Changing-look AGN are enigmatic sources in which the broad optical lines, produced by material around the supermassive black hole, appear or disappear. We recently witnessed, for the first time, the creation of such broad-lines in the nearby AGN 1ES 1927+654 (Trakhtenbrot et al. 2019). We followed up the source also in the X-ray band, where it displayed a behaviour never observed before in an AGN. After the event in fact the primary X-ray power-law continuum, ubiquitously found in AGN, almost completely disappeared (red points in Fig. 1). This implies that something dramatic happened to the X-ray corona, and possibly to the innermost regions of the accretion disk. Over the following months the source dimmed, showing an even softer
Fig.1 X-ray spectral evolution of 1ES1927+654 during the event. The drastic transformation of the X-ray properties is particularly clear when comparing to the pre-outburst spectrum from May 2011 (grey points).
X-ray continuum (orange and violet points), before brightening again. Between July and October the X-ray flux of the source increased by four orders of magnitude (Fig. 2)! As the source re-brightened the power-law continuum reappeared, dominating the overall X-ray flux (green and blue points in Fig. 1). This implies that after its initial disappearance, the X-ray corona was recreated. This event could be explained by the interaction between the debris from a tidally-disrupted stars and the accretion disk, with the innermost regions of the accretion flow being depleted by the enhanced accretion rate, caused by the dissipation of angular momentum during the shock (see Fig. 3). The letter to ApJ reporting on these findings can be found here, while the NASA press release on these results is available here. In mid-2020 I have also submitted another article to ApJS focussed on this observational campaign.
Fig. 2 Long-term light curve of 1ES 1927+654 in the X-rays (top panel) and optical/UV (bottom panel). The middle panels show how the hardness ratio and the contribution of the power-law component changed with time.
Fig. 3 Schematic cartoon showing a possible explanation for the dramatic event observed in 1ES1927+654.