A NuSTAR observation of the fast symbiotic nova V745 Sco in outburst

Abstract

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 1.6 × 10^(−11) erg cm^(−2) s^(−1). The emission measure indicates an average electron density of the order of 10^7 cm^(−2). 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 cm^(−2) s^(−1) on the gamma-ray flux of the nova on the tenth day of the outburst.

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