Detection and characterisation of submm transient sources with a large single-dish telescope

The exploration of the time-variable astronomical sky at submm wavelengths is rapidly becoming more feasible with large sky surveys by Cosmic Microwave Background telescopes with tens of thousands of detectors. Observations with the Atacama Cosmology Telescope and South Pole Telescope have already detected some transients, and Simons Observatory and CCAT are expected to detect many more in the near future. Follow-up observations to characterise these transients, and surveying to uncovering fainter populations, will need high sensitivity and large fields of view at submm wavelengths, which could be provided by large single dish telescopes such as AtLAST.

💡 Research Summary

The paper presents a compelling case for using a next‑generation, large (≈50 m) single‑dish sub‑millimetre (sub‑mm) telescope—specifically the AtLAST concept—to discover and characterise transient sources at sub‑mm wavelengths. The authors begin by noting that time‑domain astronomy in the sub‑mm regime has moved from a niche activity to a realistic prospect thanks to the unprecedented detector counts (tens of thousands) on modern Cosmic Microwave Background (CMB) experiments. Existing facilities such as the Atacama Cosmology Telescope (ACT) and the South Pole Telescope (SPT) have already reported a handful of sub‑mm transients, and the Simons Observatory (SO) is expected to deliver tens to hundreds of detections per year as its Large Aperture Telescope (LAT) surveys the sky at 30–313 GHz. Future projects like CMB‑S4 and CCAT‑prime will only increase this rate.

The authors categorise the expected transient population into Galactic and extragalactic sources. Galactic transients include flaring low‑mass stars (timescales < 3 days), protostars with variable accretion (months to years), and moving Solar‑System objects such as asteroids, which appear as transient sources because of their rapid apparent motion. Sub‑mm observations are uniquely valuable for protostars because dust emission at the shortest sub‑mm wavelengths penetrates the dense envelopes that obscure optical/IR light. Extragalactic transients are dominated by gamma‑ray bursts (GRBs), supernovae (SNe), tidal disruption events (TDEs), and the enigmatic fast blue optical transients (FBO‑Ts). The latter are of particular interest because their origins remain uncertain; sub‑mm detections could reveal whether they are dust‑enshrouded or have strong synchrotron components. TDEs, especially those embedded in dusty nuclei, are expected to produce bright sub‑mm afterglows that are invisible in the optical.

Three complementary detection strategies are outlined. First, a large single‑dish telescope can conduct its own multi‑epoch surveys, either targeting specific star‑forming regions or performing wide‑area scans for other science goals. The JCMT Transient Survey of protostars is cited as a prototype; a 50 m telescope with a 2° field of view (FoV) could extend such work by orders of magnitude in depth and sky coverage. Second, the telescope can act as a rapid‑response instrument, following up transients flagged by CMB surveys (ACT, SPT, SO, CCAT‑prime). Because CMB experiments monitor large fractions of the sky continuously, they will generate a steady stream of candidate events. Prompt sub‑mm follow‑up would improve localisation (critical for host identification), provide multi‑frequency light curves as the source fades, and enable archival searches in the original CMB data to reconstruct earlier phases. Third, the telescope can respond to alerts from other wavelengths—optical alerts from the Rubin Observatory LSST, radio alerts from ASKAP, MeerKAT, or the future SKA, and even gravitational‑wave triggers from LIGO/Virgo/KAGRA or the planned LISA and Einstein Telescope. While ALMA offers exquisite angular resolution, its oversubscription and single‑band observing mode limit its utility for large‑scale, time‑critical follow‑up. A fast‑mapping, multi‑band single dish can fill this niche.

Technical requirements are discussed in detail. The ideal instrument must combine a large collecting area (≈50 m aperture), a broad instantaneous frequency coverage (≈30–950 GHz), a wide FoV (≥2°), and a focal‑plane populated with tens of thousands of detectors to achieve high mapping speed. High‑altitude siting is essential to minimise atmospheric opacity, especially at the highest frequencies. Flexible scheduling (rapid ToO response) is also mandatory. The Large Millimetre Telescope (LMT) satisfies some of these criteria (aperture, partial frequency coverage) but lacks the required FoV and detector count. By contrast, the AtLAST design—featuring a 50 m primary, a 2° FoV, and up to 60 k detectors—matches the science case. A potential complication is contamination from thermal emission of low‑Earth‑orbit satellite constellations, which can mimic transient signals. The authors argue that early implementation of sub‑mm observations is prudent, as satellite constellations are proliferating and mitigation (e.g., cross‑referencing satellite ephemerides) will become increasingly complex.

In the concluding section, the authors assert that a 50 m class sub‑mm single‑dish telescope would dramatically increase the detection rate of sub‑mm transients, improve localisation, and enable detailed spectral and temporal characterisation. Such a facility would complement existing and upcoming observatories (ALMA, SO, CCAT‑prime, Rubin LSST, SKA, gravitational‑wave detectors) and synergise with planned infrared missions like PRIMA. Beyond transient science, the telescope would also serve a broad range of astrophysical programmes, reinforcing its value as a flagship ground‑based facility for the 2030s and beyond.