共查询到20条相似文献，搜索用时 15 毫秒
1.
N. A. Silant’ev M. Yu. Piotrovich Yu. N. Gnedin T. M. Natsvlishvili 《Astronomy Letters》2010,36(8):549-553
We compare two methods for estimating the magnetic field near a black hole—based on a statistical formula that relates the
magnetic field on the horizon of the black hole to its mass and on a direct magnetic field determination from optical polarimetric
observations. An optically thick magnetized accretion disk is usually formed around the black hole in active galactic nuclei.
The linear polarization of the radiation emerging from the disk is related to the local magnetic field and the polarization
in Milne’s classical problem. The magnetic field in the region from where the radiation with the observed wavelength comes
can be estimated by measuring this polarization. On the other hand, this field can be estimated from the above statistical
formula if the field near the black hole is extrapolated into the accretion disk using a standard power law. Comparison of
the magnetic field obtained from the observed polarization with the extrapolated field can serve to estimate the accuracy
of the latter and the exponent in the adopted extrapolation law. As an example, we compare these two methods for the source
NGC 4258. 相似文献
2.
M. Yu. Piotrovich Yu. N. Gnedin N. A. Silant'ev T. M. Natsvlishvili S. D. Buliga 《Astronomische Nachrichten》2015,336(10):1013-1016
Magnetic fields in an accretion disk around the central black hole can modify the size of the innermost stable circular orbit (ISCO) and can produce a difference to the classical Novikov‐Thorne radius. We estimated the ISCO magnetic field strength from the polarimetric observations of the accretion‐disk radiation. This estimate is obtained taking into account the effect of the Faraday rotation of the polarization plane at the distance of the mean free path of photons between successive electron scattering events. We present the new method for estimating the ISCO radius in the accretion disk, i.e. in the nearest vicinity of a central black hole. Our estimates confirmed the Frolov, Shoom & Tzounis (2014) and Ranea‐Sandoval & Garcia (2015) conclusion that the magnetic field in the accretion disk decreases the size of the innermost stable circular orbit. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
3.
Y. N. Gnedin S. D. Buliga N. A. Silant’ev T. M. Natsvlishvili M. Y. Piotrovich 《Astrophysics and Space Science》2012,342(1):137-145
In this paper we demonstrate that the wavelength dependence of polarization degree and position angle allows us to derive the distribution of magnetic field in accretion disc. The polarized radiation arises due to scattering of emission light by electrons in a magnetized optically thick accretion disc. Faraday rotation of polarization plane is taken into consideration. Through wavelength dependence of polarization it is possible to derive the value of the magnetic Prandtl number in the accretion disc plasma. The power law index of the polarization wavelength dependence is related with the radial distribution of magnetic field in an accretion disc. This allows us to test the various models of an accretion disc around the central black hole. 相似文献
4.
V. L. Afanasiev N. V. Borisov Yu. N. Gnedin T. M. Natsvlishvili M. Yu. Piotrovich S. D. Buliga 《Astronomy Letters》2011,37(5):302-310
We present the results of our spectropolarimetric observations for a number of active galactic nuclei (AGNs) carried out at
the 6-m telescope with the SCORPIO focal reducer. The derived wavelength dependences of the polarization have been analyzed
by taking into account the Faraday rotation of the polarization plane on the photon mean free path in a magnetized accretion
disk. As a result, based on traditional accretion disk models, we have determined the magnetic field strength and distribution
and a number of physical parameters of the accreting plasma in the region where the optical radiation is generated. 相似文献
5.
The observed reduction in the fraction of quasars with polarized radiation as the redshift increases is explained by an increase in the magnetic-field strength in accretion disks at high z. The emerging Faraday rotation causes the radiation to be depolarized. This mechanism allows the magnetic fields in accretion disks to be estimated at several hundred gauss. We give simple asymptotic formulas that describe the Faraday depolarization in optically thick accretion disks. 相似文献
6.
