We present X-ray, optical, and radio follow-up observations of EP250304a, an extragalactic fast X-ray transient (EFXT) discovered by the Einstein Probe. Its X-ray light curve exhibits two broad pulses with comparable peak fluxes within the first ∼1 ks, a feature rarely seen among low-luminosity gamma-ray bursts or EFXTs. Optical follow-up observations were carried out using the Korea Microlensing Telescope Network, the Thai Robotic Telescope, the Las Cumbres Observatory 1 m global network, the Gemini Multi-Object Spectrograph on Gemini south telescope, and the Global Supernova Network. The fast-cooling phase (within 3 days) of optical data can be well fitted by a shocked cocoon model. However, during the supernova phase (SN 2025fhm, from 3 to 88 days), the late-time light curve cannot be explained solely by radioactive ^56Ni decay, as demonstrated by a grid of simulations using the one-dimensional Lagrangian radiation hydrodynamics code SNEC, which reveals a significant energy excess at late epochs. To account for this excess, a central engine like a rapidly spinning, highly magnetized neutron star is needed to provide additional energy injection. This model yields a best-fit spin period of ∼12.60 ms and magnetic field strength of ∼ 3.52×10^15 G, and it successfully explains both the late-time bolometric light curve and the early X-ray pulse structures. Our results indicate that EP250304a/SN 2025fhm is likely powered by a central magnetar rather than by radioactive decay alone, offering new insights into the energy budget and physical origin of EFXTs and their associated supernovae.

Advancing Astrophysics with the SKA II (AASKAII), written by our science community, outlines the transformative scientific advances that will be enabled by the SKA telescopes. In the decade since the publication of the previous edition, telescope designs have matured, construction has commenced, and the SKA Organisation has evolved into the SKA Observatory (SKAO). At the same time, observations from SKA precursor and pathfinder telescopes have provided new insights into longstanding scientific challenges while revealing entirely new phenomena. Published in advance of the first science verification campaign for the SKA Observatory, this volume looks ahead to the coming decades of discovery and innovation in radio astronomy. AASKAII spans the broad range of scientific research enabled by the SKA telescopes, SKA-Mid and SKA-Low. The contributions are organised into six thematic categories according to their scientific focus. The opening section presents overview chapters from the SKA Science Working Groups, around which our community is organised. Each overview provides the broader context that connects the contributions in this volume to the key scientific questions being pursued by their respective communities.

The African continent holds the key to unlocking the full potential of global Very Long Baseline Interferometry (VLBI). Strategic placement of radio telescopes across Africa provides the crucial north-south and intermediate baselines that are currently missing from the global VLBI network. This expansion will dramatically enhance imaging fidelity and resolution. In this chapter, we propose a vision for a continental African VLBI Network (AVN) that will operate in close synergy with SKA-Mid, enabling transformational science across all cosmic scales. While only the Hartebeesthoek Radio Astronomy Observatory (HartRAO) in South Africa and the Ghana Radio Astronomy Observatory (GRAO) are currently operational, several partner countries are in the process of refurbishing or converting existing antennas. Here, we advocate for the expansion of this network through the deployment of a limited number of SKA-Mid-type telescopes across the continent, creating an "African arm" of SKA-VLBI. With a maximum baseline of 9000 km (from Rabat, Morocco to Cassis, Mauritius), the proposed continental facility will surpass for example, the resolution that will be achieved by the next generation Very Large Array (ngVLA) by 10

The accretion-ejection mechanism in Active Galactic Nuclei (AGN) remains a central open problem in astrophysics, tied to the role of AGN feedback in galaxy formation and evolution. Radio-quiet AGN dominate the observed AGN population. Lacking luminous jets, their radio emission traces a rich set of processes spanning the host galaxy kpc scales down to the vicinity of the supermassive black hole: star formation, AGN-driven winds and shocks, free-free emission from photo-ionized gas, low-power jets, and coronal activity close to the inner accretion disk. The Square Kilometre Array (SKA) will probe these processes across a wide frequency range with unprecedented sensitivity, wide-field survey capability, and, critically, high-resolution VLBI imaging. Flux, spectral, and polarization monitoring will constrain dynamics and environmental coupling, while mapping nuclear regions on sub-pc to kpc scales will disentangle compact cores from host emission, resolving the diversity of radio activity across accretion regimes and jet powers from the local Universe to the cosmic dawn. At the full AA4 deployment, the SKA-MID phased into global VLBI arrays will deliver sub-milliarcsecond imaging and μJy sensitivity over 0.35–15 GHz, enabling the first population-level census of radio-quiet AGN nuclei. Earlier AA∗ operations will support pilot studies of the brightest nearby systems.

