Multi-messenger astrophysics

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Discussions around tools and discoveries in the novel domain of multi-messenger and time domain astrophysics. We'll highlight recent publications, discuss tools to faciliate observations and generally talk about the cool science behind the most violent explosions in the universe.

  1. The Spectroscopic Revolution: Multi-Messenger Astronomy and the WST

    2D AGO

    The Spectroscopic Revolution: Multi-Messenger Astronomy and the WST

    In this episode, we explore the **Wide-field Spectroscopic Telescope (WST)**, a proposed 12-meter class facility that aims to revolutionize our understanding of the cosmos in the 2030s and 2040s. While imaging surveys like LSST and Euclid provide a "video" of the sky, the WST provides the physical "voice" needed to interpret those images through high-speed, massive-scale spectroscopy. **Key Topics Covered:** * **The Technological Leap:** Discover how the WST’s unique design allows for **simultaneous Multi-Object Spectroscopy (MOS) and Integral Field Spectroscopy (IFS)**, featuring a 12-meter aperture and a massive 3.1 square degree field of view. * **The "Spectroscopic Alert" Era:** How the WST will close the gap between millions of nightly photometric alerts and our limited capacity to follow them up, turning spectroscopy into a primary discovery tool for supernovae, exocomets, and binary black holes. * **Mapping the Milky Way:** Learn how "chemical tagging" will allow astronomers to reconstruct the history of our galaxy by analyzing the chemical fingerprints of millions of stars. * **Cosmology and the Cosmic Web:** Exploring the "Dark Universe," from measuring the mass of neutrinos to charting the expansion of the universe using the 3D topology of the Lyman-alpha forest. * **Multi-Messenger Synergies:** How the WST will work alongside gravitational wave detectors (LISA, Einstein Telescope) and neutrino observatories (IceCube-Gen2) to pinpoint the most violent events in the universe. **Featured Reference:** 1. **Mainieri, V., Anderson, R. I., Brinchmann, J., et al. (2024). *The Wide-field Spectroscopic Telescope (WST) Science White Paper*.** This foundational document provides a comprehensive overview of the facility's **12-meter aperture**, its unique simultaneous **Multi-Object Spectroscopy (MOS) and Integral Field Spectroscopy (IFS)** capabilities, and its broad science cases ranging from cosmology to Galactic archaeology. 2. **Melo, A., Sanchez-Saez, P., Ivanov, V. D., et al. (2025). *Spectroscopic Alerts for the Time-Domain Era*.** This article introduces the paradigm-shifting concept of **"Spectroscopic Alerts,"** which are real-time notifications triggered by physical changes in a source's spectrum, allowing the WST to act as a primary **discovery instrument** for transient phenomena. 3. **Schüssler, F., Bisero, S., Cornejo, B., et al. (2026). *Multi-Messenger Studies with High-Energy Neutrinos and Gamma Rays: The WST Opportunity*.** This reference highlights the WST's role in **multi-messenger astrophysics**, specifically its ability to rapidly survey large sky areas to classify the electromagnetic counterparts of **high-energy neutrinos** and very-high-energy **gamma rays**. Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: G.Gausachs/WST

    16 min
  2. Asymmetric Blasts: Inside the Ejecta of GRB 180728A / SN 2018fip

    5D AGO

    Asymmetric Blasts: Inside the Ejecta of GRB 180728A / SN 2018fip

    In this episode, we dive into the fascinating discovery of **GRB 180728A**, one of the nearest and most energetic long-duration gamma-ray bursts ever recorded at a low redshift. While most nearby bursts are low-energy events, this explosion released a massive **$2.5 \times 10^{51}$ erg of isotropic energy**, placing it in a rare class of cosmological powerhouses found right in our relative "backyard". We explore the detailed analysis of its associated supernova, **SN 2018fip**, and what it reveals about the complex nature of stellar collapses. **Key Topics Covered:** * **A Rare High-Energy Event:** Learn why GRB 180728A is significant, sitting at a redshift of **z = 0.1171** and ranking as one of the most energetic nearby bursts after the famous GRB 030329 and the record-breaking "BOAT" (GRB 221009A). * **The Supernova Mystery:** Despite the high energy of the gamma-ray burst itself, the associated supernova SN 2018fip was **intrinsically fainter** than many typical events, showing that the energy of a burst doesn't always correlate with the brightness of its supernova. * **The Shape of the Blast:** Discover why researchers believe this wasn't a simple spherical explosion. The sources suggest a **two-component ejecta** model: a narrow, high-velocity component (> 20,000 km/s) and a slower, more massive inner component. * **The Neighborhood:** We take a look at the **host galaxy**—a low-mass, blue, star-forming irregular dwarf galaxy typical for these types of cosmic events. * **Advanced Observations:** Insights into how astronomers used instruments like the **X-shooter** on the Very Large Telescope to track the explosion for 80 days. **Featured Reference:** Rossi, A., Izzo, L., Maeda, K., et al. (2026). **"GRB 180728A and SN 2018fip: the nearest high-energy cosmological gamma-ray burst with an associated supernova."** *Astronomy & Astrophysics*. Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: Anna Serena Esposito

