Multi-messenger astrophysics

Join us as we explore the groundbreaking observations of **GRB231117A**, a short gamma-ray burst (SGRB) located at a redshift of z = 0.257. This event, detected by the Neil Gehrels Swift Observatory, was quickly followed up by the Australia Telescope Compact Array (ATCA) just 1.3 hours post-burst, providing **unprecedented early radio detection**.**In this episode, we discuss:*** **Early Radio Afterglow:** How ATCA's rapid response revealed a dynamic early radio afterglow with **flaring, scintillating, and plateau phases**.* **Cosmic Scintillation:** The fascinating phenomenon of interstellar scintillation, which allowed scientists to place the **earliest upper limit on a GRB blast wave size to date**, constraining it to less than 1 × 10^16 cm within 10 hours of the burst.* **Energy Injection Unveiled:** Multi-wavelength modeling of GRB231117A's afterglow revealed a period of **significant energy injection** occurring between approximately 0.02 and 1 day post-burst.* **The Violent Collision Hypothesis:** This energy injection is best explained by a **violent collision of two relativistic shells**. We delve into how a **reverse shock** propagating through the injection shell accounts for the early radio plateau, while an observed **X-ray flare** is consistent with a shock passing through the leading impulsive shell.* **Late-Time Evolution:** Beyond the initial energy injection, the blast wave transitioned to a **classic decelerating forward shock**, exhibiting an electron distribution index of p = 1.66 ± 0.01 and a jet-break around 2 days. The final collimation-corrected energy was calculated to be approximately 5.7×10^49 erg, about **18 times the initial impulsive energy**.* **Probing Central Engines:** This study highlights the **critical importance of rapid and sensitive radio follow-up** for exploring the complex behavior of GRB central engines and their relativistic outflows.This deep dive into GRB231117A offers direct insight into the powerful mechanisms behind these cosmic explosions and paves the way for future discoveries with next-generation radio telescopes.**Article Reference:**Anderson, G. E., Lamb, G. P., Gompertz, B. P., et al. (2025). The radio flare and multi-wavelength afterglow of the short GRB231117A: energy injection from a violent shell collision. *Draft version August 21, 2025*, arXiv:2508.14650v1.Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: Nancy Atkinson

Join us as we explore the latest research into Fast Radio Bursts (FRBs), mysterious, intense pulses of radio emission lasting only milliseconds. These cosmic phenomena are not just fleeting signals; they are powerful probes of the ionized gas across the universe and valuable tools for cosmological studies. In this episode, we delve into an investigation of FRB properties and their host galaxies, aiming to understand how the environment surrounding an FRB influences its observed characteristics.What we discuss:• The Phenomenon of Scattering: Learn how FRBs' paths through ionized media cause "scattering," a frequency-dependent broadening of their pulse profiles. This scattering is thought to primarily originate within their host galaxies.• Key Correlations Found with FRB Scattering: ◦ Compactness and Stellar Surface Density: The study found a highly significant positive correlation between an FRB's scattering timescale (τ) and the stellar surface density (or compactness) of its host galaxy. This suggests that more compact (denser) host galaxies may contain more ionized content, leading to greater scattering of the FRB signal. ◦ Mass-Weighted Age: A highly significant positive correlation was also found between scattering timescale and the mass-weighted age of stars in the host galaxy. This implies that older stellar populations might contribute to increased scattering, though it's not driven by the overall galaxy mass. ◦ Gas-Phase Metallicity: There's a weakly significant positive correlation between scattering timescale and the gas-phase metallicity. Higher metallicity gas could mean more ionizing photons and electrons within the galaxy, contributing to scattering. This might be connected to compactness and mass-weighted age, as these properties can also correlate with metallicity.• Surprising Absences of Correlation for Scattering: ◦ The study found no correlation between FRB scattering and host galaxy stellar mass or star formation rate. ◦ Crucially, there was no correlation found with the galaxy's inclination angle or optical disc axis ratio (b/a) for scattering. This finding challenges previous suggestions of an inclination bias in FRB detection.