We present imaging and spectroscopic observations of supernova SN 2025wny, associated with the lens candidate PS1 J0716+3821. Photometric monitoring from the Lulin and Maidanak observatories confirms multiple point-like images, consistent with SN 2025wny being strongly lensed by two foreground galaxies. Optical spectroscopy of the brightest image with the Nordic Optical Telescope and the University of Hawaii 88-inch Telescope allowed us to determine the redshift to be z SN = 2.008 ± 0.001, based on narrow absorption lines originating in the interstellar medium of the supernova host galaxy. At this redshift, SN 2025wny shows a very high rest-frame UV flux and broad spectral features even weeks after the explosion, which is consistent with superluminous supernovae of Type I. We find a high ejecta temperature and depressed spectral lines compared to other similar objects. We also measured, for the first time, the redshift of the fainter of the two lens galaxies (the ‘perturber’) to be z p = 0.375 ± 0.001, which is fully consistent with the DESI spectroscopic redshift of the main deflector at z d = 0.3754. Thus, SN 2025wny represents the first confirmed galaxy-scale strongly lensed supernova with time delays likely in the range of days to weeks, as judged from the image separations. This makes SN 2025wny suitable for cosmography, offering a promising new system for independent measurements of the Hubble constant. Following a tradition in the field of strongly lensed supernovae, we give SN 2025wny the nickname SN Winny.

We present the photometric and spectroscopic analysis of the short-duration GRB 250221A ( T-90 = 1 . 80 +/- 0 . 32 s), using a data set from the optical facilities COLIBR & Iacute;, the Harlingten 50 cm Telescope, and the Very Large Telescope. We complement these observations with data from the Neil Gehrels Swift Observatory and the Einstein Probe , as well as radio observations from the Very Large Array. GRB 250221A is among the few short GRBs with direct afterglow spectroscopy, which gives a secure redshift determination of z = 0 . 768 and allows the unambiguous identification of the host as a galaxy with a star formation rate of similar to 3 M-circle dot yr(-1). The X-ray and optical light curves up to T-0 + 3 & times; 10(4) s (where T-0 refers to the GRB trigger time) are well described by forward-shock synchrotron emission in the slow-cooling regime within the standard fireball framework. However, at T-0 similar to 5 & times; 10(4) s, both the X-ray and optical bands exhibit an excess over the same interval, which we interpret as evidence of energy injection into a jet with a half-opening angle of theta(j )= 11.5(degrees) through a refreshed shock powered by late central engine activity or a radially stratified ejecta. The burst properties (duration, spectral hardness, peak energy, and location in the Amati plane) all favour a compact binary merger origin. However, our modelling of the afterglow suggests a dense circumburst medium ( n similar to 80 cm(-3)), which is more typical of a collapsar environment.

Abstract COLIBRÍ, the French–Mexican Ground Follow-up Telescope (FM-GFT) for SVOM, is a 1.3-meter rapid-response optical facility specifically developped for prompt, multi-band observations of GRB afterglows and for delivering sub-arcsecond localizations of optical counterparts for detailed follow-up studies. The telescope operates through a fully automated system that manages the entire workflow—from alert reception to counterpart identification. Commissioning results confirm that the telescope meets design specifications, and this paper presents a comprehensive performance assessment of the system’s capabilities.

Gamma-ray bursts (GRBs) are the most luminous electromagnetic explosions in the Universe, and offer unique laboratories for studying relativistic jets, compact-object formation, particle acceleration, and the high-redshift Universe. The early optical emission of GRBs, particularly within seconds to minutes after the burst, carries crucial information about the central engine, jet magnetization, bulk Lorentz factor, and circumburst environment. We present a comprehensive review of the early optical phenomenology of GRBs and the essential role played by ground-based robotic optical telescopes to observe the fleeting early-time phenomena through rapid, automated responses to real-time GRB alerts and high-cadence photometry. We examine the key early optical features of GRBs, including prompt optical emission coincident with the γ-ray phase, bright reverse shock optical flashes, the onset of external forward shock afterglow, and superimposed optical flares, plateaus, and discuss the diagnostic power of each in constraining jet physics. We discuss the physical mechanisms underlying these phenomena and their implications for GRB physics (e.g., estimating the initial Lorentz factor Γ_0, magnetization, and the density profile). Early optical observations have constrained the initial bulk Lorentz factor Γ_0 ∼ 100–1000, weak-to-moderate ejecta magnetization for events with prominent reverse shocks, the circumburst density profile, and the geometry of the magnetic field in the ejecta through polarimetry. We also provide the technical capabilities and landmark contributions of major robotic facilities, and discuss future prospects in the era of SVOM, Einstein Probe, Rubin/LSST, ULTRASAT, TeV observatories, and multi-messenger alerts.

