Amit Kumar

Symbiotic binaries contain a white dwarf accreting material from a red giant star typically through a wind. We present the results of a search for outbursts from symbiotic binaries using photometric data obtained using the GOTO all-sky survey taken from 2023 onwards. After identifying ten candidate outbursting systems, we used ATLAS photometry to characterise their photometric behaviour before 2023, leaving five systems whose photometric behaviour was consistent with an outburst. The ATLAS data also provide colour information that gives insight into the nature of the outburst. The outburst from LMC N67 is the first reported Z And-type outburst from a symbiotic binary in the Large Magellanic Cloud. In OGLE SMC-LPV-4044 and HK Sco we found previously unreported outbursts. The ongoing outbursts of QW Sge and V4141 Sgr started in 2024 and have already been reported. By better identifying and characterising Z And-type outbursts from many systems, it will be possible to better understand the physics of these events, which are still not fully understood.

Context . Time-domain photometric monitoring is essential for characterizing cometary evolution, particularly for rare interstellar objects with limited observing opportunities. Aims . We aimed to characterize the pre-perihelion photometric behavior and dust activity of the interstellar comet 3I/ATLAS and to test the capability of the Black Hole Target and Observation Manager (BHTOM) platform and telescope network for coordinated high-cadence non-sidereal observations. Methods . We obtained 70 days of time-series photometry of 3I/ATLAS from 2025 July 4 to September 11 using 16 telescopes and 1554 images. The data were processed and calibrated with the BHTOM pipeline. High-cadence multiband imaging was used to measure the rotation period and color evolution, while the dust activity was quantified via Afρ measurements. Results . We present a pre-perihelion light curve of 3I/ATLAS from R _h = 3.18–2.19 au, which exhibited a steady brightening by ∼3 magnitudes with no evidence of anomalous behavior. We measured a rotation period of P _rot = 15.98 ± 0.08 h. The colors were statistically non-changing, with only a weak, non-significant tendency for 3I/ATLAS to become bluer at 3.5 > R _h > 2.2 au. The relative dust production increased from A (0°) fρ ∼600 to 1100 cm, and the upper limit on the dust mass-loss rate increased from ≤217 kg s ⁻¹ to ≤328 kg s ⁻¹ . We measured an activity index of n = −1.24 ± 0.02, which is consistent with a well-developed dust coma.

Wide-field and high-cadence sky surveys are the first step in the chain of discovery and characterisation of astrophysical transients such as supernovae, kilonovae, and tidal disruption events, each linked to the varied demise of stellar systems. The Gravitational-wave Optical Transient Observer (GOTO) is a telescope array of thirty-two 40 cm unit telescopes split over two almost antipodal sites. It performs a regular time-domain sky-survey in the optical to ~20 mag in addition to immediate scheduling of follow-up observations at the locations of external multi-wavelength and -messenger triggers. To facilitate the timely recovery of optical counterparts to these triggers, as well as the presence of serendipitous discoveries of astrophysical transients in the regular sky-survey, a low-latency data pipeline and workflow was developed. The implementation of this workflow is described herein and the quality of GOTO data delivered by it assessed, alongside its performance for prompt transient recovery. Utilising difference image analysis to identify candidate discoveries, the process is typically complete ~7 minutes after shutter close on the telescope. We further describe later processing of these candidates – both automated and human-in-the-loop – including reporting to the wider community and the triggering of more detailed observations, with a focus on immediate, intra-night characterisation. The workflow is meeting the needs of GOTO to promptly discover, report and characterise infant transients. Nevertheless, areas for further development and improvements are also highlighted.

Context. The long gamma-ray burst GRB 180728A at a redshift of z = 0.1171 stands out due to its high isotropic energy of E _γ, iso ≈ 2.5 × 10 ⁵¹ erg, in contrast with most events at redshift z < 0.2, but it is comparable to the bulk of luminous bursts more common at higher redshift. Aims. We aim to study the properties of GRB 180728A’s prompt emission, afterglow, and associated supernova (SN 2018fip), comparing them with other GRB-SN events. Methods. This study employs a dense photometric and spectroscopic follow-up of the afterglow and the SN up to 80 days after the burst. We used image subtraction to remove the presence of a nearby bright star, and modelled both the afterglow and the supernova. Results. This event lies on the E _p, i – E _γ, iso plane occupied by classical collapsar events, and the prompt emission is one of the most energetic at z < 0.2 after GRB 030329 and GRB 221009A. The afterglow of GRB 180728A is less luminous than that of most long GRBs, showing a shallow early phase that steepens after about 5 hours (0.2 days). The GRB exploded in an irregular low-mass blue star-forming galaxy, which is typical of low- z collapsar events. Because of the relatively faint afterglow, the light curve bump of SN 2018fip dominates the optical emission already after approximately 3 days and is one of the best sampled to date. The strong suppression below ∼4000 Å and a largely featureless continuum in the early 6–9 day spectra favour aspherical two-component ejecta with a high-velocity collimated component (> 20 000 km s ⁻¹ ), that is dominant early on and a more massive low-velocity component that dominates at much later epochs. Conclusions. Our findings indicate that asymmetries need to be considered in order to better understand GRB-SNe. In any case, SN 2018fip shares many characteristics with typical GRB-SNe. Its kinetic energy is below the common range of 10 ⁵² –10 ⁵³ erg and does not correlate with the high energy of the GRB, highlighting the complexity and diversity of the GRB-SN energy budget partition.

