Context. The Large Array Survey Telescope (LAST) is a wide-field visual-band survey designed to explore the variable and transient sky with high cadence. Its raw data stream is automatically processed in near real time at the observatory site, producing sciencequality images, catalogs, and transient alerts. Transient alerts are then reported to the Transient Name Server (TNS). Aims. The LAST pipeline comprises two major components: (i) processing and calibration of single images followed by the coaddition of 20 × 20 s exposures, producing single-image and coadded-image catalogs; and (ii) the subtraction of coadded images from calibrated reference images followed by transient detection. In this work, we present a detailed description and validation of the second component of the pipeline. Methods. Transient detection is based on the algorithm for proper image subtraction (ZOGY). We combined ZOGY subtraction with the TRANSLIENT statistic for sub-pixel motion discrimination, together with a sequence of deterministic filtering steps, to produce a clean stream of transient candidates without the use of machine learning. Results. Using commissioning data, the pipeline achieves a preliminary 5σ limiting magnitude of 20.3-20.7 mag, a single-epoch transient detection efficiency of ∼80%, and a purity of ≳90% at a signal-to-noise ratio of ≥7.5σ.

The origins of sub-luminous (L_γ,iso < 10^49.5 erg s^-1) gamma-ray bursts (GRBs) associated with broad-lined Type Ic supernovae (Ic-BL SNe) are poorly understood, in part due to the low discovery rate and faint afterglows. Here we present the identification of the optical afterglow of Fermi-GBM-detected GRB 260310A (AT 2026fgk) as a rapidly rising (>1mag d^-1), red (g-r=0.4 mag) transient using the Gravitational-wave Optical Transient Observatory, Large Array Survey Telescope, and Zwicky Transient Facility (ZTF) data streams. We present multiwavelength follow-up observations from the first 50 days, which reveal that GRB 260310A/AT 2026fgk was sub-luminous (L_γ,iso=10^48.8erg s^-1); it was the most nearby (z=0.153) afterglow identified blindly by an optical survey; and that it is one of the brightest afterglows ever observed at X-ray, optical, and radio (cm to mm) wavelengths. We spectroscopically confirm an underlying Ic-BL SN with properties typical of GRB-SNe (M_ej≈3 M_⊙, E_ K≈ 10^52erg). With basic modeling of the afterglow, including the long optical rise (≈10^3s), we infer either a low initial Lorentz factor (Γ_0≈40) or a slightly off-axis viewing angle (≲3^∘). The host galaxy's mass and star formation rate are similar to the hosts of other sub-luminous GRBs. ZTF's flux-limited survey gives a volumetric rate of AT 2026fgk-like events of 0.30^+1.37_-0.29Gpc ^-3 yr^-1, which is consistent with the on-axis, high luminosity (L_ γ,iso>10^49.5 erg s^-1) long-GRB rate. The similarity in the rates strongly constrains the prevalence of low-Γ_0 bursts and the beaming of the initial relativistic material in GRBs.

Context. The origins of astrophysical high-energy neutrino flux remain uncertain. Core-collapse supernovae (CCSNe) with strong circumstellar material (CSM) interactions (i.e., Type IIn, a.k.a. SNe IIn) are compelling candidates for explaining efficient hadronic acceleration and neutrino production. Aims. We investigate the possible association between the SNe IIn SN2025cbj and the IceCube high-energy neutrino IceCube-250421A. We assess whether the observed properties of the SN would enable an appreciable neutrino yield. Methods. We combined rapid optical follow-ups with LAST and archival ZTF photometry with spectroscopy from LT/SPRAT and MMT/BINOSPEC to characterize the SN’s evolution and CSM interaction. We estimated the explosion and peak times from an early light-curve fitting and quantified the chance-coincidence probability with resampling simulations that scramble neutrino right ascensions, while preserving declinations and error contours. Using a simple post-shock-breakout interaction model in a dense wind, we estimated the expected muon-neutrino yield for IceCube’s real-time Bronze stream. Results. The spectra of SN2025cbj obtained after the neutrino epoch exhibit persistent narrow Balmer lines superposed on broad Lorentzian electron-scattering wings, consistent with a sustained dense CSM interaction. For the multimessenger association, resampling the simulations against the TNS catalog gives a chance-coincidence p-value of p ≃ 0.24 for observing k ≥ 1 events (and p ≃ 0.078 against the ZTF-BTS catalog). These values are sensitive to the size of the SN and neutrino samples and do not indicate a statistically significant multimessenger association. A post-breakout interaction scenario predicts an expected Nνμ ∼ 10−3 events in the IceCube Bronze alert stream over 96 days per this one candidate. We discuss the implications of these numbers and the possible biases that could affect these results.