Abstract The SVOM (Space-based Variable Objects Monitor) mission, launched into low Earth orbit on 22 June 2024, is a French-Chinese multi-wavelength observatory dedicated to the study of the transient sky. Inspired by the Neil Gehrels Swift Observatory, it consists of an autonomous rapid-slewing satellite, linked in real time to several ground-based telescopes. The space segment comprises two X-ray/gamma-ray wide-field instruments (ECLAIRs and GRM) with real-time triggering capabilities combined with two narrow-field telescopes in X-ray (MXT) and in visible (VT). In addition, the SVOM collaboration has also developed a unique visible and NIR ground-based follow-up system to promptly respond to the gamma-ray transients detected on board. The core program of SVOM will provide new insights into the Gamma-Ray Burst physics by providing a homogeneous dataset covering both the prompt and afterglow emissions, as well as better studying the low luminosity and soft Gamma-Ray Burst populations. As a versatile satellite platform with fast slewing capabilities, SVOM also proposes a Target-of-Opportunity program and a General Program consisting in pointed observations scheduled over the year that will both significantly contribute to the multi-messenger and time-domain astronomy.

We present imaging and spectroscopic observations of supernova SN 2025wny, associated with the lens candidate PS1 J0716+3821. Photometric monitoring from the Lulin and Maidanak observatories confirms multiple point-like images, consistent with SN 2025wny being strongly lensed by two foreground galaxies. Optical spectroscopy of the brightest image with the Nordic Optical Telescope and the University of Hawaii 88-inch Telescope allowed us to determine the redshift to be z(SN) = 2.008 +/- 0.001, based on narrow absorption lines originating in the interstellar medium of the supernova host galaxy. At this redshift, SN 2025wny shows a very high rest-frame UV flux and broad spectral features even weeks after the explosion, which is consistent with superluminous supernovae of Type I. We find a high ejecta temperature and depressed spectral lines compared to other similar objects. We also measured, for the first time, the redshift of the fainter of the two lens galaxies (the 'perturber') to be z(p) = 0.375 +/- 0.001, which is fully consistent with the DESI spectroscopic redshift of the main deflector at z(d) = 0.3754. Thus, SN 2025wny represents the first confirmed galaxy-scale strongly lensed supernova with time delays likely in the range of days to weeks, as judged from the image separations. This makes SN 2025wny suitable for cosmography, offering a promising new system for independent measurements of the Hubble constant. Following a tradition in the field of strongly lensed supernovae, we give SN 2025wny the nickname SN Winny.

A long-standing question in the death of massive stars is the role of relativistic jets. While many gamma-ray bursts and some fast X-ray transients seem to be associated with broad-lined type Ic supernovae, the opposite is not true. The lack of observable jet emission in those Ic-BL SNe can be explained by invoking off-axis jets, choked jets that inject all their energy into the stellar envelope, baryon-loaded jets for which the prompt high-energy emission is strongly suppressed, or non-jetted SNe. The lack of exact explosion time in the majority of SNe presents an obstacle to distinguish between these scenarios. Here we report the properties of SN 2026gzf associated with the X-ray thermal Einstein Probe shock-breakout EP260321a at z=0.0343. The absence of compelling shocked cocoon and radio emission up to 54 days, combined with initial expansion velocities of 30,000 km/s and a circumstellar shell of 0.07 M_⊙, favour a scenario for SN 2026gzf in which a jet was choked in the circumstellar shell. Our high-spatial resolution images of the SN environment show that the progenitor was located between two highly star-forming regions with a metallicity lower than any previously known Ic-BL SN. As the first case of a Ic-BL SN associated with high-energy prompt emission without the signature of a jet, SN 2026gzf provides a unique perspective to understand the successful launch of relativistic jets during the deaths of massive stars.