    12 min
  3. The Superkilonova Symphony: Merging Stars Inside Exploding Ones (AT2025ulz and S250818k)

    JAN 12

    The Superkilonova Symphony: Merging Stars Inside Exploding Ones (AT2025ulz and S250818k)

    In this episode, we dive into a groundbreaking discovery that may have revealed a brand-new category of cosmic explosion: the Superkilonova. On August 18, 2025, gravitational-wave detectors picked up a signal, S250818k, indicating a merger between two neutron stars—but with a twist. The estimated "chirp mass" was surprisingly low, suggesting that at least one of the objects was below the mass of our Sun, a finding that challenges standard models of stellar evolution. The Optical Mystery: The Zwicky Transient Facility (ZTF) quickly identified a matching optical transient, AT2025ulz, in the same region. While its first week of behavior looked like a classic "kilonova" (the expected glow from a neutron star merger), it soon evolved into something much more complex. Spectroscopic and photometric data eventually showed it was most similar to a Type IIb stripped-envelope supernova, which is the explosion of a massive star that has lost most of its outer hydrogen. The Superkilonova Theory: How can an event be both a neutron star merger and a supernova? The researchers explore a fascinating theoretical model known as a Superkilonova. In this scenario, a rapidly spinning massive star collapses, and its core either fissions into two pieces or its surrounding disk fragments into subsolar-mass neutron stars. These fragments then merge almost immediately inside the supernova explosion. Key Highlights: A "Veritable Symphony": The potential for a single event to produce gravitational waves from a merger while simultaneously displaying the light of a core-collapse supernova.New Stellar Pathways: If confirmed, this proves that neutron stars can form via accretion-disk fragmentation, or it might even be evidence of primordial black holes.Multimessenger Challenges: Why scientists need more than just light to solve these puzzles, relying instead on a "panchromatic dataset" including X-rays, radio waves, and gravitational strain. Article Reference Kasliwal, M. M., et al. (2025). "ZTF25abjmnps (AT2025ulz) and S250818k: A Candidate Superkilonova from a Subthreshold Subsolar Gravitational-wave Trigger." The Astrophysical Journal Letters, 995:L59 (18pp). Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: Caltech/K. Miller and R. Hurt (IPAC)

    17 min
  4. MeerKAT’s Deep Gaze: Unveiling the Radio Ghosts of V4641 Sgr

    JAN 9

    MeerKAT’s Deep Gaze: Unveiling the Radio Ghosts of V4641 Sgr

    In this episode, we dive into a groundbreaking discovery made with the **MeerKAT radio telescope**: a massive, symmetric **"bow-tie" shaped radio structure** surrounding the black hole system **V4641 Sgr**. While this microquasar has been known since 1999 for its erratic outbursts and superluminal jets, this new research reveals the long-term impact these black holes have on their galactic neighborhoods, stretching across nearly **35 parsecs (about 114 light-years)** of space. **Key Topics Discussed:** * **The System:** V4641 Sgr is a low-mass X-ray binary (LMXB) featuring a **6.4 solar mass black hole** and a B-type stellar companion. It is famous for its "superluminal" jets that appear to move faster than the speed of light due to their orientation and velocity. * **The "Bow-Tie" Discovery:** Using deep imaging techniques, astronomers found a faint, diffuse radio structure that mirrors the size and position of extended X-ray emission recently detected by the XRISM satellite. * **Particle Acceleration:** The sources suggest the radio and X-ray emission are likely caused by **synchrotron radiation**. This implies that electrons are being accelerated to energies of **more than 100 TeV**—even tens of parsecs away from the central black hole. * **The Proper Motion Mystery:** Interestingly, the black hole is slightly offset from the center of the bow-tie. The researchers explain this through the **proper motion of the system**; by tracing the black hole's path backward, they estimate it was at the center of this structure roughly **10,000 years ago**. * **The Gamma-Ray Disconnect:** While large-scale gamma-ray "bubbles" have also been detected around this system, they are oriented differently and are much larger than the radio bow-tie. We explore why these different "colors" of light reveal different chapters of the black hole's history. **Why This Matters:** This discovery adds V4641 Sgr to a growing list of **"microquasars"**—stellar-mass black holes that act as smaller-scale analogs to the supermassive black holes found in the centers of galaxies. It reinforces the idea that these systems are significant contributors to **galactic cosmic rays** and powerful drivers of change in the interstellar medium. *** ### **Reference** Grollimund, N., Corbel, S., Fender, R., et al. (2026). **"Large-scale radio bubbles around the black hole transient V4641 Sgr."** *Astronomy & Astrophysics*, manuscript no. aa57124-25. Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: N. Grollimund et al.

