# # Program information file # PROGRAM_ID 2024A044 PROGRAM_TITLE Infrared Spectroscopic Reverberation Mapping of a GRAVITY/VLTI-targeted Active Galactic Nucleus PROGRAM_INV1 Vardha N. Bennert PROGRAM_INV2 Tommaso Treu PROGRAM_INV3 Matilde Signorini PROGRAM_INV4 Hermine Landt PROGRAM_INV5 Yan-Rong Li PROGRAM_SCICAT extra-galactic PROGRAM_ABSTRACT_BEG The broad-line region [BLR] holds the key to measuring black hole [BH] mass and physics of active galactic nuclei [AGNs]. Typically unresolved, two recent observational breakthroughs have made it possible to determine size, geometry and kinematics of the BLR: [1] GRAVITY NIR interferometry on the ESO VLT resolves the BLR in nearby AGNs [spectro-astrometry; SA]; [2] high-quality reverberation mapping [RM] data allows to derive velocity-resolved lags. While GRAVITY measures the angular size of the BLR, RM measures its physical size. Thus, RM is an independent and complimentary measurement and the combination of both methods ['SARM'] allows to measure absolute geometric distance to the AGN. Once precision and accuracy of the distance measurement have been established, the long term goal is to use SARM to measure the Hubble constant independent of all other methods. To pursue these goals, we here propose a NIR RM campaign of Ark120. Systematic uncertainties will be minimized by the unique design of the proposed campaign: [i] observing simultaneously to GRAVITY and [ii] using the same NIR broad lines as GRAVITY. Independently of GRAVITY, a successful RM campaign based on NIR broad emission lines would be the first published study of its kind. An optical RM campaign conducted simultaneously will be used to compare optical and NIR BLR variability and physics to understand differences and systematic uncertainties. PROGRAM_ABSTRACT_END