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

TL;DR

Nova V1716 Sco underwent a luminous supersoft X-ray phase beginning around $93$ days after outburst and lasting about $9$ months, monitored by Swift XRT, NICER, and Chandra. The high-resolution Chandra LETG spectra reveal that the WD atmosphere dominates the continuum, with $T_{ m eff} ightarrow(7.5$--$7.8) imes10^{5}$ K and a WD mass near $1.2$ $M_ extodot$, while emission lines from shocked ejecta indicate ongoing interaction with circumstellar material. A robust timing signal at approximately $77.9$ s is detected, likely representing the WD rotation, with NICER data suggesting apparent period drifts that are artifacts of short exposures and variable modulation amplitude; long Chandra exposures show a stable period. The irregular SSS variability is decoupled from the periodic signal and is attributed to clumpy ejecta and evolving ionization in the surrounding medium. These findings imply a massive WD near the Chandrasekhar limit and underscore the complexity of the SSS phase in novae, with implications for WD spin, magnetic accretion scenarios, and ejecta dynamics.

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.

Figures (9)

• Figure 1: Upper panel: light curve of Nova V1716 Sco over 300 days of Swift observations in the 0.3-10 keV range. The epochs of the Chandra exposures are denoted with dashed red lines. Lower panel: light curve obtained with NICER, with count rate is in the range 0.2-0.8 keV, the energy band in which the flux is significant.
• Figure 2: Selected light curves to show the variability over time scales of minutes and of hours. The NICER data from day 97 post-outburst (PO), in 5 second bins (top panel), and two Chandra-HRC (zero order) light curves, respectively obs. 28048 (2023 August, middle panel, binned every 20 s) and 28049 (2023 October, lower panel, binned every 20 s). There is still some aperiodic variability in the NICER data of the same day, but the average in each pulsation does not vary by more than 30% while we observe fluctuations as large as $>100$% over timescales of days after the first peak.
• Figure 3: Comparison of high vs low count rate NICER spectra: on the left, the spectrum measured on day 96 PO(high count rate) versus the one measured on day 102 (low count rate), and on the right, spectrum of day 118 (low count rate) compared to that of day 135 (high count rate; see Table \ref{['tab:NICEROBS']}). In red, on the left, model fits with a temperature of T=778,000 K and N(H)=4.4 $\times 10^{21}$ cm$^{2}$ for the low count rate and N(H)=3.4 $\times 10^{21}$ cm$^{2}$ for the high count rate, respectively; on the right, a model fit with a fixed value of the column density, N(H)=4.7 $\times 10^{21}$ cm$^{2}$, and T=782,000 K and 703,000 K for the high and low count rate, respectively. The logarithmic scale on the Y axis shows the deviation from a pure atmosphere model, that in the grating observations turned out to be due mostly to emission lines.
• Figure 4: Top panel: comparison of the $\pm{1}$ orders average spectra of August 27 (black) and October 20 (red). Middle panel: comparison of the August 26 (red) and August 27 (black) $\pm{1}$ orders spectra, with the second shifted by 0.375 cts s$^{-1}$. Bottom panel: comparison of the October 20 (red), 21(blue), 22 (black) $\pm{1}$ order spectra with successive shifts by 0.375 cts s$^{-1}$. Note how the difference is significant only between August and October, but not in spectra taken on sequential days.
• Figure 5: Fit to the spectrum taken on 2023 October 21 with the model described in the text. The blue line indicates the fit with only the atmosphere model. The BVAPEC model (included in the red line) adds emission lines of N VII and of oxygen, but it does not significantly improve the fit, as most emission lines are not fitted. A thorough analysis on the origin of emission lines - confirming the result on the atmospheric temperature - is presented in detail in the Spectral Paper.