<title>Feasibility study of a Laue lens for hard x rays for space astronomy</title>

Abstract

We report on the feasibility study of a Laue lens for hard X-rays (> 60 keV) based on mosaic crystals, for astrophysical applications. In particular we discuss the scientific motivations, its functioning principle, the procedure followed to select the suitable crystal materials, the criteria adopted to establish crystal dimensions and their distribution on the lens in order to obtain the best lens focusing capabilities, and the criteria for optimizing the lens effective area in a given passband. We also discuss the effects of misalignments of the crystal tiles due to unavoidable mechanical errors in assembling the lens. A software was developed to face all these topics and to evaluate the expected lens performance. SCIENTIFIC MOTIVATIONSThe role of hard X-ray astronomy (> 10 keV) is now widely recognized. The numerous results obtained with the most recent satellite missions (BeppoSAX , Rossi-XTE ) on many classes of X-ray celestial sources have demonstrated the importance of the broad band (0.1 ÷ 300 keV) spectroscopy in order to derive an unbiased picture of the celestial source physics, like to establish the source geometry, the physical phenomena occurring in the emission region, the radiation production mechanisms, an unbiased separation of the contribution of thermal emission phenomena from the phenomena due to the presence of high energy plasmas (thermal or not thermal) and/or magnetic fields and/or source rotation.In spite of the excellent performance of the high energy instrument PDS (Phoswich Detection System) aboard BeppoSAX , 1 the most sensitive instrument ever flown in the 15-200 keV energy band, even in the case of the strongest Galactic (e.g., Cyg X-1 in soft state, Her X-1) and extragalactic (e.g., 3C373, MKN 3) X-ray sources, the statistical quality of the measured spectra becomes poor in the highest part of the instrument passband (> 80 keV). Thus the development of focusing optics in this band and, more generally, in the entire passband covered by the BSAX/PDS and possibly beyond it, is of key importance to overcome the limitations of the direct viewing telescopes (with or without masks) and to allow the study of the high energy spectra of the celestial sources with the same detail which is achieved at lower energies (< 10 keV), where focusing optics are available.In fact, X-ray mirrors, based on the external reflections with very high focal lengths (≥ 50 m), or 'supermirrors' based on Bragg diffraction from multilayers of bi-strates made of high and low Z materials (e.g., Joensen et al. 2 ), can overcome the sensitivity problem up to about 70 keV. For higher energy photons, an efficient focusing is a much more challenging task.Goal of our project is the development of a focusing telescope which efficiently focus hard X-/gamma-rays in a broad continuous band, from 70 keV to ≥ 300 keV, by exploiting the Bragg diffraction from mosaic crystals in Laue configuration.

Gamma-ray lens development status for a European gamma-ray imager

“…1) affects the lens performance. A higher spread gives a larger effective area, 3 but also produces a larger defocusing of the reflected photons in the focal plane, as can be seen in Fig. 3, in which we show how the radius of the circle which contains 50% of the reflected photons in the focal plane (Half Power Radius, HPR) changes with the focal length for various values of the mosaic spread.…”

“…3-5 They concern a theoretical feasibility study devoted to establish the best design of a Laue lens telescope along with its sensitivity expectations, 3,4 Monte Carlo simulations of the expected optical properties of Laue lenses, reflectivity measurements of mosaic crystal samples of Cu(111). 5 Here we summarize the most relevant results.…”

HAXTEL: A Laue lens telescope development project for a deep exploration of the hard X-ray sky (> 60 keV)

“…Cu (111) shows the best reflectivity above 100 keV. Reprinted from [3] lens passband (E min , E max ), where…”

“…We summarize here the results of a theoretical feasibility study devoted to establish the best design of a Laue lens telescope along with its sensitivity expectations [3].…”

Laue Gamma-Ray Lenses for Space Astrophysics: Status and Prospects

“…It can be seen [11,[29][30][31] that, even if a higher mosaicity gives a larger lens effective area (see Figure 9), a higher spread also produces a larger defocusing of the reflected photons in the focal plane and thus a lower lens sensitivity.…”