Spectro-Timing Analysis of a highly variable narrow-line Seyfert 1 galaxy NGC 4748 with AstroSat and XMM-Newton

TL;DR

This study analyzes the spectral and timing properties of the variable NGC 4748 galaxy using AstroSat and XMM-Newton data, revealing rare spectral behavior changes and insights into accretion modes and disk reflection processes.

Contribution

It provides the first detailed comparison of spectral states and variability in NGC 4748 across different observations, highlighting changes in accretion modes and reflection features.

Findings

Detected 'softer when brighter' behavior in 2014 data

Observed 'harder when brighter' behavior in 2017 data

Identified soft X-ray excess likely due to cool Comptonization

Abstract

We present a detailed timing and spectral study of an extremely variable narrow-line Seyfert~1 galaxy NGC 4748 using observations in the year 2017 and 2014 performed with AstroSat and XMM-Newton, respectively. Both observations show extremely variable soft and hard X-ray emission that are correlated with each other. In the 2014 data set, the source retains its general behaviour of "softer when brighter" while the 2017 observation exhibits a "harder when brighter" nature. Such changing behaviour is rare in AGNs and is usually observed in the black hole binary systems. The "harder when brighter" is confirmed with the anti-correlation between the photon index and the 0.3-10 keV power-law flux. This suggests a possible change in the accretion mode from standard to the advection-dominated flow. Additionally, both the observations show soft X-ray excess below 2 keV over the power-law continuum. This excess was fitted with a single or multiple blackbody component(s). The origin of soft excess during the 2017 observation is likely due to the cool Comptonization as the photon index changes with time. On the other hand, the broad iron line and delayed UV emission during the 2014 observation strongly suggest that X-ray illumination onto the accretion disk and reflection and reprocessing play a significant role in this AGN.