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1. Galactic Extinction: How Many Novae Does It Hide and How Does It Affect the Galactic Nova Rate?

   https://doi.org/10.3847/1538-4357/ac1f1a  

   Kawash, A.; Chomiuk, L.; Rodriguez, J. A.; Strader, J.; Sokolovsky, K. V.; Aydi, E.; Kochanek, C. S.; Stanek, K. Z.; Mukai, K.; De, K.; et al (November 2021, The Astrophysical Journal)

   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. 

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2. A Population of Heavily Reddened, Optically Missed Novae from Palomar Gattini-IR: Constraints on the Galactic Nova Rate

   https://doi.org/10.3847/1538-4357/abeb75  

   De, Kishalay; Kasliwal, Mansi M.; Hankins, Matthew J.; Sokoloski, Jennifer L.; Adams, Scott M.; Ashley, Michael C.; Babul, Aliya-Nur; Bagdasaryan, Ashot; Delacroix, Alexandre; Dekany, Richard; et al (April 2021, The Astrophysical Journal)

   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. 

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3. The ASAS-SN catalogue of variable stars X: discovery of 116 000 new variable stars using G -band photometry

   https://doi.org/10.1093/mnras/stac3801  

   Christy, C. T.; Jayasinghe, T.; Stanek, K. Z.; Kochanek, C. S.; Thompson, T. A.; Shappee, B. J.; Holoien, T. W-S; Prieto, J. L.; Dong, Subo; Giles, W. (December 2022, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT The All-Sky Automated Survey for Supernovae (ASAS-SN) is the first optical survey to monitor the entire sky, currently with a cadence of ≲ 24 h down to g ≲ 18.5 mag. ASAS-SN has routinely operated since 2013, collecting ∼ 2 000 to over 7 500 epochs of V- and g-band observations per field to date. This work illustrates the first analysis of ASAS-SN’s newer, deeper, and higher cadence g-band data. From an input source list of ∼55 million isolated sources with g < 18 mag, we identified 1.5 × 106 variable star candidates using a random forest (RF) classifier trained on features derived from Gaia, 2MASS, and AllWISE. Using ASAS-SN g-band light curves, and an updated RF classifier augmented with data from Citizen ASAS-SN, we classified the candidate variables into eight broad variability types. We present a catalogue of ∼116 000 new variable stars with high-classification probabilities, including ∼111 000 periodic variables and ∼5 000 irregular variables. We also recovered ∼263 000 known variable stars. 

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4. 60 Microlensing Events from the Three Years of Zwicky Transient Facility Phase One

   https://doi.org/10.3847/1538-4357/acba8f  

   Medford, Michael S.; Abrams, Natasha S.; Lu, Jessica R.; Nugent, Peter; Lam, Casey Y. (April 2023, The Astrophysical Journal)

   Abstract Microlensing events have historically been discovered throughout the Galactic bulge and plane by surveys designed solely for that purpose. We conduct the first multiyear search for microlensing events on the Zwicky Transient Facility (ZTF), an all-sky optical synoptic survey that observes the entire visible northern sky every few nights. We discover 60 high-quality microlensing events in the 3 yr of ZTF-I using the bulk lightcurves in the ZTF Public Data Release 5.19 of our events are found outside of the Galactic plane (∣b∣ ≥ 10°), nearly doubling the number of previously discovered events in the stellar halo from surveys pointed toward the Magellanic Clouds and the Andromeda galaxy. We also record 1558 ongoing candidate events as potential microlensing that can continue to be observed by ZTF-II for identification. The scalable and computationally efficient methods developed in this work can be applied to future synoptic surveys, such as the Vera C. Rubin Observatory’s Legacy Survey of Space and Time and the Nancy Grace Roman Space Telescope, as they attempt to find microlensing events in even larger and deeper data sets. 

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5. The ASAS-SN catalogue of variable stars – V. Variables in the Southern hemisphere

   https://doi.org/10.1093/mnras/stz2711  

   Jayasinghe, T.; Stanek, K. Z.; Kochanek, C. S.; Shappee, B. J.; Holoien, T. W-S; Thompson, T. A.; Prieto, J. L.; Dong, S.; Pawlak, M.; Pejcha, O.; et al (September 2019, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT The All-Sky Automated Survey for Supernovae (ASAS-SN) provides long baseline (∼4 yr) light curves for sources brighter than V ≲ 17 mag across the whole sky. As part of our effort to characterize the variability of all the stellar sources visible in ASAS-SN, we have produced ∼30.1 million V-band light curves for sources in the Southern hemisphere using the APASS DR9 (AAVSO Photometric All-Sky Survey Data Release) catalogue as our input source list. We have systematically searched these sources for variability using a pipeline based on random forest classifiers. We have identified $${\sim } 220\, 000$$ variables, including $${\sim } 88\, 300$$ new discoveries. In particular, we have discovered $${\sim }48\, 000$$ red pulsating variables, $${\sim }23\, 000$$ eclipsing binaries, ∼2200 δ-Scuti variables, and $${\sim }10\, 200$$ rotational variables. The light curves and characteristics of the variables are all available through the ASAS-SN variable stars data base (https://asas-sn.osu.edu/variables). The pre-computed ASAS-SN V-band light curves for all the ∼30.1 million sources are available through the ASAS-SN photometry data base (https://asas-sn.osu.edu/photometry). This effort will be extended to provide ASAS-SN light curves for sources in the Northern hemisphere and for V ≲ 17 mag sources across the whole sky that are not included in APASS DR9. 

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