418 research outputs found
Polarization and long-term variability of Sgr A* X-ray echo
We use a model of the molecular gas distribution within ~100 pc from the
center of the Milky Way (Kruijssen, Dale & Longmore) to simulate time evolution
and polarization properties of the reflected X-ray emission, associated with
the past outbursts from Sgr A*. While this model is too simple to describe the
complexity of the true gas distribution, it illustrates the importance and
power of long-term observations of the reflected emission. We show that the
variable part of X-ray emission observed by Chandra and XMM from prominent
molecular clouds is well described by a pure reflection model, providing strong
support of the reflection scenario. While the identification of Sgr A* as a
primary source for this reflected emission is already a very appealing
hypothesis, a decisive test of this model can be provided by future X-ray
polarimetric observations, that will allow placing constraints on the location
of the primary source. In addition, X-ray polarimeters (like, e.g., XIPE) have
sufficient sensitivity to constrain the line-of-sight positions of molecular
complexes, removing major uncertainty in the model.Comment: 17 pages, 10 figures, accepted for publication in MNRA
Can Sgr A* flares reveal the molecular gas density PDF?
Illumination of dense gas in the Central Molecular Zone (CMZ) by powerful
X-ray flares from Sgr A* leads to prominent structures in the reflected
emission that can be observed long after the end of the flare. By studying this
emission we learn about past activity of the supermassive black hole in our
Galactic Center and, at the same time, we obtain unique information on the
structure of molecular clouds that is essentially impossible to get by other
means. Here we discuss how X-ray data can improve our knowledge of both sides
of the problem. Existing data already provide: i) an estimate of the flare age,
ii) a model-independent lower limit on the luminosity of Sgr A* during the
flare and iii) an estimate of the total emitted energy during Sgr A* flare. On
the molecular clouds side, the data clearly show a voids-and-walls structure of
the clouds and can provide an almost unbiased probe of the mass/density
distribution of the molecular gas with the hydrogen column densities lower than
few . For instance, the probability distribution
function of the gas density can be measured this way. Future high
energy resolution X-ray missions will provide the information on the gas
velocities, allowing, for example a reconstruction of the velocity field
structure functions and cross-matching the X-ray and molecular data based on
positions and velocities.Comment: 13 pages, 7 figures; Accepted for publication in MNRA
Not that long time ago in the nearest galaxy: 3D slice of molecular gas revealed by a 110 years old flare of Sgr A*
A powerful outburst of X-ray radiation from the supermassive black hole Sgr
A* at the center of the Milky Way is believed to be responsible for the
illumination of molecular clouds in the central ~100 pc of the Galaxy (Sunyaev
et al., 1993, Koyama et al., 1996). The reflected/reprocessed radiation comes
to us with a delay corresponding to the light propagation time that depends on
the 3D position of molecular clouds with respect to Sgr A*. We suggest a novel
way of determining the age of the outburst and positions of the clouds by
studying characteristic imprints left by the outburst in the spatial and time
variations of the reflected emission. We estimated the age of the outburst that
illuminates the Sgr A molecular complex to be ~110 yr. This estimate implies
that we see the gas located ~10 pc further away from us than Sgr A*. If the Sgr
B2 complex is also illuminated by the same outburst, then it is located ~130 pc
closer than our Galactic Center. The outburst was short (less than a few years)
and the total amount of emitted energy in X-rays is erg, where is the mean hydrogen density of the
cloud complex in units of . Energetically, such fluence can
be provided by a partial tidal disruption event or even by a capture of a
planet. Further progress in more accurate positioning and timing of the
outburst should be possible with future X-ray polarimetric observations and
long-term systematic observations with Chandra and XMM-Newton. A few
hundred-years long X-ray observations would provide a detailed 3D map of the
gas density distribution in the central pc region.Comment: 10 pages, 7 figures, accepted for publication in MNRA
Probing 3D Density and Velocity Fields of ISM in Centers of Galaxies with Future X-Ray Observations
Observations of bright and variable "reflected" X-ray emission from molecular
clouds located within inner hundred parsec of our Galaxy have demonstrated that
the central supermassive black hole, Sgr A*, experienced short and powerful
flares in the past few hundred years. These flares offer a truly unique
opportunity to determine 3D location of the illuminated clouds (with ~10 pc
accuracy) and to reveal their internal structure (down to 0.1 pc scales). Short
duration of the flare(s), combined with X-rays high penetration power and
insensitivity of the reflection signal to thermo- and chemo-dynamical state of
the gas, ensures that the provided diagnostics of the density and velocity
fields is unbiased and almost free of the projection and opacity effects. Sharp
and sensitive snapshots of molecular gas accessible with aid of future X-ray
observatories featuring large collecting area and high angular (arcsec-level)
and spectral (eV-level) resolution cryogenic bolometers will present invaluable
information on properties of the supersonic turbulence inside the illuminated
clouds, map their shear velocity field and allow cross-matching between X-ray
data and velocity-resolved emission of various molecular species provided by
ALMA and other ground-based facilities. This will highlight large and
small-scale dynamics of the dense gas and help uncovering specifics of the ISM
lifecycle and high-mass star formation under very extreme conditions of
galactic centers. While the former is of particular importance for the SMBH
feeding and triggering AGN feedback, the latter might be an excellent test case
for star formation taking place in high-redshift galaxies.Comment: White paper submitted to the Astro2020 Decadal Surve
Voltammetric determination of sugar in wines and wine materials using a copper electrode
A method for voltammetric determination of sugar in wines and wine materials is described using a metallic copper electrode in a 0.5 M NaOH solution. The method is time-saving and shows a sufficiently high reproducibility (sr ≤ 0.04). © 1999 Kluwer Academic/Plenum Publishers
Solving physically nonlinear equilibrium problems for sandwich plates with a transversally soft core
© 2015, Pleiades Publishing, Ltd. A generalized statement of the problem of determining the stress-strain state of sandwich plates with a transversally soft core in the presence of constraints is proposed. Its correctness is discussed. The generalized problem is stated as the problem of finding a saddle point of some functional. The existence and uniqueness of a solution is proved. An iterative method for solving the problem is proposed and its convergence is studied
Transition to «green» economy in Russia: Current and long-term challenges
Nowadays, many believe that there is no way to ecological stability other than transition to «environmentally oriented economy». In urban areas, the main sources of pollutants are industrial enterprises and automobile transport. To reduce the adverse environmental impacts one needs special methods of air quality control. Specifically, research in this field is aimed at developing of control systems for the city transport in order to predict the environmental response to changing traffic parameters and take appropriate measures to improve the situation. In this work it is demonstrated how the method of transport system control, based on simulation modeling, has been implemented. The optimization experiment has been performed on a simulation model adjusting the parameters of parts of a city road network for adequate decision making. Model experimenting has made it possible to establish the optimal traffic density and average current rates, without exceeding the pollution quotas, and calculate the consequences of changing in the number of vehicle car fleet on city roads. The experiment was carried out in the city of Naberezhnye Chelny, Russia
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