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We report on the search for optical counterparts to IceCube neutrino alerts released between 2016 April and 2021 August with the All-Sky Automated Survey for SuperNovae (ASAS-SN). Despite the discovery of a diffuse astrophysical high-energy neutrino flux in 2013, the source of those neutrinos remains largely unknown. Since 2016, IceCube has published likely astrophysical neutrinos as public real-time alerts. Through a combination of normal survey and triggered target-of-opportunity observations, ASAS-SN obtained images within 1 h of the neutrino detection for 20 per cent (11) of all observable IceCube alerts and within one day for another 57 per cent (32). For all observable alerts, we obtained images within at least two weeks from the neutrino alert. ASAS-SN provides the only optical follow-up for about 17 per cent of IceCube’s neutrino alerts. We recover the two previously claimed counterparts to neutrino alerts, the flaring-blazar TXS 0506 + 056 and the tidal disruption event AT2019dsg. We investigate the light curves of previously detected transients in the alert footprints, but do not identify any further candidate neutrino sources. We also analysed the optical light curves of Fermi 4FGL sources coincident with high-energy neutrino alerts, but do not identify any contemporaneous flaring activity. Finally, we derive constraints on the luminosity functions of neutrino sources for a range of assumed evolution models.

 

We present the largest and most homogeneous collection of near-infrared (NIR) spectra of Type Ia supernovae (SNe Ia): 339 spectra of 98 individual SNe obtained as part of the Carnegie Supernova Project-II. These spectra, obtained with the FIRE spectrograph on the 6.5 m Magellan Baade telescope, have a spectral range of 0.8–2.5μm. Using this sample, we explore the NIR spectral diversity of SNe Ia and construct a template of spectral time series as a function of the light-curve-shape parameter, color stretchs_BV. Principal component analysis is applied to characterize the diversity of the spectral features and reduce data dimensionality to a smaller subspace. Gaussian process regression is then used to model the subspace dependence on phase and light-curve shape and the associated uncertainty. Our template is able to predict spectral variations that are correlated withs_BV, such as the hallmark NIR features: Mgiiat early times and theH-band break after peak. Using this template reduces the systematic uncertainties inK-corrections by ∼90% compared to those from the Hsiao template. These uncertainties, defined as the meanK-correction differences computed with the color-matched template and observed spectra, are on the level of 4 × 10⁻⁴mag on average. This template can serve as the baseline spectral energy distribution for light-curve fitters and can identify peculiar spectral features that might point to compelling physics. The results presented here will substantially improve future SN Ia cosmological experiments, for both nearby and distant samples.

 

We examine the early phase intrinsic (B - V)₀ color evolution of a dozen SNe Ia discovered within three days of the inferred time of first light (t _first) and have (B - V)₀ color information beginning within five days of t _first. The sample indicates there are two distinct early populations. The first is a population exhibiting blue colors that slowly evolve, and the second population exhibits red colors and evolves more rapidly. We find that the early blue events are all 1991T/1999aa-like with more luminous, slower declining light curves than those exhibiting early red colors. Placing the first sample on the Branch diagram (i.e., ratio of Si II λλ5972, 6355 pseudo-Equivalent widths) indicates that all blue objects are of the Branch shallow silicon (SS) spectral type, while all early red events except for the 2000cx-like SN 2012fr are of the Branch Core Normal (CN) or CooL (CL) type. A number of potential processes contributing to the early emission are explored, and we find that, in general, the viewing-angle dependance inherent in the companion collision model is inconsistent with all of the SS objects with early-time observations being blue and exhibiting an excess. We caution that great care must be taken when interpreting early phase light curves as there may be a variety of physical processes that are possibly at play and significant theoretical work remains to be done. 

The Carnegie Supernova Project-II (CSP-II) was an NSF-funded, four-year program to obtain optical and near-infrared observations of a “Cosmology” sample of ˜100 Type Ia supernovae located in the smooth Hubble flow (0.03 ≲ z ≲ 0.10). Light curves were also obtained of a “Physics” sample composed of 90 nearby Type Ia supernovae at z ≤ 0.04 selected for near-infrared spectroscopic timeseries observations. The primary emphasis of the CSP-II is to use the combination of optical and near-infrared photometry to achieve a distance precision of better than 5%. In this paper, details of the supernova sample, the observational strategy, and the characteristics of the photometric data are provided. In a companion paper, the near-infrared spectroscopy component of the project is presented.