Massive stars die as core-collapse supernovae, whose optical light emerges days after the implosion. Theory predicts that the initial collapse-driven shock, upon breaking through the star and dense circumstellar medium, emits a brief thermal flash of soft X-rays and ultraviolet. Yet these elusive first signals have remained largely undetected, owing to limited wide-field soft X-ray monitoring. Here we report the discovery of a soft X-ray flash, EP260321a, followed days later by a broad-lined supernova from an envelope-stripped progenitor. Its X-ray spectrum, best modeled with blackbody, establishes it as the long-sought archetypal shock breakout. The burst's duration and energetics place the breakout at a radius of 300 solar radii, tracing a dense surrounding shell and revealing abrupt mass ejection within the final month before collapse.

We present a comprehensive study of the faint counterjet in the radio galaxy 3C 84 using 15 years of 15 and 43 GHz data observed with the Very Long Baseline Array between 2009 and 2024. High-dynamic-range imaging reveals profound morphological and brightness asymmetry between the prominent approaching jet and the highly attenuated receding jet. The primary finding is that the counterjet provides a clean probe of subparsec external free-free absorption (FFA). The northern counterjet components N1 and N2 show strongly inverted 15-43 GHz spectra, whereas the approaching hotspot C3 is persistently optically thin, requiring a frequency-dependent absorbing screen in front of the receding flow. Taking the spectral index of alpha = -0.82 from C3, we infer that the FFA opacity tau 15 GHz changed from similar to 3.0 to similar to 1.9 for N1 and from similar to 3.7 to similar to 2.4 for N2, corresponding to emission measures of (1.7-3.3) & times; 109 pc cm-6 and electron densities of 104-105 cm-3 for plausible path lengths. Distinct opacity histories of N1 and N2 indicate a radially and azimuthally structured absorber, consistent with the ionized inner skin of a clumpy torus or torus-associated wind. Proper motions of C3, N1, and N2 imply a characteristic parsec-scale viewing angle of order 20 degrees and a pattern speed of similar to 0.45-0.52c under an assumption of bilateral symmetry. Evidence for 7-11 yr transverse quasi-periodic modulation is present, but remains tentative given the limited temporal baseline and possible core-reference systematics. The counterjet of 3C 84 therefore acts as a time-dependent tomographic probe of the environment of the inner active galactic nuclei and provides a useful template for linking jet propagation to circumnuclear gas on subparsec scales.

High-frequency very long baseline interferometry (VLBI) polarimetry probes synchrotron-emitting plasma closer to the central engines of radio-loud active galactic nuclei (AGNs), but observations above 43 GHz are technically demanding. We present 22-GHz European VLBI Network observations of the z=4.31 quasar J1510+5702 and J1606+3124, whose published spectroscopic redshift, z=4.56, is uncertain; a photometric estimate gives zphot=0.9±0.1. For the published z>4 redshifts, 22 GHz corresponds to rest-frame frequencies above 118 GHz. Polarized emission is detected in J1510+5702, and a low-level polarized signal is recovered from the brightest feature of J1606+3124. Adopting z=4.56, that feature has a brightness temperature of Tb,VLBI=(7.4±0.8)·1010 K, allowing a mildly Doppler-boosted interpretation, while the young compact-source scenario also remains viable. The core of J1510+5702 has Tb,VLBI=(1.08±0.15)·1012 K, implying a Doppler factor of ∼22 under the equipartition assumption. This component has a ∼3.5% fractional polarization. These observations show that cm-wavelength VLBI can access rest-frame millimeter-band polarization in bright z>4 jets.

