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81.
The magnetic field in an accretion disk is estimated assuming that all of the angular momentum within prescribed accretion
disk radii is removed by a jet. The magnetic field estimated at the base of the jet is extrapolated to the blazar emission
region using a model for a relativistic axisymmetric jet combined with some simplifying assumptions based on the relativistic
nature of the flow. The extrapolated magnetic field is compared with estimates based upon the synchrotron and inverse Compton
emission from three blazars, MKN 501, MKN 421 and PKS 2155-304. The magnetic fields evaluated from pure synchrotron self-Compton
models are inconsistent with the magnetic fields extrapolated in this way. However, in two cases inverse Compton models in
which a substantial part of the soft photon field is generated locally agree well, mainly because these models imply magnetic
field strengths consistent with an important Poynting Flux component. This comparison is based on estimating the mass accretion
rate from the jet energy flux. Further comparisons along these lines will be facilitated by independent estimates of the mass
accretion rate in blazars and by more detailed models for jet propagation near the black hole. 相似文献
82.
Ya-Juan Lei Li-Ming Song Jin-Lu Qu Cheng-Min Zhang 《Astrophysics and Space Science》2007,310(3-4):307-315
The rms (root mean square) variability is the parameter for understanding the emission temporal properties of X-ray binaries
(XRBs) and active galactic nuclei (AGN). The rms–flux relation with Rossi X-ray Timing Explorer (RXTE) data for the dips and nondip of black hole Cyg X-1 has been investigated in this paper. Our results show that there exist
the linear rms–flux relations in the frequency range 0.1–10 Hz for the dipping light curve. Moreover, this linear relation
still remains during the nondip regime, but with the steeper slope than that of the dipping case in the low energy band. For
the high energy band, the slopes of the dipping and nondipping cases are hardly constant within errors. The explanations of
the results have been made by means of the “Propagating Perturbation” model of Lyubarskii (Lyubarskii, Y.E., Mon. Not. Roy.
Astron. Soc. 292, 679–685 (1997)). 相似文献
83.
Linda Schmidtobreick Claus Tappert Alessandro Ederoclite Ivo Saviane Elena Mason 《Astrophysics and Space Science》2006,304(1-4):303-305
We report preliminary results of an observing run at the 1--m telescope at CTIO during which we followed RR Pic over several nights in February 2005. The resulting light curves show no sign of an eclipse. Apart from the 3.48 h period, which is usually interpreted as the orbital one, we find an additional period at P=3.78 h, which we interpret as the superhump period of the system.We furthermore present high–resolution Doppler maps in Hα, Hβ and He II, which we derived from observations at the NTT, La Silla in February 2004. They show strong variations in the emission distribution from one day to the next. While Hα and Hβ emission clearly show the accretion disc with some additional isolated sources, the He II emission is confined to an elongated region at low velocities. 相似文献
84.
The structure and development of foredunes on a locally prograding coast: insights from ground-penetrating radar surveys, Norfolk, UK 总被引:5,自引:0,他引:5
The internal structure of coastal foredunes from three sites along the north Norfolk coast has been investigated using ground‐penetrating radar (GPR), which provides a unique insight into the internal structure of these dunes that cannot be achieved by any other non‐destructive or geophysical technique. Combining geomorphological and geophysical investigations into the structure and morphology of these coastal foredunes has enabled a more accurate determination of their development and evolution. The radar profiles show the internal structures, which include foreslope accretion, trough cut and fill, roll‐over and beach deposits. Foredune ridges contain large sets of low‐angle cross‐stratification from dune foreslope accretion with trough‐shaped structures from cut and fill on the crest and rearslope. Foreslope accretion indicates sand supply from the beach to the foreslope, while troughs on the dune crest and rearslope are attributed to reworking by offshore winds. Bounding surfaces between dunes are clearly resolved and reveal the relative chronology of dune emplacement. Radar sequence boundaries within dunes have been traced below the water‐table passing into beach erosion surfaces. These are believed to result from storm activity, which erodes the upper beach and dunes. In one example, at Brancaster, a dune scarp and erosion surface may be correlated with erosion in the 1950s, possibly the 1953 storm. Results suggest that dune ridge development is intimately linked to changes in the shoreline, with dune development associated with coastal progradation while dunes are eroded during storms and, where beaches are eroding, a stable coast provides more time for dune development, resulting in higher foredune ridges. A model for coastal dune evolution is presented, which illustrates stages of dune development in response to beach evolution and sand supply. In contrast to many other coastal dune fields where the prevailing wind is onshore, on the north Norfolk coast, the prevailing wind is directed along the coast and offshore, which reduces the landward migration of sand dunes. 相似文献
85.
R. K. Manchanda 《Journal of Astrophysics and Astronomy》2000,21(1-2):39-52
We report the spectral measurement of GRS 1915+105 in the hard X-ray energy band of 20–140keV. The observations were made
on March 30th, 1997 during a quiescent phase of the source. We discuss the mechanism of emission of hard X-ray photons and
the evolution of the spectrum by comparing the data with earlier measurements and an axiomatic model for the X-ray source. 相似文献
86.
We present the results of a detailed analysis of RXTE observations of classω (Klein-Woltet al. 2002) which show an unusual state transition between high-soft and low-soft states in the Galactic microquasar GRS 1915 +
105. Out of about 600 pointed RXTE observations, the source was found to exhibit such state transition only on 16 occasions.
