NuSTAR hard X-ray detection and optical observations of Nova Scorpii 2023

ATel #16018; Kirill Sokolovsky (UIUC), Elias Aydi, Laura Chomiuk, Jay Strader (MSU), Jennifer Sokoloski (Columbia), Justin Linford (NRAO), Koji Mukai (NASA/GSFC), D. A. H. Buckley (SAAO/UCT), J. Mikolajewska (NCAC), M. Orio (UoW/INAF), Krzysztof Stanek, Christopher Kochanek (OSU), Franz-Josef Hambsch (Center for Backyard Astrophysics), Mohammad Odeh (UAE), Andrew Pearce, Filipp Romanov, Ian Sharp, Arie Verveer, Brad Young (AAVSO Observer)
on 27 Apr 2023; 21:05 UT
Credential Certification: Kirill Sokolovsky (kirx@scan.sai.msu.ru)

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Nova Scorpii 2023 (PNV J17224490-4137160, V1716 Sco) was discovered by A. Pearce on 2023-04-20.6780 UT (CBET #5245, ATel #16003, #16004, #16006, #16007). Pre-discovery ASAS-SN survey images (Shappee et al. 2014, ApJ, 788, 48; Kochanek et al. 2017, PASP, 129, 104502) show the nova to be bright (saturated) on 2023-04-20.410 - the date we adopt as the eruption start, t0. According to the AAVSO lightcurve, the nova peaked at a visual magnitude of 7.0 around 2023-04-21.253 (t0 + 0.843 d). Fermi/LAT detected E >100 MeV gamma-ray emission from the nova a day after the eruption (ATel #16002), while the simultaneous Swift/XRT observation detected no 0.3-10 keV X-rays (ATel #16005).

NuSTAR observed the nova in the hard 3-78 keV X-rays between 2023-04-21.89 (t0 + 1.5 d) and 2023-04-23.42 (t0 + 3.0 d) for a total exposure of 70 ks. This is the earliest post-eruption observation of a nova with NuSTAR. While the NuSTAR images were affected by stray light, likely from the nearby neutron-star-hosting low mass X-ray binary 4U 1708-40, the nova was clearly detected. Its background-subtracted count rate was smoothly increasing from 0.02 to 0.04 cts/s per focal plane module over the course of the NuSTAR exposure. Preliminary analysis suggests that the X-ray spectrum is consistent with that of a heavily absorbed thermal plasma with kT= 31 +/-13 keV and N_H= (8.2 +/-1.5) x10^23 cm^-2 (assuming solar abundances). The derived kT is higher than values found in all novae previously observed by NuSTAR. The unabsorbed 3-78 keV flux is 3.1 x10^-12 erg/cm^2/s (luminosity 2.4 x10^34 * (d / 8 kpc)^2 erg/s).

Optical photometry was obtained during the NuSTAR exposure by multiple ground-based observers and shared via the AAVSO International Database. The observations reveal a generally smooth decline at a rate of 0.66 mag/day (2 magnitudes in 3 days) from V= 7.4 measured on t0 + 1.9 d. The deviations from this average decline trend are within 0.04 mag. This is in contrast to the previous simultaneous NuSTAR-AAVSO observations of another nova V1674 Her, where irregular variations were clearly observed in both hard X-ray and optical bands 11 days after the eruption (Sokolovsky et al. 2023, MNRAS, 521, 5453). Further AAVSO observations indicate that the decline has slowed down as it actually took about t2= 5 days for V1716 Sco to decline by two magnitudes.

On 2023-04-26.93 (t0 + 6.5 d) we used the High Resolution Spectrograph (HRS; Crause et al. 2014, Proc. SPIE, 91476) mounted on the 11m Southern African Large Telescope to obtain a 1200 s spectrum of V1716 Sco as part of the SALT Large Science Program on Transients. The observations were obtained in the HR mode of HRS which covers the spectral range of 3800-8900 A at a resolution of R=67000. The data were reduced with the SALT HRS MIDAS pipeline (Kniazev et al. 2016, MNRAS 459, 3068). The spectrum shows broad emission lines of Balmer, Fe II, and O I. The Balmer lines show P Cygni absorption features at around -2500 km/s and -3300 km/s, the velocities that are slightly larger than the ones reported in ATel #16003, #16004, and #16007.

NuSTAR X-ray and AAVSO optical lightcurves of V1716 Sco