SN 2024aecx is a nearby (∼11 Mpc) Type IIb SN discovered within ∼1 day after explosion. In this paper we report high-cadence photometric (typically 0.5 ∼ 1 day) and spectroscopic follow-up observations, conducted from as early as 0.27 day post discovery out to the nebular phase at 158.4 days. We analyze the environment of SN 2024aecx and derive a new distance (11.3 ± 1.1 Mpc), metallicity and host extinction. The light curve exhibits a hot and luminous shock-cooling peak at the first few days, followed by a main peak with very rapid postmaximum decline. The earliest spectra are blue and featureless, while from 2.3 days after discovery prominent P-Cygni profiles emerge. At nebular phase, the emission lines exhibit asymmetric and double-peaked profiles, indicating asphericity and/or early dust formation in the ejecta. Nebular spectral modelling indicates a blueshifted O-rich clump moving toward observer, and the [O i ]/[Ca ii ] line ratio suggests an intermediate-mass progenitor. We simulated the progenitor and explosion using a two-component model of shock cooling and radioactive ⁵⁶ Ni heating; our model favors an extended, low-mass H-rich envelope with M _e = 0.04 ± 0.01 M _⊙ and a low ejecta mass of M ej = 1.5 5 − 0.14 + 0.18 M ⊙ . And the nebular-phase spectra and light-curve modelling both suggest that it most likely originated from an intermediate-mass binary progenitor system. The comprehensive monitoring of SN 2024aecx, coupled with the detailed characterization of its local environment, establishes it as a benchmark event for probing the progenitors and explosion mechanisms of Type IIb SNe.

SN 2025coe is a calcium-strong transient located at an extremely large projected offset ∼39.3 kpc from the center of its host, the nearby early-type galaxy NGC 3277 at a distance of ∼25.5 Mpc. In this paper, we present multiband photometric and spectroscopic observations spanning ∼100 days postdiscovery. Its multiband light curves display multiple distinct peaks: (1) an initial peak at t ≈ 1.6 day attributed to shock cooling emission, (2) a secondary peak of M _{R , peak} ≈ −15.8 mag at t ≈ 10.2 days powered by radioactive decay, and (3) a possible late-time bump at t ≈ 42.8 days likely caused by ejecta–circumstellar material/clump interaction. Spectral evolution of SN 2025coe reveals a fast transition to the nebular phase within 2 months, where it exhibits an exceptionally high [Ca II ]/[O I ] ratio larger than 6. Modeling of the bolometric light curve suggests an ejecta mass of M ej = 0.2 9 − 0.15 + 0.14 M ⊙ , a ⁵⁶ Ni mass of M 56 Ni = 2 . 4 − 0.05 + 0.06 × 1 0 − 2 M ⊙ , and a progenitor envelope with mass M e = 1 . 4 − 1.2 + 6.9 × 1 0 − 3 M ⊙ and radius R e = 13 . 5 − 11.1 + 64.1 R ⊙ . The tidal disruption of a hybrid HeCO white dwarf (WD) by a low-mass CO WD provides a natural explanation for the low ejecta mass, the small fraction of ⁵⁶ Ni, and the presence of an extended, low-mass envelope.

We present the analysis of the luminous Type II Supernova (SN) 2021tsz, which exploded in a low-luminosity galaxy. It reached a peak magnitude of −18.88 ± 0.13 mag in the r band and exhibited an initial rapid decline of 4.05 ± 0.14 mag (100 d) ⁻¹ from peak luminosity till ∼30 d. The photospheric phase is short, with the SN displaying bluer colours and a weak H α absorption component–features consistent with other luminous, short-photospheric phase Type II SNe. A distinct transition from the photospheric to the radioactive tail phase in the V band–as is common in hydrogen-rich Type II SNe–is not visible in SN 2021tsz, although a modest ∼1 mag drop is apparent in the redder filters. Hydrodynamic modelling suggests the luminosity is powered by ejecta-circumstellar material (CSM) interaction during the early phases (< 30 days). Interaction with 0.6 M _⊙ of dense CSM extending to 3100 R _⊙ reproduces the observed luminosity, with an explosion energy of 1.3 × 10 ⁵¹ erg. The modelling indicates a pre-SN mass of 9 M _⊙ , which includes a hydrogen envelope of 4 M _⊙ , and a radius of ∼1000 R _⊙ . Spectral energy distribution analysis and strong-line diagnostics revealed that the host galaxy of SN 2021tsz is a low-metallicity, dwarf galaxy. The low-metallicity environment and the derived high mass loss from the hydrodynamical modelling strongly support a binary progenitor system for SN 2021tsz.

Type Iax supernovae (SNe Iax) are one of the most common subclasses of thermonuclear supernova and yet their sample size, particularly those observed shortly after explosion, remains relatively small. In this paper, we present photometric and spectroscopic observations of two SNe Iax discovered shortly after explosion, SN 2024bfu and SN 2025qe. Both SNe were observed by multiple all-sky surveys, enabling tight constraints on the moment of first light and the shape of the early light curve. Our observations of SN 2025qe begin <2 d after the estimated time of first light and represent some of the earliest observations of any SN Iax. Spectra show features consistent with carbon absorption throughout the evolution of SN 2025qe, potentially indicating the presence of unburned material throughout the ejecta. We gather a sample of SNe Iax observed by the Asteroid Terrestrial-impact Last Alert System (ATLAS), Gravitational-wave Optical Transient Observer (GOTO), and Zwicky Transient Facility (ZTF) shortly after explosion and measure their rise times and early light curve power-law rise indices. We compare our results to a sample of normal SNe Ia and find indications that SNe Iax show systematically shorter rise times, consistent with previous work. We also find some indication that SNe Iax show systematically lower rise indices than normal SNe Ia. The low rise indices observed among SNe Iax are qualitatively consistent with extended $^{56}$Ni distributions and more thoroughly-mixed ejecta compared to normal SNe Ia, similar to predictions from pure deflagration explosions.