Analysis of x-ray characteristics of narrow line Seyfert 1 Galaxies from nuclear spectroscopic telescope array data

Abstract

Narrow line Seyfert 1 (NLS1) galaxies are among the most variable active galactic nuclei (AGN) in the sky. Hard X-ray spectral and timing analysis of two of these galaxies, NGC 4051 and ARK 564, using NuSTAR data is presented. The data was processed using nupipeline task in HEASoft and spectral, light curve and response files were generated using nuproducts module. X-ray spectra of the two sources were fitted in XSPEC with a reflection model (Pexrav) plus a Gaussian component (Gauss), corrected for line-of-sight galactic absorption using photo-electric absorption component (Phabs). The intrinsic powerlaw continuum is of slope Γ > 1.59, which is slightly steeper than the typical Seyfert 1 galaxies. Strong emission lines (EW > 100 eV) were detected at ∼ 6.4 keV in the two X-ray sources. The X-ray spectra of these sources above 10 keV exhibit a Compton hump, produced by Compton backscattering of hard X-ray photons by reprocessing optically- thick accretion disc, and is prominent at high energies. The deduced 3-79 keV luminosity of the two studied X-ray sources differ by 2 orders of magnitude (∼ 6.12×1041 ergs-1 for NGC 4051 and ∼ 2.47×1043 erg s-1 for ARK 564), thus ARK 564 is brighter than NGC 4051 in hard X-ray energies. High-amplitude X-ray variability of NLS1 galaxies was analysed by means of cross power spectral density (CPSD) in this study since multiple instrument simultaneous observations were made by NuSTAR. The average CPSD for each of the two sources was generated using High Energy Data Reduction Interface from Command Shell and the fitting performed in XSPEC. The spectra were fitted quite well in the frequency range ∼ 10-3 to ∼ 0.2 Hz, using four broad Lorentzian models with ARK 564 revealing more variability power estimates (total rms = 63.59%) than NGC 4051 (total rms = 37.3%). The coherence of variablity (Q < 2) of the X-ray sources shows no evidence of quasi-periodic oscillations (QPOs) possibly owing to the short period NuSTAR observations. Only two NLS1 galaxies have been considered. Further analysis of a large sample of these objects and the typical Seyfert 1 galaxies needs to be done in order to draw meaningful deductions about evolution of their hard X-ray spectra and timing properties and to distinguish between the two classes.