2023MNRAS.522.4841E

We present near-infrared spectroscopy of the 2022 eruption of the recurrent nova U Sco, over the period from 5.4 to 45.6 d after outburst. This is the most intensive infrared study of this nova. Our observations started early after the outburst and extended almost to the end of the 'Super Soft' X-ray phase. A major find is the presence of coronal lines from day 9.41, one of the earliest appearances of these in any nova, classical, or recurrent. The temperature of the coronal gas is 7 × 10⁵ K. There is also evidence for the presence of much cooler ($\lesssim 2.5\times 10^4$ K) gas. Remarkable changes are seen in the He I 1.083 $\, \mu$ m line, the strength of which declines, then recovers, in anticorrelation with the X-ray behaviour. We conclude that shock ionization is the dominant excitation mechanism for the coronal line emission. There is evidence in the infrared spectra for the presence of black body emission at ∼20 000 K, which we tentatively identify with the irradiated secondary, and for free-free/free-bound emission. For the previously determined binary inclination of 82.7°, the implied ejection velocities are as high as 22 000 km s^–1. These velocities appear unprecedented in nova outflows, and are comparable to those seen in supernovae, thereby marking U Sco as a truly remarkable object.