• Rotation Measure and Galaxy Orientation: ◦ A strong anti-correlation was identified between the absolute Faraday rotation measure (|RMex|) of an FRB and the optical disc axis ratio (b/a) of its host galaxy. This means that FRBs from more edge-on galaxies tend to exhibit greater rotation measures, likely because the signal travels through a larger amount of the galaxy's magnetic field. ◦ The absence of other strong correlations for RM suggests the immediate environment of the FRB progenitor might play a significant role in determining RM, but the host galaxy's orientation is still important.• Polarization Insights: ◦ While some weak correlations were seen for circular polarization fractions, these were often driven by single outlier datapoints and are not considered broadly significant across the sample. No strongly significant correlations were found for linear polarization.• The Modest Sample Size: The researchers emphasize that while several correlations are statistically robust, the sample size is still relatively modest. Further high-time resolution FRB detections and detailed host galaxy follow-up are essential to confirm these initial findings.Source Article:• Glowacki, M., Bera, A., James, C. W., et al. (2020). An investigation into correlations between FRB and host galaxy properties. Cambridge Large Two, 1–21.Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: ICRAR

Join us as we explore the groundbreaking discovery of FRB 20240304B, the most distant Fast Radio Burst (FRB) ever detected, offering unprecedented insights into the early universe.In this episode, we discuss:• What are Fast Radio Bursts (FRBs)? These enigmatic, millisecond-duration radio signals provide unique information about the plasma permeating our universe, revealing details about magnetic fields and gas distributions.• The Record-Breaking Discovery: FRB 20240304B was detected by the MeerKAT radio telescope and precisely localized to a host galaxy using the James Webb Space Telescope (JWST).• A Journey Back in Time: This FRB originates at a redshift of 2.148 ± 0.001, meaning it occurred just 3 billion years after the Big Bang. This discovery doubles the redshift reach of localized FRBs and marks the first FRB detected at "cosmic noon," a peak era of galaxy formation.• The Host Galaxy's Secrets: FRB 20240304B was traced to a low-mass, clumpy, star-forming galaxy with low metallicity, estimated to be very young with a stellar formation timescale of around 51.7 million years. This makes it atypical compared to previously observed FRB host galaxies.• Unveiling the Progenitor: The host galaxy's properties – its low stellar mass, active star formation, and low metallicity – strongly favor short-delay time progenitor models, such as those involving young magnetars born in supernovae. This supports the idea that FRB birth rates could trace the cosmic star-formation history.• Probing the Cosmic Web: The sightline of FRB 20240304B intersects cosmic structures like the Virgo Cluster and a foreground galaxy group, revealing complex magnetic field environments over vast scales. These structures contribute significantly to the FRB's dispersion measure (DM).• A Critical Milestone: This detection highlights the power of FRBs as cosmological probes, allowing astronomers to trace the distribution of ionized matter and gain insights into galaxy evolution during the universe's most active era. MeerKAT's unique sensitivity was crucial, demonstrating its capability to explore the z > 2 universe.Reference: Caleb, M., Nanayakkara, T., Stappers, B., et al. (2024). A fast radio burst from the first 3 billion years of the Universe. Excerpts from "2508.01648v1_FRB20240304B.pdf".Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: Danielle Futselaar for MeerTRAP

In this episode, we dive into the exciting future of **multi-messenger astronomy**, specifically focusing on the detection and characterization of binary neutron star (BNS) mergers.* **The Dawn of Multi-Messenger Astrophysics:** Our understanding of cosmic events was revolutionized by the extraordinary joint detection of gravitational waves (GWs) and light from a BNS merger on August 17, 2017 (GW170817). This single event confirmed theoretical hypotheses about short gamma-ray bursts (SGRBs) originating from BNS mergers and provided insights into kilonovae (KNe) – the thermal radiation powered by radioactive decay of heavy elements.* **Next-Generation Observatories:** The upcoming third-generation GW observatories, such as the **Einstein Telescope (ET)** and **Cosmic Explorer (CE)**, are poised to dramatically increase detection rates, potentially observing hundreds of thousands of BNS mergers annually, reaching distances beyond redshift (z) ~ 3.