AstroSat view of spectral transition in the changing-look active galaxy NGC1566 during the declining phase of the 2018 outburst

TL;DR

This study uses AstroSat and XMM-Newton observations to analyze the spectral variability of the changing-look galaxy NGC1566 during its 2018 outburst, revealing significant changes in soft X-ray excess linked to accretion disk instabilities.

Contribution

First detailed multi-epoch X-ray spectral analysis of NGC1566's outburst, demonstrating the evolution of soft excess and linking it to accretion disk radiation pressure instability.

Findings

Soft X-ray excess increased by over 200 times during outburst

Spectral transition occurred at a few percent of Eddington luminosity

Soft excess disappeared as the galaxy returned to quiescence

Abstract

NGC1566 is a changing-look active galaxy that exhibited an outburst during 2017-2018 with a peak in June 2018. We triggered AstroSat observations of NGC 1566 twice in August and October 2018 during its declining phase. Using the AstroSat observations along-with two XMM-Newton observations in 2015 (pre-outburst) and June 2018 (peak-outburst), we found that the X-ray power-law, the soft X-ray excess, and the disk components showed extreme variability during the outburst. Especially, the soft excess was negligible in 2015 before the outburst, it increased to a maximum level by a factor of $>200$ in June 2018, and reduced dramatically by a factor of $\sim7.4$ in August 2018 and become undetectable in October 2018. The Eddington fraction ($L/L_{Edd})$ increased from $\sim0.1\%$ (2015) to $\sim 5\%$ (June 2018), then decreased to $\sim 1.5\%$ (August 2018) and $0.3\%$ (October 2018). Thus, NGC 1566 made a spectral transition from a high soft-excess state to a negligible soft-excess state at a few $\%$ of the Eddington rate. The soft-excess is consistent with warm Comptonization in the inner disk that appears to have developed during the outburst and disappeared towards the end of the outburst over a timescale comparable to the sound crossing time. The multi-wavelength spectral evolution of NGC 1566 during the outburst is most likely caused by the radiation pressure instability in the inner regions of the accretion disk in NGC 1566.