Equipartition magnetic fields can dramatically affect the polarization of radiation emerging from accretion disc atmospheres in active galactic nuclei. We extend our previous work on this subject by exploring the interaction between Faraday rotation and absorption opacity in local, plane-parallel atmospheres with parameters appropriate for accretion discs. Faraday rotation in pure scattering atmospheres acts to depolarize the radiation field by rotating the polarization planes of photons after last scattering. Absorption opacity in an unmagnetized atmosphere can increase or decrease the polarization compared to the pure scattering case, depending on the thermal source function gradient. Combining both Faraday rotation and absorption opacity, we find the following results. If absorption opacity is much larger than scattering opacity throughout the atmosphere, then Faraday rotation generally has only a small effect on the emerging polarization because of the small electron column density along a photon mean free path. However, if the absorption opacity is not too large and it acts alone to increase the polarization, then the effects of Faraday rotation can be enhanced over those in a pure scattering atmosphere. Finally, while Faraday rotation often depolarizes the radiation field, it can in some cases increase the polarization when the thermal source function does not rise too steeply with optical depth. We confirm the correctness of the analytic calculation by Silant'ev of the high magnetic field limit of the pure scattering atmosphere, which we incorrectly disputed in our previous paper. 相似文献
7.
M. Yu. Piotrovich Yu. N. Gnedin T. M. Natsvlishvili N. A. Silant’ev 《Astronomy Letters》2010,36(6):389-395
We analyze the spectropolarimetric observations of 12 candidates for quasars from the spectroscopic database of the SDSS Catalog.
The magnetic fields of these objects are estimated in the context of a theory that includes the Faraday rotation of the polarization
plane on the mean free path of a photon in the outflow from an accretion disk. As a result, we have determined the column
density in the outflow, N
H ∼ 6 × 1023 cm−2, and the radial, B ∼ 1 G, and toroidal, B ∼ 600 G, magnetic fields. 相似文献
8.
T.G. Arshakian R. Stepanov R. Beck M. Krause D. Sokoloff 《Astronomische Nachrichten》2011,332(5):524-536
Future radio observations with the Square Kilometre Array (SKA) and its precursors will be sensitive to trace spiral galaxies and their magnetic field configurations up to redshift z ≈ 3. We suggest an evolutionary model for the magnetic configuration in star‐forming disk galaxies and simulate the magnetic field distribution, the total and polarized synchrotron emission, and the Faraday rotation measures for disk galaxies at z ≲ 3. Since details of dynamo action in young galaxies are quite uncertain, we model the dynamo action heuristically relying only on well‐established ideas of the form and evolution of magnetic fields produced by the mean‐field dynamo in a thin disk. We assume a small‐scale seed field which is then amplified by the small‐scale turbulent dynamo up to energy equipartition with kinetic energy of turbulence. The large‐scale galactic dynamo starts from seed fields of 100 pc and an averaged regular field strength of 0.02 μG, which then evolves to a “spotty” magnetic field configuration in about 0.8 Gyr with scales of about one kpc and an averaged regular field strength of 0.6 μG. The evolution of these magnetic spots is simulated under the influence of star formation, dynamo action, stretching by differential rotation of the disk, and turbulent diffusion. The evolution of the regular magnetic field in a disk of a spiral galaxy, as well as the expected total intensity, linear polarization and Faraday rotation are simulated in the rest frame of a galaxy at 5GHz and 150 MHz and in the rest frame of the observer at 150 MHz. We present the corresponding maps for several epochs after disk formation. Dynamo theory predicts the generation of large‐scale coherent field patterns (“modes”). The timescale of this process is comparable to that of the galaxy age. Many galaxies are expected not to host fully coherent fields at the present epoch, especially those which suffered from major mergers or interactions with other galaxies. A comparison of our predictions with existing observations of spiral galaxies is given and discussed (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
9.
