Rudy Wijnands

Close binaries with active chromospheres have recently been shown to emit circularly polarized radio emission at low frequencies, motivating detailed studies of their orbital properties. We present high-resolution ( R = 85,000) optical spectroscopy of TYC 2834-1385-1, a previously unclassified radio source, obtained with the HERMES spectrograph on the Mercator Telescope. We identify the source as a double-lined spectroscopic binary and measure radial velocities from both metallic absorption and Ca ii emission lines. We find an orbital period of P = 3.601 ± 0.002 days and nearly equal radial-velocity semi-amplitudes for the two components. The system exhibits strong Ca ii H and K emission, which is consistent with a BY Dra-type binary containing two similar late-G/early-K main-sequence stars, with a preliminary classification of K0V. This represents one of the first spectroscopic confirmations of a chromospherically active binary following its identification in a low-frequency radio survey.

Wide-field optical transient surveys collectively discover ∼50 new transients per night, making rapid, multi-filter and/or spectroscopic follow-up of all but the most exceptional events unfeasible. Consequently, the (early) light-curve properties of most supernovae (SNe), particularly sub-luminous events, are often poorly characterised. Here, we showcase recovery of key parameters by combining publicly available data from multiple, contemporaneous surveys. We built a catalogue of 125 SNe by cross-matching the MeerLICHT (ML) transient survey with the Transient Name Server list of spectroscopically classified SNe detected between 2017 August and 2022 October. For a subset of 14 SNe with good multi-survey photometric coverage, we combined ML- q , ZTF- g , r , and ATLAS- c , o data to construct composite early-time light curves. We then applied an expanding fireball ( t ² ) model to constrain their explosion dates and rest-frame rise times. We find that this simple model provides a reliable description for the early emission of most SNe in our sample and successfully constrains their rise time, a key parameter for characterising SN categories and probing progenitor origins. This multi-survey approach, only using standard survey products, provides a powerful method to characterise large numbers of SNe that would otherwise be lost, a technique that will be essential for maximising the scientific return from current and future surveys such as GOTO, BlackGEM, the Vera C. Rubin Observatory’s LSST, and Global Open Transient Telescope Array (GOTTA).

The Near-Ultraviolet eXplorer (NUX) is a proposed ground-based observing facility designed to expand nearultraviolet (NUV) astronomy by enabling observations in the 300–350nm range from Earth. The full system is intended to consist of four modified 36 cm Celestron RASA telescopes with custom NUV-optimized optics and UV-sensitive CMOS detectors, providing a total field of view of about 67 square degrees for high-cadence searches for fast, hot transients. To validate this concept, a single-telescope prototype, Proto-NUX, was constructed. Its optical design and development are described in previous work. Initial sea-level tests in the Netherlands showed that the instrument could be operated successfully and that the main technical requirements were met. In March 2026, Proto-NUX was deployed at the Pic du Midi Observatory in the French Pyrenees, at an altitude of 2877m, for its first high-altitude on-sky commissioning campaign. The campaign tested the full system under realistic observing conditions, including deployment, remote operation, calibration procedures, detector behaviour, image quality, tracking, ghosting, and simultaneous optical monitoring. The observations demonstrate that Proto-NUX is a viable platform for ground-based NUV observations, but also show that the system was not yet operating at its expected optical limit. In particular, the delivered NUV point-spread function was broader than expected, with likely contributions from residual collimation error, defocus, chromatic effects, and internal reflections. The detector dark current was also found to be a significant contributor to the noise budget. We discuss these limitations and identify the main improvements needed for future Proto-NUX campaigns and for scaling the concept to the full NUX facility.

The Near-UV-eXplorer (NUX) is a proposed ground-based, wide-field telescope array with a field of view of ∼70 square degrees, designed to operate over the 300–350 nm wavelength range and to achieve a target sensitivity of 20 mag in 150 s (5 σ ). Its main scientific objective is the detection and characterization of hot, rapidly evolving transients in the near-UV (NUV). Proto-NUX is a pathfinder instrument for NUX, based on an off-the-shelf 36 cm Celestron RASA wide-field astrograph that has been modified to enhance throughput and image quality in the targeted NUV band. The main objectives of Proto-NUX are: (1) to quantify the NUV sensitivity of the prototype and assess the feasibility of the full NUX facility; and (2) to characterize atmospheric extinction in the NUV, including its temporal variability and its dependence on zenith angle. Using three filter configurations, we aim to measure the wavelength dependence of the atmospheric extinction and to disentangle the contributions from Rayleigh scattering (dominating at wavelengths >325 nm) and molecular ozone-dominated absorption (dominating <315 nm). On-site testing is scheduled for 2026 at the Pic du Midi Observatory (France, 2877 m altitude) in order to evaluate on-sky performance under high-altitude observing conditions.