    16 min
  5. Breaking the Redshift Barrier: H.E.S.S. and the Distant Blazar PKS 0346−27

    JAN 6

    Breaking the Redshift Barrier: H.E.S.S. and the Distant Blazar PKS 0346−27

    In this episode, we dive into a groundbreaking discovery in high-energy astrophysics: the detection of the blazar PKS 0346−27 at a redshift of $z = 0.991$. This makes it one of the most distant objects ever detected in very-high-energy (VHE) gamma-rays ($E > 100$ GeV). We explore how the H.E.S.S. (High Energy Stereoscopic System) telescopes in Namibia managed to capture this elusive signal despite the thick "fog" of Extragalactic Background Light (EBL) that usually absorbs such distant photons. Key Discussion Points: The Record-Breaking Detection: Why reaching a redshift of approximately 1 is a major milestone for gamma-ray astronomy and what it tells us about the evolution of the universe.A Tale of Two Flares: The strange two-day delay between the high-energy flare caught by the Fermi-LAT satellite and the very-high-energy flare detected by H.E.S.S..The Physics of the Jet: We break down the debate between leptonic and hadronic models. While electrons are the usual suspects, the data from PKS 0346−27 strongly favors a proton-synchrotron model, even though it requires jet power that temporarily exceeds the source’s Eddington limit.Multi-Wavelength Cooperation: How a global team used data from H.E.S.S., Fermi-LAT, the Swift Observatory, and the ATOM telescope to build a complete picture of this cosmic event.The "Synchrotron Mirror" Hypothesis: Exploring how stationary clouds near the black hole might be reflecting radiation back into the jet to create "orphan" VHE flares. Technical Insight: The researchers found that a traditional leptonic model (based on electrons) would require "implausible" parameters, such as a Doppler factor exceeding 80, to explain the flare. This push toward hadronic models suggests that relativistic protons may play a much larger role in the most powerful jets in the universe than previously confirmed. Featured Article: H.E.S.S. Collaboration, et al. (2026). "H.E.S.S. detection and multi-wavelength study of the $z \sim 1$ blazar PKS 0346−27." Astronomy & Astrophysics manuscript no. 0346. Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: Stefan Schwarzburg

    16 min
  6. The Super PeVatron: LHAASO Unlocking the High-Energy Secrets of Cygnus X-3

    12/23/2025

    The Super PeVatron: LHAASO Unlocking the High-Energy Secrets of Cygnus X-3

    In this episode, we dive into a groundbreaking discovery from the **Large High Altitude Air Shower Observatory (LHAASO)**. For decades, the microquasar **Cygnus X-3** has been "an astronomical puzzle," but new data has finally confirmed its status as a **Super PeVatron**—a cosmic engine capable of accelerating protons to tens of petaelectronvolt (PeV) energies. **Key Discussion Points:** **The Iconic Microquasar:** Cygnus X-3 is a unique high-mass X-ray binary consisting of a compact object (a black hole or neutron star) and a massive **Wolf–Rayet donor star**. It features a relativistic jet and a remarkably short 4.8-hour orbital period. **Breaking the Energy Barrier:** LHAASO detected variable gamma-rays reaching up to **3.7 PeV**, the highest-energy photons ever recorded from such an astrophysical source. **The Hardest Spectrum:** The source exhibits the **hardest ultra-high-energy (UHE) spectrum** ever detected by LHAASO, with a distinct "hump" or spectral hardening around 1 PeV.**Protons vs. Electrons:** While lower-energy GeV gamma-rays are often produced by electrons, researchers explain that **leptonic origins are robustly excluded** for these PeV emissions due to intense synchrotron cooling. Instead, the signal likely comes from **photomeson processes**, where protons accelerated in the jet collide with the dense ultraviolet and X-ray photon fields of the binary system.**Temporal Puzzles:** We discuss the **month-scale variability** of the signal and the 3.2$\sigma$ evidence for orbital modulation, which strongly suggests the PeV radiation is born deep within the innermost regions of the jet. The Big Picture: This discovery provides the first compelling evidence that a microquasar can act as a **super-PeVatron**, generating transient PeV gamma-ray emission in close proximity to the central engine. This shifts our understanding of how cosmic rays are accelerated within our own galaxy. ### Article Reference **Title:** *Cygnus X-3: A variable petaelectronvolt gamma-ray source* **Authors:** The LHAASO Collaboration **Journal:** *National Science Review (NSR)* **Source PDF:** 2512.16638v1.pdf Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: LHAASO Collaboration

