Gamma Ray Fresnel lenses - why not?

Fresnel lenses offer the possibility of concentrating the flux of X-rays or gamma-rays flux falling on a geometric area of many square metres onto a focal point which need only be a millimetre or so in diameter (and which may even be very much smaller). They can do so with an efficiency that can approach 100%, and yet they are easily fabricated and have no special alignment requirements. Fresnel lenses can offer diffraction-limited angular resolution, even in a domain where that limit corresponds to less than a micro second of arc. Given all these highly desirable attributes, it is natural to ask why Fresnel gamma ray lenses are not already being used, or at least why there is not yet any mission that plans to use the technology. Possible reasons (apart from the obvious one that nobody thought of doing so) include the narrow bandwidth of simple Fresnel lenses, their very long focal length, and the problems of target finding. It is argued that none of these is a "show stopper" and that this technique should be seriously considered for nuclear astrophysics.Comment: Presented at "Gamma Wave 2005", Bonifacio, September 2005. To be published in "Experimental Astronomy

Rapid accretion state transitions following the tidal disruption event AT2018fyk

High Energy Astrophysic

A black hole x-ray binary at ∼100 Hz : multiwavelength timing of MAXI J1820+070 with HiPERCAM and NICER

We report on simultaneous sub-second optical and X-ray timing observations of the low mass X-ray binary black hole candidate MAXI J1820+070. The bright 2018 outburst rise allowed simultaneous photometry in five optical bands (ugrizs) with HiPERCAM/GTC (Optical) at frame rates over 100 Hz, together with NICER/ISS observations (X-rays). Intense (factor of two) red flaring activity in the optical is seen over a broad range of timescales down to ∼ 10 ms. Cross-correlating the bands reveals a prominent anti-correlation on timescales of ∼ seconds, and a narrow sub-second correlation at a lag of ≈ +165 ms (optical lagging X-rays). This lag increases with optical wavelength, and is approximately constant over Fourier frequencies of ∼ 0.3–10 Hz. These features are consistent with an origin in the inner accretion flow and jet base within ∼ 5000 Gravitational radii. An additional ∼ +5 s lag feature may be ascribable to disc reprocessing. MAXI J1820+070 is the third black hole transient to display a clear ∼ 0.1 s optical lag, which may be common feature in such objects. The sub-second lag variation with wavelength is novel, and may allow constraints on internal shock jet stratification models