The calm before the (next) storm: no third outburst in 2019--2020, and ongoing monitoring of the transient AGN IC 3599

TL;DR

This study monitored the Seyfert galaxy IC 3599 over several years, finding no outbursts in 2019-2020 as predicted by the star stripping model, but supporting the accretion disk instability explanation for its variability.

Contribution

The paper provides long-term X-ray monitoring data that favors the accretion disk instability model over the star stripping scenario for IC 3599's outbursts.

Findings

No outburst in 2019-2020, contradicting the star stripping model.

Observed ongoing variability and a significant flare in December 2022.

Spectral hardening after the 2022 flare suggests temporary corona formation.

Abstract

We report on follow-up observations of the Seyfert 1.9 galaxy IC 3599 with the NASA Neil Gehrels Swift mission. The detection of a second X-ray outburst in 2010 by Swift after the first discovery of a bright X-ray outburst in 1990 by ROSAT led to the suggestion of two very different explanations: The first one assumed that IC 3599 exhibits outbursts due to repeated partial tidal stripping of a star, predicting another outburst of IC 3599 in 2019/2020. The second, alternative scenario assumed that the event observed in X-rays is due to an accretion disk instability which would suggest a much longer period between the large outbursts. Our continued monitoring campaign by Swift allowed us to test the first scenario which predicted a repetition of high amplitude flaring activity in 2019/2020. We do not find any evidence of dramatic flaring activity with factors of 100 since the last X-ray outburst seen in 2010. These observations support the accretion disk scenario. Further, while IC 3599 remains in low emission states, the long-term X-ray light curve of IC 3599 reveals ongoing strong variability of a factor of a few. The most remarkable event is a mini flare of a factor of 10 in X-rays in December 2022. After that flare, the otherwise supersoft X-ray spectrum shows an exceptional hardening, reminiscent of a temporary corona formation.