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

TL;DR

This study reports NuSTAR X-ray observations of the fast nova V745 Sco, revealing shock-heated plasma characteristics, ruling out gamma-ray emission, and providing insights into the nova's circumstellar environment shortly after outburst.

Contribution

First NuSTAR observation of V745 Sco capturing its X-ray emission, characterizing plasma properties, and constraining gamma-ray flux shortly after outburst.

Findings

X-ray emission consistent with shock-heated plasma at 2.7 keV

No significant hard X-ray emission above 20 keV

Gamma-ray flux upper limit established at 10^{-11} erg cm^{-2} s^{-1}

Abstract

The fast recurrent nova V745 Sco was observed in the 3-79 keV X-rays band with NuSTAR 10 days 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 $\times$ 10$^{-11}$ erg cm$^{-2}$ s$^{-1}$. The emission measure indicates an average electron density of order of 10$^7$ cm$^{-3}$. 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.