Thermal Conduction in Clusters of Galaxies

We estimate the thermal conductivity of a weakly collisional magnetized plasma with chaotic magnetic field fluctuations. When the fluctuation spectrum extends over two or more decades in wave-vector, we find that thermal conduction is very efficient; the conduction coefficient is only a factor ~5 below the classical Spitzer estimate. We suggest that conduction could play a significant role in cooling flows in clusters of galaxies.Comment: 4 pages, 2 figures. Accepted for publication in ApJ Letter

Long-time evolution of magnetic fields in relativistic GRB shocks

We investigate the long-time evolution of magnetic fields generated by the two-stream instability at ultra- and sub-relativistic astrophysical collisionless shocks. Based on 3D PIC simulation results, we introduce a 2D toy model of interacting current filaments. Within the framework of this model, we demonstrate that the field correlation scale in the region far downstream the shock grows nearly as the light crossing time, lambda(t) ~ ct, thus making the diffusive field dissipation inefficient. The obtained theoretical scaling is tested using numerical PIC simulations. This result extends our understanding of the structure of collisionless shocks in gamma-ray bursts and other astrophysical objects.Comment: 5 pages. 2 figures. Submitted to ApJ

Can Cluster Evaporation Explain the Missing Thermal Energy in Galaxy Clusters?

Resent observations of a number of galaxy clusters using the Sunyaev-Zel'dovich effect indicate that about 1/3 of baryonic mass is missing from the hot intracluster medium (ICM), which is significantly larger than the fraction of stars and cool gas, which account for only about 10%. Here we address the question whether the remaining $22\pm10$ can be accounted for by thermal evaporation of gas from clusters. We have found that evaporation can occur only from the cluster ``surface'', $r\sim r_{\rm vir}$, and not from it's interior. We evaluated particle diffusion through the magnetized ICM for several scenarios of ISM turbulence and found that diffusivity is suppressed by at least a factor of 100 or more, compared to the Spitzer value. Thus, only particles from radii r\ga0.9r_{\rm vir} can evaporate. Diffusion of particles from inside the cluster, r\la0.9r_{\rm vir}, takes longer than the Hubble time. This lowers the cluster-averaged fraction of the evaporated hot gas to few percent or less. However, if the missing hot component {\it is indeed} due to evaporation, this strongly constrains the magnetic field structure in the cluster envelope, namely either (i) the gas is completely unmagnetized ($B\le10^{-21}$ gauss) in the cluster halo or (ii) the magnetic fields in the ICM are rather homogeneous and non-turbulent.Comment: ApJL, accepted version; 4 pages, 2 figure

Radiation of electrons in Weibel-generated fields: a general case

Weibel instability turns out to be the a ubiquitous phenomenon in High-Energy Density environments, ranging from astrophysical sources, e.g., gamma-ray bursts, to laboratory experiments involving laser-produced plasmas. Relativistic particles (electrons) radiate in the Weibel-produced magnetic fields in the Jitter regime. Conventionally, in this regime, the particle deflections are considered to be smaller than the relativistic beaming angle of 1/$\gamma$ ($\gamma$ being the Lorentz factor of an emitting particle) and the particle distribution is assumed to be isotropic. This is a relatively idealized situation as far as lab experiments are concerned. We relax the assumption of the isotropy of radiating particle distribution and present the extension of the jitter theory amenable for comparisons with experimental data.Comment: Proceedings of International Conference on HEDP/HEDLA-0

Cluster magnetic fields from large-scale-structure and galaxy-cluster shocks

The origin of the micro-Gauss magnetic fields in galaxy clusters is one of the outstanding problem of modern cosmology. We have performed three-dimensional particle-in-cell simulations of the nonrelativistic Weibel instability in an electron-proton plasma, in conditions typical of cosmological shocks. These simulations indicate that cluster fields could have been produced by shocks propagating through the intergalactic medium during the formation of large-scale structure or by shocks within the cluster. The strengths of the shock-generated fields range from tens of nano-Gauss in the intercluster medium to a few micro-Gauss inside galaxy clusters.Comment: 4 pages, 2 color figure

