Benjamin Rusholme

The hotspots of powerful radio galaxies are the sites of high-energy particle acceleration, but radio observations give us limited information on the electron energy distributions that they generate, which affects our overall understanding of their physics and energetics. Optical synchrotron counterparts to radio galaxy hotspots, which probe high electron energies, have been known for many years, but have only been studied in a small number of objects selected as being very bright in the radio. By combining Low-Frequency Array (LOFAR) observations with the Euclid Q1 data release in the Euclid Deep Field North, we identify 77 powerful radio galaxies with hotspots and show that a small but significant fraction of them, 7–15%, may have detected infrared and optical counterparts to their hotspots. The hotspot spectral energy distributions in our clearest cases are consistent with particle acceleration up to (but not beyond) 1 TeV in energy on the assumption of field strengths close to equipartition, whereas non-detected hotspots either do not reach such high electron energies or would have to have very different electron energy spectra or significantly sub-equipartition magnetic field strengths. Two new candidate optical hotspots are clearly detected matching high-resolution LOFAR data; one is remarkable as both the highest-redshift optical hotspot known to date ( z ≈1.6) and also the first optical detection of the hotspot of the inner lobe of a restarting source. We discuss approaches to performing a large-scale study of hotspots with the full wide-area Euclid surveys, particularly when combined with the international baselines of LOFAR, enabling for the first time a statistical analysis of high-energy particle acceleration in hotspots.

We present the discovery of two disky titans in the first data release of the Euclid satellite. These sources are massive ( M _* > 10 ¹¹ M _⊙ ) star-forming (SFR ∼ 20 M _⊙ yr ⁻¹ ) disks located in strong over-dense environments at intermediate redshifts ( z ∼ 0.75). They represent a small fraction of the massive galaxies in over-dense regions (just four candidates in over 20 deg ² analysed in this study). Their existence is puzzling, given the abundance of passive, bulge-dominated sources commonly found at the centre of groups and clusters at low redshifts. First, our analysis shows that these objects are located in massive groups ( M _h ∼ 10 ^13.8 M _⊙ ), where rapid accretion of cold gas should be prevented by the formation of a static hot halo. Despite this, a millimetre follow-up with NOEMA shows significant cold gas reservoirs ( M H ₂ ∼ 10 ^10.3 M _⊙ ) within these sources. Second, our morphological analysis indicates a massive, passive bulge in these galaxies. This bulge is expected to stabilise the disk against fragmentation, thereby suppressing further star formation. However, these sources lie on the Schmidt–Kennicutt relation or even slightly above. Building on these observations, we propose a scenario where these disky titans are the product of a merger-induced rejuvenation episode, in which the most massive galaxy in a group accretes cold gas from another member and briefly restarts star formation. Such a scenario is supported by a comparison with the TNG300 simulation and easily explains the persistence of star formation activity in massive galaxies in over-dense environments as temporary stages of more complex evolution. More generally, our study showcases the ability of Euclid to find rare objects, thanks to the unprecedented statistic offered by its surveys and the scientific potential residing in the synergy between Euclid and other facilities observing at longer wavelengths.

Calcium-strong supernovae (Ca-strong SNe) are a class of thermonuclear transients characterised by rapid photometric evolution (rise times of ≤16 d) and nebular-phase spectra dominated by strong [Ca II] λλ 7291,7324 emission, compared to the strength of [O I] λλ 6300,6364 emission. Despite their unique spectroscopic signatures and association with remote environments at large physical offsets from their host galaxy, their origins remain uncertain. Using new discoveries from surveys such as the Zwicky Transient Facility, supplemented by archival data, we compiled a sample of 46 Ca-strong SNe, including 8 newly classified objects – the largest sample presented to date. We present a comprehensive optical photometric and spectroscopic study of Ca-strong SNe, aimed at better constraining their diversity, progenitor systems, and explosion mechanisms. This includes Gaussian decomposition of the [Ca II] and [O I] nebular emission lines to measure bulk velocities and line profile evolution from peak brightness to deep nebular-phase epochs. Of the 35 Ca-strong SNe with spectra near peak light, we find that 32 objects exhibit significant [Ca II] emission within 10 days of peak. Forbidden [Ca II] emission is typically expected to emerge after the ejecta have expanded, cooled, and reached a sufficiently low density for forbidden transitions to occur. The prevalence of these early features in Ca-strong SNe, however, remains unexplained by current explosion models. In several objects, we identify complex, multi-component line profiles that suggest a transition between two distinct velocity regimes. These transitional features, combined with the common presence of early forbidden emission, lead us to favour a scenario in which extended material surrounds the progenitor and pre-dates the SN itself. Our results suggest that the diverse spectroscopic behaviours of Ca-strong SNe are consistent with white dwarfs exploding in extended environments polluted by prior activity of the progenitor system.