Context . SDSS J143016.05+230344.4 ( z = 0.08105) has been proposed as a candidate pre-coalescence supermassive black hole binary and shows remarkable multiwavelength variability. Its radio evolution provides a direct probe of the compact emitting region and of the physical origin of the late-time activity. Aims . We aim to localize the variable radio emission, characterize its spectral evolution, and constrain whether the radio brightening is produced by a newly emerging compact component, external absorption, or dissipation in a structured circumnuclear environment. Methods . We analyzed 4.7–22.2 GHz very long baseline interferometry (VLBI) observations obtained between 2022 February and 2024 February, together with quasi-simultaneous connected-array spectra covering 0.7–16.5 GHz. We combined the VLBI brightnesstemperature and compactness constraints with spectral decomposition and equipartition-based estimates of source size and ambient density structure. Results . At all epochs, the radio emission is dominated by a single unresolved milliarcsecond core with T B ≳ 10 7 K, constraining the variable emission to ≲0.3 pc. The broadband spectra require two synchrotron self-absorbed components: a persistent low-frequency component with ν p,steady ≈ 0.74 GHz and S p,steady ≈ 1.22 mJy, and a flare component whose turnover evolves from (6.35 GHz, 0.18mJy) in February–May 2022 to (8.61 GHz, 0.38 mJy) in December 2022, and then to (5.83 GHz, 0.25 mJy) in March–April 2023. The flare contribution at 15 GHz reaches ~80% and matches the near-epoch VLBI recovery fraction, showing that the high-frequency brightening arises from a newly formed compact synchrotron component. A second brightening of the 15.2 GHz VLBI core is detected between September 2023 and February 2024, while the source remains unresolved. Equipartition scalings imply characteristic radii of ~5 × 10 −4 pc for the flare and ~9 × 10 −3 pc for the steady component, and indicate a steep inner circumnuclear density profile, n ∝ R −1.7 . Conclusions . The delayed radio flare is best explained by dissipation in an outflow or jet-base disturbance propagating through a structured circumnuclear medium.

We present high-sensitivity Very Long Baseline Interferometry (VLBI) observations of four ultraluminous X-ray sources (ULXs): Holmberg II X-1, IC 342 X-1, NGC 6946 X-1, and NGC 925 X-1. No compact emission was detected on milliarcsecond scales, with rms noise levels reaching approximately 5–20 μJy. The corresponding 5σ flux density upper limits reach ∼ 26 μJy, implying radio luminosity limits L_ R≲ 2 × 10^33 erg s^-1. This disfavors any persistently bright hard-state-like compact core at our sensitivity level. The previously reported VLBI core in Holmberg II X-1 exhibits significant long-term variability, broadly consistent with an overall decline over the past decades. This behavior is consistent with emission from optically-thin ejecta undergoing adiabatic expansion. The VLBI non-detections may reflect intrinsically weak/intermittent compact emission, and/or low–surface–brightness structure that is resolved out by VLBI, and/or absorption/propagation effects such as free–free absorption in dense, ionized winds.

We present multi-wavelength observations of the nearby spiral galaxy NGC 5938 (Araish) to investigate the origin of its radio emission, specifically the contribution from active galactic nucleus (AGN) activity and star formation. Using Evolutionary Map of the Universe (EMU) data, we detect extended radio emission extending outwards to the galactic axis, with a steep non-thermal spectral index ( $\alpha = -1.2 \pm 0.2$ ) indicative of synchrotron radiation from an AGN jet. The jet has a physical extent of $\approx 8.2$ kpc (angular length of 64 $<^>{\prime\prime}$ ). Multi-wavelength data from The Dark Energy Camera Plane Survey 2 (DECaPS2), Wide-field Infrared Survey Explorer (WISE) and extended Roentgen Survey with an Imaging Telescope Array (eROSITA) provide further support for this interpretation. The colour-colour diagram presenting WISE infrared observations suggests the presence of dust and young stars that trace the galaxy's disk structure. Our analysis reveals a radio jet, alongside star formation traced by infrared emission, demonstrating the complex interplay of AGN activity and star formation in this well-resolved galaxy. Intriguingly, the spatial relationship reveals the brighter X-ray emission to be largely adjacent to and enveloping the extended radio emission. This suggests that the radio jet, while extending at a significant angle to the galactic disk, is confined by the larger X-ray gas/halo, similar to other systems (i.e. ESO 295-IG022, Centaurus A) and may indicate jet collimation and channelling effects.

