Abstract Gamma-ray bursts (GRBs) are the most luminous stellar explosions in the Universe, in which relativistic jets are involved. GRB 221009A stands out as the brightest GRB ever observed, providing a unique opportunity to probe the properties of an ultrarelativistic jet. Here, we report our very long baseline interferometry (VLBI) observation on the extremely bright GRB 221009A from 5 to 26 days after the burst. An intriguing two-stage evolution of the source size of this GRB is revealed by combining this data with later VLBI data. The observed size initially increases slowly as ~ tobs0.15 during the first month, but expands with a much faster rate as ~ tobs2.21 at later times. This unprecedented two-stage size evolution cannot be explained by the existing theoretical models invoking a non-spreading top-hat jet or a conventional structured jet and thus challenges our current understanding of this GRB jet. The transition may suggest the beginning of rapid sideways expansion of a narrow jet as the relativistic jet decelerates enough or the emergence of a new emission component. Our findings unveil the complex jet dynamics of GRB 221009A, highlighting the necessity of direct measurements of the size and geometry of the emission region in studying low-redshift GRBs in the multi-wavelength era.

This paper presents an overview of the QUARKS survey, which stands for “Querying Underlying mechanisms of massive star formation with ALMA-Resolved gas Kinematics and Structures.” The QUARKS survey is observing 139 massive clumps covered by 156 pointings at Atacama Large Millimeter/submillimeter Array (ALMA) Band 6 (λ ∼ 1.3 mm). In conjunction with data obtained from the ALMA-ATOMS survey at Band 3 (λ ∼ 3 mm), QUARKS aims to carry out an unbiased statistical investigation of massive star formation process within protoclusters down to a scale of 1000 au. This overview paper describes the observations and data reduction of the QUARKS survey, and gives a first look at an exemplar source, the mini-starburst Sgr B2(M). The wide-bandwidth (7.5 GHz) and high-angular-resolution (∼0.″3) observations of the QUARKS survey allow for the resolution of much more compact cores than those could be done by the ATOMS survey, and to detect previously unrevealed fainter filamentary structures. The spectral windows cover transitions of species including CO, SO, N2D+, SiO, H30 α, H2CO, CH3CN, and many other complex organic molecules, tracing gas components with different temperatures and spatial extents. QUARKS aims to deepen our understanding of several scientific topics of massive star formation, such as the mass transport within protoclusters by (hub-)filamentary structures, the existence of massive starless cores, the physical and chemical properties of dense cores within protoclusters, and the feedback from already formed high-mass young protostars.

The star-forming clumps in star-bursting dwarf galaxies provide valuable insights into understanding the evolution of dwarf galaxies. In this paper, we focus on five star-bursting dwarf galaxies featuring off-centered clumps in the Mapping Nearby Galaxies at Apache Point Observatory survey. Using the stellar population synthesis software Fitting Analysis using Differential evolution Optimization, we obtain the spatially resolved distribution of the star formation history, which allows us to construct the g-band images of the five galaxies at different ages. These images can help us to probe the evolution of the morphological structures of these galaxies. While images of a stellar population older than 1 Gyr are typically smooth, images of a stellar population younger than 1 Gyr reveal significant clumps, including multiple clumps which appear at different locations and even different ages. To study the evolutionary connections of these five galaxies to other dwarf galaxies before their star-forming clumps appear, we construct the images of the stellar populations older than three age nodes, and define them to be the images of the “host” galaxies. We find that the properties such as the central surface brightness and the effective radii of the hosts of the five galaxies are in between those of dwarf ellipticals (dEs) and dwarf irregulars (dIrrs), with two clearly more similar to dEs and one more similar to dIrrs. Among the five galaxies, 8257-3704 is particularly interesting, as it shows a previous starburst event that is not quite visible from its gri image, but only visible from images of the stellar population at a few hundred million years. The star-forming clump associated with this event may have appeared at around 600 Myr ago and disappeared at around 40 Myr ago.

ABSTRACT This paper presents GPFC, a novel Graphics Processing Unit (GPU) Phase Folding and Convolutional Neural Network (CNN) system to detect exoplanets using the transit method. We devise a fast-folding algorithm parallelized on a GPU to amplify low signal-to-noise ratio transit signals, allowing a search at high precision and speed. A CNN trained on two million synthetic light curves reports a score indicating the likelihood of a planetary signal at each period. While the GPFC method has broad applicability across period ranges, this research specifically focuses on detecting ultrashort-period planets with orbital periods less than one day. GPFC improves on speed by three orders of magnitude over the predominant Box-fitting Least Squares (BLS) method. Our simulation results show GPFC achieves 97 per cent training accuracy, higher true positive rate at the same false positive rate of detection, and higher precision at the same recall rate when compared to BLS. GPFC recovers 100 per cent of known ultrashort-period planets in Kepler light curves from a blind search. These results highlight the promise of GPFC as an alternative approach to the traditional BLS algorithm for finding new transiting exoplanets in data taken with Kepler and other space transit missions such as K2, TESS, and future PLATO and Earth 2.0.