* **The Wide-field Spectroscopic Telescope (WST):** This proposed 12-meter-class spectroscopic facility, expected to operate in the 2040s in the southern hemisphere, will be crucial for exploiting the unique information from joint GW and electromagnetic (EM) detections. WST will employ both **Integral Field Spectroscopy (IFS)** and **Multi-Object Spectroscopy (MOS)**, enabling simultaneous acquisition of multiple spectra over wide fields of view.* **Detecting Faint Counterparts:** * WST is designed to detect **Kilonovae (KNe)** up to **z ~ 0.4** and apparent magnitudes as faint as **mAB ~ 25 (with fibres) to ~25.5 (with IFS)**. The optimal time for KN detection observations is **12–24 hours after the merger**. * **GRB afterglows** can be observed at even higher redshifts, beyond z = 1, particularly for on-axis or slightly off-axis systems (viewing angles Θview ≲ 15°). Timely follow-up, within a few hours of the GRB prompt detection, is critical due to their rapid decay.* **Observing Strategies and Challenges:** * The vast majority of next-generation GW events will have **large sky localization regions** and **faint EM counterparts**, making their identification challenging. * **Galaxy-targeted searches** with WST involve identifying galaxies within the 3D error volume of the GW signal, leveraging high multiplexing capabilities. These searches benefit greatly from complete galaxy catalogues with redshift information up to z ≤ 0.5. * **Synergy with photometric surveys**, like the Vera Rubin Observatory, allows WST to target transient candidates identified by these wide-field facilities. * **The "Golden Events"**: BNS mergers detected by ET+CE at z < 0.3 (or ET-alone at z < 0.2) with sky localizations better than 10 deg² are ideal for WST, as it can cover all galaxies in the error volume with limited exposures (e.g., 3 one-hour exposures for 10 deg² or 1 one-hour exposure for 1 deg²).* **Addressing Offsets and Host Galaxies:** Many EM counterparts are not expected to be at the exact center of their host galaxies. The use of **mini-IFUs or "fibre bundles"** is proposed as an extremely valuable solution to cover regions around host galaxy centers and detect counterparts with larger offsets. Spectral subtraction techniques can also be used to separate the transient's spectrum from the host galaxy's.* **The Future is Multi-Messenger:** This research underscores the need for **new instruments** that are developed with multi-messenger science as a core design case, enabling rapid data reduction and analysis for timely alerts to the astronomical community.**Reference:**Bisero, S., Vergani, S. D., Loffredo, E., et al. (2025). Multi-messenger observations of binary neutron star mergers: synergies between the next generation gravitational wave interferometers and wide-field, high-multiplex spectroscopic facilities. *Astronomy & Astrophysics*.Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: ESO

Dive into the fascinating world of cosmic rays with the latest research from the IceCube Neutrino Observatory! This episode explores new measurements of high-energy muons in extensive air showers, shedding light on the mysterious mass composition of cosmic rays and the challenges of simulating their interactions in Earth's atmosphere.**What we discuss:*** **Measuring High-Energy Muons:** Learn about the first measurement of the **mean number of muons with energies above 500 GeV** in near-vertical extensive air showers. These "TeV muons" are crucial because they originate predominantly in the early stages of shower development and their number depends strongly on the energy and mass of the primary cosmic ray.* **The IceCube Neutrino Observatory's Unique Capabilities:** Discover how this research uses a **coincident detection method** combining IceTop, the surface detector array, with the large-volume in-ice detector located 1.5 km to 2.5 km below the surface. The thick ice absorbs lower-energy muons, allowing for a pure measurement of high-energy muons.* **Cosmic Ray Energies and Hadronic Models:** The study focuses on cosmic rays with primary energies between **2.5 PeV and 100 PeV**. The results were analyzed using various hadronic interaction models, including **Sibyll 2.1, QGSJet-II.04, and EPOS-LHC**, which are vital for simulating air shower development.