Spectral connection between far UV and soft X-ray emission from active galactic nuclei using AstroSat

TL;DR

This study addresses how UV disk emission in Seyfert 1 AGN connects to the soft X-ray excess and whether warm or hot coronal processes dominate. It employs simultaneous UV/X-ray spectroscopy from AstroSat with physically grounded corona models OPTXAGNF and FAGNSED to fit disk seed photons and their Comptonization, covering Eddington ratios from $0.01$ to $1$. Key results include a compact warm corona for SWIFT J1835.0+3240 with $R_{warm} - R_{hot} \,\lesssim \,18\,r_g$ and larger warm regions ($\gtrsim 32\,r_g$) for the other sources, plus a spin constraint for PG 0804+761 of $a^{\star}=0.76_{-0.20}^{+0.08}$. UV/X-ray spectral variability appears driven by thermal Comptonization in the hot corona, with SWIFT J1835.0+3240 showing potential state transition from high/soft to low/hard; these results demonstrate that joint UV/X-ray spectroscopy with physically motivated corona models can illuminate disk–corona geometry and accretion state evolution in AGN.

Abstract

We present the UV/X-ray joint spectral analyses of four Seyfert~1 galaxies (PG 0804+761, NGC 7469, SWIFT J1921.1-5842, and SWIFT J1835.0+3240) using the data acquired with the Ultraviolet Imaging Telescope and Soft X-ray Telescope onboard AstroSat. We model the intrinsic UV/X-ray continuum with the accretion disk, warm and hot Comptonization using the OPTXAGNF and FAGNSED models, where the disk seed photons are Comptonized in the warm and hot corona. The Eddington ratio of the four Seyferts ranges from 0.01 to 1. In the case of SWIFT J1835.0$+$3240, we infer a compact warm corona ($ R_{warm} - R_{hot} \lesssim 18 r_{g}$) while, PG 0804+761, NGC 7469, and SWIFT J1921.1-5842 may exhibit a larger warm Comptonizing region ($\gtrsim 32r_{g}$). We could constrain the spin parameter in PG 0804+761, $a^{\star} = 0.76_{-0.20}^{+0.08}$ (1$σ$ error), with the FAGNSED model. In SWIFT J1835.0+3240 and SWIFT J1921.1-5842, the UV/X-ray spectral variability may be driven by the thermal Comptonization of the disk seed photons in the hot corona. Furthermore, the observed spectral hardening with the decrease in disk temperature and accretion rate compared to earlier observations may indicate a state transition in SWIFT J1835.0+3240 from a high/soft to a low/hard state.