Multi-wavelength Variability of the Broad Line Radio Galaxy 3C 120

TL;DR

This study presents a multi-year, multi-wavelength monitoring of the broad-line radio galaxy 3C 120, revealing correlated X-ray and optical variability, a characteristic power spectrum, and insights into its accretion state and emission origins.

Contribution

It provides the first detailed analysis of 3C 120's multi-wavelength variability, including power spectrum characterization and implications for accretion and emission mechanisms.

Findings

X-ray and optical emissions are highly variable and correlated, with X-ray leading by 28 days.

The X-ray power spectrum has a broken power law with a break at 6.5 days, consistent with expectations based on the galaxy's properties.

3C 120 exhibits high/soft variability state similar to other AGNs, with X-ray emission likely originating near the accretion disk.

Abstract

We present results from a multi-year monitoring campaign of the broad-line radio galaxy 3C 120, using the Rossi X-ray Timing Explorer (RXTE) for nearly five years of observations. Additionally, we present coincident optical monitoring using data from several ground-based observatories. Both the X-ray and optical emission are highly variable and appear to be strongly correlated, with the X-ray emission leading the optical by 28 days. The X-ray power density spectrum is best fit by a broken power law, with a low-frequency slope of -1.2, breaking to a high-frequency slope of -2.1, and a break frequency of log nu_b=-5.75 Hz, or 6.5 days. This value agrees well with the value expected based on 3C 120's mass and accretion rate. We find no evidence for a second break in the power spectrum. Combined with a moderately soft X-ray spectrum (Gamma=1.8) and a moderately high accretion rate (mdot / mdot_Edd ~ 0.3), this indicates that 3C 120 fits in wellwith the high/soft variability state found in most other AGNs. Previous studies have shown that the spectrum has a strong Fe K-alpha line, which may be relativistically broadened. The presence of this line, combined with a power spectrum similar to that seen in Seyfert galaxies, suggests that the majority of the X-ray emission in this object arises in or near the disk, and not in the jet.