X-ray observational signature of a black hole accretion disc in an active galactic nucleus RXJ1633+4718

TL;DR

This paper reports the discovery of a very soft X-ray excess in a Seyfert galaxy, interpreted as direct emission from a black hole accretion disc, providing observational evidence of such discs in active galactic nuclei.

Contribution

It presents the first observational evidence of X-ray emission directly from a black hole accretion disc in an AGN, based on spectral analysis of RXJ1633+4718.

Findings

Ultra-soft X-ray excess with a temperature of ~32.5 eV.

Emission consistent with the Wien tail of a multi-temperature blackbody disc.

Derived black hole parameters agree with optical estimates.

Abstract

We report the discovery of a luminous ultra-soft X-ray excess in a radio-loud narrow-line Seyfert1 galaxy, RXJ1633+4718, from archival ROSAT observations. The thermal temperature of this emission, when fitted with a blackbody, is as low as 32.5(+8.0,-6.0)eV. This is in remarkable contrast to the canonical temperatures of ~0.1-0.2keV found hitherto for the soft X-ray excess in active galactic nuclei (AGN), and is interestingly close to the maximum temperature predicted for a postulated accretion disc in this object. If this emission is indeed blackbody in nature, the derived luminosity [3.5(+3.3,-1.5)x10^(44)ergs/s] infers a compact emitting area with a size [~5x10^(12)cm or 0.33AU in radius] that is comparable to several times the Schwarzschild radius of a black hole at the mass estimated for this AGN (3x10^6Msun). In fact, this ultra-steep X-ray emission can be well fitted as the (Compton scattered) Wien tail of the multi-temperature blackbody emission from an optically thick accretion disc, whose parameters inferred (black hole mass and accretion rate) are in good agreement with independent estimates using optical emission line spectrum. We thus consider this feature as a signature of the long-sought X-ray radiation directly from a disc around a super-massive black hole, presenting observational evidence for a black hole accretion disc in AGN. Future observations with better data quality, together with improved independent measurements of the black hole mass, may constrain the spin of the black hole.