* EP241021a was discovered as a soft X-ray trigger but was not detected at gamma-ray frequencies.

* The prompt soft X-ray emission spectrum is consistent with **non-thermal radiation**, suggesting a **mildly relativistic outflow with a bulk Lorentz factor Γ≳ 4**.

* The optical and near-infrared light curve shows a **two-component behavior**: an initial fading component (∼ t⁻¹) followed by a **rapid rise (steeper than ∼ t⁴)**, peaking at an absolute magnitude of **Mr ≈−22 mag**, before quickly returning to the initial decay. This peak magnitude is **the most luminous optical emission associated with an FXT**, surpassing EP240414a.

* Standard supernova models cannot explain either the **absolute magnitude or the rapid timescale (< 2 days rest frame)** of the rebrightening.

* The X-ray, optical, and near-infrared spectral energy distributions indicate a **red color (r− J ≈ 1 mag)** and suggest a **non-thermal origin (∼ ν⁻¹)** for the broadband emission.

* Considering a gamma-ray burst (GRB) as a possible scenario, the authors favor a **refreshed shock as the cause of the rebrightening**. This is consistent with the inferred mildly relativistic outflow.

* The results suggest a **likely link between EP-discovered FXTs and low-luminosity gamma-ray bursts**.

The source also compares EP241021a to another peculiar EP transient, **EP240414a**, which showed a roughly similar multi-wavelength behavior. Both events share features like the lack of gamma-ray emission, multiple optical emission components, a relatively flat X-ray light curve, and luminous, late-peaking radio emission. However, EP241021a has a **more luminous peak in its second optical component** and **longer timescales** for its light curve variations. Unlike EP240414a, which showed spectroscopic evidence of a supernova, **no clear supernova features were identified in the HET spectra of EP241021a**.

The authors explore various interpretations for the rebrightening, including off-axis structured jets and refreshed shocks. They disfavor a simple forward shock from an off-axis structured jet due to the steep rise observed but suggest that a **reverse shock from off-axis material in a shallow structured jet** or a **refreshed shock** are more plausible explanations. The consistency of the temporal and spectral indices with standard afterglow closure relations in a wind environment (expected for a massive star progenitor) supports the refreshed shock scenario.

The paper concludes that both EP241021a and EP240414a are likely produced by the **death of a massive star**. The non-thermal prompt emission necessitates at least a mildly relativistic outflow. The rapid optical rebrightening is challenging for supernova models and may be due to refreshed shocks or a reverse shock from off-axis material, both favoring a mildly relativistic outflow and non-thermal synchrotron radiation. The authors emphasize the need for future observations of similar events to better understand their nature.

Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: Chinese Academy of Sciences (CAS).

**Introduction:**

What are Fast Radio Bursts (FRBs)? These millisecond bursts from distant galaxies have astrophysicists intrigued. We explore repeating FRBs (R-FRBs) and theories about their origins, including magnetars.

**AGILE's High-Energy Hunt:**

The Italian AGILE satellite, with its SuperAGILE (18-60 keV), MCAL (0.35-100 MeV), and GRID (0.03-50 GeV) detectors, searched for X- and gamma-ray counterparts to a sample of R-FRBs.

**The Search and Non-Detection:**

AGILE observed several bursts from R-FRBs with low dispersion measure (DMexc < 300 pc cm−3). However, no astrophysical signals were identified in the X- and gamma-ray bands.

**Upper Limits and Magnetar Models:**

The study derived upper limits on the flux, particularly with MCAL, which are now the most stringent in the 0.4-30 MeV range. Researchers compared these findings to the galactic magnetar SGR 1935+2154 (the source of FRB 200428) to test magnetar emission models for FRBs.

**Key Findings:**

* **No high-energy counterparts were detected by AGILE for the observed R-FRB sample**.

* **Stringent upper limits were placed on high-energy emission**, especially by MCAL.

* The study compared R-FRB energies with those extrapolated from **SGR 1935+2154**, providing constraints on the magnetar model.

