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Potential magnetic pole inversion in a changing look Active Galactic Nuclei

by Sibasish Laha (NASA Goddard Space Flight Center, USA) on behalf of co-authors of the paper

Laha 1
The light curves of the X-ray, UV and Radio
parameters of the central engine of the AGN
1ES1927+654, as observed by Swift, VLBI and VLBA.

Recent discoveries of so-called changing-look AGN (CL-AGN), have given us a rare view of extreme changes in the AGN state spanning a few months to years, which may give insight into major open questions about the nature of AGN fueling. CL-AGN are rare, with only a few dozen candidates in the literature. One of the most dramatic cases is 1ES 1927+654, a nearby (74.2 Mpc) and previously well-studied type 2 AGN which was observed to undergo a dramatic optical outburst beginning in December 2017. In this work (Laha et al., 2022) we report the evolution of the radio, optical, UV and X-rays from the pre-flare state through mid-2021 with a suite of new and archival multiwavelength observations, including archival EVN and new VLBA observations.

During the outburst of 1ES 1927+654 which peaked in March 2018, the optical and UV fluxes increased by four magnitudes, and broad emission lines newly appeared for the first time in the optical/UV. By July 2018 the X-ray coronal emission (2 − 10 keV) had completely vanished, only to reappear a few months later in October 2018. Interestingly, the optical and UV emission dropped monotonically after the peak in 2018, following a power-law with t−0.91±0.04 (See Figure 1). After around 1200 days since the start of the flare, the UV and X-rays reached their pre-flare values (as observed in 2011). New and archival VLBI observations spanning this period show that the core radio flux (at ≤ 1pc scale) was at its minimum at the time when the X-ray emission was also at a minimum.

Although the initial papers (Trakhtenbrot et al., 2019; Ricci et al., 2020) reporting this unique event suspected a TDE as the source of the CL event due to the power-law decay signature, our multiwavelength observations spanning a period from pre- to post-flare states suggest otherwise. Clearly the event was triggered by a sudden change in the accretion rate. But what made the accretion rate change? We conjecture that it could be a unique case of magnetic polarity inversion in the accretion disk surrounding the SMBH (Scepi et al., 2021), see Figure 2. We see a similar phenomenon in the Sun every 11 years.

The uncorrelated evolution of the optical/UV and X-ray suggests that two separate physical parameters are changing during this event. We suggest that the optical/UV are related to a change in the mass accretion rate at some large radii, Ṁ(ropt), and that the X-rays come from very close to the black hole and are related to a change in the magnetic flux onto the black hole, ΦBH.

One of the most crucial observations supporting our conjecture is the dip in the core-radio flux, occurring nearly at the same time as that of the X-ray dip, favoring the flux inversion scenario.

Laha 2
Cartoon representing the magnetic flux inversion event. From left to right are the pre-flare to post-flare states of the central engine. The initial direction of the magnetic field is depicted by yellow lines, while the reversed polarity is shown by red lines.


Published in Laha et al 2022 ApJ 931 5