NuSTAR and Swift upper limits on the early X-ray emission from Nova Scorpii 2024 (V1723 Sco, PNV J17261813-3809354)

ATel #16444; Kirill Sokolovsky (UIUC), Gerardo Juan Manuel Luna (CONICET/UNAHUR), Elias Aydi, Laura Chomiuk, Peter Craig, Isabella Molina, Jay Strader (MSU), Jennifer Sokoloski (Columbia), Justin Linford (NRAO), Koji Mukai (NASA/GSFC), Andrew Pearce (Nedlands, Western Australia)
on 14 Feb 2024; 05:09 UT
Credential Certification: Kirill Sokolovsky (kirx@scan.sai.msu.ru)

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The eruption of Nova Scorpii 2024 (V1723 Sco, PNV J17261813-3809354) was discovered by A. Pearce and independently Y. Sakurai with the first detection on 2024-02-08.827 UT and spectroscopically confirmed as a classical nova in the following days (CBET #5346, ATel #16440, #16442). The nova was detected as a bright gamma-ray transient by Fermi-LAT starting on 2024-02-09.50 (ATel #16439, #16441).

We observed V1723 Sco with NuSTAR for 63ks between 2024-02-10.299 and 2024-02-11.767. No X-ray source is visible at the nova position with a combined two-module count rate upper limit of 0.0015 cts/s. Assuming kT= 4 keV thermal plasma emission (like the one reported for nova V1674 Her by Sokolovsky et al. 2023, MNRAS, 521, 5453) and the total line of sight HI column density of 8.48x10^21 cm^-2 (Kalberla et al. 2005 A&A, 440, 775; https://www.astro.uni-bonn.de/hisurvey/AllSky_profiles/index.php ) this translates to the unabsorbed 3.0-78 keV flux limit of 6x10^-14 ergs/cm^2/s (1x10^-13 ergs/cm^2/s when extrapolating to the 0.3-10.0 keV band of Swift/XRT).

V1723 Sco was observed with Swift for 6.3ks between 2024-02-10 and 2024-02-13. Swift/XRT detected no X-ray source at the position of the nova with an upper limit of 0.001 cts/s. Assuming the same kT= 4 keV thermal plasma emission and HI column density of 8.48x10^21 cm^-2 this translates to the unabsorbed 0.3-10.0 keV flux limit of 1x10^-13 ergs/cm^2/s, similar to the extrapolated NuSTAR limit.

We also used an optical image obtained on 2024-02-11.7409 with a remotely-controlled 0.5m telescope at Siding Spring Observatory (iTelescope T30) to measure astrometric position of V1723 Sco relative to the surrounding Gaia DR3 stars: 17:26:18.078 -38:09:36.31 +/-0.2" J2000. This supports the earlier-reported identification of the nova with a Gmag=19.4 star Gaia DR3 5974053153713533184 located 0.13" form the measured position. Given the density of Gaia DR3 sources in this field, the probability of chance coincidence is 0.002 if we adopt 0.2" as the nova position uncertainty.

We thank the NuSTAR and Neil Gehrels Swift Observatory teams for the very rapid scheduling of these observations.