Hard X-ray emission of the Earth's atmosphere: Monte Carlo simulations

We perform Monte Carlo simulations of cosmic ray-induced hard X-ray radiation from the Earth's atmosphere. We find that the shape of the spectrum emergent from the atmosphere in the energy range 25–300 keV is mainly determined by Compton scatterings and photoabsorption, and is almost insensitive to the incident cosmic ray spectrum. We provide a fitting formula for the hard X-ray surface brightness of the atmosphere as would be measured by a satellite-borne instrument, as a function of energy, solar modulation level, geomagnetic cut-off rigidity and zenith angle. A recent measurement by the INTEGRAL observatory of the atmospheric hard X-ray flux during the occultation of the cosmic X-ray background by the Earth agrees with our prediction within 10 per cent. This suggests that Earth observations could be used for in-orbit calibration of future hard X-ray telescopes. We also demonstrate that the hard X-ray spectra generated by cosmic rays in the crusts of the Moon, Mars and Mercury should be significantly different from that emitted by the Earth's atmosphere.     

An Upper Limit on Nickel Overabundance in the Supercritical Accretion Disk Wind of SS 433 from X-ray Spectroscopy

We analyze a long (with a total exposure time of 120 ks) X-ray observation of the unique Galactic microquasar SS 433 carried out by the XMM-Newton space observatory with the goal of searching for the fluorescent line of neutral (or weakly ionized) nickel at energy 7.5 keV. We consider two models for the formation of fluorescent lines in the spectrum of SS 433: (1) through the reflection of radiation from a putative central X-ray source off the optically thick neutral gas of the supercritical disk “funnel” walls; and (2) due to the scattering of the radiation coming from the hottest parts of the jets in the optically thin wind of the system. We show that for these two cases the flux of the Ni I K_α fluorescent line is expected to be 0.45 of the flux of the Fe I K_α fluorescent line at 6.4 keV for the relative nickel overabundance Z_Ni/Z = 10 observed in the jets of SS 433. For the continuum model without the absorption edge of neutral iron, we have found an upper limit on the flux of the narrow Ni I K_α fluorescent line of 0.9 × 10^?5 phot s^?1 cm^?2 (90% confidence level). In the continuum model with the absorption edge we have determined an upper limit on the flux of the Ni I K_α line at the level of 2.5×10^?5 phot s^?1 cm^?2. At the same time, the flux of the fluorescent iron line has been measured to be 9.9 _8.4 ^11.2 × 10^?5 phot s^?1 cm^?2. This result implies that the nickel overabundance in the accretion disk wind should be at least a factor of 1.5 times smal than the corresponding nickel overabundance observed in the jets of SS 433.     

Modeling Ice-Jam Floods in the Frameworks of an Intelligent System for River Monitoring

Water Resources - The article presents the results of adaptation and application of two-dimensional hydrodynamic model STREAM_2D to reproduce the characteristics of ice-jam-induced floods in the...     

Optical Spectroscopy of Quasars Discovered by SRG/eROSITA with a 2.5-m Telescope at the Caucasus Mountain Observatory of SAI MSU

Astronomy Letters - Based on observations with the transient double-beam spectrograph (TDS) at the 2.5-m telescope of the Caucasus Mountain Observatory of SAI MSU, we have determined the redshifts...     

Gas heating inside radio sources to mildly relativistic temperatures via induced compton scattering

The measured brightness temperatures of the low-frequency synchrotron radiation from intense extragalactic sources reach 10¹¹–10¹² K. If there is some amount of nonrelativistic ionized gas within such sources, it must be heated through induced Compton scattering of the radiation. If cooling via inverse Compton scattering of the same radio radiation counteracts this heating, then the plasma can be heated up to mildly relativistic temperatures kT～10–100 keV. In this case, the stationary electron velocity distribution can be either relativistic Maxwellian or quasi-Maxwellian (with the high-velocity tail suppressed), depending on the efficiency of Coulomb collisions and other relaxation processes. We derive several simple approximate expressions for the induced Compton heating rate of mildly relativistic electrons in an isotropic radiation field, as well as for the stationary electron distribution function and temperature. We give analytic expressions for the kernel of the integral kinetic equation (one as a function of the scattering angle, and the other for an isotropic radiation field), which describes the photon redistribution in frequency through induced Compton scattering in thermal plasma. These expressions can be used in the parameter range [in contrast to the formulas written out previously in Sazonov and Sunyaev (2000), which are less accurate].