The Galactic Nova Rate: Estimates from the ASAS-SN and Gaia Surveys

Abstract

We present the first estimate of the Galactic nova rate based on optical transient surveys covering the entire sky. Using data from the All-Sky Automated Survey for Supernovae (ASAS-SN) and Gaia—the only two all-sky surveys to report classical nova candidates—we find 39 confirmed Galactic novae and 7 additional unconfirmed candidates discovered from 2019 to 2021, yielding a nova discovery rate of ≈14 yr−1. Using accurate Galactic stellar mass models and three-dimensional dust maps and incorporating realistic nova light curves, we have built a sophisticated Galactic nova model to estimate the fraction of Galactic novae discovered by these surveys over this time period. The observing capabilities of each survey are distinct: the high cadence of ASAS-SN makes it sensitive to fast novae, while the broad observing filter and high spatial resolution of Gaia make it more sensitive to highly reddened novae across the entire Galactic plane and bulge. Despite these differences, we find that ASAS-SN and Gaia give consistent Galactic nova rates, with a final joint nova rate of 26 ± 5 yr−1. This inferred nova rate is substantially lower than found by many other recent studies. Critically assessing the systematic uncertainties in the Galactic nova rate, we argue that the role of faint, fast-fading novae has likely been overestimated, but that subtle details in the operation of transient alert pipelines can have large, sometimes unappreciated effects on transient recovery efficiency. Our predicted nova rate can be directly tested with forthcoming red/near-infrared transient surveys in the southern hemisphere.

Rapidly evolving Galactic plane outbursts in NEOWISE: Revisiting the Galactic nova rate with the first all-sky search in the mid-infrared

“…However, the WISE sample of novae, being discovered in the midinfrared bands over a broad-sky area including the Galactic plane, is tightly clustered around the central Galactic plane in regions of heavy extinction. This trend corroborates previous suggestions (De et al 2021;Kawash et al 2022) that infrared searches are substantially more sensitive to the (most frequent) nova eruptions near the Galactic plane, even though these events have been historically overlooked in optical surveys.…”

“…As this work focuses on an archival search for events, spectroscopic confirmation of the events remains highly unfeasible. Therefore, although our search identifies more events than other recent systematic searches (De et al 2021;Kawash et al 2022), there is likely contamination from other fast evolving outbursts (e.g. X-ray binaries and young stars) near the Galactic plane (noting that the sample of Mróz et al 2015 as well as Kawash et al 2022 also consist of several unconfirmed events).…”

“…In particular, they showed that previous optical searches were systematically missing a substantial population of dust-obscured novae, resulting in inconsistencies between observed and estimated nova rates in optical searches. More recently, Kawash et al (2021b) and Kawash et al (2022) used novae discovered in the ASASSN and Gaia surveys to carry out a systematic analysis of the nova rate, providing a rate of 26 ± 5 yr −1 . With the estimated optical nova rate nominally inconsistent with the higher NIR rate, further progress requires additional investigation of independent surveys and observing wavebands to assess these discrepancies.…”

The evolution of CNO elements in galaxies

“…The current nova rate in the Galaxy is also largely uncertain. Recent studies report rates of 43:7 þ19:5 À8:7 yr À1 (De et al 2021), 26 AE 5 yr À1 (Kawash et al 2022), and 28 þ5 À4 yr À1 (Rector et al 2022). The past Galactic nova rate is basically unknown, though some population synthesis studies suggest the existence of an anticorrelation between metallicity and the number of novae produced: for instance, according to Kemp et al (2022), the number of novae at Z ¼ 0:03 is roughly half that at Z ¼ 10 À4 .…”

The multi-wavelength view of shocks in the fastest nova V1674 Her

“…While the optical continuum light of a nova fades on a time-scale of days to months, the warm ejected envelope remains the source of optical line and radio continuum emission for months and years after the eruption (Strope et al 2010;Chomiuk et al 2021b). About 30 such events occur in the Galaxy each year, with only 10 events per year typically observed while others remain hidden by dust extinction (Shafter 2017;De et al 2021;Kawash et al 2022;Rector et al 2022).…”