Hot Settling Accretion Flow onto a Spinning Black Hole

We study the structure and properties of hot MHD accretion onto a Kerr black hole. In such a system, the hole is magnetically coupled to the inflowing gas and exerts a torque onto the accretion flow. A hot settling flow can form around the hole and transport the angular momentum outward, to the outer edge of the flow. Unlike other hot flows, such as advection- and convection-dominated flows and inflow-outflow solutions (ADAFs, CDAFs, and ADIOS), the properties of the hot settling flow are determined by the spin of the central black hole, but are insensitive to the mass accretion rate. Therefore, it may be possible to identify rapidly spinning BHs simply from their broad-band spectra. Observationally, the hot settling flow around a Kerr hole is somewhat similar to other hot flows in that they all have hard, power-law spectra and relatively low luminosities. Thus, most black hole candidates in the low/hard and, perhaps, intermediate X-ray state may potentially accrete via the hot settling flow. However, a settling flow will be somewhat more luminous than ADAFs/CDAFs/ADIOS, will exhibit high variability in X-rays, and may have relativistic jets. This suggests that galactic microquasars and active galactic nuclei may be powered by hot settling flows. We identify several galactic X-ray sources as the best candidates.Comment: 7 pages, 1 figure. Submitted to Ap

Chandra Observations of the Dwarf Nova WX Hyi in Quiescence

We report Chandra observations of the dwarf nova WX Hyi in quiescence. The X-ray spectrum displays strong and narrow emission lines of N, O, Mg, Ne, Si, S and Fe. The various ionization states implied by the lines suggest that the emission is produced within a flow spanning a wide temperature range, from T ~ 10^6 K to T >~ 10^8 K. Line diagnostics indicate that most of the radiation originates from a very dense region, with n ~ 10^{13}-10^{14} cm^{-3}. The Chandra data allow the first tests of specific models proposed in the literature for the X-ray emission in quiescent dwarf novae. We have computed the spectra for a set of models ranging from hot boundary layers, to hot settling flows solutions, to X-ray emitting coronae. WX Hyi differs from other dwarf novae observed at minimum in having much stronger low temperature lines, which prove difficult to fit with existing models, and possibly a very strong, broad O VII line, perhaps produced in a wind moving at a few x 10^3 km/s. The accretion rate inferred from the X-rays is lower than the value inferred from the UV. The presence of high-velocity mass ejection could account for this discrepancy while at the same time explaining the presence of the broad O VII line. If this interpretation is correct, it would provide the first detection of a wind from a dwarf nova in quiescence.Comment: accepted to ApJ; 19 pages, 3 figures, 1 tabl

THE STUDY OF PATH-FOLLOWING ACCURACY OF ROBOTIC SINGLE-ROTOR HELICOPTER

In this paper we study the accuracy of a single-rotor robotic small-scale helicopter flight along a complex path. The control algorithms for the autopilot are synthesized using the position-trajectory control approach. We use hardware-software complex to test the helicopter autopilot. The simulation in hardware-software complex is used to debug the autopilot software and complex study of autopilot control algorithms in early development stages without full-scale experiments. The paper shows results of the simulation of single-rotor small-scale helicopter flight

Ocean-bottom seismographs based on broadband MET sensors: architecture and deployment case study in the Arctic

The Arctic seas are now of particular interest due to their prospects in terms of hydrocarbon extraction, development of marine transport routes, etc. Thus, various geohazards, including those related to seismicity, require detailed studies, especially by instrumental methods. This paper is devoted to the ocean-bottom seismographs (OBS) based on broadband molecular–electronic transfer (MET) sensors and a deployment case study in the Laptev Sea. The purpose of the study is to introduce the architecture of several modifications of OBS and to demonstrate their applicability in solving different tasks in the framework of seismic hazard assessment for the Arctic seas. To do this, we used the first results of several pilot deployments of the OBS developed by Shirshov Institute of Oceanology of the Russian Academy of Sciences (IO RAS) and IP Ilyinskiy A.D. in the Laptev Sea that took place in 2018–2020. We highlighted various seismological applications of OBS based on broadband MET sensors CME-4311 (60 s) and CME-4111 (120 s), including the analysis of ambient seismic noise, registering the signals of large remote earthquakes and weak local microearthquakes, and the instrumental approach of the site response assessment. The main characteristics of the broadband MET sensors and OBS architectures turned out to be suitable for obtaining high-quality OBS records under the Arctic conditions to solve seismological problems. In addition, the obtained case study results showed the prospects in a broader context, such as the possible influence of the seismotectonic factor on the bottom-up thawing of subsea permafrost and massive methane release, probably from decaying hydrates and deep geological sources. The described OBS will be actively used in further Arctic expeditions