A new generation of optical intensity interferometers are emerging in recent years taking advantage of the existing infrastructure of Imaging Atmospheric Cherenkov Telescopes (IACTs). The MAGIC SII (Stellar Intensity Interferometer) in La Palma, Spain, has been operating since its first successful measurements in 2019 and its current design allows it to operate regularly. The current setup is ready to follow up on bright optical transients, as changing from regular gamma-ray observations to SII mode can be done in a matter of minutes. A paper studying the system performance, first measurements and future upgrades has been recently published. MAGIC SII’s first scientific results are the measurement of the angular size of 22 stars, 13 of which with no previous measurements in the B band. More recently the Large Sized Telescope prototype from the Cherenkov Telescope Array Observatory (CTAOLST1) has been upgraded to operate together with MAGIC as a SII, leading to its first correlation measurements at the beginning of 2024. MAGIC+CTAO-LST1 SII will be further upgraded by adding the remaining CTAOLSTs at the north site to the system (which are foreseen to be built by the end of 2025). MAGIC+CTAO-LST1 SII shows a feasible technical solution to extend SII to the whole CTAO.
The two MAGIC 17-m diameter Imaging Atmospheric Cherenkov Telescopes have been equipped to work also as an intensity interferometer with a deadtime-free, 4-channel, GPU-based, real-time correlator. Operating with baselines between ∼40 and 90 m the MAGIC interferometer is able to measure stellar diameters of 0.5 - 1 mas in the 400-440 nm wavelength range with a sensitivity roughly 10 times better than that achieved in the 1970’s by the Narrabri Stellar Intensity Interferometer. Besides, active mirror control allows to split the primary mirrors into sub-mirrors. This allows to make simultaneous calibration measurements of the zero-baseline correlation or to simultaneously collect six baselines below 17 m with almost arbitrary orientation, corresponding to angular scales of ∼1-50 mas. We plan to perform test observations adding the nearby Cherenkov Telescope Array (CTA) LST-1 23 m diameter telescope by next year. All three telescope pairs will be correlated simultaneously. Adding LST-1 is expected to increase the sensitivity by at least 1 mag and significantly improve the u-v plane coverage. If successful, the proposed correlator setup is scalable enough to be implemented to the full CTA arrays.
MAGIC is a system of two 17m diameter Imaging Atmospheric Cherenkov Telescopes (IACTs) situated on the observatory of the Roque de los Muchachos on the island of La Palma, 2200m asl. MAGIC is currently in operation for the observation of VHE gamma-ray sources. Here we report the installation of a system to perform intensity interferometry observations with these telescopes. We investigated the feasibility of this technique for these telescopes to be used for high resolution observation of stellar objects. The optics and electronics of MAGIC are designed for single photoelectron resolution and a sub-nano second timing precision and they are ideally suited for optical interferometry observations. During easter 2019 we have installed a simple setup using the central PMT pixels in the camera to test the working principle that MAGIC can be used for intensity interferometry observations. We demonstrate MAGIC is able to reach resolutions of about 0.7 mas with a baseline of 85m between the telescopes. Thanks to the large mirror area of MAGIC of about 239 m 2 we achieve a sensitivity of roughly 10 times better than that achieved in the 1970's with the Narrabri interferometer. Currently, a professional readout with GPU realtime analysis has been developed that will be used in the near future for observations.
The MAGIC telescopes are a system of two imaging atmospheric Cherenkov telescopes designed to observe very-high-energy γ rays. MAGIC utilizes a large reflective surface and photodetectors with ultrafast time response to capture Cherenkov photons. These features, together with the dedicated system installed in the central photomultiplier tube of their camera, so-called central pixel system (CPS), turn MAGIC into a suitable telescope to study high-speed optical astronomy in the millisecond (ms) regime. We report on the status of the CPS currently mounted in the MAGIC-II camera, its performance and calibration to demonstrate the sensitivity of MAGIC-II to ms optical pulses, for both transient and periodic signals, and discuss its potential over several science cases.
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