Broadband X-Ray Observations of the 2018 Outburst of the Changing-Look Active Galactic Nucleus NGC 1566

TL;DR

This study analyzes the 2018 outburst of NGC 1566, revealing a significant increase in X-ray emission, complex soft excess, and insights into the black hole's spin and possible merging scenario through broad-band spectral and timing analysis.

Contribution

It provides the first detailed broadband spectral and timing analysis of NGC 1566 during its outburst, including modeling of the soft excess and black hole spin estimation.

Findings

X-ray intensity increased by 25-30 times during outburst.

Soft X-ray excess is complex and likely from warm Comptonization.

Black hole spin parameter estimated at ~0.2.

Abstract

We study the nature of the changing-look Active Galactic Nucleus NGC 1566 during its June 2018 outburst. During the outburst, the X-ray intensity of the source rises up to ~25-30 times compared to its quiescent state intensity. We perform timing and spectral analysis of the source during pre-outburst, outburst, and post-outburst epochs using semi-simultaneous observations with the XMM-Newton, Nuclear Spectroscopic Telescope Array (NuSTAR), and Neil Gehrels Swift Observatories. We calculate variance, normalized variance, and fractional rms amplitude in different energy bands to study the variability. The broad-band 0.5-70 keV spectra are fitted with phenomenological models, as well as physical models. A strong soft X-ray excess is detected in the spectra during the outburst. The soft excess emission is found to be complex and could originate in the warm Comptonizing region in the inner accretion disc. We find that the increase in the accretion rate is responsible for the sudden rise in luminosity. This is supported by the q-shape of the hardness-intensity diagram that is generally found in outbursting black hole X-ray binaries. From our analysis, we find that NGC 1566 most likely harbours a low-spinning black hole with the spin parameter a* ~ 0.2. We also discuss a scenario where the central core of NGC 1566 could be a merging supermassive black hole.