The study of exoplanets is a rapidly developing field, driven by the discoveries of Kepler and TESS, among others. The recent detection of Jovian planetary companions of low-mass stars demonstrates that VLBI observations will be an excellent tool for indirect detection of planetary companions through precise radio astrometry of the host star. The anticipated sensitivity of SKA-VLBI and its capability to form multi-beam Tied Array Beams and MultiView analysis will allow us to achieve an order of magnitude increase in astrometric precision, providing much finer details for a wider range of exoplanets and hosts, which will revolutionize the field of exoplanets. Precise micro-arcsecond astrometric observations are crucial for detecting not only Jupiter-like planets, but also lower-mass planets. SKA-VLBI astrometric observations in L and C bands will open the possibility of indirect detection of thousands of planetary companions to radio-bright ultra cool dwarfs, M dwarfs and young stars. When a companion is also detected, the astrometric fit of the data will provide the dynamical masses of the components. In the case of binary systems with planets, fitting the astrometric data will provide the individual masses of stars and planets, as well as the mutual inclination angle of the system, which will show whether the planet is moving in prograde or retrograde orbit around its host star. The search for exoplanets at radio wavelengths will be complementary to other techniques and will allow for the detection of a population of exoplanets that is difficult to reach using other techniques.

Tidal disruption events (TDEs) probe the birth and evolution of black hole accretion flows and jets on human timescales. Radio emission traces shocks and outflows from thermal TDEs and powerful relativistic jets in the rare jetted class. SKA Mid, phased for VLBI and used together with global networks, will deliver milliarcsecond imaging, tens of microarcsecond astrometry, and microJy sensitivity, enabling: (i) proper motion measurements that discriminate off axis relativistic jets from subrelativistic winds; (ii) resolved morphologies and magnetic field diagnostics via polarimetry; and (iii) precise nuclear localization to distinguish SMBH vs. IMBH and to reveal recoiling or binary systems. SKA's wide frequency coverage (0.35 to 15.4 GHz) and 1h continuum sensitivities of 3 to 10 microJy per beam, together with multibeam tiedarray VLBI and a transient buffer for rapid triggers, are transformational. LSST, Einstein Probe, and SVOM will increase TDE alerts to hundreds per year, and late time radio flares appear common, ensuring rich SKA VLBI samples. We provide observing strategies, detection forecasts, and predictions, e.g., about 5 proper motion detections of jetted (or off axis) TDEs per year and routine core shift constraints at the microarcsecond level. This program will establish TDEs as laboratories for exploring jet launching, particle acceleration (including neutrinos), black hole accretion history and demographics, and properties of circumnuclear medium.

We present 4.7–22.2 GHz Very Long Baseline Interferometry (VLBI) monitoring of the candidate pre-coalescence supermassive black hole binary SDSS J143016.05+230344.4 (z=0.08105) from 2022 February to 2024 February, together with quasi-simultaneous 0.7–16.5 GHz connected-array spectra. At all epochs, the radio emission is dominated by a single unresolved milliarcsecond core with T_ B≳10^7 K, confining the variable emission to ≲0.3 pc. The spectra require two self-absorbed synchrotron components: a persistent low-frequency component with ν_ p,steady≈0.74 GHz and S_ p,steady≈1.22 mJy, and a flare component whose turnover evolves from (6.35 GHz,0.18 mJy) in 2022 February–May to (8.61 GHz,0.38 mJy) in 2022 December and then to (5.83 GHz,0.25 mJy) in 2023 March–April. The 15 GHz flare fraction peaks at ≃80% and matches the near-epoch VLBI recovery fraction, showing that the high-frequency brightening arises from a new compact synchrotron component. A second 15.2 GHz VLBI-core brightening is detected from 2023 September to 2024 February while the source remains unresolved. Equipartition scalings imply characteristic radii of R_ eq∼5×10^-4 pc for the flare and ∼9×10^-3 pc for the steady component, and a steep inner circumnuclear density profile, n∝ R^-1.7. The delayed radio peak is consistent with dissipation of an outflow or jet-base disturbance in a structured circumnuclear medium, while a uniform free–free absorber is disfavored.

