X-ray Observations of Nova Scorpii 2023 (V1716 Sco) in Outburst

TL;DR

This paper presents detailed X-ray observations of Nova Scorpii 2023, revealing a prolonged supersoft X-ray phase, variability, and a stable 77.9-second modulation likely linked to white dwarf rotation, providing insights into nova evolution and white dwarf properties.

Contribution

First comprehensive X-ray analysis of Nova Scorpii 2023, including timing, spectral, and variability studies, revealing white dwarf characteristics and nova evolution details.

Findings

Supersoft X-ray phase lasted about 9 months.

Detected a stable 77.9-second periodic modulation.

Indications of a 1.2 solar mass white dwarf.

Abstract

Nova Scorpii 2023 was first detected as a luminous supersoft X-ray source (SSS) 93 days after outburst and continued emitting soft X-rays for over two months, until it was too close to the Sun to observe. The nova was monitored with the Swift X-ray Telescope (XRT) and the Neutron Star Interior Composition Explorer (NICER) on the International Space Station, and in long exposures with the Chandra High Resolution Camera (HRC) and Low Energy Transmission Grating (LETG) on days 128, 129, and 183-185 after optical maximum. Swift detected a rapidly decaying SSS when observations resumed, constraining the constant bolometric luminosity phase to 9 months. The SSS flux was irregularly variable. A nearly three-fold increase in flux was observed between August and October 2023 in the 15 to 35 Angstrom range, from 3.5 x 10^(-11) to 9.4 x 10^(-11) erg cm^(-2) s^(-1). The SSS duration and effective temperature derived from the October LETG spectra indicate a massive white dwarf with temperature fitting nova evolutionary tracks for a 1.2 solar mass WD; emission lines superimposed on the WD continuum are attributed to surrounding shocked ejecta. We present a timing study based on Chandra and archival NICER data. The irregular variability timescale was days, but a 77.9 second periodic modulation in the SSS flux with varying amplitude was measured in many observations. Our analysis shows that this period was stable; short drifts derived with NICER, but not in long, uninterrupted Chandra exposures, are artifacts of measuring variable amplitude modulation. We suggest the modulations are associated with the WD rotation.