* **The "Muon Puzzle" and Inconsistencies:** We'll explore the intriguing **"Muon Puzzle,"** a known discrepancy between measurements and simulations of muons in air showers. This new study compares high-energy (TeV) muon measurements with previous low-energy (GeV) muon measurements from IceTop alone. While Sibyll 2.1 shows excellent agreement between the two measurements, **the EPOS-LHC model reveals a significant tension**, indicating it doesn't consistently describe both low and high-energy muon data.* **Implications for Particle Physics and Astronomy:** Understanding these hadronic interactions is crucial for inferring cosmic-ray mass composition, calculating atmospheric neutrino flux, and exploring particle physics beyond human-made accelerators. Solving the Muon Puzzle is considered one of the most pressing problems in high-energy cosmic-ray physics.**Article Reference:**"Measurement of the mean number of muons with energies above 500 GeV in air showers detected with the IceCube Neutrino Observatory" by R. Abbasi et al. (IceCube Collaboration). Dated June 25, 2025.Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: D. Soldin

Join us as we dive into the latest astronomical discovery! Scientists have identified a **new candidate pulsar wind nebula (PWN)**, named XMMU 034124.2+525720, which may be directly linked to **1LHAASO J0343+5254u**, a powerful "PeVatron" in our galaxy.**What are PeVatrons?** They are the most energetic astrophysical objects in our galaxy, producing cosmic rays (CRs) with energies exceeding 1 PeV (10^15 eV), far surpassing what terrestrial accelerators can achieve. Understanding them is key to solving the mystery of the most energetic galactic cosmic rays and gamma rays.This potential PWN, discovered through extensive **XMM-Newton observations**, exhibits key characteristics typical of other very high-energy PWNs like the "Eel" and "Boomerang" nebulae. Its X-ray emission shows an **extended, asymmetric morphology** and a **power-law spectrum (ΓX = 1.9)** that becomes notably softer farther from its center.Using **multiwavelength modeling**, researchers demonstrated that a **fully leptonic model**—involving electron synchrotron radiation and inverse-Compton (IC) scattering of ambient photons—can explain the observed X-ray and gamma-ray emission, especially if there are **elevated infrared (IR) photon fields** in the region. While this model largely accounts for the LHAASO gamma-ray flux, future observations will help explore if hadronic processes in nearby molecular clouds also contribute to the gamma-ray emission and potential neutrino flux.Though XMM-Newton observations didn't definitively resolve a central pulsar or detect X-ray pulsations, this discovery marks a crucial step in understanding galactic PeVatrons. Future, higher-resolution X-ray observations with missions like Chandra and NuSTAR, along with dedicated radio searches for a pulsar, are planned to solidify this PWN classification and provide deeper insights into these extreme cosmic accelerators.**Article Reference:**DiKerby, S., Zhang, S., Ergin, T., et al. 2025, *Discovery of a Pulsar Wind Nebula Candidate Associated with the Galactic PeVatron 1LHAASOJ0343+5254u*, The Astrophysical Journal, 983:21.Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: Stephen DiKerby et al., 2025 ApJ 983 21

In this episode, we dive into a fascinating new study that performs the **first direct consistency check** between two crucial measurements from the Large High Altitude Air Shower Observatory (LHAASO): the **cosmic-ray (CR) proton spectrum at the "knee"** and the **Galactic diffuse gamma-ray emission**.The "knee" in the cosmic ray spectrum (around a few PeV) is thought to mark the maximum energy reached by Galactic CR accelerators. Diffuse gamma-ray emission, primarily from CR interactions with interstellar gas, provides a complementary view of the same underlying particle population.The study reveals a **persistent mismatch**:* The **predicted gamma-ray flux robustly overshoots the LHAASO data** in both inner and lateral Galactic regions.* This discrepancy is evident in **both normalization and spectral shape**.* This is particularly puzzling because while an excess of gamma-rays has been discussed before, **evidence of a deficit in observed emission represents a new and more puzzling feature**.Key insights from the research:* The disagreement **challenges conventional scenarios** linking the local cosmic-ray sea to Galactic gamma-ray emission.* It **calls for a revision of current cosmic ray models** in the TeV-PeV sky.* The mismatch is **not attributed to the hadronic interaction model** used for calculations; using alternative models would actually increase the tension.* The findings suggest a **possible tension between the LHAASO gamma-ray observations and the CR proton flux measured by LHAASO itself**.