Long-period radio transients (LPTs) are a recently identified phenomenon that challenge our current understanding of compact objects and coherent radio emission mechanisms. These objects emit radio pulses similar to those of pulsars, but at much longer periods -- on the order of minutes to hours. With duty cycles of only a few percent, individual pulses have been observed to last between 10 and 1000 seconds. This places LPTs in a timescale gap between the two main techniques used in transient radio searches: time-series analysis at millisecond to second timescales, and image-plane searches sensitive to variability on the scale of days. As a result, LPTs remained undetected until recently, and only a handful are currently known. To increase the sample of known LPTs, we conducted a dedicated search using 200 hours of archival data from the ASKAP Evolutionary Map of the Universe survey, covering 750 deg(2) of sky at the shortest possible imaging time step of 10-seconds. This represents the first large-scale search using ASKAP data at second-scale resolution. Although no LPTs were detected, we identified flares from six stars, at least one had never been detected in the radio regime before. We placed a lower limit on the transient surface density of 2.21x10(-6 )deg(-2) at a 10-second timescale, with a sensitivity of 16.9 mJy. Our findings evaluate the feasibility of detecting radio transients using 10-second imaging with ASKAP and provide insights into improving detection pipelines and observation strategies for LPTs.

We present the discovery of EP250827b/SN 2025wkm, an X-ray Flash (XRF) discovered by the Einstein Probe (EP), accompanied by a broad-line Type Ic supernova (SN Ic-BL) at z = 0.1194. EP250827b possesses a prompt X-ray luminosity of ∼ 10^45 erg s^-1, lasts over 1000 seconds, and has a peak energy E_p < 1.5 keV at 90% confidence. SN 2025wkm possesses a double-peaked optical light curve (LC), though its bolometric luminosity plateaus after its initial peak for ∼ 20 days, consistent with a central engine injecting additional energy into the explosion. Its spectrum transitions from a blue to red continuum with clear blueshifted broad absorption features consistent with a SN Ic-BL classification. We do not detect any transient radio emission and rule out the existence of an on-axis, energetic jet ≳ 10^50erg assuming a typical LGRB circumburst constant density (n ≈ 10^-3–10^-1 cm^-3) and microphysical parameters (ε_ e = 0.1 and ε_ B = 0.01). In the model we invoke, the collapse gives rise to a long-lived magnetar, potentially surrounded by an accretion disk. Magnetically–driven winds from the magnetar and the disk mix together and break out with a velocity ∼ 0.35c and interact with an extended circumstellar medium with radius ∼ 10^13 cm, generating X-ray breakout emission through non-thermal free-free processes. The disk outflows and magnetar winds power blackbody photospheric emission as they cool adiabatically and thermalize, producing the first SN peak. The spin-down luminosity of the magnetar and radioactive decay of ^56Ni powers the late-time emission. We end by discussing the landscape of XRF-SNe within the context of EP's recent discoveries.

