NuSTARConstrainstheOriginofGammaRays inRSOph Marina Orio1,2, G. Juan M. Luna3 1. U Wisconsin and INAF-Padova, 2. orio@astro.wisc.edu, 3. CONICET, Buenos Aires and Hurlingham Universities Introduction RS Oph is arguably the best known recurrent symbiotic nova. Classical and recurrent novae occur in white dwarf binaries. The outbursts are attributed to a thermonuclear runaway on the surface of the WD, that has accreted ma- terial from the binary companion. A radiation- driven wind follows, depleting the accreted enve- lope [1, 2]. The designation “recurrent” implies that the outburst is observed more than once over human life timescales, and in RS Oph case outbursts were recorded in 1898, 1933, 1958, 1967, 1985, and 2006. RS Oph is also a sym- biotic system, hosting a red giant M0-2 III mass donor companion [3, 4] in a binary with a 453.6 day orbital period [5]. The NuSTAR observation We obtained a Director Discretionary Time NuSTAR 40 ks exposure in the 3-79 keV range on 2021 August 17, day 10 of the outburst ( it was impossible to schedule the exposure earlier, when the source may have been harder). The X-ray ﬂux was expected from colliding winds, presumably the high velocity ejecta impacting the slow red giant wind. One of the most in- teresting facts of this phase is the gamma-ray ﬂux, measured with Fermi in the GeV range and with HESS up to tens of GeV [6]. During the NuSTAR observation, several short exposures were done with NICER. Most important, for the ﬁrst time, a nova was observed at the same time also in gamma rays with Fermi and with Cherenkov telescopes [6]. References [1] S. Starrﬁeld, F. X. Timmes, C. Iliadis, W. R. Hix, W. D. Arnett, C. Meakin, and W. M. Sparks. Hy- drodynamic Studies of the Evolution of Recurrent, Symbiotic and Dwarf Novae: the White Dwarf Com- ponents are Growing in Mass. Baltic Astronomy, 21:76–87, January 2012. [2] William M. Wolf, Lars Bildsten, Jared Brooks, and Bill Paxton. Hydrogen Burning on Accreting White Dwarfs: Stability, Recurrent Novae, and the Post- nova Supersoft Phase. ApJ, 777(2):136, November 2013. [3] Danuta Dobrzycka, Scott J. Kenyon, Daniel Proga, Joanna Mikolajewska, and Richard A. Wade. The Hot Component of RS Ophiuchi. AJ, 111:2090, May 1996. [4] G. C. Anupama and J. Mikołajewska. Recurrent no- vae at quiescence: systems with giant secondaries. A&A, 344:177–187, April 1999. [5] E. Brandi, C. Quiroga, J. Mikołajewska, O. E. Ferrer, and L. G. García. Spectroscopic orbits and variations of RS Ophiuchi. A&A, 497(3):815–825, April 2009. [6] H. E. S. S. Collaboration. Time-resolved hadronic particle acceleration in the recurrent nova RS Ophi- uchi. Science, 376(6588):77–80, April 2022. [7] Indrek Vurm and Brian D. Metzger. High-energy Emission from Nonrelativistic Radiative Shocks: Ap- plication to Gamma-Ray Novae. ApJ, 852(1):62, Jan- uary 2018. [8] Gary Wegner, Joseph P. Cassinelli, and George Wallerstein. Shockwave Models for the Outburst of the Repeating Nova, RS Ophiuchi. In Bulletin of the American Astronomical Society, volume 1, page 209, March 1969. [9] S. Orlando, J. J. Drake, and J. M. Laming. Three- dimensional modeling of the asymmetric blast wave from the 2006 outburst of RS Ophiuchi: Early X-ray emission. ApJ, 493(3):1049–1059, January 2009. Conclusions The low maximum temperature measured with NICER rules out that the site of the X-ray emission is the same as the site of the gamma-ray emission. In fact, the energetics of novae are such that the shock temperature may be in the hard X-ray range, but gamma rays due either to inverse Compton eﬀect (leptonic mechanism) or to particle acceleration in the shock (causing pion decay; hadronic mechanism) are only associated with a very hard X-ray spectrum, with conspicuous ﬂux up to 80 keV. The site of the gamma-ray emission was thus optically thick even to very hard X-rays. In the 40-79 keV range, we have upper limits of 3 ×10−4 cts s−1 in NuSTAR. Assuming an initial X-ray ﬂux above 10−9 erg cm−2, consistent with Fermi