2015MNRAS.448L..35O

The fast recurrent nova V745 Sco was observed in the 3-79 keV X-rays band with NuSTAR 10 d after the optical discovery. The measured X-ray emission is consistent with a collisionally ionized optically thin plasma at temperature of about 2.7 keV. A prominent iron line observed at 6.7 keV does not require enhanced iron in the ejecta. We attribute the X-ray flux to shocked circumstellar material. No X-ray emission was observed at energies above 20 keV, and the flux in the 3-20 keV range was about 1.6x10^–11 erg/ cm2/s. The emission measure indicates an average electron density of the order of 10⁷/ cm2. The X-ray flux in the 0.3-10 keV band almost simultaneously measured with Swift was about 40 times larger, mainly due to the luminous central supersoft source emitting at energy below 1 keV. The fact that the NuSTAR spectrum cannot be fitted with a power law, and the lack of hard X-ray emission, allow us to rule out Comptonized gamma-rays, and to place an upper limit of the order of 10^–11 erg/cm2/s on the gamma-ray flux of the nova on the tenth day of the outburst.