An official website of the United States government
Abstract There is a long-standing discrepancy between the observed Galactic classical nova rate of ∼10 yr −1 and the predicted rate from Galactic models of ∼30–50 yr −1 . One explanation for this discrepancy is that many novae are hidden by interstellar extinction, but the degree to which dust can obscure novae is poorly constrained. We use newly available all-sky three-dimensional dust maps to compare the brightness and spatial distribution of known novae to that predicted from relatively simple models in which novae trace Galactic stellar mass. We find that only half (53%) of the novae are expected to be easily detectable ( g ≲ 15) with current all-sky optical surveys such as the All-Sky Automated Survey for Supernovae (ASAS-SN). This fraction is much lower than previously estimated, showing that dust does substantially affect nova detection in the optical. By comparing complementary survey results from the ASAS-SN, OGLE-IV, and Palomar Gattini IR surveys using our modeling, we find a tentative Galactic nova rate of ∼30 yr −1 , though this could be as high as ∼40 yr −1 , depending on the assumed distribution of novae within the Galaxy. These preliminary estimates will be improved in future work through more sophisticated modeling of nova detection in ASAS-SN and other surveys.
more »
« less
A Population of Heavily Reddened, Optically Missed Novae from Palomar Gattini-IR: Constraints on the Galactic Nova Rate
Abstract The nova rate in the Milky Way remains largely uncertain, despite its vital importance in constraining models of Galactic chemical evolution as well as understanding progenitor channels for Type Ia supernovae. The rate has been previously estimated to be in the range of ≈10–300 yr −1 , either based on extrapolations from a handful of very bright optical novae or the nova rates in nearby galaxies; both methods are subject to debatable assumptions. The total discovery rate of optical novae remains much smaller (≈5–10 yr −1 ) than these estimates, even with the advent of all-sky optical time-domain surveys. Here, we present a systematic sample of 12 spectroscopically confirmed Galactic novae detected in the first 17 months of Palomar Gattini-IR (PGIR), a wide-field near-infrared time-domain survey. Operating in the J band (≈1.2 μ m), which is significantly less affected by dust extinction compared to optical bands, the extinction distribution of the PGIR sample is highly skewed to a large extinction values (>50% of events obscured by A V ≳ 5 mag). Using recent estimates for the distribution of Galactic mass and dust, we show that the extinction distribution of the PGIR sample is commensurate with dust models. The PGIR extinction distribution is inconsistent with that reported in previous optical searches (null-hypothesis probability <0.01%), suggesting that a large population of highly obscured novae have been systematically missed in previous optical searches. We perform the first quantitative simulation of a 3 π time-domain survey to estimate the Galactic nova rate using PGIR, and derive a rate of ≈ 43.7 − 8.7 + 19.5 yr −1 . Our results suggest that all-sky near-infrared time-domain surveys are well poised to uncover the Galactic nova population.
more »
« less
- Award ID(s):
- 1816100
- PAR ID:
- 10333369
- Author(s) / Creator(s):
- ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; more »
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 912
- Issue:
- 1
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 19
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
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.more » « less
-
Abstract Despite their long history, a complete inventory of Galactic novae has remained incomplete due to severe dust extinction. Recent ground-based near-infrared (NIR) as well as space-based mid-infrared (MIR) searches are providing a new window into discovering heavily obscured Galactic novae. Even with its slow cadence, archival photometric searches for large-amplitude, fast fading outbursts in the NEOWISE survey were previously used to identify Galactic nova candidates between 2014 and 2020. In this paper, we demonstrate the efficacy of the method with the discovery and NIR spectroscopy of WTP 21aaeuqd—a bright (peak W2 AB flux ≈10.5 mag), rapidly evolving (t2 ≈ 120 days) MIR Galactic outburst (at ). The reduced spectrum exhibits broad (≈800 km s−1) emission lines of H iand C i, confirming its classification as a missed Fe iinova. This result confirms that archival infrared photometric searches are an effective way to identify nova candidates missed in optical searches.more » « less
-
ABSTRACT Classical novae in the Milky Way have now been well-established as high-energy GeV $$\gamma$$-ray sources. In novae with main-sequence companions, this emission is believed to result from shocks internal to the nova ejecta, as a later fast wind collides with an earlier slow outflow. To test this model and constrain the $$\gamma$$-ray production mechanism, we present a systematic study of a sample of recent Galactic novae, comparing their $$\gamma$$-ray properties ($$\gamma$$-ray luminosity and duration) with their outflow velocities, peak V-band magnitudes, and the decline times of their optical light curves ($$t_2$$). We uniformly estimate distances in a luminosity-independent manner, using spectroscopic reddening estimates combined with three-dimensional Galactic dust maps. Across our sample, $$\gamma$$-ray luminosities ($>$100 MeV) vary by three orders of magnitude, spanning $$10^{34}{{-}}10^{37}$$ erg s$$^{-1}$$. Novae with larger velocity of the fast outflow (or larger differential between the fast and slow outflow) have larger $$\gamma$$-ray luminosities, but are detectable for a shorter duration. The optical and $$\gamma$$-ray fluxes are correlated, consistent with substantial thermal emission in the optical from shock-heated gas. Across six novae with $$\gamma$$-ray and infrared light curves, evidence for dust formation appears soon after the end of the detected $$\gamma$$-ray emission. Dusty and non-dusty novae appear to have similar $$\gamma$$-ray luminosities, though novae that have more material processed by the shocks may be more likely to form dust. We find that the properties of the $$\gamma$$-ray emission in novae depend heavily on the ejecta properties, and are consistent with expectations for internal shocks.more » « less
-
Abstract We demonstrate proof of concept of a new strategy for studying infrared (IR) transients enabled by the newly launched SPHEREx space mission, by leveraging its synergy with the NEOWISE space mission. With its 15 yr baseline and all-sky mid-IR coverage, NEOWISE provides an excellent avenue to discover thousands of slowly evolving IR outbursts. With its all-sky spectrophotometric coverage and mid-IR sensitivity matching NEOWISE, SPHEREx is uniquely positioned to provide low-resolution IR spectra for the vast majority of these outbursts, several of which are too obscured for ground-based spectroscopic classification. As a demonstration of this approach, we present SPHEREx spectra for eight Galactic transients identified in NEOWISE. This sample includes two previously known FU Orionis-type (FUOr) outbursts whose SPHEREx spectra exhibit clear signatures of cool molecular absorption and three known classical novae showing strong emission lines in SPHEREx. Using these sources as templates, we identify two new FUOrs and one previously missed Galactic nova. Our results highlight the potential of SPHEREx for systematic explorations of the relatively underexplored dynamic IR sky.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.3847/1538-4357/abeb75