**Conclusion:**

While AGILE didn't detect high-energy counterparts for this R-FRB sample, its observations provide valuable constraints for theoretical models, especially those involving magnetars. The archival AGILE data still holds potential for future discoveries.

**Reference:**

Casentini, C., Verrecchia, F., Tavani, M., Pilia, M., & Pacciani, L. (2025). AGILE observations of a sample of repeating Fast Radio Burst sources. *Draft version March 13, 2025*.

Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: AGILE collaboration

Welcome to this episode about the **Einstein Telescope (ET)**, a planned **third-generation gravitational-wave observatory** [see source].

* **ET will revolutionize gravitational-wave astronomy** with **higher sensitivity** and a **broader frequency range** compared to current detectors [see source].

* This allows deeper insights into **Fundamental Physics** (tests of General Relativity, search for dark matter), **Cosmology** (more precise Hubble constant measurement, early Universe studies), and the **Astrophysics of Compact Objects** (black holes, neutron stars, their formation and evolution) [see source].

* A key focus is exploring the **physics of extreme matter** in neutron stars by observing mergers [see source].

* **Multi-messenger astronomy** will be significantly advanced through improved event localization in combination with electromagnetic and neutrino telescopes [see source].

* **Data analysis** of the expected large data volumes and **overlapping signals** presents a significant challenge, for which new methods are being developed [see source].

**Reference:**

* arXiv:2503.12263

Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: Marco Kraan, Nikhef

* **Introduction:** Astronomers have discovered a new celestial object, PSR J0311+1402, a radio pulsar with an unusual spin period of **41 seconds**. This discovery bridges the gap between normal pulsars (millisecond to seconds) and long-period radio transients (LPTs) (minutes to hours).

* **The Discovery:** PSR J0311+1402 was first detected by the **Australian Square Kilometre Array Pathfinder (ASKAP)** during commissioning tests of the CRACO system in January 2024. It exhibited pulses with a duration of about 0.5 seconds.

* **Intermediate Nature:** Unlike normal pulsars and LPTs, PSR J0311+1402's **41-second spin period** falls in a previously under-explored range. Traditional pulsar searches were less sensitive to these periods, and image-based LPT searches missed shorter pulses.

* **Pulsar-like Properties:** Despite its long period, PSR J0311+1402 shows characteristics similar to normal pulsars, including **low linear (∼25%) and circular (∼5%) polarisation** and a **steep spectral index (∼ −2.3)**. It also has a double or potentially triple-peaked pulse profile.

* **Below the Death Line:** Intriguingly, its spin-down properties place PSR J0311+1402 **below the pulsar death line**, a theoretical boundary where radio emission is expected to cease due to insufficient particle production. This challenges current understanding of pulsar emission mechanisms.

* **Relation to Long-Period Transients (LPTs):** Known LPTs have much longer periods and often exhibit radio luminosities too high to be powered by rotation alone, along with high polarisation. PSR J0311+1402's properties, such as its luminosity being potentially powered by rotation and its low polarisation, suggest it is more likely a pulsar. Its duty cycle also aligns better with the trend observed in typical pulsars.

* **Implications:** The discovery suggests the existence of a **previously undetected population of neutron stars with intermediate spin periods**. Finding more such objects will help bridge the gap between pulsars and LPTs and improve our understanding of neutron star evolution.

* **Future Research:** Ongoing observations and timing studies are crucial to refine PSR J0311+1402's spin-down properties and shed light on its emission mechanism and evolutionary state. The ASKAP CRACO system is expected to discover more such intermediate period objects.

**Reference:**

* Wang, Y., Uttarkar, P. A., Shannon, R. M., et al. (2025). The discovery of a 41-second radio pulsar PSR J0311+1402 with ASKAP. *arXiv preprint arXiv:2503.07936*.

Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: Alex Cherney

**Introduction**

* The podcast discusses the search for **diffuse photons** with energies above tens of PeV, using data from the **Pierre Auger Observatory**.

* These photons are produced by interactions between cosmic rays and interstellar matter or background radiation.

