AT 2023prq: A Classical Nova in the Halo of the Andromeda Galaxy

AT 2023prq (aka ZTF23aaxzvrr) was discovered by the Zwicky Transient Facility (ZTF; Kulkarni 2018) on 2023 August 15.45 UT with an ${r}^{{\prime} }$-band magnitude of 17.13 at α = 1^h00^m2228, $\delta =+42^\circ 05^{\prime} 13\buildrel{\prime\prime}\over{.} 57$ (J2000; Ho et al. 2023). It was observed by the GROWTH-India Telescope (GIT; Kumar et al. 2022) and the Himalayan Chandra Telescope (HCT) on August 17.95; the 1.82 m Plaskett Telescope on August 18.49 (Kendurkar & Balam 2023) and the Asteroid Terrestrial-impact Last Alert System (ATLAS; Tonry et al. 2018) on August 18.51. The last non-detections at the nova's location occurred ∼0.5 day prior to discovery: g > 18.98 mag on August 14.45 for ZTF and >19.59 mag on August 14.51 for ATLAS ("cyan" filter). We followed AT 2023prq 4.19 days post-discovery using the ${u}^{{\prime} }{{BVr}}^{{\prime} }{i}^{{\prime} }$ filters of IO:O on the Liverpool Telescope (LT; Steele et al. 2004) across seven epochs until August 31.1.

A spectrum of AT 2023prq, used for classification, was obtained by Perley et al. (2023) on August 18.19 (3.2 days post-discovery) using SPRAT on the LT. As reported in Basu et al. (2023), on August 19, a spectrum was obtained with the HFOSC on HCT. We obtained optical spectroscopy of AT 2023prq using SPRAT on the LT on August 19.14.

Photometry from ZTF, Plaskett Telescope, ATLAS and LT is shown in the top left panel of Figure 1. We take the time of eruption as 2023 August 14.98 ± 0.47; this is the midpoint between the last non-detection on August 14.51 by ATLAS and discovery on August 15.45.

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Taking the brightest data point, ${r}^{{\prime} }=17.13$ from ZTF, as the peak apparent magnitude of the nova, a distance to M31 of 778 kpc (Stanek & Garnavich 1998) and an extinction of ${A}_{{r}^{{\prime} }}=0.268$, we derive an absolute peak magnitude of ${M}_{{r}^{{\prime} }}\sim -7.6$.

Considering the ${r}^{{\prime} }$ filter from ZTF and LT ${r}^{{\prime} }$ filter as approximately the same to linearly interpolate the data, we estimate decline times of ${t}_{2,{r}^{{\prime} }}=3.4\pm 0.5$ days and ${t}_{3,{r}^{{\prime} }}=4.8\pm 0.5$ days, indicating that AT 2023prq belongs to "very-fast" speed class. This supports the ${t}_{2,{r}^{{\prime} }}\sim 3.1$ days decline time derived by Basu et al. (2023) and their speed classification.

Spectra obtained with LT are shown in the bottom panel of Figure 1. The classification spectrum is labeled t = 3.2 days (Perley et al. 2023) and the spectrum obtained on August 19.14 is labeled t = 4.2 days.

The spectrum taken 3.2 days post-discovery exhibits Balmer emission lines (Hα at 6563 Å and Hβ at 4861 Å) as reported by Perley et al. (2023), but also Hγ at 4340 Å and He i at 5876 Å. There are possibly faint emission lines of He ii at 4686 Å, [N i] at 5199 Å and He i at 7065 Å. Therefore the spectrum of AT 2023prq is consistent with belonging to the He/N spectroscopic class. During this epoch, the Hα line FWHM is 2100 ± 300 km s⁻¹.

While the spectrum obtained with HCT HFOSC ∼4 days after discovery displayed He i at 5876 Å alongside Hα and Hβ emission lines (Basu et al. 2023), only Balmer lines are evident within the LT spectrum at a similar epoch (4.2 days post-discovery). The Hα FWHM in the LT spectrum at this epoch decreased to 1700 ± 200 km s⁻¹ (close to the FWHM of 2300 km s⁻¹ reported by Basu et al. 2023).