Abstract
The classical nova, AT 2023prq, was discovered on 2023 August 15 and is located at a distance of 46 kpc from the Andromeda galaxy (M31). Here we report photometry and spectroscopy of the nova. The "very fast" ( days) and low luminosity (
) nature of the transient along with the helium in its spectra would indicate that AT 2023prq is a "faint-and-fast" He/N nova. Additionally, at such a large distance from the center of M31, AT 2023prq is a member of the halo nova population.
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1. Introduction
Classical novae (CNe) are cataclysmic eruptions emanating from accreting white dwarfs. Hydrogen-rich matter accreted from the donor eventually reaches a critical pressure and undergoes a thermonuclear runaway at the accreted layer's base, resulting in the material being expelled as a nova eruption (Starrfield et al. 1972).
During eruption, CNe can reach absolute magnitudes up to MV ≈ − 10.5 (Aydi et al. 2018). Studying CNe in intergalactic regions can aid measurements of intracluster light (Neill et al. 2005) and help with tracing populations of intergalactic stars (Shara 2006).
While "hostless" novae, residing far from (but still associated with) a specific nearby galaxy, are expected (Shara 2006), the number known is considerably low: e.g., six in the intracluster regimes of the Fornax Cluster (Neill et al. 2005); four in the halo of M31 (Meinunger 1973) and two in the M31 Giant Stellar Stream (Darnley et al. 2020). Here we report on AT 2023prq, a nova found in the halo of M31, and one of the most distant novae found from its host.
2. Observations
2.1. Photometry
AT 2023prq (aka ZTF23aaxzvrr) was discovered by the Zwicky Transient Facility (ZTF; Kulkarni 2018) on 2023 August 15.45 UT with an -band magnitude of 17.13 at α = 1h00m22
28,
(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
filters of IO:O on the Liverpool Telescope (LT; Steele et al. 2004) across seven epochs until August 31.1.
2.2. Spectroscopy
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.
3. Results
3.1. Photometry
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.
Figure 1. Top left: light curve of AT 2023prq. Top right: 45 × 4
5 DSS image of M31 and its surroundings. AT 2023prq is shown (star) with the two tidal stream CNe (AT 2016dah and AT 2017fyp) from Darnley et al. (2020) for comparison. Bottom: spectra of AT 2023prq with emission lines labeled.
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Standard image High-resolution imageTaking the brightest data point, from ZTF, as the peak apparent magnitude of the nova, a distance to M31 of 778 kpc (Stanek & Garnavich 1998) and an extinction of
, we derive an absolute peak magnitude of
.
Considering the filter from ZTF and LT
filter as approximately the same to linearly interpolate the data, we estimate decline times of
days and
days, indicating that AT 2023prq belongs to "very-fast" speed class. This supports the
days decline time derived by Basu et al. (2023) and their speed classification.
3.2. Spectroscopy
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).
4. Discussion
AT 2023prq is situated at an angular distance of ∼34 from the center of M31. If the nova is 778 kpc away, so in the same plane of the sky as the center of M31, this indicates a projected distance from M31 of 45.9 kpc. Whereas, if the nova is in the inclined disk of M31, it would be 67.7 kpc from the center (∼14 K-band scale lengths). AT 2023prq is not a Galactic nova as it is too faint and does not spectroscopically resemble a dwarf nova. While not possible to rule out, and unlikely, the nova may be a long distance behind M31 (up to ∼4 Mpc if comparative to the brightest ever nova). Based on the above assumptions, AT 2023prq is likely to belong to the halo of M31, indicating the CN is another example of a "hostless" nova (Shara 2006).
Indeed, if AT 2023prq is at the same distance as M31, then the "very-fast" ( days) decline time and low absolute magnitude (
) would place this object in the "faint-and-fast" category of novae (Kasliwal et al. 2011; Healy et al. 2019). Furthermore, on account of He emission lines and relatively high ejecta velocities derived from the Hα emission line, AT 2023prq, unlike other spectroscopically classified halo novae, belongs to the He/N spectroscopic class.
Acknowledgments
M.W.H.-K. acknowledges a PDRA position funded by the UK STFC with grant No. ST/S505559/1.
Facilities: ATLAS - , GIT - , HCT - Himalayan Chandra Telescope, LT - , ZTF - , 1.82-m Plaskett Telescope - .
Software: IRAF (Tody 1986), Starlink (Currie et al. 2014), SAOImageDS9 (Smithsonian Astrophysical Observatory 2000).