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

Abstract Nova Scorpii 2023 (V1716 Sco) was first detected as a luminous supersoft X-ray source (SSS) 93 days after outburst and continued emitting soft X-rays for over 2 months, until it was too close to the Sun to observe. The nova was monitored with the Swift X-ray Telescope and the Neutron Star Interior Composition Explorer (NICER) on the International Space Station, and in long exposures with the Chandra High Resolution Camera 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 phase of constant bolometric luminosity to ∼9 months. The SSS flux was irregularly variable. A nearly threefold increase in flux was observed between 2023 August and October in the 15–35 Å range, from 3.5 × 10 −11 to 9.4 × 10 −11 erg cm −2 s −1 . The SSS duration and effective temperature derived from the October LETG spectra indicate a massive white dwarf (WD) with temperature fitting nova evolutionary tracks for a ≈1.2 M ⊙ 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 s 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.