We present a comprehensive multi-wavelength study of GRB 260310A / SN 2026fgk, a nearby ($z=0.153$), long-duration gamma-ray burst (GRB) with an exceptionally underluminous prompt $γ$-ray emission and a Comptonized spectrum. It is located at the edge of a blue host galaxy with a projected distance of 15 kpc, which is one of the largest offsets reported for a long GRB. The bright optical afterglow, with dense coverage from COLIBRÍ, likely peaked at a few to several hours post-burst, followed by a shallow decay not expected from canonical afterglow models. Both the optical and X-ray light curves show a brief chromatic plateau from $4-7$ days and a more standard decay thereafter only terminated with a rebrightening at $\sim20$ days. We demonstrate that this feature is best described by a combination of emission from the Ic-BL supernova, as identified in GTC spectra, and a late-time refreshed shock. The broadband optical to X-ray spectral energy distribution is well described by synchrotron emission from the forward shock, while the radio observations demand an additional emission component. We model the afterglow using (a) an on-axis uniform jet from a dirty fireball with late-time energy injection and (b) a misaligned jet with power-law angular structure, both having material emitting along our line-of-sight (LOS) moving with an initial Lorentz factor of $Γ_0\sim20-35$. Had this GRB occurred at a more typical redshift ($z\gtrsim0.5$), its prompt emission would likely have remained undetected by current $γ$-ray monitors while its optical afterglow would still have been readily detectable, placing it observationally among orphan afterglows or gamma-ray quiet fast X-ray transients.

We present the Infra-Red Telescope (IRT), which is part of the payload of the THESEUS mission, one on the three phase A candidate missions for the M7 slot of ESA (launch date 2037). The IRT is a 0.7 m class telescope with an off-axis Korsch optical design, with imaging capabilities in the 0.7-1.8 microns range over a 15 x 15 arc min field of view. The IRT also provides slit-less low resolution spectroscopy (R 400) over a limited field of view of 2 x 2 arc min, in the 0.8-1.6 microns range. The goal of the IRT is to identify the near infrared counterparts to the Gamma-Ray Bursts (GRBs) detected by the two other telescopes on board THESEUS (the XGIS and the SXI), and to measure on board its photometric redshift in near real-time. The position and the redshift will be transmitted immediately to ground to allow for deeper follow-up by the large telescopes (ELT, VLT, ...). If the source is bright enough, spectroscopy will be performed to characterize the GRB environment.

The Space-based multiband astronomical Variable Objects Monitor (SVOM) detected its first short gamma-ray burst (GRB), GRB 240715A, in flight, which was jointly observed by Fermi. Based on observational data of SVOM/GRM and Fermi/GBM, we perform a comprehensive temporal and spectral analysis of individual pulses in the prompt emission of this burst, and novel characteristics are revealed. First, opposite evolutions of spectral lag are found in the first and third pulse of this burst. Second, the large negative lag of the first pulse is an outlier in the short GRB sample, especially when the pulse duration is considered. Spectral analysis shows that the negative lag of the first pulse is caused by the evolution of the spectrum index and is irrelevant to E _peak , which is inconsistent with the previous study. The intrinsic mechanism is probably attributed to electron cooling in the decaying magnetic field, which leads to the continuous hardening of the spectrum index and results in negative lag. Furthermore, spectral analysis also shows that the third pulse is more likely to be described by a quasi-thermal spectrum, indicating the existence of photospheric emission. It is difficult to explain how the synchrotron radiation appears before photospheric emission in a single GRB, and some assumptions are discussed.

We present the practical experiences and results obtained in the manufacturing of tools and equipment needed for handling the telescope mirrors, the corrective process of the operation, and the maintenance of the 1.3m Colibri telescope. These tools and equipment include those used for mounting and dismounting the M1, M2, and M3 mirrors, as well as the DDRAGO/CAGIRE instrument of the Colibri telescope, from the observing room to the ground level outside the building, and to the vacuum chamber for aluminization of the mirrors. This also includes tools to balance the instrument and those used in the cleaning process of the mirrors. Our designs addressed the challenges of handling and maintaining the components in the limited space available in the building and at the dome of the Colibri fast alt-az telescope.

DDRAGO is the first light instrument for the 1.3-m COLIBRI robotic telescope at the Observatorio Astronomico Nacional, San Pedro Martir, Baja California, Mexico (OAN-SPM). COLIBRI was developed by France and Mexico in support of the Sino-French SVOM satellite with its ECLAIRs instrument, designed to provide initial follow-up of GRBs. DDRAGO will also support a much wider program of observations of transient and multi-messenger sources. It is a wide-field multi-channel imager consisting of two parts: DDRAGO and CAGIRE. DDRAGO has blue and red channels, and it also delivers an infrared beam to the CAGIRE imager which will be installed soon after. Here we briefly recall the design and discuss the prototyping, fabrication, integration, and verification of DDRAGO. The installation and commissioning of the instrument at the OAN will start shortly.