The forthcoming launch of the Ultraviolet Transient Astronomy Satellite (ULTRASAT) will transform our understanding of the transient ultraviolet sky by increasing our ability to identify transients due to its unprecedented 204 deg ² field of view. While rapid (extragalactic) transients are a priority science area for the mission, flaring stars and AGN can often contaminate searches for such objects. To prepare for these challenges, the Zwicky Transient Facility (ZTF)-ULTRASAT experiment observed five fields at high cadence over three nights, in close proximity to ULTRASAT’s three northern high-cadence fields. A real-time filter identified seven transient candidates, of which five were persistent variable sources and two were spurious. Periods and amplitudes derived from the ZTF Source Classification Project ( SCoPe ) showed that three candidates were RR Lyrae stars with short periods and high amplitudes, while the remaining two displayed flaring behavior. We demonstrate that short-timescale, high-amplitude variables can systematically mimic transient alerts in high-cadence UV surveys, and we provide a concrete strategy to mitigate this contamination using pre-existing machine learning catalogs.

We present the discovery and characterization of SN 2018erx (ZTF18abkmbpy), a fast-evolving, unusually red, interacting stripped-envelope supernova. Spectroscopically, SN 2018erx exhibits prominent broad C ii emission with characteristic widths of ∼3800 km s ⁻¹ , consistent with interaction with a carbon-rich circumstellar medium (CSM) and a Type Icn core-collapse SN classification. Photometrically, SN 2018erx evolves exceptionally rapidly, rising from half-maximum to peak in 2.1 days and declining back in 3.1 days, placing it among the fastest-evolving core-collapse supernovae observed. Semi-analytical CSM-interaction modeling favors a compact, shell-like CSM with M _CSM ≈ 0.3 M _⊙ , an inner radius of R ₀ ≈ 0.7 au, and a very low ejecta mass of M _ej ≈ 0.11 M _⊙ . The radioactive yield is likewise small, with a ⁵⁶ Ni mass ≲(3–5) × 10 ⁻³ M _⊙ , placing SN 2018erx at the low end of the H-poor stripped-envelope SN distribution. At +29 days after peak, we detect a near-infrared excess consistent with pre-existing local circumstellar dust and infer a hot-dust mass of M _d ∼ 10 ⁻⁶ –10 ⁻⁵ M _⊙ . Taken together, the rapid evolution, strong local reddening, carbon-rich emission, and evidence for pre-existing dust point to a multi-component circumstellar environment consisting of a dense inner interaction region produced by enhanced pre-SN mass loss ( M ̇ ∼ 1 0 − 4 –10 ⁻³ M _⊙ yr ⁻¹ ) and an outer dusty layer at R ≳ 4 × 10 ¹⁶ cm, produced by an earlier mass-loss episode roughly 10–200 yr before core collapse. These properties favor an ultra-stripped core-collapse explosion of a low-mass He star in a binary system that underwent late-time Case BC mass transfer or Si-flash-induced mass loss. Fallback-modified Wolf–Rayet collapse or a merger-driven mass-loss episode remain plausible alternatives. These observations provide rare insight into the mass-loss history of stripped-envelope SNe, indicate that SN 2018erx-like events may be efficient sites of dust formation, and suggest that dust-enshrouded explosions of this kind may be systematically underrepresented in optical surveys.

