Multi-messenger astrophysics

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

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

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

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

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