Exploring Hard X-ray Properties of $γ$-ray Emitting Narrow Line Seyfert-I Galaxies through NuSTAR Observations

TL;DR

This study analyzes hard X-ray properties of six gamma-ray–emitting Narrow-Line Seyfert 1 galaxies using NuSTAR (3–79 keV) and optical data from ZTF to probe disk–jet coupling. Across 14 epochs, X-ray spectra are well described by a single power law, with moderate variability ($F_{var}\sim$10–20%) and an overall photon index pattern linking to blazar subclasses, particularly LBL/IBL. The work reveals correlations between X-ray and gamma-ray properties and anti-correlations with jet power and disk luminosity, along with strong optical variability and a predominantly redder-when-brighter color behavior, suggesting a significant disk contribution alongside jet emission. Collectively, the results position gamma-ray–detected NLSy1 galaxies closer to FSRQs than BL Lacs in jet-dominated high-energy behavior and provide empirical constraints on disk–jet coupling and the blazar-like nature of these systems.

Abstract

We studied the six gamma-ray-detected Narrow Line Seyfert 1 (NLSy1) galaxies using the hard X-ray observations from Nuclear Spectroscopic Telescope Array (NuSTAR) and optical g- \& r-band from Zwicky Transient Facility (ZTF). The X-ray spectra corresponding to all objects are well-fitted with a power-law spectral model, and a strong "redder-when-brighter" trend is seen, which is mostly seen in Blazars. The X-ray light curves were produced for all the available observations, and the F$_{var}$ is estimated. In 1H 0323+342, we found that F$_{var}$ lies between 9$\%$ to 22$\%$, suggesting significant variability in the source. Similarly, for PKS 2004-447, we found F$_{var}$ lies between 10$\%$ to 21$\%$. We see a strong X-ray and $γ$-ray spectral index correlation among these objects, suggesting that these are produced through a similar process. Comparing the X-ray spectral index with other class objects, we see that NLSy1 galaxies are similar to LBL and IBL types. We see a negative trend of X-ray flux with the $γ$-ray luminosity in these objects, suggesting an anti-correlation between them. A similar trend is seen between the X-ray flux, total jet power, and disk luminosity. The X-ray spectral index also shows a negative trend with total jet power and disk luminosity. The optical variability amplitude (in magnitude) lies between 0.90 to 2.32, and the fractional variability varies from 13\% to 40\%. The color-magnitude plot shows mostly the redder-when-brighter (RWB) trend, suggesting $γ$-NLSy1 are much closer to FSRQs than BL Lacs. Our results, overall, summarize how the various parameters in gamma-ray-detected NLSy1 are connected.

Figures (13)

• Figure 1: NuSTAR X-ray spectra of $\gamma$-NLSy1 fitted with power-law (PL) model.
• Figure 2: NuSTAR spectra of PKS 2004-447 fitted with a single power-law model.
• Figure 3: NuSTAR spectra for various observations of 1H 0323+342.
• Figure 4: X-ray (NuSTAR) lightcurves of 1H 0323+342.
• Figure 5: X-ray (NuSTAR) light curve for PKS 2004-447.