The broad band SEDs of four 'hypervariable' AGN

TL;DR

This study analyzes the spectral energy distributions of four hypervariable AGN over a decade, exploring their X-ray and optical properties, black hole masses, and the nature of their variability, suggesting extrinsic effects like microlensing may cause their outbursts.

Contribution

It provides the first detailed SED analysis of hypervariable AGN, comparing high and low states, and discusses the potential extrinsic origin of their variability.

Findings

Hypervariable AGN may be X-ray loud relative to UV luminosity.

SED properties are closer to normal in low states.

Large outbursts might be caused by extrinsic effects such as microlensing.

Abstract

We present an optical to X-ray spectral analysis of four 'hypervariable' AGN (HVAs) discovered by comparing Pan-STARRS data to that from SDSS over a 10 year baseline (Lawrence et al 2016). There is some evidence that these objects are X-ray loud for their corresponding UV luminosities, but given that we measured them in a historic high state, it is not clear whether to take the high-state or low-state as typical of the properties of these HVAs. We estimate black hole masses based on MgII and H$\alpha$ emission line profiles, and either the high and low state luminosities, finding mass ranges $\log(M_{\rm BH}/M_{\odot}) = 8.2-8.8$ and $\log(M_{\rm BH}/M_{\odot}) = 7.9-8.3$ respectively. We then fit energy conserving models to the SEDs, obtaining strong constraints on the bolometric luminosity and $\alpha_{\rm OX}$. We compare the SED properties with a larger, X-ray selected AGN sample for both of these scenarios, and observe distinct groupings in spectral shape versus luminosity parameter space. In general, the SED properties are closer to normal if we assume that the low-state is representative. This supports the idea that the large slow outbursts may be due to extrinsic effects (for example microlensing) as opposed to accretion rate changes, but a larger sample of HVAs is needed to be confident of this conclusion.