Astronomy Letters

We show that Compton scattering by electrons of the hot intergalactic gas in galaxy clusters should lead to peculiar distortions of the cosmic background X-ray and soft gamma-ray radiation—an increase in its brightness at hv ≲ 60−100 keV and a drop at higher energies. The background distortions are proportional to the cluster gas surface density, in contrast to the intensity of the thermal gas radiation proportional to the density squared, which allows the most important cluster parameters to be measured. The spectral shape of the background distortions and its dependence on the gas temperature, optical depth, and surface density distribution law have been studied using detailed Monte Carlo calculations and conformed by analytical estimations. In the cluster frame the maximum of the background decrease due to the recoil effect occurs at hv ∼ 500−600 keV. The photoionization of hydrogen- and helium-like iron and nickel ions leads to additional distortions in the background spectrum—a strong absorption line with the threshold at hv ∼ 9 keV (and also to an absorption jump at hv ≳ 2 keV for cold clusters). The absorption of intrinsic thermal radiation from the cluster gas by these ions also leads to such lines. In nearby (z ≲ 1) clusters the line at hv ∼ 2 keV is noticeably enhanced by absorption in the colder (∼10⁶ K) plasma of their peripheral (≲3 Mpc) regions; moreover, the absorption line at hv ∼ 1.3 keV, which does not depend on the properties of the hot cluster gas, splits off from it. The redshift of distant clusters shifts the absorption lines in the background spectrum (at ∼2, ∼9, and ∼500 keV) to lower energies. Thus, in contrast to the microwave background radiation scattering effect, this effect depends on the cluster redshift z, but in a very peculiar way. When observing clusters at z ≳ 1, the effect allows one to determine how the X-ray background evolved and how it was “gathered” with z. To detect the effect, the accuracy of measurements should reach ∼0.1%. We consider the most promising clusters for observing the effect and discuss the techniques whereby the influence of the thermal gas radiation hindering the detection of background distortions should be minimal.

We continue the study begun in Karasev et al. (2018) and present the results of our optical identifications of four hard X-ray sources from the INTEGRAL sky surveys. Having first improved the positions of these objects in the sky with the X-ray telescope (XRT) of the Swift observatory, we have identified their counterparts using optical and infrared sky survey data. Then, we have obtained optical spectra for the putative counterparts with the RTT-150 Russian-Turkish telescope and the AZT-33IK telescope. This has allowed the nature of the objects under study to be established. The sources IGR J11079+7106 and IGR J12171+7047 have turned out to be extragalactic in nature and be Seyfert 1 and 2 galaxies, respectively, with the second object being characterized by a large absorption column density. The source IGR J18165−3912 is most likely an intermediate polar with a very high luminosity. The fourth source, IGR J20596+4303, is a chance superposition of two objects—a Seyfert 2 galaxy and a cataclysmic variable.

The possibility of obtaining dynamical estimates of the total masses in the two Trojan asteroid groups of Jupiter is investigated. The compact Greek (L4) and Trojan (L5) groups contain several tens of thousands of asteroids near the stable Lagrange points moving in a 1: 1 resonance with the orbital motion of Jupiter. The dynamical mass estimates

\(\begin{array}{*{20}{c}} {{M_{L4}} = (8.63 \pm 0.51) \times {{10}^{ - 6}}{M_ \oplus },} \\ {{M_{L5}} = (5.46 \pm 0.54) \times {{10}^{ - 6}}{M_ \oplus }} \end{array}\)