Superluminous supernovae (SLSNe) are one of the most luminous stellar explosions known, yet they remain poorly understood. Because they are intrinsically rare, efficiently identifying them in the large alert streams produced by modern time-domain surveys is essential for building larger observational samples and enabling spectroscopic follow-up. We present NOMAI, a machine-learning classifier designed to identify SLSN candidates directly from photometric alerts in the ZTF stream, using light curves accumulated over at least 30 days. It does not require any spectroscopic redshift and is running in real time within the Fink broker. ZTF light curves were transformed into a set of physically motivated features derived primarily from model-fitting procedures using SALT2 and Rainbow, a blackbody-based multiband fitting framework. These features were used to train an XGBoost classifier on a curated dataset of labeled ZTF sources constructed using literature samples of SLSNe, along with TNS and internal ZTF labeled sources. The final dataset contains 5280 unique sources, including 225 spectroscopically classified SLSNe. Averaged over 100 bootstrap realizations, the classifier reaches 66% completeness and 58% purity. Deployed within the Fink broker, NOMAI has been running continuously since 18 December 2025 on the ZTF alert stream and publicly reports SLSN candidates every night by automatically posting them to dedicated communication channels. Based on this, we also report the first two-month evaluation period, where the classifier successfully recovered 22 of the 24 active SLSNe reported on the Transient Name Server, providing an upper limit on its real-time completeness. The achieved performances, particularly the high completeness in real-time operations, demonstrate that the classifier provides a valuable tool for experts to efficiently scan the alert stream and identify promising candidates. In the near future, NOMAI is intended to be adapted to operate on the Legacy Survey of Space and Time conducted by the Vera C. Rubin Observatory, which is expected to uncover an unprecedented number of transients, making machine-learning based photometric classification essential.

Double-degenerate white dwarf (WD) merger remnants can exhibit extreme magnetic fields exceeding 10 ⁸ G and rapid rotation, but their spectral energy distributions and high-energy emission mechanisms remain poorly characterised. ZTF J1901+1458 stands out as the most compact and strongly magnetised object discovered in this class to date. Intriguingly, recent Chandra observations have revealed that the white dwarf is also a source of soft X-ray emission that is too bright and hard to be of photospheric origin. We analysed new phase-resolved ultraviolet (UV) spectroscopy from the Hubble Space Telescope, together with optical and near-infrared photometry and spectroscopy, using new magnetic atmosphere models to determine its effective temperature, radius, mass, average surface magnetic-field strength, and cooling age. The spectral break at ≈3000 Å, observed in several highly magnetised WDs, is well reproduced by our new models, which account for the effect of magnetic opacities on the atmospheric structure. Our best-fit parameters for the WD yield a cooler effective temperature ( T _eff = 27,445 ⁺⁶⁸⁰ ₋₁₃₉₀ K) and a larger radius than previously reported. Furthermore, the near-infrared data exclude the presence of a stellar or brown dwarf companion hotter than ≈700 K. We jointly analysed published Chandra /Advanced CCD Imaging Spectrometer Imaging array (ACIS-I) data and new XMM-Newton /European Photon Imaging Camera (EPIC) X-ray spectra. The faint X-ray emission, L _X = (1.44 ± 0.13)×10 ²⁷ erg s ⁻¹ , is highly pulsed at the rotation period of the WD, and the soft spectrum can be modelled by a power-law model with photon index Γ = 2.43 ^+0.17 _−0.15 . We suggest that the X-rays are powered by accretion or by interaction between the WD magnetosphere and circumstellar material. A rapidly rotating magnetic field could power a weak wind along open field lines and extract material from the surface of the WD. Alternatively, low-level accretion of fallback material from the past merger event or the tidal disruption of a planetary body could supply the circumstellar material.