* One intriguing explanation is that the **CR spectrum measured locally might not be the same as the one responsible for the observed gamma-ray emission** throughout the Galaxy, possibly having a different "knee" location (e.g., around 300 TeV).* Uncertainties also exist due to the **lack of helium flux measurements** between 100 TeV and a few PeV.This research highlights the critical importance of evaluating the consistency between these two types of measurements and opens new avenues for understanding cosmic ray propagation in our Galaxy.**Article Reference:**Espinosa Castro, L. E., Villante, F. L., Vecchiotti, V., Evoli, C., & Pagliaroli, G. (2025). *LHAASO Protons versus LHAASO Diffuse Gamma Rays: A Consistency Check*. arXiv preprint arXiv:2506.06593.Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: LHAASO

In this episode, we dive into the mysterious world of Fast Radio Bursts (FRBs) and the ongoing quest to understand their origins. We discuss a systematic search for **past supernovae (SNe) and other historical optical transients** at the positions of FRB sources, exploring a leading theory that links FRBs to **magnetars**.The study **found no statistically significant associations** within the 5σ FRB localization uncertainties between the observed CHIME-KKO or literature FRBs and optical transients, *except* for a previously identified potential optical counterpart to FRB 20180916B, named AT 2020hur. AT 2020hur, however, occurred *after* the FRB was first detected, making it inconsistent with the "past SN" progenitor model, though it remains a potential association under other theories.**Chance Coincidences:** The probability of a chance coincidence (Pcc) between an FRB and a transient was found to be **low (Pcc < 0.1)**. It's estimated that it would take **~22,700 subarcsecond-localized FRBs** to yield one chance association, which translates to roughly **30–60 years** at the projected CHIME/FRB Outrigger detection rate. This means that any robust match found in the near future is highly likely to be a **physical association**.**Implications of Transparency Time:** The research estimates that **5–7% of matched optical transients** (if all were SNe) are old enough to be associated with a detectable FRB, assuming the 6.4-10 year transparency timescale. More broadly, **23–30% of all cataloged SNe and 32–41% of CCSNe** are currently old enough to have detectable FRB emission.**The Future with Rubin Observatory:** The upcoming **Vera C. Rubin Observatory (LSST)** is expected to dramatically increase the number of known SNe and the volume over which they can be detected. This will significantly **increase the rate of potential FRB-SN associations** at redshifts below z~1, where most FRBs are discovered.**Flexible Framework:** The systematic search machinery developed for this work is publicly available and flexible, allowing it to be applied to a wide range of transient timescales, FRB localization sizes, and different optical transient populations in future searches.**Reference Article:*** DONG, Y., KILPATRICK, C. D., FONG, W., et al. (2025). **Searching for Historical Extragalactic Optical Transients Associated with Fast Radio Bursts**. arXiv e-prints, arXiv:2506.06420v1.Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: NASA - JPL/Caltech

In this episode, we discuss a significant new detection of the Geminga pulsar, a middle-aged, radio-quiet gamma-ray pulsar. The **Large-Sized Telescope (LST-1)**, the first of the Cherenkov Telescope Array Observatory (CTAO) Northern Array, has detected Geminga at energies down to 20 GeV.Key takeaways from the study:* The LST-1 detected the Geminga pulsar using 60 hours of data.* The **second emission peak (P2)** of Geminga was detected with a high significance of **12.2σ** in the energy range between 20 and 65 GeV. This is a doubled significance compared to previous results by the MAGIC Collaboration, achieved with less observation time and a single telescope.* The first peak (P1) was detected at a lower significance level of 2.6σ.* The LST-1 analysis has an estimated energy threshold as low as 10 GeV for pulsar analysis, although the peak in reconstructed energy was around 20 GeV.* The best-fit model for the P2 spectrum was a power law with a spectral index of Γ = 4.5 ± 0.4 (statistical uncertainty). When considering systematic uncertainties, the spectral index is Γ = (4.5 ± 0.4stat)+0.2sys −0.6sys. This is compatible with previous results from the MAGIC Collaboration.* A joint fit of LST-1 and Fermi-LAT data preferred a power law with a sub-exponential cut-off (PLSEC) over a pure exponential cut-off (PLEC), although the PLSEC model didn't fully match the LST-1 points.