Multi-epoch UV–X-Ray Spectral Study of NGC 4151 with AstroSat

• Published in: The Astrophysical journal Vol. 975; no. 1; pp. 73 - 87
• Main Authors: Kumar, Shrabani, Dewangan, G. C., Gandhi, P., Papadakis, I. E., Mithun, N. P. S., Singh, K. P., Bhattacharya, D., Zdziarski, A. A., Stewart, G. C., Bhattacharyya, S., Chandra, S.
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01.11.2024; IOP Publishing
• Subjects: Absorption; Accretion disks; Bulk density; Cosmic dust; Dust; Emissions; Galaxies; Hubble Space Telescope; Interstellar extinction; Seyfert galaxies; Space telescopes; Sublimation; Ultraviolet emission; Ultraviolet spectroscopy; X ray absorption; X ray reflection; X ray sources; X ray spectra; X-ray active galactic nuclei; X-ray astronomy; X-rays
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ad77ca

Abstract We present a multiwavelength spectral study of NGC 4151 based on five epochs of simultaneous AstroSat observations in the near-ultraviolet (NUV) to hard X-ray band (∼0.005–80 keV) during 2017–2018. We derived the intrinsic accretion disk continuum after correcting for internal and Galactic extinction, contributions from broad- and narrow-line regions, and emission from the host galaxy. We found a bluer continuum at brighter UV flux, possibly due to variations in the accretion disk continuum or the UV reddening. We estimated the intrinsic reddening, E ( B − V ) ∼ 0.4, using high-resolution Hubble Space Telescope (HST)/STIS spectrum acquired in 2000 March. We used thermal Comptonization, neutral and ionized absorption, and X-ray reflection to model the X-ray spectra. We obtained the X-ray absorbing neutral column varying between N H ∼1.2 and 3.4 × 10 23 cm −2 , which are ∼100 times larger than that estimated from UV extinction, assuming the Galactic dust-to-gas ratio. To reconcile this discrepancy, we propose two plausible configurations of the obscurer: (a) a two-zone obscurer consisting of dust-free and dusty regions, divided by the sublimation radius, or (b) a two-phase obscurer consisting of clumpy, dense clouds embedded in a low-density medium, resulting in a scenario where a few dense clouds obscure the compact X-ray source substantially, while the bulk of UV emission arising from the extended accretion disk passes through the low-density medium. Furthermore, we find a positive correlation between the X-ray absorption column and NUV − far-UV color and UV flux, indicative of enhanced winds possibly driven by the “bluer-when-brighter” UV continuum.