Recent observations have revealed rare, previously unknown flashes of cosmic radio waves lasting from milliseconds to minutes, with a periodicity of minutes to an hour. These transient radio signals must originate from sources in the Milky Way and from coherent emission processes in astrophysical plasma. They are theorized to be produced in the extreme and highly magnetized environments around white dwarfs or neutron stars. However, the astrophysical origin of these signals remains contested, and multiple progenitor models may be needed to explain their diverse properties. Here we present the discovery of a transient radio source, ILT J1101 + 5521, whose roughly minute-long pulses arrive with a periodicity of 125.5 min. We find that ILT J1101 + 5521 is an M dwarf–white dwarf binary system with an orbital period that matches the period of the radio pulses, which are observed when the two stars are in conjunction. The binary nature of ILT J1101 + 5521 establishes that some long-period radio transients originate from orbital motion modulating the observed emission, as opposed to an isolated rotating star. We conclude that ILT J1101 + 5521 is probably a polar system where magnetic interaction has synchronized the rotational and orbital periods of the white dwarf. Magnetic interaction and plasma exchange between two stars has been theorized to generate sporadic radio emission, making ILT J1101 + 5521 a potential low-mass analogue to such mechanisms.

The main science aim of the BlackGEM array is to detect optical counterparts to gravitational wave mergers. Additionally, the array will perform a set of synoptic surveys to detect Local Universe transients and short timescale variability in stars and binaries, as well as a six-filter all-sky survey down to ∼22nd mag. The BlackGEM Phase-I array consists of three optical wide-field unit telescopes. Each unit uses an f/5.5 modified Dall-Kirkham (Harmer-Wynne) design with a triplet corrector lens, and a 65 cm primary mirror, coupled with a 110Mpix CCD detector, that provides an instantaneous field-of-view of 2.7 square degrees, sampled at 0.″564 pixel⁻¹. The total field-of-view for the array is 8.2 square degrees. Each telescope is equipped with a six-slot filter wheel containing an optimised Sloan set (BG-u, BG-g, BG-r, BG-i, BG-z) and a wider-band 440–720 nm (BG-q) filter. Each unit telescope is independent from the others. Cloud-based data processing is done in real time, and includes a transient-detection routine as well as a full-source optimal-photometry module. BlackGEM has been installed at the ESO La Silla observatory as of 2019 October. After a prolonged COVID-19 hiatus, science operations started on 2023 April 1 and will run for five years. Aside from its core scientific program, BlackGEM will give rise to a multitude of additional science cases in multi-colour time-domain astronomy, to the benefit of a variety of topics in astrophysics, such as infant supernovae, luminous red novae, asteroseismology of post-main-sequence objects, (ultracompact) binary stars, and the relation between gravitational wave counterparts and other classes of transients.

We have analyzed Chandra and Suzaku observations of the globular cluster Terzan 6, made when the recurrent transient GRS 1747–312 was in quiescence. Our analysis reveals the presence of a second eclipsing, bursting neutron star low-mass X-ray binary in the central regions of the cluster, in addition to GRS 1747–312. The new source, which we name Terzan 6 X2, is located only ∼0.″7 away from GRS 1747–312 in the 2021 Chandra images. The detection of a 5.14 ks long eclipse in the light curve of X2 at a time not predicted by the ephemeris of GRS 1747–312 confirms that it is an unrelated source. Using the Suzaku light curve from 2009, which, in addition to a type I X-ray burst, also showed an eclipse-like feature, we constrain the orbital period to be longer than 16.27 hr. The 0.5–10 keV luminosities of X2 vary in the range of ∼0.24–5.9 × 10³⁴ erg s⁻¹ on timescales of months to years. We have identified a plausible optical counterpart of X2 in Hubble Space Telescope F606W and F814W images. This star varied by 2.7 mag in V ₆₀₆ between epochs separated by years. In the cluster color–magnitude diagram, the variable counterpart lies in the blue-straggler region when it was optically bright, about 1.1–1.7 mag above the main-sequence turn-off. From the orbital period–density relation of Roche-lobe filling stars, we find the mass-donor radius to be ≳0.8 R _⊙.

Very-faint X-ray binaries (VFXBs) are a sub-class of black holes and neutron stars in binaries that appear to be accreting at a very low rate. In addition to providing interesting constraints on poorly understood forms of accretion, elucidating the nature of VFXBs is particularly interesting for binary evolution and population modeling. Through near-infrared (nIR) spectroscopy, we here investigate the nature of the bursting neutron star and VFXB 1RXH J173523.7–354013 (J1735), which persistently accretes at an X-ray luminosity of L_X ∼ 10³⁴ − 10³⁵ erg s⁻¹. Our analysis shows that the nIR emission is dominated by that of the companion star, which we find to be a late G or early K-type giant, making this the second neutron star identified as a VFXB found to have a giant companion. We discuss how several of the system properties are difficult to reconcile with a wind-fed symbiotic X-ray binary. We therefore also propose an alternative scenario wherein J1735 is a wide binary system (supported by the discovery of a 7.5 d modulation in the nIR light curves) with a quiescent luminosity of L_X ∼ 10³⁴ − 10³⁵ erg s⁻¹, in which the donor star is overflowing its Roche lobe. This raises the possibility that J1735 may, every century or more, exhibit very long and very bright outbursts during which it reaches accretion rates around the Eddington limit like the neutron star Z sources.