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