# # Program information file # PROGRAM_ID 2024B028 PROGRAM_TITLE Infrared Spectroscopic Reverberation Mapping of GRAVITY/VLTI-targeted AGN Ark120 PROGRAM_INV1 Vardha Bennert PROGRAM_INV2 Matilde Signorini PROGRAM_INV3 Yan-Rong Li PROGRAM_INV4 Tommaso Treu PROGRAM_INV5 Hermine Landt 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, reverberation mapping [RM] of the Doppler-broadened emission lines has been used to determine its size through variability. We here propose a NIR RM campaign of Ark120 for which GRAVITY [ESO/VLT] has recently resolved the BLR through spectroastrometry [SA]. GRAVITY SA probes the BLR structure only perpendicular to the line of sight, while a combination with RM can give a full 3D view of the BLR, particularly important for accurately measuring BH mass. Thus, RM is an independent and complimentary measurement. Moreover, GRAVITY measures the angular size of the BLR, RM measures its physical size and the combination of both methods ['SARM'] allows to measure absolute geometric distance to the AGN. Systematic uncertainties will be minimized by the unique design of the proposed campaign: [i] observing quasi 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 and allows to study BLR photoionization and structure. An optical RM campaign conducted simultaneously will be used to compare optical and NIR BLR variability and physics to understand differences and systematic uncertainties. 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. PROGRAM_ABSTRACT_END