* While no curvature was detected in the LST-1-only spectrum, combining LST-1 and Fermi-LAT data showed a statistical preference for a curved log parabola model at lower minimum energies (10-20 GeV).* Theoretical models, such as the synchro-curvature (SC) model from Harding et al. (2021), can explain the dominance of the SC component at high energies and the non-detection of the first peak above 20 GeV, although improvements are needed to match the LST-1 SED better.* These results demonstrate the LST-1's excellent capabilities for observing pulsars at the upper end of their spectra and its overlap with the Fermi-LAT energy range. Future observations with the full CTAO Northern Array are expected to improve sensitivity and allow for more detailed studies of the pulsar peaks and spectra.**Reference:*** K. Abe et al. (CTAO LST Project). Detection of the Geminga pulsar at energies down to 20 GeV with the LST-1 of CTAO. *Astronomy & Astrophysics* manuscript no. aa54350-25 ©ESO 2025 May 29, 2025.Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: Iván Jiménez (IAC)

Astronomers have made a significant discovery, detecting X-ray emission from a rare type of cosmic object known as a **Long-Period Radio Transient (LPT)** for the very first time.The object, designated **ASKAP J1832−0911**, is extraordinarily bright in radio, reaching flux densities of 10–20 Jy.Crucially, it exhibits **coincident radio and X-ray emission**, both pulsing with a regular period of **44.2 minutes** (2,656.2412 seconds in radio, 2,634 seconds in X-rays). This combination of properties – long period, bright coherent radio, and variable X-ray emission – makes ASKAP J1832−0911 **unlike any other known object in our galaxy**. Its luminosity is **highly variable**, with both radio and X-ray emission decreasing dramatically over a few months following a 'hyper-active' phase. This variability suggests that the lack of previous X-ray detections from other LPTs might be due to not observing them during such brief bright phases. The object is estimated to be located at a distance of approximately **4.5 kpc**. Current data suggest potential classifications like an old magnetar or an ultra-magnetized white dwarf, though both interpretations present **theoretical challenges** for existing models. It is not consistent with a classical rotation-powered pulsar or a typical isolated white dwarf.The discovery of X-ray emission from ASKAP J1832−0911 demonstrates that LPTs can be **more energetic** than previously believed. It also establishes a new class of hour-scale periodic X-ray transients linked to exceptionally bright radio emission.Reference Article: "Detection of X-ray emission from a bright long-period radio transient" by Ziteng Wang et al..Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: Alex Cherney

A recent study utilized **15 years of observations** from the **Fermi Large Area Telescope (LAT)** to analyze the gamma-ray emission from the Sun in its quiet state, meaning when it's not flaring. This is the first study to separately analyze the flux variation of the two distinct components of this quiet-state gamma-ray emission over solar cycles.According to theoretical understanding, the Sun's steady-state gamma-ray emission arises from interactions with Galactic cosmic rays (CRs). There are two main components:* The hadronic component, which is primarily confined to the **solar disk**. It's thought to be produced by CR cascades in the solar atmosphere. This component's flux is expected to **anticorrelate with solar activity** (like sunspot number, SSN) and **correlate with the flux of cosmic rays**.* The **leptonic component**, which is spatially **extended** beyond the solar disk. This is theorized to be an Inverse Compton (IC) component, where CR electrons scatter off solar photons. Like the disk component, its intensity was expected to **vary with the solar cycle**, being highest during solar minimum and lowest during solar maximum, thus anticorrelating with SSN and correlating with CR flux (specifically CR electron flux).Previous Fermi-LAT observations had shown that the overall solar gamma-ray flux varies with solar activity, anticorrelating with SSN and changing by nearly a factor of two between solar maximum and minimum. However, these studies had not separated the contributions of the disk and extended components.This new work analyzed Fermi-LAT data from August 2008 to June 2023, carefully selecting data and using an "off-source" method to evaluate background contamination. They were