The problem of steady-state accretion to nonrotating black holes is examined. Advection is included and generalized formulas
for the radiation pressure in both the optically thick and thin cases are used. Special attention is devoted to models with
a high accretion rate. Global solutions for accretion disks are studied which describe a continuous transition between an
optically thick outer region and an optically thin inner region. It is shown that there is a maximum disk temperature for
the model with a viscosity parameter α = 0.5. For the model with α = 0.1, no optically thin regions are found to exist for
any accretion rate. 相似文献
10.
本文讨论了具有磁场的几何厚吸积盘结构和辐射。结果表明:磁场的存在明显地影响吸积盘的总光度和形状。这种模型可以解释类星体的辐射幂谱分布和高度偏振现象。它是类星体主要辐射区域的一种合理模型。 相似文献
11.
We have investigated the influence of X-ray irradiation on the vertical structure of the outer accretion disk in low-mass
X-ray binaries by performing a self-consistent calculation of the vertical structure and X-ray radiation transfer in the disk.
Penetrating deep into the disk, the field of scattered X-ray photons with energy E ≳ 10 keV exerts a significant influence on the vertical structure of the accretion disk at a distance R ≳ 1010 cm from the neutron star. At a distance R ∼ 1011 cm, where the total surface density in the disk reaches Σ0 ∼ 20 g cm−2, X-ray heating affects all layers of an optically thick disk. The X-ray heating effect is enhanced significantly in the presence
of an extended atmospheric layer with a temperature T
atm ≈ (2–3) × 106 K above the accretion disk. We have derived simple analytic formulas for the disk heating by scattered X-ray photons using
an approximate solution of the transfer equation by the Sobolev method. This approximation has a ≲10% accuracy in the range
of X-ray photon energies E < 20 keV. 相似文献
12.
13.
14.
We consider the reflection of non-polarized radiation from the point-like sources above the accretion discs both the optically thick and optically thin. We investigate the dependence of the polarization of reflected radiation on the aperture angle of incident radiation. The aperture angle is determined by the radius of accretion disc and the height of the source above the disc. For optically thick accretion discs we show that, if the aperture angle is smaller 70 grad, then the wave electric field oscillations of reflected radiation are parallel to the accretion disc plane. For aperture angle greater than 70 grad the electric field oscillations are parallel to the plane “normal to accretion disc—the line of sight”. The latter also holds for reflection from the optically thin accretion disc independent of the aperture angle value. 相似文献
15.
M. G. Revnivtsev R. A. Burenin A. Yu. Tkachenko I. M. Khamitov T. Ak A. Merloni M. N. Pavlinsky R. A. Sunyaev 《Astronomy Letters》2012,38(4):238-248
We present the results of our studies of the aperiodic optical flux variability for SS Cyg, an accreting binary systemwith
a white dwarf. The main set of observational data presented here was obtained with the ANDOR/iXon DU-888 photometer mounted
on the RTT-150 telescope, which allowed a record (for CCD photometers) time resolution up to 8 ms to be achieved. The power
spectra of the source’s flux variability have revealed that the aperiodic variability contains information about the inner
boundary of the optically thick flow in the binary system. We show that the inner boundary of the optically thick accretion
disk comes close to the white dwarf surface at the maximum of the source’s bolometric light curve, i.e., at the peak of the
instantaneous accretion rate onto the white dwarf, while the optically thick accretion disk is truncated at distances 8.5
× 109 cm ∼10R
WD in the low state. We suggest that the location of the inner boundary of the accretion disk in the binary can be traced by
studying the parameters of the power spectra for accreting white dwarfs. In particular, this allows the mass of the accreting
object to be estimated. 相似文献
16.
