This is the first part of the Cosmic Neutrino Era series. Each part covers one milestone in IceCube’s establishment of high-energy cosmic neutrino astronomy. We begin with the events that started it all: Bert and Ernie, the two PeV-scale cascades detected in 2011-2012.
The setup
By 2011, IceCube had been taking physics-quality data for over a year with its full 86-string configuration. The collaboration was searching for astrophysical neutrinos in two energy regimes:
TeV-PeV muon tracks: produced by $\nu_\mu$ interactions outside the detector with the muon entering the detector. Good pointing (sub-degree resolution at high energy). But difficult to separate from atmospheric muon backgrounds for events arriving from above.
PeV cascades: produced by $\nu_e$, $\nu_\tau$, or $\nu_\mu$ neutral-current interactions inside the detector. Good energy resolution but poor pointing (10-15° at PeV energies). Contained events have a clean signature with no atmospheric-muon background.
The collaboration had developed a search strategy for starting cascades — events where the neutrino interaction vertex is inside the detector. This eliminates atmospheric muon backgrounds essentially completely.
Bert (August 8, 2011)
The first event was named “Bert” by the IceCube collaboration. Detection date: August 8, 2011 (in the 2010-2011 data set, analyzed retrospectively).
Reconstructed energy: ~1.04 PeV ($1.04 \times 10^{15}$ eV).
Event topology: Cascade — a roughly spherical light burst lasting about 1 microsecond, with most photomultiplier hits clustered near the interaction vertex. The cascade is consistent with an electron neutrino producing an electromagnetic-and-hadronic shower in the ice.
Direction: Reconstructed direction has ~15° uncertainty. Generally consistent with isotropic origin.
Atmospheric background: Essentially zero. The expected number of atmospheric neutrino events at this energy in the detector is ~0.01 per year.
Ernie (January 3, 2012)
The companion event, dubbed “Ernie.”
Reconstructed energy: ~1.14 PeV.
Event topology: Also cascade — similar to Bert in overall appearance.
Direction: Similarly poor pointing.
The two events were named together for the obvious Sesame Street reference. The IceCube collaboration has continued naming subsequent extreme events with similar humorous conventions (Big Bird, etc.).
Why these mattered
Two PeV-scale events in a single year was unexpected:
Atmospheric flux at PeV: The atmospheric neutrino flux falls steeply with energy. At PeV energies, the predicted atmospheric-only background rate is at most ~0.05 events per year in IceCube — and that’s including the “prompt” charm-decay component that has not been firmly established.
Two events in one year: With expected background of 0.05 per year, the probability of getting two events in a single year is ~ (0.05)² / 2 = $10^{-3}$. The events were therefore at the 3-4σ level evidence for a real astrophysical population, just from these two events alone.
The 2013 announcement
In April 2013, IceCube published a Physical Review Letters paper announcing “first observation of PeV-energy neutrinos.” The paper presented Bert and Ernie plus a third PeV event (“Big Bird,” December 2012) plus additional sub-PeV cascade events suggesting a continuous astrophysical flux.
Combined statistical significance: well above 3σ for an astrophysical contribution. The two-PeV-event coincidence plus the additional lower-energy excess events established astrophysical neutrinos as real.
What we learned
Cosmic neutrinos exist at PeV energies. The astrophysical universe accelerates protons to at least $10^{7}$ GeV scales — beyond anything in human accelerators.
The flux is roughly isotropic — at the few-tens-of-degree resolution available with cascades. No single direction dominated.
The flavor mix is consistent with 1:1:1 — the expected mix after long-baseline oscillation of cosmic-accelerator-produced flavors. This was an important confirmation that the events were indeed cosmic in origin, not some exotic background.
The energy spectrum is roughly $E^{-2.5}$ — consistent with cosmic-ray-accelerator predictions for the underlying sources.
What we did not learn
Specific sources: Cascade pointing is too poor. Bert, Ernie, and Big Bird could not be associated with any known astrophysical objects.
Source classes: Multiple kinds of accelerators (blazars, AGN, gamma-ray bursts, tidal disruption events) were consistent with the observed flux. Distinguishing required more events and better pointing.