able to distinguish the two components and study their flux variations over Solar Cycle 24 and the beginning of Cycle 25.The key findings from this analysis reveal both confirmation of expectations and **significant surprises**:* For the **disk component**, the results align well with theoretical predictions. Its flux variation: * **Anticorrelates strongly with the sunspot number (SSN)**. * **Correlates strongly with the flux of cosmic-ray protons** measured near Earth. * Correlates with the gamma-ray flux from the Moon, supporting similar production mechanisms. * The variation is **independent of energy** above 250 MeV. This confirms that the hadronic emission mechanism for the disk component has been correctly identified.* For the **spatially extended component**, the behavior was **unexpectedly complex**. * It showed the expected anticorrelation with SSN and correlation with the disk component **only until approximately mid-2012**. * **After 2013, there was no longer any significant correlation or anticorrelation observed** between the extended component's flux variation and either the SSN or the cosmic-ray electron flux. Correlation coefficients over the entire period are below 0.3. * Like the disk component, the extended component's variation was also found to be independent of energy above 250 MeV.This **surprising lack of correlation for the extended component after 2013** is a major finding. The change in behavior coincides with the start of the **reversal of the Sun's polar magnetic field**, which began at the end of 2012. This suggests that the transport and modulation of cosmic rays, particularly electrons, in the **inner heliosphere (close to the Sun)** may be **unexpectedly complex** and possibly different for electrons and protons.Paper: https://arxiv.org/abs/2505.06348Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: Solar Dynamics Observatory/GSFC/NASA

The research investigates how supernovae exploding into dense circumstellar environments, specifically those with dense shells of material, can potentially accelerate particles to energies of a few PeV, thus acting as "PeVatrons" and contributing to the "knee" feature in the cosmic ray spectrum.Supernova remnants (SNRs) have long been considered prime candidates for the sources of Galactic Cosmic Rays (CRs) up to energies of a few PeV. However, despite decades of gamma-ray astronomy, there hasn't been clear observational proof that standard SNR models can accelerate particles beyond approximately 100 TeV. Young SNRs like Tycho and Casiopeia A, initially expected to be strong accelerators, show even lower cutoff energies.The presented study explores a different scenario: supernovae that expand into **much denser circumstellar material**, including dense shells ejected by the progenitor star shortly before explosion. These dense shells are thought to be present around massive stars like Luminous Blue Variables (LBVs), which can undergo brief episodes of very high mass-loss rates (up to 1 M⊙/yr). Type IIn supernovae, associated with LBVs, make up about 5% of core-collapse supernovae.The researchers used spherically symmetric 1D simulations with their time-dependent acceleration code called **RATPaC** (Radiation Acceleration Transport Parallel Code). This code simultaneously solves the transport equations for cosmic rays, magnetic turbulence, and the hydrodynamical flow of the thermal plasma in the test-particle limit. Unlike models that assume a steady state for magnetic turbulence, RATPaC accounts for the time needed for turbulence to build up, which often leads to lower maximum energies in standard scenarios.**The key finding is that the interaction of the supernova shock front with these dense circumstellar shells can significantly boost the maximum energy** of the accelerated particles.Specifically, the simulations show that:* **Interactions with shells that occur earlier post-explosion lead to a greater increase in maximum energy (Emax)**.* If the interaction happens within the first **5 months (approximately 140 days)** after the explosion, the **Emax can increase to more than 1 PeV**.