[Paper Review] (Very) Fast astronomical photometry for meter-class telescopes

L. Zampieri, G. Naletto+8 more|arXiv (Cornell University)|Aug 9, 2019

Adaptive optics and wavefront sensing27 references5 citations

TL;DR

This paper presents Aqueye+ and IFI+Iqueye, high-time-resolution photometers for meter-class telescopes, enabling sub-nanosecond photon timing for optical intensity interferometry and fast photometry. The instruments detect photon arrival times with ~100 ps accuracy, enabling the first kilometer-baseline optical intensity interferometry and detection of quasi-periodic oscillations (QPOs) at 268 ± 12 mHz in the X-ray binary MAXI J1820+070.

ABSTRACT

Our team at the INAF-Astronomical Observatory of Padova and the University of Padova is engaged in the design, construction and operations of instruments with very high time accuracy in the optical band for applications to High Time Resolution Astrophysics and Quantum Astronomy. Two instruments were built to perform photon counting with sub-nanosecond temporal accuracy, Aqueye+ and Iqueye. Aqueye+ is regularly mounted at the 1.8m Copernicus telescope in Asiago, while Iqueye was mounted at several 4m class telescopes around the world and is now attached through the Iqueye Fiber Interface at the 1.2m Galileo telescope in Asiago. They are used to perform coordinated high time resolution optical observations and, for the first time ever, experiments of optical intensity interferometry on a baseline of a few kilometers. I will report on recent technological developments and scientific results obtained within the framework of this project.

Motivation & Objective

• Enable very fast optical photometry with sub-nanosecond time resolution on meter-class telescopes for High Time Resolution Astrophysics.
• Develop and deploy instruments capable of photon arrival time tagging with ~100 ps relative accuracy and <500 ps absolute accuracy to UTC.
• Perform coordinated high-time-resolution observations and first kilometer-baseline optical intensity interferometry using fiber-fed Iqueye at the 1.2 m Galileo telescope.
• Monitor the Crab pulsar for long-term stability of radio-optical delay and detect QPOs in X-ray transient MAXI J1820+070.
• Explore the origin of low-frequency X-ray QPOs in black hole X-ray binaries through multiwavelength correlation and modeling.

Proposed method

• Use single-photon avalanche diodes (SPADs) with ~50% quantum efficiency and 30–50 ps time resolution to detect individual photons.
• Implement a pupil-splitting optical design dividing the telescope pupil into four segments, each focused on a separate SPAD to increase effective area and count rate.
• Time-tag each detected photon with ~100 ps relative accuracy and <500 ps absolute accuracy using a high-stability timing system synchronized to UTC.
• Store photon event lists in mass memory for post-processing with variable time bins (ns to minutes), enabling flexible spectral and timing analysis.
• Use the Iqueye Fiber Interface (IFI) to couple Iqueye to the 1.2 m Galileo telescope via a fiber-fed system with a 1:2 demagnification and beam splitting for field imaging.
• Perform cross-correlation of photon data from two distant telescopes to enable baseline-limited optical intensity interferometry experiments.

Experimental results

Research questions

• RQ1 Can sub-nanosecond optical photometry be achieved on meter-class telescopes to enable high-time-resolution astrophysics?
• RQ2 Can optical intensity interferometry be successfully performed over a few-kilometer baseline using fiber-fed instruments?
• RQ3 Do optical and infrared emissions from black hole X-ray binaries like MAXI J1820+070 exhibit quasi-periodic oscillations (QPOs) correlated with X-ray QPOs?
• RQ4 What is the origin of low-frequency X-ray QPOs in black hole X-ray binaries, and can optical QPOs help distinguish between models involving disc precession or jet modulation?
• RQ5 Is the radio-optical delay of the Crab pulsar stable over time, indicating no significant change in the geometry of emission regions?

Key findings

• The Aqueye+ and IFI+Iqueye instruments achieved sub-nanosecond photon time tagging with ~100 ps relative accuracy and <500 ps absolute accuracy, enabling high-precision timing.
• Optical intensity interferometry was successfully demonstrated over a few-kilometer baseline using Iqueye at the 1.2 m Galileo telescope via fiber coupling.
• In the X-ray transient MAXI J1820+070, a broad QPO-like feature was detected at 268 ± 12 mHz with a full-width-half-maximum of 150 ± 39 mHz and fractional rms variability of 1.9 ± 0.2%.
• A second, less significant QPO was detected at 151 ± 6 mHz with a width of 33 ± 16 mHz and fractional rms of 1.2 ± 0.3%.
• Acceptable fits to the power spectrum were obtained with harmonically related QPOs at 1:2, 2:3, or 3:5 frequency ratios, suggesting possible physical coupling.
• No significant variation in the Crab pulsar's radio-optical delay was detected over 10 years of monitoring, indicating stable relative geometry of emission regions.