Broad-band X-ray observations of the 2018 outburst of the changing-look active galactic nucleus NGC 1566

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.

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

“…We allowed all the parameters to vary freely. We found statistically good fit only for a highly spinning black-hole (a > 0.6) and strong reflection (R ∼ 1.3), which is inconsistent with the black-hole spin of a ≤ 0.2 and reflection fraction of R < 0.2 derived from the broad iron line alone (see Parker et al 2019;Jana et al 2021). Generally the smooth soft X-ray excess component when described as the blurred reflection model requires maximum blackhole spin (see Mallick et al 2018;Jiang et al 2019).…”

“…The main parameters of the 111.4/102 a FWHM in km s −1 b Flux in 10 −12 ergs cm −2 s −1 c Flux in 1300-1800 Å in 10 −11 ergs cm −2 s −1 optxagnf model are, mass of the black-hole (M BH ), comoving distance to the source (D in Mpc), dimensionless accretion rate (log ṁ = L/L Edd ), dimensionless black-hole spin (a), inner and outer disk radii (r cor and r out ), temperature (kT w ) and optical depth (τ w ) of the warm corona, X-ray power-law photon-index (Γ) and fraction of the power below r cor which is emitted in the hard comptonization component (f pl ). We fixed the black-hole mass at M BH = 8.32 × 10 6 M (Woo & Urry 2002) following the previous studies (see Parker et al 2019;Oknyansky et al 2020;Jana et al 2021), the black-hole spin a = 0 for a slowly spinning blackhole (Parker et al 2019;Jana et al 2021), and the outer disk radius r out = 10 5 R g , where R g = GM BH /c 2 is the gravitational radius. The comoving distance of the source is uncertain, the reported distance ranges from 5.5 Mpc (Sorce et al 2014)…”

Thermal Comptonization in a changing corona in the changing-look active galaxy NGC 1566

“…We analyze the simultaneous NUV/X-ray data on NGC 1566 acquired by XMM-Newton (Jansen et al 2001), NuSTAR (Harrison et al 2013), and the Neil Gehrels Swift Observatory (Gehrels et al 2004). Jana et al 2021 have analyzed these X-ray data but they did not use the simultaneous NUV data. Parker et al 2019 andDewangan 2022 have analyzed the XMM-Newton data acquired on 26 June 2018 (outburst peak).…”

“…The parameters with (f) sign are fixed. Next, we used the relxill (García et al 2014) and the xillver (García et al 2013) model to fit the blurred and distant reflection features, respectively, as reported by Parker et al (2019), Jana et al (2021), andTri-pathi &Dewangan (2022). The main parameters of the relxill model are emissivity index (q assuming a single emissivity profile ∝ R −q , where R is the radial distance), the black-hole spin (a), inclination angle of the accretion disk (θ), inner and outer disk radii (R in and R out in R g ), photon-index (Γ hot ) of the incident X-ray power-law component, ionization parameter (log(ξ rel / ergs cm s −1 )), iron abundance (A F e in the solar unit), high energy cut-off (E cut in keV), reflection fraction (RF rel ), and normalization (N rel ).…”

“…During the outburst, the accretion disk, the soft X-ray excess, and the X-ray power-law fluxes increased by factors of ∼ 30, > 200, and ∼ 25, respectively, and the accretion disk and the soft excess fluxes have been found to be correlated with the X-ray power-law flux (see Tripathi & Dewangan 2022). Using the multi-epoch X-ray (0.5-70 keV) data of NGC 1566, Jana et al 2021 found that the temperature of the hot corona to increase from ∼ 60 keV to ∼ 100 keV during the declining phase of the outburst from June 2018 to August 2019. Here, we study broadband UV-X-ray spectral variability with improved thermal Comptonisation model and accounting for both blurred and distant reflection emission, and then investigate the effect of drastically varying accretion disk and soft X-ray excess emission on the coronal properties during the declining phase of the 2018 outburst.…”