An Investigation of 5-year Simultaneous X-ray and Radio Light Curves of the Dwarf Seyfert Galaxy UGC 6728

TL;DR

The study analyzes serendipitous five-year, simultaneous X-ray and radio light curves of the dwarf Seyfert galaxy UGC 6728, showing an X-ray flare followed by a gradual rise and decline, with the radio flux dimming in tandem only to recover after the X-ray activity subsides. The authors test a magnet reconnection–driven plasmoid model, as proposed for NGC 2992, against a coronal-flaring scenario, and find that the lack of X-ray spectral evolution during the events supports a plasmoid obscuration of a non-coronal radio source (likely a compact jet or extended outflow). Energetics of the X-ray flare are consistent with magnetic disk activity, and the inferred properties of the obscuring material (optical depth, electron density) align with broad-line region conditions, though the long occultation timescale implies a radio source larger than the corona. While coronal flaring remains a possible contributor, the magnetic reconnection scenario best reconciles the joint timing and spectral behavior given the sparse sampling, suggesting that time-domain radio monitoring could illuminate plasmoid ejection and inner-disk physics in AGN. Future wide-field, high-cadence radio surveys may enable systematic exploration of such events and tighten constraints on AGN central engine structures.

Abstract

We present serendipitous simultaneous radio and X-ray light curves of the dwarf Seyfert galaxy UGC 6728 spanning 5 years. The X-ray light curve exhibits a flaring period, followed by a gradual rise and decline. Throughout these events, the X-ray hardness ratio and spectrum do not change significantly. The radio flux is constant, as far as can be determined from its sparse sampling, until the end of the X-ray flare, then decreases by a factor of two by the midpoint of the gradual X-ray rise before returning to baseline at the end of the X-ray decline. We interpret this behavior in light of a similar event recently reported in NGC 2992, in which there is a temporary obscuration of the radio source by a blob of plasma ejected by a magnetic reconnection in the accretion disk. The energetics of the X-ray flare are consistent with those expected from magnetic disk activity. As in NGC 2992, the X-ray spectrum does not evolve during the obscuration event. We also discuss the possibility that the observed phenomena are due to normal AGN coronal flaring and variability, which is plausible but unlikely given the lack of spectral variation.

Figures (4)

• Figure 1: Radio, X-ray, and X-ray hardness ratio curves. Each point includes a 1$\sigma$ error bar, which are sometimes comparable to the marker size. A black dashed line is provided across all panels at the position of the first post-flare radio data point for ease of comparison. The region of the radio light curve where the C-band upper limits were observed is blown up on the right. These observations were simultaneous with the X-band observations in the same program. Both bands detected only upper limits on MJD 60161. Grey regions in the X-ray light curves represent the flare, middle, and end spectral extraction regions as described in Section \ref{['sec:xray_data']}.
• Figure 2: Left: Normalized light curves, with radio fluxes normalized to the first radio data point and X-ray normalized to the first X-ray point after the flare. Right: Same as first panel, but zoomed in on the X-ray flare. Colors and normalizations are chosen to match those presented for NGC 2992 by Fernandez2022 to facilitate comparison with their Figure 11.
• Figure 3: X-ray spectra of the X-ray flare and the middle and end of the radio-dimming event, as defined in Section \ref{['sec:xray_data']}. The best-fitting model for each spectrum, from the parameters given in Table \ref{['t:xrayspec']}, is shown as a solid step-line. The lower panel shows the fit residuals.
• Figure 4: Dependence of electron density (top) and total electron column density (bottom) on the path length through the obscuring cloud for three representative temperatures.