Long-term monitoring of Ark 120 with Swift

TL;DR

This six-month Swift monitoring of Ark 120 reveals significant X-ray and UV variability, spectral changes, and supports a disk reprocessing model with delays consistent with light-travel times over large disk regions.

Contribution

First long-term simultaneous UV and X-ray monitoring of Ark 120, confirming disk reprocessing and establishing a relation between delays and black hole mass.

Findings

X-ray flux varies by large amplitudes over days

UV variations are smoother over weeks

X-ray spectral steepening correlates with brightness

Abstract

We report the results of a six-month Swift monitoring campaign of Ark120, a prototypical bare Seyfert1 galaxy. The lack of intrinsic absorption combined with the nearly contemporaneous coverage of the UV and X-ray bands makes it possible to investigate the link between the accretion disk and the Comptonization corona. Our observations confirm the presence of substantial temporal variability, with the X-rays characterized by large-amplitude flux changes on timescales of few days, while the variations in the UV bands are smoother on timescales of several weeks. The source also shows spectral variability with the X-ray spectrum steepening when the source is brighter. We do not detect any correlation between the UV flux and the X-ray spectral slope. A cross correlation analysis suggests positive delays between X-rays and the UV emission, favoring a scenario of disk reprocessing. Although the strength of the correlation is moderate with a delay not well constrained (7.5+-7 days), it is nevertheless indicative of a large disk reprocessing region, with a separation between the X-ray and the UV emitting regions of the order of 1000 rG. The Ark120 correlation results are in agreement with those obtained in similar monitoring studies. When combined together, the observations can be well described by a linear relation between the X-ray/UV delay and the black hole mass. Within the context of the simplest scenario where these delays correspond to light-travel times, the implied distance between the X-ray source and the UV disk reprocessing region is of the order of many hundreds of gravitational radii.