Multi-messenger astrophysics

Astro-COLIBRI
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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. Fast Radio Bursts & Magnetar X-Rays: A Peculiar Discovery

    6 HRS AGO

    Fast Radio Bursts & Magnetar X-Rays: A Peculiar Discovery

    In this episode, we dive into the deep cosmos to explore a recent astronomical breakthrough linking Fast Radio Bursts (FRBs)—enigmatic, millisecond-long cosmic transients—to extreme stellar objects known as magnetars. We unpack the discovery of **MXB 221120**, a peculiar magnetar X-ray burst detected by the GECAM observatory on November 20, 2022, which originated from the galactic magnetar SGR J1935+2154 and coincided with an FRB. Discover why this specific burst has astronomers buzzing. Unlike previously observed bursts, MXB 221120 is a massive outlier featuring an unusually long duration and a high blackbody temperature. Most surprisingly, it is the **first FRB-associated X-ray burst from this magnetar to exhibit a purely thermal spectrum**. This discovery fundamentally challenges current theoretical models, which previously assumed that these events are dominated by non-thermal emissions due to resonant Compton scattering. We will also explore a strange ~18 Hz Quasi-Periodic Oscillation (QPO) detected within the burst. We discuss how this frequency might actually be the seismic "ringing" of a low-order crustal torsional eigenmode—essentially, the sound of the magnetar's crust cracking from a singular dissipation of intense internal magnetic energy. Episode Reference: Tan, W.-J., Wang, Y., Wang, C.-W., et al. (2026). "GECAM discovery of a peculiar magnetar X-ray burst (MXB 221120) from SGR J1935+2154 associated with a fast radio burst." *Astronomy & Astrophysics*, April 3, 2026. Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: CAS

    22 min
  2. Starbursts and Seyferts: The Mystery of the Missing Gamma Rays

    MAR 30

    Starbursts and Seyferts: The Mystery of the Missing Gamma Rays

    In this episode, we dive deep into the fascinating world of "composite" galaxies—cosmic beasts that host both an actively feeding supermassive black hole (a Seyfert nucleus) and regions of intense star formation (a starburst component). We explore recent research from the High Energy Stereoscopic System (H.E.S.S.) observatory, which conducted deep observations of three nearby composite galaxies: NGC 1068, the Circinus galaxy, and NGC 4945. The big question driving the research: Can we detect very high-energy (VHE) gamma rays from the extreme environments at the centers of these galaxies? Surprisingly, H.E.S.S. detected no significant VHE gamma-ray signals from any of the three targets. Tune in to find out why this lack of detection is actually highly revealing! We discuss how these newly established upper limits on gamma-ray fluxes are helping astrophysicists test and constrain major theories, including: Jet-Driven Bubbles: How the outflows in these galaxies compare to the giant "Fermi bubbles" found in our own Milky Way. Cosmic Ray Calorimeters & UHECRs: Whether these galaxies act as traps for cosmic rays, and if they could be the source of mysterious ultra-high-energy cosmic rays (UHECRs) hitting Earth. The Neutrino Connection: How the absence of gamma rays in NGC 1068 perfectly complements the detection of high-energy neutrinos by the IceCube observatory, suggesting that gamma rays are being heavily absorbed by a dense X-ray photon field right next to the supermassive black hole. Reference to the Article: H.E.S.S. Collaboration, Acharyya, A., Aharonian, F., et al. (2026). "H.E.S.S. observations of composite Seyfert–starburst galaxies." Astronomy & Astrophysics (Preprint online version: March 24, 2026). Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: NASA/ESA/A. van der Hoeven

    23 min
  3. 15 years hunting for GRBs with H.E.S.S.

    MAR 27

    15 years hunting for GRBs with H.E.S.S.

    In this episode, we dive into the explosive world of Gamma-Ray Bursts (GRBs)—brief, intense pulses of sub-MeV gamma rays that are considered excellent laboratories for studying particle acceleration, capable of releasing up to $10^{51} - 10^{54}$ ergs of isotropic equivalent energy. We explore the newly published second H.E.S.S. gamma-ray burst catalogue, which details a massive 15-year observational campaign spanning from 2004 to 2019. We discuss how the High Energy Stereoscopic System (H.E.S.S.) followed up on 89 different GRB alerts, yet found no *new* very-high-energy (VHE) signals beyond previously published detections. But as we will learn, a "non-detection" is actually a massive win for astrophysics! The resulting upper limits form the largest available dataset for GRBs at VHE. We break down why catching these signals is so incredibly difficult, exploring the technical challenge of rapidly repointing ground-based telescopes before the early afterglow fades and how Extragalactic Background Light (EBL) absorbs high-energy gamma rays from distant sources before they ever reach Earth. We also unpack the standard Synchrotron Self-Compton (SSC) emission models and explain how the upper limits set by H.E.S.S. perfectly align with current physics, proving that VHE-detected GRBs are not a distinct, weird population of stars, but simply the ones that are closest to us and possess naturally luminous X-ray emission. Finally, we look to the future with the next-generation Cherenkov Telescope Array Observatory (CTAO), which features a lower energy threshold that will revolutionize our ability to detect fainter and more distant GRBs. Reference: Acharyya, A. et al., "The second H.E.S.S. gamma-ray burst catalogue: 15 years of observations with the H.E.S.S. telescopes." *Astronomy & Astrophysics*, accepted 2026. Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: H.E.S.S./Vikas Chander

