Observations of a Possible Transient Magnetically Arrested Accretion State in a Nearby Quasar: OQ 208

OQ 208 is a nearby, partially obscured quasar ( z = 0.077) that is a young, bright, parsec-scale radio source. We assemble archival and new high-frequency Very Long Baseline Array (VLBA) and Very Large Array (VLA) observations and optical spectra to form a dataset spanning 39 yr. Radio light curves covering 58 yr were also compiled. We utilize new spectrophotometry to calibrate previous spectroscopy using forbidden narrow lines that are expected to be stable on much longer timescales. VLBA and VLA observations of a light-year-scale bright nuclear flare at 15.4 and 22 GHz reveal a rise (fade) beginning in mid-1996 (early 2000). Quasi-contemporaneously, from 1997 February 7 to 2000 June 3, the H α broad-line equivalent widths (EWs) and fluxes dropped dramatically. In the context of the tendency of radio-loud quasars to have a depressed extreme-ultraviolet (EUV) continuum (the main source of ionizing flux for H α ) relative to radio-quiet quasars at matched UV luminosity (the EUV deficit of radio-loud quasars), this may not be a coincidence. Analytic models previously developed to explain the relationship between jet power and the EUV deficit are consistent with (but not direct observational proof of) the small EWs being a consequence of transient magnetically arrested accretion states from ∼1997 to 2001. The 22 GHz VLBA nucleus gradually fades; in 2023 the flux density was <5% of its value in 2000. The environs of the nucleus also fade at 22 GHz, but in a time-delayed fashion.