties of individual objects belonging to this class are often extreme and associated with accretion at high Eddington ratios. Here we present a study on a sample of 14 NLSy1 galaxies selected at hard X-rays (> 20 keV) from the fourth INTEGRAL/IBIS catalogue. The 20-100 keV IBIS spectra show hard X-ray photon indeces flatly distributed (Gamma_{20-100 keV} ranging from ~1.3 to ~3.6) with an average value of <Gamma_{20-100 keV}> = 2.3+/-0.7, compatible with a sample of hard X-ray BLSy1 average slope. Instead, NLSy1 show steeper spectral indeces with respect to BLSy1 when broad-band spectra are considered. Indeed, we combine XMM-Newton and Swift/XRT with INTEGRAL/IBIS data sets to obtain a wide energy spectral coverage (0.3-100 keV). A constraint on the high energy cut-off and on the reflection component is achieved only in one source, Swift J2127.4+5654 (E_{cut-off} ~ 50 keV, R=1.0{+0.5}_{-0.4}). Hard X-ray selected NLSy1 do not display particularly strong soft excess emission, while absorption fully or partially covering the continuum is often measured as well as Fe line emission features. Variability is a common trait in this sample, both at X-ray and at hard X-rays. The fraction of NLSy1 in the hard X-ray sky is likely to be ~15%, in agreement with estimates derived in optically selected NLSy1 samples. We confirm the association of NLSy1 with small black hole masses with a peak at 10^{7} M_{dot} in the distribution, however hard X-ray NLSy1 seem to occupy the lower tail of the Eddington ratios distribution of classical NLSy1.