Nova Scorpii 2023 shows months-long X-ray variability after stellar outburst

Using NASA’s Swift spacecraft, an international team of astronomers have performed X-ray observations of a classical nova named Nova Scorpii 2023. Results of the observational campaign, published October 21 on the pre-print server arXiv, deliver important insights into the properties and behavior of this eruption.

A nova is a star experiencing a sudden increase in brightness and slowly returning to its original state, a process that could last many months. Such an outburst, which emits radiation across all wavelengths, is the result of the accretion process in a close binary system containing a white dwarf and its non-degenerate stellar companion.

Nova Scorpii 2023, also known as V1716 Sco, was detected when it erupted on April 20, 2023 at magnitude 8.0. Follow-up observations of this outburst spectroscopically confirmed it as a classical (Fe II) nova.

Since its outburst, Nova Scorpii 2023 has been monitored regularly with Swift, which detected X-ray emission from this source.

New X-ray findings reveal nova behavior

Now, a group of astronomers led by John T. Worley of the University of Wisconsin-Madison, presents new results of this monitoring that shed more light on the nova’s nature. Their study was complemented by data from the Neutron Star Interior Composition Explorer (NICER) and the Chandra X-ray Observatory.

The study found that the super soft X-ray source (SSS) phase of Nova Scorpii 2023 started a little after three months after the onset of the optical eruption and lasted between three and six additional months.

The collected data indicate that the SSS flux was variable, both aperiodically and periodically. For instance, between late July and September 2023 there was large irregular variability over timescales of days. This aperiodic variability persisted but decreased in amplitude in October 2023.

White dwarf properties and variability explained

Therefore, based on these findings, the astronomers assume that they caught Nova Scorpii 2023 in February 2024 at the very beginning of cooling-off and the total SSS phase duration is likely to have been close to half a year. This suggests a constant bolometric luminosity phase of total duration close to nine months, which is typical for a white dwarf with a mass of about 1.2 solar masses.

The effective temperature of the white dwarf in Nova Scorpii 2023 was estimated to be 750,000-780,000 K, which points to a mass at a level of 1.1–1.2 solar masses. This is consistent with other estimates of massive white dwarfs in X-ray luminous novae.

The astronomers added that the irregular variability of Nova Scorpii 2023 may be triggered by clumps in the ejecta, and interactions of the outflow with already emitted circumstellar material, which causes varying ionization stages in the surrounding medium. They also found that there is no correlation between the irregular variability and the amplitude of the short, periodic modulations.

“The lack of correlation of the amplitude of the SSS pulsation with the total SSS flux seems to indicate two different origins for the two types of variability, possibly with the periodic modulation decreasing in amplitude when the polar caps are not entirely visible,” explain the researchers.

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