* The measurement of a diffuse photon flux can help us understand the distribution of cosmic rays in the Galaxy and probe models of super-heavy dark matter.

**The Pierre Auger Observatory**

* The observatory uses a surface detector (SD) and an underground muon detector (UMD).

* The SD array consists of **water-Cherenkov detectors (WCDs)**, and the UMD uses **buried scintillators**.

* The study focuses on data from a 2 km² area with 19 WCDs and 11 UMD stations.

* The combination of SD and UMD measurements allows for a more accurate analysis of air showers.

**The Search for Photons**

* Primary photons are difficult to distinguish from the background of charged cosmic rays.

* Photon-initiated air showers are mostly electromagnetic, while hadron-initiated showers have more muons.

* The analysis uses a **muon content estimator (Mb)** to discriminate between photon and hadron events.

* The study uses **15 months of data** collected during the construction of the array.

* A photon-equivalent energy scale is developed for comparing events initiated by different primary species.

**Results and Implications**

* No photon candidate events were identified in the data.

* Upper limits on the integral photon flux were set between 13.3 and 13.8 km−2 sr−1 yr−1 above tens of PeV.

* These limits are the only ones based on measurements from the **Southern Hemisphere** in this energy domain.

* The analysis extends the Pierre Auger Observatory photon search program to lower energies.

* The results provide constraints on models of super-heavy dark matter.

* Future data from the observatory is expected to improve the upper limit by a factor of ~20.

**Article Reference:**

* A. Abdul Halim et al., Search for a diffuse flux of photons with energies above tens of PeV at the Pierre Auger Observatory, 2024,

Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: Pierre Auger Observatory

**Introduction:**

* This episode discusses the search for the sources of high-energy neutrinos using the example of the blazar B3 2247+381.

* The IceCube Neutrino Observatory detects astrophysical neutrinos, and scientists are working to find their origins by looking for correlations between neutrino alerts and electromagnetic radiation from objects like blazars.

**The IceCube Alert and B3 2247+381:**

* IceCube detected a multiplet of muon neutrino events, which appeared to be coming from the direction of the blazar B3 2247+381 between May and November 2022.

* This triggered a multiwavelength observational campaign, including observations by the VERITAS telescope.

* The Gamma-ray Follow-Up (GFU) program is a method used by IceCube to enable follow up investigations of known gamma-ray sources for which IceCube has detected a cluster of candidate neutrino events.

**VERITAS and Multiwavelength Observations:**

* VERITAS did not detect B3 2247+381 during the time period of the neutrino alert.

* The source was in a low-flux state in the optical, ultraviolet, and gamma-ray bands during the neutrino event.

* B3 2247+381 was detected in the hard X-ray band with NuSTAR during this time.

* Data from Swift-XRT, Swift-UVOT, ASAS-SN, ATLAS, and the 48” optical telescope at the FLWO were also used in this study.

* The multiwavelength spectral energy distribution (SED) was modeled using a one-zone leptonic synchrotron self-Compton (SSC) radiation model.

**Analysis and Findings:**

* The observed neutrino excess had a significance of 3.2σ but was likely not fully corrected for trials. The corresponding false alert rate was 0.0355 per year.

* The neutrino events associated with B3 2247+381 had energies primarily between 0.5 TeV and 6 TeV, making them likely to be atmospheric neutrino background.

* The lack of detection by VERITAS, combined with the low multiwavelength flux levels during the neutrino alert period, suggests that B3 2247+381 is an unlikely source of the IceCube multiplet.

* The neutrino excess is likely a background fluctuation.

* The study highlights some of the challenges in searching for neutrino-emitting blazars, such as the limited localization precision of the IceCube Observatory and the effect of weather on IACT observations.

* The one-zone leptonic model reasonably fits the SED, suggesting that no hadronic component is needed to explain the data.

**Conclusion:**

* This study is an example of a follow-up to an IceCube alert within the framework of the GFU program.

* Further multiwavelength observations, especially during flaring periods, and improved understanding of instrument uncertainties, are needed to identify neutrino sources.

