# Low frequency X-ray timing with Gaussian processes and reverberation in the radio-loud AGN 3C 120

## TL;DR

This paper introduces a Gaussian process-based framework for low-frequency X-ray timing analysis, enabling reverberation measurements in AGN using data from satellites like NICER and NuSTAR, and applies it to the radio-loud AGN 3C 120.

## Contribution

It develops a novel Gaussian process method for Fourier-domain timing analysis that works with gapped data and measures reverberation on longer timescales than traditional techniques.

## Key findings

- First detection of X-ray reverberation in 3C 120.
- Measured time lag of the iron K line indicating corona height.
- Method allows reverberation studies with satellite data in low-Earth orbit.

## Abstract

A framework is developed to perform Fourier-domain timing analysis on X-ray light curves with gaps, employing Gaussian processes to model the probability distribution underlying the observed time series from which continuous samples can be drawn. A technique is developed to measure X-ray reverberation from the inner regions of accretion discs around black holes in the low frequency components of the variability, on timescales longer than can be probed employing standard Fourier techniques. This enables X-ray reverberation experiments to be performed using data from satellites in low-Earth orbit such as NICER, NuSTAR and the proposed X-ray timing mission STROBE-X, and enables long timescale reverberation around higher mass AGN to be measured by combining multiple observations. Gaussian processes are applied to observations of the broad line radio galaxy 3C120 spanning two orbits with XMM-Newton to measure the relative time lags of successive X-ray energy bands. The lag-energy spectrum between 5E-6 and 3E-5Hz, estimated using Gaussian processes, reveals X-ray reverberation from the inner accretion disc for the first time in this radio-loud AGN. Time lags in the relativistically broadened iron K line are significantly detected. The core of the line lags behind the continuum by (3800 +/- 1500)s, suggesting a scale height of the corona of (13 +/- 8)rg above the disc. The ability to compare the structure of coronae in radio loud AGN to their radio quiet counterparts will yield important insight into the mechanisms by which black holes are able to launch jets.