Program: GN-2021A-Q-402

Interferometric observations of the broad Paschen alpha emission line in the quasar 3C 273 with the newly deployed GRAVITY instrument at the VLT revealed that line-emitting gas in the vicinity of the central supermassive black hole is in ordered rotation (Sturm et al. 2018, Nature). However, the radius measured for the Paschen alpha broad-line region (BLR) is a factor of ~2 smaller than previous and recent estimates of the H beta BLR radius based on long-term optical reverberation campaigns. Nevertheless, the recent analysis of Zhang et al. of a 10-year long light-curve of 3C 273 shows that detrending (i.e. accounting for red noise) can bring the lag down to a value consistent with the interferometric radius. Here we propose to continue our spectroscopic near-IR reverberation campaign on 3C 273 started with Gemini on 2019 January 1 to cover a total period of about 3.5 years. This will offer us the unique opportunity to compare BLR radius measurements of the same emission line (Paschen alpha) using two very different methods. GRAVITY is expected to resolve also the inner radius of the obscuring dusty structure in 3C 273. The large wavelength coverage of our near-IR GNIRS spectra readily gives us also the hot dust spectral energy distribution. This is a long-term poor weather proposal since 3C 273 is a relatively bright source.