* Future neutrino detectors are expected to improve sensitivity to high-energy neutrino events.

**Reference:**

* Acharyya, A., et al. "VERITAS and multiwavelength observations of the Blazar B3 2247+381 in response to an IceCube neutrino alert." *Draft version February 7, 2025*

Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: CfA/Rick Peterson

**Introduction:**

* A recent ultra-high-energy neutrino event, named KM3-230213A, was detected by the KM3NeT/ARCA detector.

* This event has sparked interest in the scientific community, as its origin is still unclear.

* The neutrino's high energy suggests it may have come from a very powerful cosmic source.

* The event was detected on February 13, 2023.

* The podcast explores two potential origins for this neutrino event: galactic sources and cosmogenic neutrinos.

**Galactic Origin:**

* The study investigates potential galactic sources such as supernova remnants (SNRs), X-ray binaries, and microquasars.

* **No nearby sources from HAWC or LHAASO were found, imposing stringent constraints on potential astrophysical sources**.

* The study also looks at known gamma-ray sources from catalogs such as 4FGL-DR4, 3HWC, and 1LHAASO.

* Researchers explored the possibility of the neutrino originating from blazars, which are active galactic nuclei (AGN) with jets pointed towards Earth.

* **Seventeen blazar candidates were identified within the 99% confidence region of the neutrino event**.

* The study examined multiwavelength data, including radio, X-ray, and gamma-ray observations, to characterize these blazars.

* **A major radio flare from blazar PMN J0606-0724 was found to be coincident with the neutrino event, with a time difference of five days**, which is considered statistically uncommon.

* The chance probability of this coincidence is estimated to be 0.26%, which suggests a possible association, but is not conclusive.

* Other blazars, such as MRC0614-083, also showed flaring activity in the X-ray band around the time of the neutrino detection.

* **It is not possible to conclusively associate the neutrino with a specific blazar due to the size of the neutrino direction uncertainty region, encompassing seventeen blazar candidates**.

**Cosmogenic Origin:**

* The study explores the possibility that the neutrino is cosmogenic, produced by the interaction of ultra-high-energy cosmic rays (UHECRs) with the cosmic microwave background (CMB) or the extragalactic background light (EBL).

* Cosmogenic neutrinos are expected from the interactions of cosmic rays with photons.

* The paper examines how the expected cosmogenic neutrino flux can be enhanced, starting from a minimal scenario.

* The study considers the effects of different models for the EBL and the photo-disintegration cross section, and concludes that these uncertainties do not significantly impact the results.

* **The study compares the spectra of neutrinos produced in the nearby and far-away Universe**.

**Conclusion:**

* The origin of KM3-230213A remains an open question.

* While a specific source cannot be pinpointed, the study provides valuable insights into potential galactic and cosmogenic origins of such high-energy neutrino events.

* Further studies and observations are needed to determine the precise origin of this neutrino.

**Reference:**

* The information presented is based on the following three articles:

* "On the Potential Galactic Origin of the Ultra-High-Energy Event KM3-230213A"

* "Characterising Candidate Blazar Counterparts of the Ultra-High-Energy Event KM3-230213A"

* "On the potential cosmogenic origin of the ultra-high-energy event KM3-230213A"

Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: KM3NeT

**Introduction**

* A recent detection by the KM3NeT/ARCA telescope of an ultra-high-energy neutrino, named KM3-230213A, is discussed. This event has an estimated energy in the hundreds of PeV, surpassing previous observations by the IceCube Neutrino Observatory.

* The observed neutrino's high energy suggests an astrophysical origin, as it's unlikely to be from atmospheric sources.

**Key Concepts**

* The study explores the compatibility of the KM3NeT event with previous data from IceCube and the Pierre Auger Observatory.

* The analysis assumes the neutrino originates from an isotropic diffuse flux, exploring scenarios such as steady sources, transient sources, cosmogenic origins, or physics beyond the Standard Model.

* The research uses both single power law (SPL) and broken power law (BPL) models to fit the neutrino flux. A single power law assumes the flux follows a consistent pattern, while a broken power law allows for a change in the pattern at a certain energy level.