* For very early interactions, around **0.1 years**, Emax can even surpass **10 PeV**.This significant energy boost is attributed to several mechanisms during and after the shock-shell interaction:1. **Enhanced Particle Escape:** The shock slows down considerably during the interaction with the dense shell, which temporarily enhances the "precursor scale" (the region upstream where particles diffuse back towards the shock, given by D(E)/v_shock). This increased scale provides more time for turbulence to grow. Enhanced particle escape also occurs during the onset of the interaction, boosting the CR current.2. **Reacceleration in a Pre-amplified Field:** After passing through the shell, the shock propagates into a medium where the magnetic field has been pre-amplified by escaping cosmic rays during the interaction phase. The shock accelerating into this region with an enhanced field boosts Emax.3. **Interaction with Reflected Shocks:** The collision with the dense shell creates reflected shocks. These can catch up with and interact with the forward shock from behind, leading to sharp increases in the forward shock's velocity and slightly boosting Emax.Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: ESO/L. Calçada

The research presents **new observations of the gamma-ray binary system LS 5039 using the High Altitude Water Cherenkov (HAWC) observatory**, revealing significant insights into the nature of this high-energy source.One of the most striking findings is that **HAWC detected gamma rays from LS 5039 extending up to 200 TeV with no apparent spectral cutoff**. This is a crucial extension of previous observations by the High Energy Stereoscopic System (H.E.S.S.), which had observed the system up to TeV energies. The spectral energy distribution (SED) presented in Figure 2 shows this extension, particularly during the inferior conjunction (INFC). The lower limit on the maximum energy measured by HAWC for LS 5039 is 208 TeV at a 68% confidence level during INFC.Furthermore, the HAWC data **confirms with a 4.7σ confidence level that the gamma-ray emission between 2 TeV and 118 TeV is modulated by the orbital motion of the binary system**. This modulation, where the emission is more significant during the inferior conjunction (INFC) compared to the superior conjunction (SUPC), strongly suggests that these high-energy photons are produced within or very near the binary orbit. The study notes that despite a longer phase interval for the SUPC data, LS 5039 was more significantly detected during INFC due to a higher flux. This modulation up to 100 TeV provides strong evidence for gamma-ray production inside the binary.These high-energy observations pose a challenge to purely **leptonic scenarios** for gamma-ray production. In a leptonic scenario, the highest energy photons would be produced by electrons inverse Compton scattering stellar photons. The detection of photons up to 200 TeV would require electrons to be accelerated to at least this energy, demanding an extremely efficient acceleration mechanism within LS 5039, especially given the dense radiation and potentially high magnetic fields within the binary system. The study suggests that achieving such high electron energies within the stellar photosphere would require an acceleration efficiency η close to 1 and a magnetic field not significantly larger than 0.1 Gauss to avoid substantial synchrotron losses.Alternatively, the HAWC radiation can be interpreted through a **hadronic scenario**. In this case, protons are accelerated to peta-electronvolt (PeV) energies and then produce gamma rays through interactions with either the dense gas (stellar winds) or the intense radiation fields within and close to the binary orbit. The timescale for proton-proton collisions and subsequent pion decay is remarkably close to the binary period, making this a viable explanation. If the gamma rays are of hadronic origin, LS 5039 would be an astronomical accelerator capable of producing PeV-scale hadrons. The required jet power to produce the observed gamma-ray luminosity through proton-proton interactions is estimated, and the study suggests that binary jets powered by either Bondi-type accretion or colliding winds could potentially provide the necessary luminosity.In conclusion, the HAWC observations provide compelling evidence for **gamma-ray emission beyond 100 TeV from LS 5039 and confirm the orbital modulation of this emission**, suggesting that the production of these very high-energy photons occurs within the binary system. These findings have significant implications for our understanding of particle acceleration and radiation processes in gamma-ray binaries, potentially hinting at a hadronic origin for the highest energy emission and establishing LS 5039 as a candidate PeVatron. Future observations at even higher energies could provide crucial evidence to further elucidate the underlying mechanisms at play.Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: J. Goodman

* 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:2503.12263Acknowledements: 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