# # Program information file # PROGRAM_ID 2023A011 PROGRAM_TITLE Spectroscopic monitoring of the infrared broad-line region and dusty torus in the active nucleus of PDS 456 PROGRAM_INV1 Hermine Landt PROGRAM_INV2 Dakshesh Kololgi PROGRAM_INV3 David Sanmartim PROGRAM_INV4 Alexandre Roman Lopes PROGRAM_INV5 PROGRAM_SCICAT extra-galactic PROGRAM_ABSTRACT_BEG Combining an angular size with a physical size measurement from reverberation mapping for the IR broad emission line region and dusty torus in AGN can constrain the geometrical distance and so the Hubble constant in the Local Universe. Furthermore, infrared spectroscopic reverberation mapping can give an independent measure of the torus radius and thus yield the chemistry of the circumnuclear dust around supermassive black holes when compared to simultaneous size estimates from thermal equilibrium considerations. Here we wish to study the variability of the Paschen broad emission lines and dusty torus in the luminous, nearby AGN PDS 456. The GRAVITY infrared interferometric instrument on ESO's VLT has recently resolved the nuclear structure in this source. Our campaign will be supported by optical/near-IR photometric monitoring and optical spectroscopy with other facilities. Therefore, we will also be able to test if the effects of collisional excitation are important in the hydrogen broad-line region and so assess to what extent Balmer emission line radii from optical reverberation mapping campaigns can be compared with geometrical Paschen emission line radii as measured in the infrared by GRAVITY. We propose to use SpeX in the SXD mode and obtain a spectrum of PDS 456 roughly every 14 days for the entire duration of this semester 2023A. PROGRAM_ABSTRACT_END