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

TL;DR

This study analyzes UV and X-ray spectra of four Seyfert 1 galaxies using AstroSat data, revealing details about their accretion disks, coronae, and potential state transitions, with constraints on black hole spin.

Contribution

First joint UV/X-ray spectral analysis of Seyfert galaxies using AstroSat, modeling accretion and corona properties, and constraining black hole spin for PG 0804+761.

Findings

Compact warm corona in SWIFT J1835.0+3240

Larger warm Comptonizing regions in others

Black hole spin constrained in PG 0804+761

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$\sigma$ 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.