Context. The Large Array Survey Telescope (LAST) is a wide-field visual-band survey designed to explore the variable and transient sky with high cadence. Its raw data stream is automatically processed in near real time at the observatory site, producing science-quality images, catalogs, and transient alerts. Transient alerts are then reported to the Transient Name Server (TNS). Aims. The LAST pipeline comprises two major components: (i) processing and calibration of single images followed by the coaddition of 20 & times; 20 s exposures, producing single-image and coadded-image catalogs; and (ii) the subtraction of coadded images from calibrated reference images followed by transient detection. In this work, we present a detailed description and validation of the second component of the pipeline. Methods. Transient detection is based on the algorithm for proper image subtraction (ZOGY). We combined ZOGY subtraction with the TRANSLIENT statistic for sub-pixel motion discrimination, together with a sequence of deterministic filtering steps, to produce a clean stream of transient candidates without the use of machine learning. Results. Using commissioning data, the pipeline achieves a preliminary 5 sigma limiting magnitude of 20.3-20.7 mag, a single-epoch transient detection efficiency of similar to 80%, and a purity of greater than or similar to 90% at a signal-to-noise ratio of >= 7.5 sigma.

A significant fraction of large-telescope observing time is devoted to single-object spectroscopy and photometry. We compare the cost-effectiveness of different telescope architectures, including single seeing- and diffraction-limited telescopes and arrays of seeing-limited or diffraction-limited telescopes. We use the survey speed per unit cost as the performance metric. For single telescopes with equal collecting area, diffraction-limited operation becomes more cost-effective once seeing-limited observations enter the background-dominated regime, typically at visible magnitudes of similar to 20. For unresolved single-object observations, arrays of seeing-limited telescopes are generally more cost-effective than a single seeing-limited telescope. When matched in survey speed, a single diffraction-limited telescope outperforms an array of small seeing-limited telescopes only for targets fainter than similar to 20-24 mag, while a single large diffraction-limited telescope becomes more cost-effective than an array of smaller diffraction-limited telescopes only for targets fainter than similar to 26-28 mag. We conclude with a discussion on aspects related to the minimum telescope size and maintenance of telescope arrays. These conclusions are sensitive to the specific assumptions and system parameters adopted for each architecture and should therefore be interpreted with appropriate caution. Our results motivate the development of telescope arrays such as Multi-Aperture Spectroscopic Telescope and LFAST and may inform the design of future optical facilities.

Optical polarimetry provides information on the geometry of the emitting region, the magnetic field configuration and the properties of dust in astrophysical sources. Current state-of-the-art instruments typically have a small field of view (FoV), which poses a challenge for conducting wide surveys. We propose the construction of the Large Array Survey Telescope Polarization Node (LAST-P), a wide-field array of optical polarimeters. LAST-P is designed for high-cadence (less than or similar to 1 day) polarization monitoring of numerous astrophysical transients, such as the early phases of gamma-ray bursts, supernovae, and novae. Furthermore, LAST-P will facilitate the creation of extensive polarization catalogs for X-ray binaries and white dwarfs, alongside a large FoV study of the interstellar medium. In survey mode, LAST-P will cover a FoV of 88.8 deg2. With a 15 x1 minutes exposure, the instrument will be capable of measuring polarization of sources as faint as Gaia Bp-magnitude similar to 20.9. The precision on the linear polarization degree will reach 0.7%, 1.5%, and 3.5% for sources with magnitudes 17, 18, and 19, respectively, for a seeing of 2 .'' 7, air mass of about 1 for observations in dark locations. We propose three distinct non-simultaneous survey strategies, among them an active galactic nuclei (AGN) strategy for long-term monitoring of similar to 200 AGN with <1 day cadence. In this paper, we present the predicted sensitivity of the instrument and outline the various science cases it is designed to explore.