    23 min
  4. The cosmic traffic jam of TDE 2025aarm

    MAR 25

    The cosmic traffic jam of TDE 2025aarm

    In this episode, we dive into the violent and fascinating cosmic phenomenon known as a Tidal Disruption Event (TDE)—what happens when a star wanders a little too close to a supermassive black hole and gets torn apart by tidal forces. We focus on a newly analyzed event, TDE2025aarm, which is the second closest TDE ever discovered, located "just" 61.48 megaparsecs away. Because it happened in our cosmic backyard, astronomers were able to get an unprecedented, highly detailed look at the event across multiple wavelengths of light, including optical, UV, and X-ray. Join us as we break down the forensic evidence of this stellar crime scene. We discuss the victims and the culprit—data suggests a lightweight star (about 16% the mass of our Sun) was shredded by a massive black hole weighing 20 million times the mass of our Sun. We also explore the mystery of the event's incredibly faint X-ray emissions. Does the data point to a "delayed accretion" scenario, where the bright light we see actually comes from stellar debris colliding with itself rather than immediately falling into the black hole? Tune in to find out! Reference: Simongini, A., Kherlakian, M., López-Oramas, A., & Becerra, J. (2026). Early emission characterization of TDE2025aarm. https://arxiv.org/pdf/2603.20123 Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: NASA / CXC / M. Weiss

    21 min
  5. TROVE: Decoding the Subsolar Gravitational Wave Event S251112cm

    MAR 23

    TROVE: Decoding the Subsolar Gravitational Wave Event S251112cm

    In this episode, we dive into a cosmic mystery that has astronomers buzzing: the detection of the gravitational wave event S251112cm. Detected in November 2025, this event is groundbreaking because it has a 100% probability of containing a compact object with a subsolar mass—an object lighter than our own Sun. Standard stellar evolution models tell us that neutron stars and black holes shouldn't be this light, as modern supernova simulations do not yield remnant objects lighter than roughly 1.17 solar masses. So, what exactly collided out there in the dark? We explore the massive, multi-telescope campaign launched by the astronomical community to find the electromagnetic "flash" of this merger. Along the way, we discuss the wild theoretical phenomena that might produce such a signal, such as primordial black holes merging within the accretion disks of active galactic nuclei (AGN), massive "super-kilonovae," or "kilonovae-within-supernovae" born from the fragmented disks of collapsing massive stars. Finally, we learn how scientists are using a new framework called TROVE (Multimessenger Tool for Rapid Object Vetting and Examination) to sift through hundreds of transient candidates to separate the true cosmic counterparts from the false alarms. Key Takeaways: The Anomaly of S251112cm: Why a subsolar mass (SSM) merger challenges our current understanding of physics, and how it opens the door to theories involving primordial black holes.The Electromagnetic Zoo: A breakdown of the exotic, theorized transients that could accompany an SSM merger, including standard kilonovae, kilonovae embedded within stripped-envelope supernovae, super-kilonovae, and bright flares in AGN disks.The Search Effort: How a global network of telescopes (including the Vera C. Rubin Observatory, Swift-XRT, and others) vetted 248 optical and X-ray candidates, and why ultimately none of them were confidently linked to S251112cm.Introducing TROVE: How the Multimessenger Tool for Rapid Object Vetting and Examination ranks candidates using location, distance, and photometry to help astronomers efficiently allocate their limited telescope time during future gravitational wave events. Episode Reference: Vieira, N., Franz, N., Subrayan, B., Kilpatrick, C. D., Sand, D. J., Fong, W., et al. (2026). Search For a Counterpart to the Subsolar Mass Gravitational Wave Candidate S251112cm. Draft version March 19, 2026. Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: Astro-COLIBRI