**Findings**

* **Initial analysis of the KM3NeT event suggests a per-flavor isotropic diffuse flux of E2Φ1f ν+ν̄(E) = 5.8+10.1 −3.7 × 10−8 GeV cm−2 s−1 sr−1, assuming an E−2 spectrum**.

* Combining the KM3NeT observation with non-observations from IceCube (IC-EHE) and Auger, the best-fit flux normalisation becomes E2Φ1f ν+ν̄ = 7.5 × 10−10 GeVcm−2s−1sr−1.

* The joint fit of all experiments under the assumption of an isotropic E−2 flux shows a preference for a break in the PeV regime when the IceCube "High-Energy Starting Events" (HESE) data is included, with a tension of 2.5σ − 3σ.

* The analysis explores if the KM3NeT event is an outlier compared to the IceCube and Auger data.

* **When considering only KM3NeT and IceCube HE measurements, the data shows a significant preference for a broken power law model, which suggests a break at a certain energy**.

* However, this model would be inconsistent with null observations from IceCube and Pierre Auger.

* The study notes that more statistics are required to resolve the tension and better characterise the neutrino landscape at ultra-high energies.

**Implications and Future Research**

* The study highlights the importance of combining data from different experiments.

* Future observations with larger detectors and increased exposures from various observatories are crucial to determine the shape of the neutrino spectrum and to differentiate between different models of neutrino production. This includes the KM3NeT/ARCA detector configuration, IceCube, Auger, and upcoming radio instruments.

**Reference:**

* The KM3NeT Collaboration, "The ultra-high-energy event KM3-230213A within the global neutrino landscape," (Dated: February 2025).

Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: KM3NeT

* **Introduction**: A recent groundbreaking discovery by the KM3NeT Collaboration has detected an exceptionally high-energy cosmic neutrino. This event, named KM3-230213A, is significant because its energy far exceeds any neutrino previously observed.

* **What are Cosmic Neutrinos?**: Cosmic neutrinos are electrically neutral particles that travel vast distances without being deflected by magnetic fields or significantly absorbed by matter. They are produced when cosmic rays interact with matter or photons, making their detection a key to understanding high-energy astrophysical processes.

* **The KM3NeT Experiment**: The KM3NeT is a deep-sea neutrino telescope located in the Mediterranean Sea. It consists of two detector arrays: ARCA, optimized for high-energy cosmic neutrinos, and ORCA, for neutrino oscillations. These detectors utilize optical sensors to detect Cherenkov light produced by charged particles resulting from neutrino interactions.

* **The Ultra-High-Energy Event**: The detected event, KM3-230213A, is a muon with an estimated energy of **120 PeV**. The neutrino that produced this muon is estimated to have had an even higher energy. The muon was detected traversing the ARCA detector on February 13, 2023.

* **How it was Detected**: The muon's trajectory was reconstructed using the arrival times and positions of the first hits recorded on the photomultiplier tubes (PMTs). The energy was estimated by counting the number of PMTs that triggered. The large amount of light detected saturated the PMTs closest to the muon trajectory, and large showers resulting from energy loss processes were observed along the track.

* **Significance**: This event may indicate a different source of cosmic neutrinos or could be the first detection of a cosmogenic neutrino, produced by interactions of ultra-high-energy cosmic rays with background photons. The detected energy significantly exceeds previous detections, suggesting new astrophysical phenomena.

* **Background and Analysis**: The possibility of the event being caused by atmospheric muons or neutrinos was considered. The probability of an atmospheric origin is extremely low, especially given the near-horizontal direction and high energy. The direction of the neutrino matches expectations for an isotropic flux of ultra-high-energy neutrinos, where downgoing neutrinos are obscured by atmospheric muons, and upgoing neutrinos are absorbed by the Earth.

* **Searches for Source**: Extensive searches were conducted for a source counterpart within a 3° radius of the event using multiwavelength data. Various catalogs of gamma-ray, X-ray, infrared, and radio sources were examined, but no conclusive source association has been made.

