High Time Resolution Astrophysics in the Extremely Large Telescope Era : White Paper

High Time Resolution Astrophysics (HTRA) concerns itself with observations on short scales normally defined as being lower than the conventional read-out time of a CCD. As such it is concerned with condensed objects such as neutron stars, black holes and white dwarfs, surfaces with extreme magnetic reconnection phenomena, as well as with planetary scale objects through transits and occultations. HTRA is the only way to make a major step forward in our understanding of several important astrophysical and physical processes; these include the extreme gravity conditions around neutron stars and stable orbits around stellar mass black holes. Transits, involving fast timing, can give vital information on the size of, and satellites around exoplanets. In the realm of fundamental physics very interesting applications lie in the regime of ultra-high time resolution, where quantum-physical phenomena, currently studied in laboratory physics, may be explored. HTRA science covers the full gamut of observational optical/IR astronomy from asteroids to {\gamma}-rays bursts, contributing to four out of six of AstroNet’s fundamental challenges described in their Science Vision for European Astronomy. Giving the European-Extremely Large Telescope (E-ELT) an HTRA capability is therefore importance. We suggest that there are three possibilities for HTRA and E-ELT. These are, firstly giving the E-ELT first light engineering camera an HTRA science capability. Secondly, to include a small HTRA instrument within another instrument. Finally, to have separate fibre feeds to a dedicated HTRA instrument. In this case a small number of fibres could be positioned and would provide a flexible and low cost means to have an HTRA capability. By the time of E-ELT first light, there should be a number of significant developments in fast detector arrays, in particular in the infra-red (IR) region.

💡 Research Summary

The white paper makes the case that High‑Time‑Resolution Astrophysics (HTRA) is an essential capability for the European Extremely Large Telescope (E‑ELT) and outlines how it can be realized. HTRA is defined as observations on timescales shorter than the conventional CCD read‑out time, typically below one second and often down to micro‑ or nanosecond levels. This regime is crucial for studying compact objects (neutron stars, black holes, white dwarfs), rapid magnetic reconnection events, and planetary transits or occultations. The authors argue that HTRA directly addresses four of the six fundamental challenges identified by AstroNet’s Science Vision: probing strong gravity, understanding supernovae and gamma‑ray bursts, elucidating black‑hole accretion, jets and outflows, and investigating energetic radiation and particles.

The paper reviews key science cases. Pulsars and magnetars exhibit optical pulsations on microsecond scales; their pulse shapes and polarisation provide unique diagnostics of magnetospheric geometry and emission mechanisms. Current optical detections are limited to eight pulsars, but the 42‑m aperture of the E‑ELT would enable signal‑to‑noise ratios >5 for objects as faint as 30 mag, potentially expanding the sample by dozens. White dwarfs, both isolated and in compact binaries, show variability on seconds to minutes (rotation, pulsations, eclipses). High‑cadence photometry and time‑resolved spectroscopy are needed to measure masses, radii, and orbital period derivatives, but conventional instruments suffer from read‑out overheads and detector noise.

Three practical pathways to embed HTRA in the E‑ELT are proposed. (1) Equip the first‑light engineering camera with a fast photometric module, providing an immediate, low‑cost capability. (2) Integrate a compact HTRA sub‑instrument within a major facility instrument such as MICADO, leveraging the adaptive‑optics corrected PSF and allowing simultaneous science. (3) Deploy dedicated fibre feeds (fewer than ten) from the telescope focus to an independent HTRA spectro‑photometer, offering flexibility and the possibility of multi‑target observations.

A major technical driver is the development of fast infrared detector arrays. While electron‑multiplying CCDs (EMCCDs) already deliver sub‑microsecond frame rates in the visible, the near‑infrared (1–2 µm) still relies on prototype HgCdTe avalanche photodiode (APD) arrays. The authors anticipate that within the next five years these devices will achieve quantum efficiencies of 30–50 %, frame rates of ≥10 kHz, and dark currents below 0.01 e⁻ pixel⁻¹ s⁻¹, making them suitable for E‑ELT‑scale HTRA work.

Beyond astrophysics, ultra‑high‑time‑resolution observations open a window on quantum‑optical phenomena (photon bunching, entanglement) that have so far been confined to laboratory settings. Observing such effects from astronomical sources would link fundamental physics with high‑energy astrophysics.

In conclusion, the paper convincingly demonstrates that HTRA is not a peripheral add‑on but a core component of the E‑ELT’s scientific mission to explore new parameter space in the temporal domain. By adopting one or more of the proposed implementation strategies and capitalising on rapid advances in fast IR detectors, the E‑ELT can deliver groundbreaking insights into compact objects, transient phenomena, and even quantum optics in the cosmos.