    14 min
  7. Chasing Cosmic Ghosts: The Global Multi-Messenger Hunt for Neutrino Sources

    12/22/2025

    Chasing Cosmic Ghosts: The Global Multi-Messenger Hunt for Neutrino Sources

    In this episode, we dive into the cutting-edge world of multi-messenger astronomy. We explore how scientists are using a global network of specialized telescopes to solve one of the greatest mysteries in physics: the origin of high-energy cosmic rays. By tracking "ghost particles" called neutrinos from the depths of the South Pole to the highest mountain peaks where gamma-ray telescopes wait, researchers are building a new map of the most violent processes in our universe. Key Discussion Points: What are Neutrinos? Learn why these secondary particles are the "smoking gun" signature of hadronic acceleration processes in space.The Multi-Messenger Approach: Why detecting neutrinos alone isn't enough and how simultaneous observations of very-high-energy (VHE) gamma-rays help pinpoint source locations.The IceCube-IACT Partnership: A look at how the IceCube Neutrino Observatory at the South Pole coordinates with the "Big Four" imaging atmospheric Cherenkov telescopes—FACT, H.E.S.S., MAGIC, and VERITAS—to react to cosmic alerts in real-time.Target-of-Opportunity (ToO) Programs: How telescopes automatically repoint within seconds or minutes to catch a glimpse of a neutrino’s source.Case Studies & Legacy Results: We review the famous coincidence of the blazar TXS 0506+056 and discuss the latest findings from follow-up observations conducted between 2017 and 2021.The Future of the Hunt: What the next generation of detectors, like IceCube-Gen2 and the Cherenkov Telescope Array Observatory (CTAO), will mean for the next decade of discovery. Featured Reference: FACT, H.E.S.S., MAGIC, VERITAS, Fermi-LAT, and IceCube Collaborations. (2025). Prompt Searches for Very-High-Energy $\gamma$-Ray Counterparts to IceCube Astrophysical Neutrino Alerts. Accepted at the Astrophysical Journal, arXiv: https://arxiv.org/abs/2512.16562 Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: IceCube/NASA

    14 min
  8. V1723 Sco and V6598 Sgr: Decoding the Fastest and Brightest Gamma-Ray Eruptions

    12/18/2025

    V1723 Sco and V6598 Sgr: Decoding the Fastest and Brightest Gamma-Ray Eruptions

    Classical novae, thermonuclear eruptions on the surface of a white dwarf in a binary system, are known sources of high-energy gamma-rays detected by the Fermi-LAT. This episode explores a multi-wavelength analysis of two recent novae, **V1723 Sco 2024** and **V6598 Sgr 2023**, aiming to constrain the mechanism behind this intense gamma-ray emission. **V1723 Sco** proved to be a very bright gamma-ray source, with emission lasting 15 days, allowing scientists to constrain the total energy and spectral properties of accelerated protons. Intriguingly, V1723 Sco also showed unexpected gamma-ray and thermal hard X-ray emission more than 40 days after its initial outburst, suggesting that particle acceleration can occur even several weeks post-eruption. In contrast, **V6598 Sgr** was detected by Fermi-LAT for only two days, marking one of the shortest gamma-ray emission durations ever recorded for a classical nova. Its brief gamma-ray signal coincided with a rapid decline in optical brightness. V6598 Sgr also exhibits peculiar characteristics, including no significant gamma-ray emission below 1 GeV and the possibility that it is an Intermediate Polar (IP) system, which may hint at a different particle acceleration region due to potentially strong magnetic fields. The detailed analysis, which combined Fermi-LAT data with optical (AAVSO) and X-ray (NuSTAR) observations, strongly supports the hypothesis that the gamma-ray generation in both novae is more consistent with the **hadronic scenario** (involving accelerated protons) than the leptonic scenario. However, the long-standing challenge remains: no non-thermal X-ray emission has been detected simultaneously with the gamma-rays. **Article Reference:** Fauverge, P., Jean, P., Sokolovsky, K., et al. (2025). *Fermi-LAT detections of the classical novae V1723 Sco and V6598 Sgr in a multi-wavelength context.* submitted to Astronomy & Astrophysics, arXiv: 2512.14198 Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: NASA's Goddard Space Flight Center/S. Wiessinger

    14 min
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Discussions around tools and discoveries in the novel domain of multi-messenger and time domain astrophysics. We'll highlight recent publications, discuss tools to faciliate observations and generally talk about the cool science behind the most violent explosions in the universe.

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