Astrophysics

With the observation of gravitational waves (GW's) at high, kilohertz frequencies by LIGO and the evidence for GW's at low, nanohertz frequencies from NANOGrav there is a new emphasis on exploring the GW landscape at intermediate frequency ranges. Beyond the two measurement methods used in these observations, i.e. laser metrology in LIGO and pulsar timing offsets in NANOGrav, we have been developing a third approach of observing astrometric GW signatures, which is very well suited to the intermediate microhertz frequency range. While astrometric GW observations have been discussed in the context of survey missions, e.g. GAIA, this presentation will exhibit a potentially superior approach using long baseline two-photon interferometry, with both space-based and ground-based platforms. The practicalities of a near-future experiment will be particularly highlighted

Accretion flows aroud black-holes, neutron stars or white dwarfs are studied since almost 60 years. Although they are ubiquitous and somewhat similar over scales reaching billions in mass and size, their study has been limited because they remain unresolved point like sources in the optical/ultraviolet and X-rays, where they emit. Two main modes of accretion have been identified in Active Galactic Nuclei. In most sources the accretion rate is low and a high pressure, low density, low collision rate, optically thin, radiatively inefficient, two temperature plasma can form (Shapiro 1976; Narayan & Yi 1994,1995). This solution is stable only for low luminosities (<1% LEDD). The Event Horizon Telescope has recently resolved such flows in Sgr A and M87, confirmed several aspects of the model and could detect particles accelerated close to the horizon of Sgr A (Wielgus, 2022) a likely signature of the Blandford-Znajek (1977) process. When the accretion rate is higher, momentum can be dissipated by viscosity and the flow proceeds via geometrically thin disk-shaped structures. These accretion disks provide feedback to their environment by accelerating winds and launching jets in their central regions. The apparent size of accretion disks are of the order of 1-40µarcsec in nearby quasars, Seyfert galaxies and galactic cataclysmic variables and of 0.1-1µarcsec in of low mass X-ray binaries in our Galaxy. Hanbury-Brown & Twiss (1954) invented intensity interferometry and measured the size of some bright stars by correlating the arrival times of photons detected by two optical telescopes. The physics has been explained as a quantum effect in the early 60s (Fano 1961) and has triggered the development of quantum optics (Glauber 1963). Its root is found in the quantum theory of statistical fluctuations in an ideal gas (Einstein 1925). The achievable signal-to-noise depends on the telescope size, the detector time resolution, and the number of spectral channels observed simultaneously. Extremely large telescope and 10ps resolution single photon detectors bring the key improvements to reach in the optical angular resolutions better than these achieved in the radio by the Event Horizon Telescope and to obtain the first images of accretion disks around galactic and extragalactic compact objects, a breakthrough. I will present the goals and the status of the QUASAR project, which started one year ago, aiming at bringing a 10ps resolution optical spectrometer on very large telescope.

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.