Time variability: The few-event sample was insufficient to identify flares or other time-variable sources.
What came next
The 2013 IceCube paper kicked off a decade of follow-up:
- Continued accumulation of PeV-scale events through the years.
- Development of real-time alerting protocols for sharing potential events with multi-messenger observers.
- The famous September 2017 IceCube-170922A alert that led to identification of TXS 0506+056.
- The 2022 identification of NGC 1068.
- The 2023 galactic plane detection.
The bigger picture
Bert and Ernie were the first concrete evidence that high-energy cosmic neutrino astronomy could work. Two events in a year, each at the PeV scale, established that the field had moved from theoretical promise to observational reality.
Subsequent IceCube data has filled out the picture: hundreds of TeV-PeV astrophysical events, multiple identified sources, the first multi-messenger identifications. The 2013 announcement is recognized as the founding moment of a new branch of astronomy.
The next part of this series turns to the most consequential single IceCube event: the 2017 alert that led to TXS 0506+056.
Frequently asked
What were Bert and Ernie?
Two cascade events detected by IceCube — Bert in August 2011 and Ernie in January 2012 — each at approximately 1 PeV reconstructed energy. The events were named after the Sesame Street characters by the IceCube team. Both were nearly contained in the detector, eliminating atmospheric muon backgrounds. They were the first high-confidence PeV-scale neutrino events ever detected.
Why were these events significant?
They established cosmic neutrinos as real. Below PeV energies, atmospheric neutrinos from cosmic-ray interactions dominate. Above PeV, the atmospheric flux is too low to explain Bert and Ernie. The events had to come from astrophysical sources — beginning the era of cosmic-neutrino astronomy.
When did IceCube publish them?
April 2013, in a Physical Review Letters paper announcing the 'first observation of PeV-energy neutrinos.' Subsequent analysis added a third PeV cascade event ('Big Bird' in December 2012), bringing the total to three PeV-scale events plus dozens of TeV-scale astrophysical-flux events.
What did these events tell us about cosmic neutrino sources?
The PeV energy implies a cosmic accelerator capable of accelerating protons to at least 10⁷ GeV. The flavor composition (cascades vs muon tracks) is consistent with an astrophysical 1:1:1 flavor ratio after long-baseline oscillation. The events themselves don't identify specific sources — the direction reconstruction has substantial uncertainty for cascades.
Cite this article 5 formats
APA
Neutrino Times Editorial Team. (2026, May 11). The Cosmic Neutrino Era — Part 1: 2013 — Bert and Ernie, the first PeV events. Neutrino Times. https://neutrino-times.com/articles/cosmic-era-part-1-bert-and-ernie/
Chicago
Neutrino Times Editorial Team. "The Cosmic Neutrino Era — Part 1: 2013 — Bert and Ernie, the first PeV events." Neutrino Times, May 11, 2026. https://neutrino-times.com/articles/cosmic-era-part-1-bert-and-ernie/.
MLA
Neutrino Times Editorial Team. "The Cosmic Neutrino Era — Part 1: 2013 — Bert and Ernie, the first PeV events." Neutrino Times, 11 May. 2026, https://neutrino-times.com/articles/cosmic-era-part-1-bert-and-ernie/.
BibTeX
@misc{neutrino-times-cosmic-era-part-1-bert-and-ernie,
author = {Neutrino Times Editorial Team},
title = {The Cosmic Neutrino Era — Part 1: 2013 — Bert and Ernie, the first PeV events},
howpublished = {Neutrino Times},
year = {2026},
month = {may},
url = {https://neutrino-times.com/articles/cosmic-era-part-1-bert-and-ernie/},
note = {Accessed: 2026-05-11}
} RIS
TY - GEN TI - The Cosmic Neutrino Era — Part 1: 2013 — Bert and Ernie, the first PeV events AU - Neutrino Times Editorial Team PY - 2026 DA - 2026-05-11 PB - Neutrino Times UR - https://neutrino-times.com/articles/cosmic-era-part-1-bert-and-ernie/ ER -