The first hours and days of the 2021 explosion of the recurrent symbiotic nova RS Ophiuchii

The accretion of matter on a massive white dwarf (WD) can lead to repeated nuclear explosions on its surface over a timescale of years to decades. The seventh explosion of the recurrent symbiotic nova RS Ophiuchi (RS Oph) was recorded on August 8, 2021. In this paper, we examine its early evolution, from 9 hours before its optical maximum until day 42. We achieved our goal by modeling the spectral energy distribution (SED) using optical spectroscopy and simultaneous BVR CI C photometry, supplemented by JHKL photometry and ultraviolet spectroscopy from previous explosions in 2006 and 1985. Our SED models revealed an early stage of development of the ejecta bipolar structure, consisting of a flared, density-enhanced equatorial disk and low-density regions in bipolar directions. The comparability of the internal shocks' luminosity in the equatorial outflow, inferred from our model parameters, with the luminosity of the warm WD pseudophotosphere during its presence in the spectrum (until about day 42) confirmed that a significant part of its radiation originates from reprocessed shock emission. We explain the formation and evolution of the bipolar ejecta structure during RS Oph explosions by the rotation of the accreting WD. Such an ejecta structure provides a natural framework for the generation of strong internal shocks and thus gamma-ray emission inside the ejecta.