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Abstract ArgusSpec is a prototype autonomous spectroscopic follow-up instrument designed to characterize flares detected by the Argus Pathfinder telescope array by taking short exposure (30 s) broadband spectra (370–750 nm) at low resolutions (R∼ 150 at 500 nm). The instrument is built from consumer off-the-shelf astronomical equipment, assembled inside a shipping container, and deployed alongside the Argus Pathfinder at a dark sky observing site in Western North Carolina. In this paper, we describe the hardware design, system electronics, custom control software suite, automated target acquisition procedure, and data reduction pipeline. We present initial on-sky test data used to evaluate system performance and show a series of spectra taken of a small flare from AD Leonis. The $35k prototype ArgusSpec was designed, built, and deployed in under a year, largely from existing parts, and has been operating on-sky since 2023 March. With current hardware and software, the system is capable of receiving an observation, slewing, performing autonomous slit acquisition, and beginning data acquisition within an average of 32 s. With Argus Pathfinder’s 1 s cadence survey reporting alerts of rising sources within 2 s of onset, ArgusSpec can reach new targets well within a minute of the start of the event. As built, ArgusSpec can observe targets down to a 20σlimiting magnitude ofm_V≈ 13 at 30 s cadence with an optical resolution ofR∼ 150 (at 500 nm). With automated rapid acquisition demonstrated, later hardware upgrades will be based on a clean-sheet optical design, solving many issues in the current system, significantly improving the limiting magnitude, and potentially enabling deep spectroscopy by the coaddition of data from an array of ArgusSpec systems. The primary science driver behind ArgusSpec is the characterization of the blackbody evolution of flares from nearby M-dwarfs. Large flares emitted by these stars could have significant impacts on the potential habitability of any orbiting exoplanets, but our current understanding of these events is in large part built on observations from a handful of active stars. ArgusSpec will characterize large numbers of flares from across the night sky, building a spectroscopic library of the most extreme events from a wide variety of stellar masses and ages. 

Abstract Astrophysical transients with rapid developments on subhour timescales are intrinsically rare. Due to their short durations, events like stellar superflares, optical flashes from gamma-ray bursts, and shock breakouts from young supernovae are difficult to identify on timescales that enable spectroscopic follow-up. This paper presents the Evryscope Fast Transient Engine (EFTE), a new data reduction pipeline that is designed to provide low-latency transient alerts from the Evryscopes—a north–south pair of ultra-wide-field telescopes with an instantaneous footprint covering 38% of the entire sky—and tools for building long-term light curves from Evryscope data.EFTEleverages the optical stability of the Evryscopes by using a simple direct image subtraction routine that is suited to continuously monitoring the transient sky at a cadence of a minute. Candidates are produced within the base Evryscope 2 minute cadence for 98.5% of images, and internally filtered usingvetnet, a convolutional neural network real–bogus classifier.EFTEprovides an extensible and robust architecture for transient surveys probing similar timescales, and serves as the software test bed for the real-time analysis pipelines and public data distribution systems for the Argus Array, a next-generation all-sky observatory with a data rate 62 times higher than that of Evryscope. 

Abstract X-ray observations of low-mass stars in open clusters are critical to understanding the dependence of magnetic activity on stellar properties and their evolution. Praesepe and the Hyades, two of the nearest, most-studied open clusters, are among the best available laboratories for examining the dependence of magnetic activity on rotation for stars with masses ≲1 M ⊙ . We present an updated study of the rotation–X-ray activity relation in the two clusters. We updated membership catalogs that combine pre-Gaia catalogs with new catalogs based on Gaia Data Release 2. The resulting catalogs are the most inclusive ones for both clusters: 1739 Praesepe and 1315 Hyades stars. We collected X-ray detections for cluster members, for which we analyzed, re-analyzed, or collated data from ROSAT, the Chandra X-ray Observatory, the Neil Gehrels Swift Observatory, and XMM-Newton. We have detections for 326 Praesepe and 462 Hyades members, of which 273 and 164, respectively, have rotation periods—an increase of 6× relative to what was previously available. We find that at ≈700 Myr, only M dwarfs remain saturated in X-rays, with only tentative evidence for supersaturation. We also find a tight relation between the Rossby number and fractional X-ray luminosity L X / L bol in unsaturated single members, suggesting a power-law index between −3.2 and −3.9. Lastly, we find no difference in the coronal parameters between binary and single members. These results provide essential insight into the relative efficiency of magnetic heating of the stars’ atmospheres, thereby informing the development of robust age-rotation-activity relations.