Multi-messenger astrophysics

• Unmasking Cosmic Explosions with the Einstein Probe

  his episode dives into the groundbreaking discoveries of the Einstein Probe, a new soft X-ray mission revolutionizing our understanding of high-energy transients in the universe.The Einstein Probe (EP), launched on January 9, 2024, has opened a new era of transient discovery in the previously largely unexplored soft X-ray band. It detects numerous fast X-ray transients, many of which surprisingly show no gamma-ray emission, making their connection to more common gamma-ray bursts (GRBs) a key mystery.Recent research, detailed in the article "The redshift distribution of Einstein Probe transients supports their relation to gamma-ray bursts," has made a significant breakthrough. Using the Astro-COLIBRI archive of transient phenomena and analyzing the redshift distributions of both EP fast X-ray transients and long-duration gamma-ray bursts, scientists found **no statistically significant difference** between them. This strong empirical connection suggests that their redshifts are drawn from the same underlying distribution and that most extragalactic EP transients are **closely related to long GRBs**, originating from the deaths of massive stars (collapsars).Further supporting this link is the agreement of EP transients with the "Amati relation," a known correlation between spectral peak energy and isotropic-equivalent energy for GRBs. Unlike long GRBs, EP transients are **clearly distinct from short-duration GRBs**, which originate from compact object mergers.The Einstein Probe is effectively **uncovering a "missing population"** of "failed jets" and "dirty fireballs" that primarily emit at soft X-ray wavelengths. These include fascinating new discoveries such as relativistic shock breakout candidates and even a candidate relativistic jetted tidal disruption event. The volumetric rates of these EP transients are estimated to be comparable to or even exceed those of standard GRBs, suggesting that weak or failed jets might be intrinsically more common than successful ones.This work highlights the crucial role of the Einstein Probe in expanding our knowledge of **massive star deaths and the mechanisms of jet formation**, revealing a parameter space of cosmic explosions previously hidden from gamma-ray-only detectors.**Read the full article:**O’Connor, B. et al. "The redshift distribution of Einstein Probe transients supports their relation to gamma-ray bursts." Draft version September 10, 2025.Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: ESA

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• Unprecedented Radio Views: Decoding GRB 231117A's Energetic Afterglow

  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

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• Beyond the Burst: How Host Galaxies Shape Fast Radio Bursts

  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

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• A Glimpse into the Early Universe with FRB 20240304B

  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

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• Einstein Telescope + WST: Spectroscopy's Role in the Gravitational Wave Era

  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

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