Context. Transforming the instrumental photometry of ground-based telescopes into a calibrated physical flux in a well-defined passband is a major challenge in astronomy. Along with the intrinsic instrumental difference between telescopes sharing the same filter, the effective transmission is continuously modified by the effects of the variable atmosphere of the Earth.Aims. We have developed a new approach to the absolute photometric calibration (i.e., tied to the CALSPEC standards) that simultaneously treats instrumental and atmospheric effects on an image-by-image basis by fitting the system transmission. This approach aims at breaking the 1% absolute photometric accuracy which limits current calibration methods for ground-based observatories.Methods. We fit the transmission, as a function of wavelength, for each image. The fit is done by comparing the instrumental fluxes of stars in the image to the synthetic photometry of the stars given their spectrum and the transmission function which have free parameters. A key element that enables this approach is the set of about 220 million low-resolution spectra measured by Gaia , which provides a large number of stellar calibrators in the image that are calibrated against the CALSPEC scale.Results. We demonstrate the method using data from the Large Array Survey Telescope (LAST). We show that the residuals between observations and synthetic photometry of the Gaia spectra in the fitted transmission have a standard deviation <1% on an image-by-image basis, with no spatial and color dependencies. The median accuracy of the zero-point throughout the image is between 3-5 mmag, depending on the total image exposure. Furthermore we show that this method provides high stability over long temporal scales.

The early evolution of a supernova (SN) can reveal information about the environment and the progenitor star. When a star explodes in vacuum, the first photons to escape from its surface appear as a brief, hours-long shock-breakout flare1,2, followed by a cooling phase of emission. However, for stars exploding within a distribution of dense, optically thick circumstellar material (CSM), the first photons escape from the material beyond the stellar edge and the duration of the initial flare can extend to several days, during which the escaping emission indicates photospheric heating3. Early serendipitous observations2,4 that lacked ultraviolet (UV) data were unable to determine whether the early emission is heating or cooling and hence the nature of the early explosion event. Here we report UV spectra of the nearby SN 2023ixf in the galaxy Messier 101 (M101). Using the UV data as well as a comprehensive set of further multiwavelength observations, we temporally resolve the emergence of the explosion shock from a thick medium heated by the SN emission. We derive a reliable bolometric light curve that indicates that the shock breaks out from a dense layer with a radius substantially larger than typical supergiants. Using ultraviolet data as well as a comprehensive set of further multiwavelength observations of the supernova 2023ixf, a reliable bolometric light curve is derived that indicates the heating nature of the early emission.

SOXS (Son Of X-Shooter) is the new ESO instrument that is going to be installed on the 3.58-m New Technology Telescope at the La Silla Observatory. SOXS is a single object spectrograph offering a wide simultaneous spectral coverage from U- to H-band. Although such an instrument may have potentially a large variety of applications, the consortium designed it with a clear science case: it is going to provide the spectroscopic counterparts to the ongoing and upcoming imaging surveys, becoming one of the main follow-up instruments in the Southern hemisphere for the classification and characterization of transients. The NTT+SOXS system is specialized to observe all transients and variable sources discovered by imaging surveys with a flexible schedule maintained by the consortium, based on a remote scheduler which will interface with the observatory software infrastructure. SOXS is realized timely to be highly synergic with transients discovery machines like the Vera C. Rubin Observatory. The instrument has been integrated and tested in Italy, collecting and assembling subsystems coming from all partners spread over six countries in three continents. The first preparatory activities in Chile have been completed at the telescope. This article gives an updated status of the project before the shipping of the instrument to Chile.

This paper presents the opto-mechanical integration and alignment, functional and optical performance verification of the NIR arm of Son Of X-Shooter (SOXS) instrument. SOXS will be a single object spectroscopic facility for the ESO-NTT 3.6-m telescope, made by two arms high efficiency spectrographs, able to cover the spectral range 350-2050 nm with a mean resolving power R similar to 4500. In particular the NIR arm is a cryogenic echelle cross-dispersed spectrograph spanning the 780-2050 nm range. We describe the integration and alignment method performed to assemble the different optomechanical elements and their installation on the NIR vacuum vessel, which mostly relies on mechanical characterization. The tests done to assess the image quality, linear dispersion and orders trace in laboratory conditions are summarized. The full optical performance verification, namely echellogram format, image quality and resulting spectral resolving power in the whole NIR arm (optical path and science detector) is detailed. Such verification is one of the most relevant prerequisites for the subsequent full instrument assembly and provisional acceptance in Europe milestone, foreseen in 2024.