This work presents the design, implementation, and commissioning of the infrastructure and support services of the 1.3meter COLIBRI robotic telescope site, located at the Observatorio Astronomico Nacional en San Pedro Martir, Instituto de Astronomia- UNAM, Baja California, Mexico. COLIBRI is a ground-based telescope, associated with the SVOM (Space Variable Object Monitor) mission dedicated to the study of gamma ray bursts. We share the progress on the building as well as the installations of the electrical systems, communications, air conditioning systems and security systems. We also share the strategies implemented to achieve the optimization of spaces in the building and the operation site, including technological challenges related to the process of enabling equipment to meet operating specifications and requirements.

The DDRAGO instrument for COLIBRI 1.3 meter telescope of the Observatorio Astronomico Nacional in San Pedro Martir, Baja California, Mexico, will be installed in mid-2024. The structural design of DDRAGO was carried out by applying specific techniques for the development of astronomical instruments. At this work is decribed the mass budget, errors, and mechanical tolerances developed to fulfill the scientific ob jectives of DDRAGO, as well as the manufacturing methods carried out to satisfy the required geometrical specifications. Also, manufacturing processes such as grinding of reference surfaces of the structural plates, CNC manufacturing, precision drilling and tapping, and anodizing are described. The mechanical assembly of the mechanical support structure was performed in two stages. The first one, for verifying the correct integration of all the structural plates with each other and with the rest of the mechanical components that are supported by it. Then, to ensure compliance with the dimensions and geometric tolerances of the assembled structure before anodizing, the general dimensional metrology was performed with the use of a coordinate machine. Once the structure was verified and accepted, anodizing was performed. An interesting aspect of this process is that we were able to measure the dimensional difference, as well as the repeatability of the assembly process, before and after anodizing. Finally, the dimensional acceptance reports of the DDRAGO instrument structure are shown and a series of guidelines for the manufacture, assembly, integration, and validation for mechanical structures in astronomical instrumentation are proposed.

The use of high energy transients such as Gamma Ray Bursts (GRBs) as probes of the distant universe relies on the close collaboration between space and ground facilities. In this context, the Sino-French mission SVOM has been designed to combine a space and a ground segment and to make the most of their synergy. On the ground, the 1.3 meter robotic telescope COLIBRI, jointly developed by France and Mexico, will quickly point the sources detected by the space hard X-ray imager ECLAIRs, in order to detect and localise their visible/NIR counterpart and alert large telescopes in minutes. COLIBRI is equipped with two visible cameras, called DDRAGO-blue and DDRAGO-red, and an infrared camera, called CAGIRE, designed for the study of high redshift GRBs candidates. Being a low-noise NIR camera mounted at the focus of an alt-azimutal robotic telescope imposes specific requirements on CAGIRE. We describe here the main characteristics of the camera: its optical, mechanical and electronics architecture, the ALFA detector, and the operation of the camera on the telescope. The instrument description is completed by three sections presenting the calibration strategy, an image simulator incorporating known detector effects, and the automatic reduction software for the ramps acquired by the detector. This paper aims at providing an overview of the instrument before its installation on the telescope.

When the SVOM mission is fully operational, data from the GRB and GW locations on the sky must be sent to ground stations to study their optical counterparts. Among these telescopes is COLIBRÍ, a Franco-Mexican robotic telescope. Its diameter is 1.3m and its focal length is f/7.2. It is mainly designed to observe the counterpart in the visible and near infrared. In this paper we describe the control system of DDRAGO, the imager component of COLIBRÍ.

The COLIBRÍ robotic observatory is being developed for observing the optical counterparts of GRBs detected by the SVOM satellite. It will be located at the Observatorio Astronómico Nacional in San Pedro Mártir, México. The project is a collaboration between France and México. For this purpose the astronomical instrument DDRAGO is under the last phase of critical design and starting its construction. The structural design techniques applied for developing DDRAGO are described. The mechanical calculations and finite element analysis of the instrument are included and translated into their respective error budget.