Short-Timescale monitoring of the X-ray, UV and broad double-peak emission line of the nucleus of NGC 1097

TL;DR

This study monitored NGC 1097's nucleus in X-ray, UV, and optical wavelengths over several months, finding minimal variability and supporting the idea that the broad double-peaked emission lines are ionized by an inner high-energy source.

Contribution

First simultaneous multi-wavelength monitoring of NGC 1097's nucleus during a low activity state, providing insights into the ionization source of broad emission lines.

Findings

Hα flux was very low with limited variability.

X-ray variations were minimal (~13%).

UV did not show significant variations.

Abstract

Recent studies have suggested that the short-timescale ($\lesssim7$ days) variability of the broad ($\sim$10,000 km s$^{-1}$) double-peaked H$\alpha$ profile of the LINER nucleus of NGC1097 could be driven by a variable X-ray emission from a central radiatively inefficient accretion flow (RIAF). To test this scenario, we have monitored the NGC1097 nucleus in X-ray and UV continuum with Swift and the H$\alpha$ flux and profile in the optical spectrum using SOAR and Gemini-South from 2012 August to 2013 February. During the monitoring campaign, the H$\alpha$ flux remained at a very low level --- 3 times lower than the maximum flux observed in previous campaigns and showing only limited ($\sim 20\%$) variability. The X-ray variations were small, only $\sim 13\%$ throughout the campaign, while the UV did not show significant variations. We concluded that the timescale of the H$\alpha$ profile variation is close to the sampling interval of the optical observations, which results in only marginal correlation between the X-ray and H$\alpha$ fluxes. We have caught the AGN in NGC1097 in a very low activity state, in which the ionizing source was very weak and capable of ionizing just the innermost part of the gas in the disk. Nonetheless, the data presented here still support the picture in which the gas that emits the broad double-peaked Balmer lines is illuminated/ionized by a source of high-energy photons which is located interior to the inner radius of the line-emitting part of the disk.