This is the first in a collection of three papers introducing the science with an ultra-violet (UV) space telescope on an approximately 130~kg small satellite with a moderately fast re-pointing capability and a real-time alert communication system approved for a Czech national space mission. The mission, called Quick Ultra-Violet Kilonova surveyor - QUVIK, will provide key follow-up capabilities to increase the discovery potential of gravitational wave observatories and future wide-field multi-wavelength surveys. The primary objective of the mission is the measurement of the UV brightness evolution of kilonovae, resulting from mergers of neutron stars, to distinguish between different explosion scenarios. The mission, which is designed to be complementary to the Ultraviolet Transient Astronomy Satellite - ULTRASAT, will also provide unique follow-up capabilities for other transients both in the near- and far-UV bands. Between the observations of transients, the satellite will target other objects described in this collection of papers, which demonstrates that a small and relatively affordable dedicated UV-space telescope can be transformative for many fields of astrophysics.

DeepSpec is a novel R similar to 650 broad-band (365nm-900nm) spectrograph for the Weizmann Multi-Aperture Spectroscopic Telescope (MAST), an array of 20 x 61 cm prime focus telescopes dedicated for spectroscopic observations, at the Weizmann astronomical observatory in Israel. MAST/DeepSpec is capable of either acting as a single 2.7m telescope or multiplexing over the entire sky using smaller flexible groups. This approach will make the combination of MAST/DeepSpec a unique facility worldwide in terms of its low cost, flexibility, and efficiency, capable of observing up to hundreds of targets per night spread over large areas. With an end-to-end throughput of > 65%, DeepSpec will be able to observe targets fainter than 20 mag in a 15-min exposure using all telescopes, or tens/hundreds of spectra per hour of 18-19 magnitude targets using smaller groups of telescopes making it an ideal instrument for time-domain astronomy. All optics and hardware have been delivered during fall 2023 and DeepSpec is currently in the assembly, integration, and testing phase. It is planned to start on-sky commissioning in late summer 2024.

SOXS (Son Of X-Shooter) is the new single object spectrograph for the ESO New Technology Telescope (NTT) at the La Silla Observatory, able to cover simultaneously both the UV-VIS and NIR bands (350-2000 nm). The instrument is currently in the integration and test phase, approaching the Preliminary Acceptance in Europe (PAE) before shipment to Chile for commissioning. After the assembly and preliminary test of the control electronics at INAF - Astronomical Observatory of Capodimonte (Napoli), the two main control cabinets of SOXS are now hosted in Padova, connected to the real hardware. This contribution describes the final electronic cabinets layout, the control strategy and the different integration phases, waiting for the Preliminary Acceptance in Europe and the installation of the instrument in Chile.

Observing a supernova explosion shortly after it occurs can reveal important information about the physics of stellar explosions and the nature of the progenitor stars of supernovae (SNe). When a star with a well-defined edge explodes in vacuum, the first photons to escape from its surface appear as a brief shock-breakout flare. The duration of this flare can extend to at most a few hours even for nonspherical breakouts from supergiant stars, after which the explosion ejecta should expand and cool. Alternatively, for stars exploding within a distribution of sufficiently dense optically thick circumstellar material, the first photons escape from the material beyond the stellar edge, and the duration of the initial flare can extend to several days, during which the escaping emission indicates photospheric heating. The difficulty in detecting SN explosions promptly after the event has so far limited data regarding supergiant stellar explosions mostly to serendipitous observations that, owing to the lack of ultraviolet (UV) data, were unable to determine whether the early emission is heating or cooling, and hence the nature of the early explosion event. Here, we report observations of SN 2023ixf in the nearby galaxy M101, covering the early days of the event. Using UV spectroscopy from the Hubble Space Telescope (HST) as well as a comprehensive set of additional multiwavelength observations, we trace the photometric and spectroscopic evolution of the event and are able to temporally resolve the emergence and evolution of the SN emission.