Revealing Thermal Comptonization of accretion-disk photons in IC4329A with AstroSat

TL;DR

This study uses AstroSat's simultaneous UV/X-ray observations of IC4329A to reveal that UV variability originates from the accretion disk and that thermal Comptonization in a warm corona explains the soft X-ray excess and spectral variability.

Contribution

First simultaneous UV/X-ray analysis of IC4329A showing intrinsic UV variability and its role as seed photons for Comptonization, revealing details of the warm corona and spectral changes.

Findings

UV variability is intrinsic to the disk, not X-ray illumination.

Soft X-ray excess likely due to thermal Comptonization in a warm corona.

X-ray spectral variability explained by cooling of the corona with increasing UV flux.

Abstract

We present five simultaneous UV/X-ray observations of IC4329A by AstroSat, performed over {a five-month} period. We utilize the excellent spatial resolution of the Ultra-Violet Imaging Telescope (UVIT) onboard AstroSat to reliably separate the intrinsic AGN flux from the host galaxy emission and to correct for the Galactic and internal reddening, as well as the contribution from the narrow and broad-line regions. We detect large-amplitude UV variability, which is unusual for a large black hole mass AGN, like IC4329A, over such a small period. In fact, the fractional variability amplitude is larger in the UV band than in the X-ray band. This demonstrates that the observed UV variability is intrinsic to the disk, and is not due to X-ray illumination. The joint X-ray spectral analyses of five SXT and LAXPC spectral data reveal a soft-X-ray excess component, a narrow iron-line (with no indication of a significant Compton hump), and a steepening power-law ($\Delta\Gamma\sim 0.21$) with increasing X-ray flux. The soft excess component could arise due to thermal Comptonization of the inner disk photons in a warm corona with $kT_e\sim 0.26$ keV. The UV emission we detect acts as the primary seed photons for the hot corona, which produces the broadband X-ray continuum. The X-ray spectral variability is well described by the cooling of this corona from $kT_e\sim42$ keV to $\sim 32$ keV with increasing UV flux, while the optical depth remains constant at $\tau\sim 2.3$.