J. M. Hutchison T. V. Cawthorne D. C. Gabuzda 《Monthly notices of the Royal Astronomical Society》2001,321(3):525-536
Very long-baseline interferometry (VLBI) observations of the quasar 4C 71.07 (0836+710) at frequencies of 5 and 8.4 GHz at two epochs are used to investigate apparent misalignments between the magnetic field and jet direction found in this source. The observed polarization angles are not consistent with Faraday rotation of synchrotron radiation from an aligned magnetic field. Internal Faraday rotation in a uniform spherical source is also ruled out by the observations, and while the misalignments could result from internal Faraday rotation in a non-uniform source, no strong signatures of this effect were found. The jet shows two distinct kinks at which the ridge-line changes direction and then reverts to its original direction. The magnetic field in these regions is parallel to the jet, and remains so as the jet bends. It seems likely that the largest remaining misalignment is associated with another such kink that is unresolved by these observations. The percentage polarization decreases near the bright knots, consistent with enhancement in brightness by compressions in the plane normal to the jet axis. The inferred rotation measure is low (100 rad m−2 ) throughout the jet, as for other quasars. However near the core, the polarization position angles suggest a rotation measure that appears to be uncharacteristically low by comparison with other quasars. 相似文献
17.
N. A. Silant’ev Yu. N. Gnedin S. D. Buliga M. Yu. Piotrovich T. M. Natsvlishvili 《Astrophysical Bulletin》2013,68(1):14-25
Estimates of magnetic fields for a number of active galactic nuclei are presented. These estimates are based on the observed polarization degrees and position angles of broad Hα lines and in the nearby continuum and on asymptotic analytical formulas for the Stokes parameters of the radiation emerging from a magnetized accretion disk (the Milne problem in a magnetized atmosphere). The characteristic observed feature of the wavelength dependence of the polarization degree inside the line—a minimum at the center and a fast increase of the position angle from one wing to another—can be explained by the superposition of resonance emission from two or more clouds located in the right (Keplerian velocity directed away from the observer) and left (Keplerian velocity directed toward the observer) parts of the orbit in the rotating magnetized accretion disk. The main component in our mechanism is the azimuthal magnetic field in the disk. The presence of a magnetic field perpendicular to the disk plane (which is usually weaker than the azimuthal field) results in the asymmetry of the distribution of the polarization degree and position angle inside the line. The inferred magnetic field strengths at the galactocentric distances where broad lines are emitted can be used to estimate the magnetic fields in the region of the centermost stable orbit and at the horizon of the central black hole, using the power-law dependence of the magnetic field strength corresponding to the standard model of the accretion disk. 相似文献
18.
19.
We revisit the vertical structure of neutrino-dominated accretion flows (NDAFs) in spherical coordinates with a new boundary
condition based on the mechanical equilibrium. The solutions show that NDAF is significantly thick. The Bernoulli parameter
and neutrino trapping are determined by the mass accretion rate and the viscosity parameter. According to the distribution
of the Bernoulli parameter, the possible outflow may appear in the outer region of the disk. The neutrino trapping can essentially
affect the neutrino radiation luminosity. The vertical structure of NDAF is like a “sandwich”, and the multilayer accretion
may account for the flares in gamma-ray bursts. 相似文献
20.
Mateusz Ruszkowski 《Astrophysics and Space Science》2003,288(1-2):133-142
I discuss the transfer of polarized synchrotron radiation in relativistic jets. I argue that the main mechanism responsible for the circular polarization properties of compact synchrotron sources is likely to be Faraday conversion and that, contrary to common expectation, a significant rate of Faraday rotation does not necessarily imply strong depolarization. The long-term persistence of the sign of circular polarization, observed in some sources, is most likely due to a small net magnetic flux generated in the central engine, carried along the jet axis and superimposed on a highly turbulent magnetic field. I show that the mean levels of circular and linear polarizations depend on the number of field reversals along the line of sight and that the gradient in Faraday rotation across turbulent regions can lead to`correlation depolarization'. The model is potentially applicable to a wide range of synchrotron sources. In particular, I demonstrate how the model can naturally explain the excess of circular over linear polarization in the Galactic Center (SgrA*) and the low-luminosity AGN M81*. 相似文献