* **Flux Measurement:** The steady isotropic flux that would produce one event like KM3-230213A is **5.8 x 10^-8 GeV cm^-2 s^-1 sr^-1**. This flux measurement exceeds current limits from IceCube and Auger, possibly indicating an upward fluctuation or a new component in the flux. This event could be from cosmogenic neutrino production or from transient emitters such as gamma-ray bursts or tidal-disruption events.

* **Conclusion**: The detection of KM3-230213A provides significant evidence for the existence of ultra-high-energy neutrinos and enhances our understanding of the universe's most energetic phenomena.

* **Reference**: The KM3NeT Collaboration. "Observation of an ultra-high-energy cosmic neutrino with KM3NeT." *Nature* (2025).

Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: KM3NeT

The Mystery of ANITA: Investigating Anomalous Radio Pulses with the Pierre Auger Observatory

* **Introduction**: The Antarctic Impulsive Transient Antenna (ANITA) has detected some unusual radio pulses that don't fit with the standard model of particle physics. These "anomalous" pulses, which appear to come from below the horizon, could potentially be caused by air showers developing in an upward direction. This podcast discusses a search using the Pierre Auger Observatory to either confirm or constrain the possibility of upward-going air showers.

* **The ANITA Anomalies**: ANITA, which flies on NASA balloons, has detected radio pulses consistent with ultra-high-energy cosmic ray air showers. Most of these pulses are reflected from the ice, but some anomalous ones have been observed with strong horizontal polarization but without the expected polarity inversion. These could be caused by upward-going air showers, possibly from tau lepton decays, but this interpretation faces significant challenges.

* **The Pierre Auger Observatory**: The Pierre Auger Observatory, a large cosmic ray detector, was used to search for these upward-going air showers. It combines a Surface Detector (SD) and a Fluorescence Detector (FD) which uses telescopes to collect the fluorescence light emitted by nitrogen as a shower develops. The search focused on data from the FD, as upward-going air showers are unlikely to trigger the SD.

* **The Search**: A dedicated search was conducted for upward-going air showers with zenith angles greater than 110 degrees and energies above 0.1 EeV. The search analyzed data collected between 2004 and 2018, using simulations of both regular cosmic ray showers and upward-going events to distinguish potential candidates from background. The analysis also employed a Global Fit (GF) reconstruction to eliminate misidentified events.

* **Background and Challenges**: A key challenge was distinguishing genuine upward-going showers from mis-reconstructed cosmic ray showers and other sources of background such as laser pulses. Several selection cuts were implemented to filter out background, including cuts based on time sequence of triggered pixels, the shower profile, and zenith angle. A discrimination variable, *l*, was defined to differentiate between upward and downward reconstructions based on the likelihood ratio.

* **Results**: After analyzing the data, only one event was found that passed all selection criteria. This was consistent with an expected background of 0.27 ± 0.12 events from mis-reconstructed cosmic ray showers. This result was used to calculate an upper bound on the integral flux of upward-going showers.

* **Implications**: The non-observation of upward-going air showers by the Pierre Auger Observatory puts constraints on the interpretation of the anomalous ANITA events as being produced by upward-going showers. The study found that the sensitivity of the Auger Observatory exceeds that of the ANITA-III flight, making the results particularly significant. The results suggest that if the ANITA events are caused by upward-going air showers, then those showers likely originated at very high altitudes, or have unusual shower profiles inconsistent with known particle decays or interactions.

* **Conclusion**: The search for upward-going air showers at the Pierre Auger Observatory did not find evidence to support the interpretation of the anomalous ANITA events as being caused by upward-going air showers. This implies either the ANITA events are caused by something else, or there is a need for new theoretical models beyond the Standard Model of particle physics to explain the data.

* **Reference**: A. Abdul Halim et al. (Pierre Auger Collaboration), "A search for the anomalous events detected by ANITA using the Pierre Auger Observatory", 2502.04513v1.pdf.

Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: Pierre Auger Observatory