    20 min
  6. Nova Shockwaves: VLBI observations of the 2019 Eruption of V3890 Sgr

    MAR 20

    Nova Shockwaves: VLBI observations of the 2019 Eruption of V3890 Sgr

    In this episode, we dive deep into the cosmos to explore the dramatic 2019 thermonuclear eruption of V3890 Sgr, a symbiotic recurrent nova located 6.8 kiloparsecs away. A recurrent nova occurs when a white dwarf accumulates enough hydrogen-rich material from its massive companion star—in this case, an M-class red giant—to trigger a massive surface explosion without destroying the binary system. Join us as we explore how astronomers mapped the anatomy of this blast using high-resolution radio imaging from Very Long Baseline Interferometry (VLBI) and gamma-ray data from the Fermi Space Telescope. We discuss: The Shape of the Blast: How the nova's ejecta collided with the red giant's stellar winds, morphing from an asymmetrical blast into a glowing, expanding shell.A Tale of Two Signals: Why the explosion's gamma-rays and radio waves originate from entirely different regions of the shockwave. We explain how gamma-rays are produced in the dense equatorial plane of the star system, while the radio waves emanate from interactions with a more spherical stellar wind. The Mysterious "Second Bump": We unpack the puzzling reappearance of radio and gamma-ray signals nearly 50 to 60 days after the initial explosion. Discover how this late-stage resurgence is driven by a massive "synchrotron halo" of relativistic particles leaking out of the primary shockwave into the surrounding space. Whether you are an astrophysics veteran or a casual space enthusiast, this episode will give you a front-row seat to one of the most fascinating stellar eruptions of the last decade! Featured Reference: Molina, I., Craig, P., Diesing, R., Chomiuk, L., Linford, J. D., Metzger, B. D., ... & Williams, M. N. (2026). Shocks in the Symbiotic Recurrent Nova V3890 Sgr: VLBI Radio Imaging and Fermi GeV Gamma-Rays. Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: I. Molina et al.

    19 min
  7. Ultra Fast Outflows: Hunting for AGN Shocks with the CTAO

    MAR 13

    Ultra Fast Outflows: Hunting for AGN Shocks with the CTAO

    In this episode, we dive into the extreme universe of Active Galactic Nuclei (AGN) and the supermassive black holes that power them. Join us as we explore the astronomical phenomenon of "Ultra Fast Outflows" (UFOs)—incredibly fast winds launched from these black holes at speeds reaching up to 76% the speed of light! We discuss how these violent outflows crash into surrounding galactic gas to form massive shockwaves, effectively turning into giant cosmic particle accelerators. While current telescopes like Fermi-LAT have struggled to definitively spot the gamma-ray signatures of these specific shocks, we break down new research revealing how next-generation instruments, like the Cherenkov Telescope Array Observatory (CTAO), might soon unveil these hidden high-energy emissions. Key Topics Covered: - What are UFOs? An introduction to sub-relativistic winds driven by Active Galactic Nuclei. - Cosmic Accelerators: How Diffusive Shock Acceleration (DSA) energizes protons to produce very-high-energy (VHE) gamma rays and neutrinos. - The Hadronic Channel: Why proton interactions (rather than electrons) are expected to be the dominant source of these gamma rays. - Future Discoveries: The most promising nearby galaxy candidates for future VHE detection, including NGC 7582, NGC 4051, and NGC 5506. Article Reference: B. Le Nagat Neher, E. Peretti, P. Cristofari, and A. Zech. "Very High Energy Gamma Rays from Ultra Fast Outflows." Astronomy & Astrophysics (March 10, 2026). Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: Google/NotebookLM

    22 min
  8. Gamma Rays and Ghost Particles: Investigating IceCube Alerts with HAWC

    MAR 4

    Gamma Rays and Ghost Particles: Investigating IceCube Alerts with HAWC

    In this episode, we dive into the cutting-edge realm of multi-messenger astronomy to explore how scientists are attempting to link high-energy neutrinos with gamma-ray emissions to uncover the origins of ultra-high-energy cosmic rays. We discuss a recent study by the HAWC collaboration, which cross-referenced 368 public astrophysical neutrino alerts from the IceCube observatory with archival gamma-ray data from the HAWC observatory in Mexico. We break down the unique capabilities of both observatories and how researchers utilized a Bayesian Block algorithm to search for spatial and temporal coincidences (flares) between the two datasets. Tune in to hear why the active galactic nuclei (AGN) Markarian 421 and Markarian 501 appeared as matches in the data, and learn why researchers ultimately suspect these exciting detections are likely false positives. We'll explain the hadronic physics behind neutrino production (like pion decay), how the data disfavors these simple models, and what this means for the future of detecting multi-messenger transient events. Key Takeaways: * The Multi-Messenger Approach: How observing both TeV gamma-rays and neutrinos can confirm if a source is accelerating cosmic rays through hadronic interactions. * The Observatories: A look at IceCube, a cubic-kilometer neutrino detector buried in Antarctic ice, and HAWC, a high-altitude water Cherenkov gamma-ray detector in Mexico. * The Findings: The study found a roughly 5% coincident detection rate between the 368 IceCube alerts and HAWC data, which matches the expected background false-positive rate. * The Markarian Mystery: While AGNs Markarian 421 and 501 were found within the containment radii of two neutrino alerts, poor spectral fit constraints and the low astrophysical probability of the alerts suggest they are false positives rather than confirmed neutrino sources. Reference: Alfaro, R., et al. (The HAWC collaboration). "Investigating IceCube Neutrino Alerts with the HAWC $\gamma$-Ray$ Observatory." Draft version February 20, 2026. *arXiv:2602.16818v1*. Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: J. Goodman, HAWC Collaboration

    19 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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