A changing-look Seyfert discovered by eROSITA reveals a two-component broad-line region

Extreme sudden changes in the flow of accreting gas onto supermassive black holes manifest themselves via large-amplitude multiband continuum variability, as well as changes to broad Balmer emission profiles, driving changing-look active galactic nuclei (AGN). X-ray flux monitoring with Spectrum Roentgen Gamma (SRG)/eROSITA revealed that in the Seyfert AGN HE 1237-2252 the soft X-ray flux dipped abruptly, by a factor of 17 within 18 months. We initiated a follow-up campaign that caught the luminosity recovery after the dip, and enabled us to study how the various accretion components, including the broad-line region (BLR) and X-ray-emitting coronae, responded during this flux recovery. Our campaign included multiband photometry, X-ray spectroscopy, and optical spectroscopy. We tracked as the accretion rate relative to Eddington increased by a factor of 7 in 3 years. Based on broad Hβ variability, HE 1237-2252 was subtype 1.0--1.2 in 2002, transitioned to subtype 1.8 by the time of the luminosity dip, and then transitioned back to subtype 1.0 within 3 months as luminosity recovered. Both transitions saw broad Hβ integrated line flux change by factors of 4--6. The broad Balmer profile is decomposed into a broad Gaussian consistent with virialized gas at 27±3 lt-dy, plus a double-peaked profile, consistent with a diskline structure at ≳5 lt-dy. The diskline component's relative contribution to the total profile increases as continuum flux rises. The lack of significant obscuration in the X-ray spectra, as well as the IR continuum dip, point to an intrinsic pause in the accretion rate as opposed to variable line-of-sight obscuration. Candidates for the underlying mechanisms include propagating cold and warm fronts in the accretion disk. The increased prominence of the diskline BLR component's emission could be due to evolution in the physical extent of the X-ray corona, and in the fraction of $>$13.6 eV photons intercepted by the diskline, as the accretion rate increases.