Radio observations of γ-ray bursts (GRBs) employing the very long baseline interferometry (VLBI) technique provide us with fundamental information on the dynamics and the geometry of the GRB blast wave. With its high angular resolution (∼milli-arcsecond), VLBI allows us to measure the apparent superluminal expansion, to characterise the structure of the jet and to constrain the viewing angle and jet opening angle. While this information is crucial to understand these transient events, such studies have been possible only for three GRBs to date, owing to both the poor sensitivity of current radio facilities and the paucity of close and bright GRBs. In this chapter, we estimate the impact that the SKA-Mid will have on these studies, when included in a VLBI network. We performed a series of dedicated simulations of VLBI observations of GRBs, considering five VLBI networks and the SKA-Mid, both in its AA* and AA4 configurations. We show that including the SKA-Mid in a global-VLBI experiment will: (i) allow us to measure the size and the expansion of a GRB up to a redshift z≃ 0.25 (at a confidence level of 3σ); (ii) constrain the size ≳2 times better than the current global-VLBI array; (iii) improve the localisation precision in Declination from 4 to 30 times; (iv) detect the apparent proper motion of GRBs seen slightly off-axis with a confidence level 3 times better than current VLBI networks. Ultimately, the SKA-Mid will open a new window on a portion of the GRB population that has been inaccessible so far.

We present a multi-wavelength investigation of radio sources in the globular cluster M22 (NGC6656) using VLA, HST, and Chandra observations. Among the eight identified counterparts, we highlight VLA22 as the most promising stellar-mass black hole (BH) candidate. Its radio and X-ray luminosities follow the established L_R-L_X correlation for quiescent black hole low-mass X-ray binaries (BH-LMXBs), while its moderately steep radio spectrum and X-ray spectral hardening further support this classification. Analysis of two potential optical counterparts-a bright main-sequence star and a faint subgiant/red giant-suggests a binary system with a relatively long orbital period. The discovery of VLA22 consistent with recent retention models that stellar-mass BH can be retained within globular clusters over Hubble timescales. Additionally, VLA19 exhibits a characteristically inverted radio spectrum (α= 0.79 ± 0.39, S_ν∝ ν^α) indicative of a compact jet, while VLA40 also aligns with the BH L_R-L_X track, though both require further observations to definitively confirm their nature.

Supernovae (SNe) drive cosmic chemical enrichment and shape galactic feedback, yet the link between progenitors and explosion outcomes remains poorly constrained because the earliest phases are rarely resolved. Radio emission traces synchrotron radiation where the fastest ejecta interact with the circumstellar medium (CSM), providing a uniquely penetrating probe of these phases. SKA-Mid phased into global VLBI will move from simple detections to routine interferometric imaging of nearby extragalactic SNe. Sub-μJy sensitivity and mas-scale SKA+VLBI imaging, complemented by visibility-domain model fitting for sub-beam radius measurements at 5-15 GHz will allow us to follow the expanding shocks of stripped-envelope SNe out to ∼25 Mpc, measure deceleration indices (m) and axial ratios to ≈ 5-10%, and directly test jet-assisted versus neutrino-driven explosion mechanisms. For interacting SNe (Type IIn/Ibn), SKA+VLBI will resolve clumpy and toroidal CSM on progenitor scales, constraining the timing and geometry of eruptive pre-explosion mass loss. Deep limits on Type Ia SNe will tightly restrict the allowed single-degenerate parameter space, while late-time imaging will search for nascent compact remnants and pulsar wind nebulae. In synergy with optical, X-ray and gravitational wave facilities, SKA+VLBI will turn nearby SNe into laboratories for time-resolved shock physics and progenitor mapping.

Extragalactic Fast X-ray Transients (EFXTs) represent an emerging class of high-energy phenomena characterized by X-ray outbursts lasting from tens to hundreds of seconds. However, for more than half of the EFXTs, their physical origins remain elusive. In this Letter, we report the discovery of EP250302a, a luminous EFXT detected by the Einstein Probe (EP) at a redshift of z = 1.131. The multi-wavelength light curves of EP250302a reveal remarkable temporal features that distinguish it from the previously known EP-detected EFXT population, most notably a needle-like X-ray flare accompanied by smooth optical rebrightening during the afterglow phase. We suggest that the distinct X-ray and optical behaviors constitute the first observed instance of late-time violent collision of two relativistic shells in an EFXT. Drawing on insights from GRB studies, such a collision process strongly indicates the reactivation of a central engine, making EP250302a-like transients a unique laboratory for probing the late-time activity and jet physics of EFXT central engines.