Asteroid Occultations with VERITAS: Observations and Considerations

Occultations, the covering up of one celestial body by another celestial body, have been used in astronomy for millennia to learn about the sun and moon. Since 2018, VERITAS has implemented a program to detect predicted asteroid occultations, where an asteroid covers up a star. VERITAS has attempted to observe over 100 occultations to date and successfully observed 20 occultations. With these occultations, VERITAS can directly measure the smallest angular diameters of any instrument or technique in the optical for stars between magnitude 9 and 13. Each angular diameter is measured by fitting the diffraction pattern observed by the central VERITAS pixel at the start and end of an occultation. Once a planned FADC upgrade is complete, VERITAS will begin a program to search for serendipitous occultations within its full field of view (3 deg). Serendipitous occultations of sub-km trans-Neptunian objects (TNOs) have the potential to constrain models of solar system formation. This work details how VERITAS predicts and observes occultations as well as the overall status of the asteroid occultation program and future steps for observing occultations of both asteroids and TNOs.

💡 Research Summary

This paper presents a comprehensive overview of the asteroid occultation observation program conducted using the VERITAS (Very Energetic Radiation Imaging Telescope Array System) imaging atmospheric Cherenkov telescopes (IACTs). Since its inception in 2018, the program has aimed to detect predicted events where an asteroid passes in front of a star, temporarily blocking its light.

The core scientific motivation is the direct measurement of stellar angular diameters at sub-milliarcsecond scales, which is challenging for other optical techniques. VERITAS achieves this by fitting the Fresnel diffraction pattern generated at the edges of the asteroid’s shadow. The high-speed data acquisition is enabled by an Enhanced Current Monitor (ECM) system attached to central camera pixels, capable of sampling rates up to 4800 Hz. To date, VERITAS has attempted over 100 predicted occultations, with 20 successful detections. Among these, 11 events displayed clear diffraction fringes, allowing for angular diameter measurements of stars between visual magnitudes 9 and 13.

The paper details the operational workflow, from prediction to analysis. Prediction tools like Occult and OccultWatcher (OW) are used, with OW showing a significantly higher practical detection rate. Observations are filtered based on feasibility criteria like dark time, elevation, and predicted probability. The authors analyze the various reasons for non-detections, including weather, scheduling conflicts, prediction inaccuracies, and pixel saturation.

A significant portion of the paper is dedicated to technical considerations and lessons learned from seven years of operation. Key insights include:

1. The choice of prediction software greatly impacts efficiency.
2. The optical bandwidth of the IACT is the primary limiting factor for angular resolution. VERITAS’s ~146 nm bandwidth sets a fundamental limit on the measurable diffraction pattern contrast.
3. The irregular shape of asteroids can attenuate diffraction fringes, mimicking the signal of a larger star, which must be accounted for in analysis.
4. Improvements in timing accuracy (both absolute timestamp precision and electronic time delay) are needed for precise asteroid shape and astrometry studies.
5. The use of post-observation analysis software like SORA is crucial for deriving accurate asteroid parameters like shadow velocity.

The conclusion highlights the unique niche of IACTs in high-time-resolution optical astronomy. With minimal overhead, they can contribute to stellar astrophysics by measuring the smallest angular diameters accessible in the optical band, comparable to stellar intensity interferometry but at a lower cost. Furthermore, the program has broader impacts: it fosters connections with amateur astronomy networks like the International Occultation Timing Association (IOTA) and opens a new window for planetary science. IACTs have the potential to study asteroid satellites, rings, and atmospheres. Most notably, the paper outlines a future upgrade plan to equip VERITAS’s entire 3.5-degree field of view with fast optical readout. This would enable a search for serendipitous occultations by sub-kilometer Trans-Neptunian Objects (TNOs), observations that could place critical constraints on the size distribution of small bodies in the outer solar system and thereby inform models of solar system formation.