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Quasinormal modes of a black hole are closely related to the dynamics of the spacetime near the horizon. In this connection, the black hole ringdown phase is a powerful probe into the nature of gravity. However, the challenge of computing quasinormal mode frequencies has meant that ringdown tests of gravity have largely remained model-independent. In this talk, I will introduce Metric pErTuRbations wIth speCtral methodS (METRICS) [1], a novel spectral scheme capable of accurately computing the quasinormal mode frequencies of black holes, including those with modifications beyond Einstein's theory or the presence of matter. I will demonstrate METRICS' accuracy in calculating quasinormal mode frequencies within general relativity, as a validation, and its application to Einstein-scalar-Gauss-Bonnet gravity [2, 3], an example of modified gravity theory to which METRICS has been applied. I will also present preliminary results from applying METRICS to dynamical Chern-Simons gravity. Finally, I will discuss potential future applications of METRICS beyond computing black hole quasinormal modes.   [1]: https://arxiv.org/abs/2312.08435 [2]: https://arxiv.org/abs/2405.12280 [3]: https://arxiv.org/abs/2406.11986

Superconducting nanowire single photon detectors (SNSPDs) are of interest for intensity interferometry measurements because they have picosecond timing resolution. In addition, they work from the UV to mid-IR, with excellent eOiciency at visible and near-IR wavelengths, and are being fabricated into ever-larger detector arrays. On behalf of my colleagues in the JPL SNSPD group, I will present on the Deep Space Optical Communication (DSOC) demonstration, in which an SNSPD array was coupled to the 5 mHale telescope at Palomar, and received data at 267 Mbps from the Psyche spacecraft, the first optical communication between Earth and interplanetary space. The DSOC infrastructure at Palomar is suitable for intensity interferometry, as demonstrated by g(2) correlation (photon bunching) measurements of the stars Rigel and Procyon. I will also describe our current work on SNSPD array readout schemes, extending detector sensitivity into the mid-IR, and improving the system timing jitter of SNSPD arrays.

In this talk we want to promote a new kind of single photon detector able to record high count rates with the prospect of making stellar intensity interferometry (SII) measurements more effective. This micro-channel plate based photomultiplier tube from Photonscore (LINPix) is nearly dead time free and offers an active area of 8mm diameter. By choosing a matching photocathode, the quantum efficiency (QE) can take values greater than 30% at the desired wavelength. Using a Hi-QE Blue photocathode in a testbench featuring a fs-pulsed laser we were able to measure the timing resolution of the LINPix at different count rates from 190kHz up to 95 MHz. We find that the timing resolution of the detector only increases marginally when increasing the laser power and stays well below 100ps. Hence, we conclude that together with the LINTag, a suitable time to digital converter from Photonscore able to process the high throughput, this system can contribute significantly to the further development of SII.

Interferometry is the use of wave interference to measure the properties of a source observed by two or more detectors. For example, the Event Horizon Telescope measures the phase and amplitude of 1.3 mm wavelength radiation at telescopes up to ten thousand kilometers apart to reveal event horizon scale images of supermassive black holes. Measuring wave phases in the optical has been demonstrated for baselines no longer than hundreds of meters. Intensity interferometry dispenses with the need to measure phases, allowing much larger baselines, and hence much higher spatial resolution. The technique has been in use for seven decades, but recent advances in detector technology have reinvigorated interest in the method. I will discuss the basics of intensity interferometry, the characteristics of the new detectors, and possible applications of broad astrophysical and cosmological interest. The latter include estimates of the Hubble constant from observations of the disks of active galactic nuclei (AGN), with possible impact on the Hubble tension. The same observations will provide detailed information on the AGN disk and line emission regions; the latter may be crucial for estimating the mass loss rates in AGN winds, which are believed to impact their host galaxies. Other possible applications include spatially resolved measurements of stellar oscillations, which, by analogy with helioseismology, would provide constraints on the run of temperature in stellar interiors, as well as the interior differential rotation.

We present the design and initial results of a stellar intensity interferometer using small 0.25 m Newtonian-style telescopes in an urban backyard setting. The primary purpose of the interferometer is to measure the angular diameters of stars. Recent advances in low jitter time-tagging equipment and Single Photon Avalanche Detectors have made the detection of second-order correlation signals, necessary for Intensity Interferometry as demonstrated by Hanbury Brown and Twiss in 1956, feasible with small telescopes. Using Sirius as a target star, we observe a strong second-order correlation spike with an integrated signal to noise ratio (SNR) ∼7 after 13.55 h of integration over a three-night period using a 3.3 m baseline. The measured signal agrees with the theoretical estimates of both coherence time, 𝜏coh = 0.74 ± 0.26 ps and SNR. We discuss the future expansion of this technique with multiple wavelengths simultaneously via a prism grating and multiple detectors.

The Multi-Aperture Spectroscopic Telescope is an array of 20x60cm prime-focus telescopes with single F/3 parabolic mirrors. The telescope array is being commissioned in the Negev Desert, with 10 telescopes expected to see first light by the end of the year. In the following talk, I will present the array, its various properties, including unique fiber coupling and imaging units, and its potential as an intensity interferometry facility.

The VERITAS Imaging Atmospheric Cherenkov Telescope array was augmented in 2019 with high-speed focal plane electronics to allow VERITAS for Stellar Intensity Interferometry (VSII) observations. Since December 2019, VSII has been used to measure angular diameters of bright (OBA) stars at an effective wavelength of 416 nm. VSII observations have also served as a testbed to explore hardware and analysis improvements to advance the instrument's sensitivity. VSII has performed more than 730 hours of moonlit observations on 56 bright stars and binary systems ($ -1.46 < m_V < 4.22$). This talk will describe the VSII observatory, highlight selected observations made by the VSII observatory, and describe ongoing improvements in detector instrumentation and analysis.

The renaissance in stellar intensity interferometry has resulted in two main types of telescope arrays: those using large "light bucket" telescopes and photomultiplier tubes, such as CTA, VERITAS, MAGIC, and others, and those that instead use smaller, more traditional astronomical telescopes with high-grade optics, such as the systems at the Cote d'Azur and Asiago Observatories. To detect and timestamp photons, these latter systems have used single-photon avalanche diode (SPAD) detectors. This talk will focus on the latter type of instrument, which is also being pursued at Southern Connecticut State University. The current status of our instrument, the Southern Connecticut Stellar Interferometer (SCSI), will be reviewed, and prospects for improved sensitivity will be discussed. Principal among these is the use of SPAD arrays, which are increasingly available, to record different wavelengths simultaneously. If a sufficient number of channels can be employed, this type of intensity interferometer can reach much fainter magnitudes than currently possible. The talk will also briefly discuss work toward wireless intensity interferometry with SCSI, which will make larger baselines easier to set up and use, and ideas for quantum-assisted intensity interferometry that might be employed with SCSI in the future.