In 2026 March, the Einstein Probe (EP) discovered its nearest ( z = 0.0343) Fast X-ray Transient (FXT), EP 260321a, the first EP FXT to provide a strong match to expectations for X-ray “shock breakout” (SBO) emission. Here, we present our multiwavelength follow-up campaign of EP 260321a and its broad-line Type Ic (Ic-BL) supernova (SN) counterpart, SN 2026gzf. We show that our radio follow-up extending over 5.8–54.5 days post-FXT rules out an on-axis jet counterpart of isotropic-equivalent kinetic energy E _K ≳ 10 ⁴⁹ erg for circumburst densities n > 10 ⁻² cm ⁻³ and assuming microphysical parameters ϵ _e = ϵ _B = 0.1. Our radio data also constrain a median mass-loss rate of M ̇ ≲ 1.2 × 1 0 − 5 M ⊙ yr − 1 for a Wolf–Rayet progenitor. In addition, we derive SN 2026gzf’s properties, including ⁵⁶ Ni mass, diffusion timescale, and expansion velocities, from our ∼nightly cadence optical data and compare them with those of optically discovered Type Ic-BL SNe, finding that SN 2026gzf is well within the 90% confidence interval across all properties. We further fit SN 2026gzf’s light curve and determine that combined emission from both interaction with CSM and ⁵⁶ Ni radioactive decay provides the best fit with plausible model parameters. Finally, using the rate of Ic-BL SNe from the Zwicky Transient Facility Bright Transient Survey and assuming all Type Ic-BL SNe produce EP 260321a-like FXTs, we infer an expected rate of EP-detected SBOs of 4.4–16 yr ⁻¹ . This is inconsistent at the 90% confidence level with current EP detection rates, potentially indicating that most Type Ic-BL SNe produce less luminous X-ray SBO signals compared to EP 260321a.

We present early (≲1 yr) multiwavelength observations and analysis of six luminous fast blue optical transients (LFBOTs). We identified these LFBOTs in Zwicky Transient Facility survey data from their fast light-curve evolution ( t _1/2 ≤ 12 days), blue colors at peak brightness ( g − r ≤ −0.2 mag), a visible host galaxy, high optical luminosity ( M _g < −20 mag), and an X-ray and/or radio detection. With the exception of AT2024aehp (ZTF24abygbss), these transients exhibit peaks in their 10 GHz radio light curves at t _rest ≈ 50–100 days, with peak radio luminosities ranging from 10 ³⁸ to 10 ⁴⁰ erg s ⁻¹ . Modeling the radio emission as synchrotron radiation indicates a fast ( v = 0.1–0.3 c ) shock in a dense ( n _e ≈ 10 ³ –10 ⁴ cm ⁻³ ) medium. The X-ray emission varies by ≈2 orders of magnitude in luminosity (10 ⁴² –10 ⁴⁴ erg s ⁻¹ ) at t _rest ∼ 20 days. Analysis of the host-galaxy photometry and spectroscopy for each transient shows that LFBOTs are predominantly nonnuclear (a few kiloparsecs offset) with star-forming host galaxies of stellar masses = 10 ⁹ –10 ¹¹ M _⊙ . Unlike all other LFBOTs to date, AT2024aehp exhibited a luminous ( M < −19 mag) plateau in the optical light curve and its 6–15 GHz radio emission brightened by over an order of magnitude from t _rest ≈ 70 to 130 days. The mostly consistent radio behavior between these LFBOTs implies a similar circumburst medium, leading us to prefer a progenitor scenario in which mass is lost in a consistent way shortly prior to the terminal event.

Tidal disruption events (TDEs) have traditionally been discovered in optical sky surveys through targeted searches of nuclear transients. However, it is expected that some TDEs will occur outside the galaxy nucleus, arising from wandering black holes (BHs) originating in galaxy mergers. Here, we present observations of TDE 2025abcr, the first optical TDE discovered in the outskirts of a host galaxy. The TDE was identified by a custom “off-nuclear” implementation of the machine learning classifier tdescore , which classifies new ZTF transients based on their lightcurves. Follow-up observations confirm that TDE 2025abcr is a TDE-H+He, occurring 9 . ″ 5 (9.3 kpc projected distance) from the nucleus of a massive galaxy ( M _⋆ = 10 ^11.18±0.03 M _⊙ ) with a central BH mass of 10 ^8.82±0.65 M _⊙ . TDE 2025abcr itself was likely disrupted by a much lighter BH (10 ^6.09±0.53 M _⊙ , as estimated with peak luminosity scaling relations). The BH was either dynamically ejected from the nucleus or lies at the center of a very faint tidally stripped dwarf galaxy undergoing a minor merger. Late-time observations of TDE 2025abcr could confirm the origin of this apparent “wandering” BH. The rate of highly offset (≳3 kpc) TDEs can be constrained to <10% of the nuclear TDE rate, but our discovery implies that many dozens of similar sources will be detected by the Vera C. Rubin Observatory each year with resolvable offsets.