An examination of the RXTE/ASM data in conjunction with the pointed observations reveals that these events appeared as a series
of quasi-regular dips in two stretches of long duration (about 20 days during each occasion) when hard X-ray and radio flux
were very low. The X-ray light curve and colour-colour diagram of the source during these observations are found to be different
from any reported so far. The duration of these dips is found to be of the order of a few tens of seconds with a repetition
time of a few hundred seconds. The transition between these dips and non-dips which differ in intensity by a factor of ∼ 3.5,
is observed to be very fast (∼ a few seconds). It is observed that the low-frequency narrow QPOs are absent in the power density
spectrum (PDS) of the dip and non-dip regions of classω and the PDS is a power law in the 0.1–10 Hz frequency range. There is a remarkable similarity in the spectral and timing
properties of the source during the dip and non-dip regions in this set of observations. These properties of the source are
distinctly different from those seen in the observations of other classes. This indicates that the basic accretion disk structure
during both dip and non-dip regions of classω is similar, but differ only in intensity. To explain these observations, we invoke a model in which the viscosity is very
close to critical viscosity and the shock wave is weak or absent. 相似文献
87.
R. K. Manchanda 《Journal of Astrophysics and Astronomy》2001,22(1):9-20
The black hole candidate Cyg X-1 was observed in ultra low state on march 30, 1997 using Large Area Scintillation counter
Experiment (LASE) in the hard X-ray energy region of 20–180 keV. During the 30 minute exposure a combined signal of 68 sigma
was obtained, however, the measured flux at 50 keV was lower by a factor of 2 than the minimum flux reported so far. Using
the recent orbital ephemeris of the source, our snap-shot observations were made at ϕ5.6 = 0.915, which corresponds to the binary minimum revealed by the ASM light curves. The daily average data from the BATSE
detectors give the source intensity level to be higher by a factor of 5. Very low flux values measured in the present experiment
suggest that the hard X-ray source may have been partially occulted by the primary companion during its transit near the X-ray
minimum. 相似文献
88.
John Chambers 《Icarus》2006,180(2):496-513
A new semi-analytic model for the oligarchic growth phase of planetary accretion is developed. The model explicitly calculates damping and excitation of planetesimal eccentricities e and inclinations i due to gas drag and perturbations from embryos. The effects of planetesimal fragmentation, enhanced embryo capture cross sections due to atmospheres, inward planetesimal drift, and embryo-embryo collisions are also incorporated. In the early stages of oligarchic growth, embryos grow rapidly as e and i fall below their equilibrium values. The formation of planetesimal collision fragments also speeds up embryo growth as fragments have low-e, low-i orbits, thereby optimizing gravitational focussing. At later times, the presence of thick atmospheres captured from the nebula aids embryo growth by increasing their capture cross sections. Planetesimal drift due to gas drag can lead to substantial inward transport of solid material. However, inward drift is greatly reduced when embryo atmospheres are present, as the drift timescale is no longer short compared to the accretion timescale. Embryo-embryo collisions increase embryo growth rates by 50% compared to the case where growth is solely due to accretion of planetesimals. Formation of 0.1-Earth-mass protoplanets at 1 AU and 10-Earth-mass cores at 5 AU requires roughly 0.1 and 1 million years respectively, in a nebula where the local solid surface density is 7 g cm−2 at each of these locations. 相似文献
89.
Edward R.D. Scott 《Icarus》2006,185(1):72-82
Thermal models and radiometric ages for meteorites show that the peak temperatures inside their parent bodies were closely linked to their accretion times. Most iron meteorites come from bodies that accreted <0.5 Myr after CAIs formed and were melted by 26Al and 60Fe, probably inside 2 AU. Rare carbon-rich differentiated meteorites like ureilites probably also come from bodies that formed <1 Myr after CAIs, but in the outer part of the asteroid belt. Chondrite groups accreted intermittently from diverse batches of chondrules and other materials over a 4 Myr period starting 1 Myr after CAI formation when planetary embryos may already have formed at ∼1 AU. Meteorite evidence precludes accretion of late-forming chondrites on the surface of early-formed bodies; instead chondritic and non-chondritic meteorites probably formed in separate planetesimals. Maximum metamorphic temperatures in chondrite groups are correlated with mean chondrule age, as expected if 26Al and 60Fe were the predominant heat sources. Because late-forming bodies could not accrete close to large, early-formed bodies, planetesimal formation may have spread across the nebula from regions where the differentiated bodies formed. Dynamical models suggest that the asteroids could not have accreted in the main belt if Jupiter formed before the asteroids. Therefore Jupiter probably reached its current mass >3-5 Myr after CAIs formed. This precludes formation of Jupiter via a gravitational instability <1 Myr after the solar nebula formed, and strongly favors core accretion. Jupiter probably formed too late to make chondrules by generating shocks directly, or indirectly by scattering Ceres-sized bodies across the belt. Nevertheless, shocks formed by gravitational instabilities or Ceres-sized bodies scattered by planetary embryos may have produced some chondrules. The minimum lifetime for the solar nebula of 3-5 Myr inferred from the total spread of CAI and chondrule ages may exceed the median lifetime of 3 Myr for protoplanetary disks, but is well within the 1-10 Myr observed range. Shorter formation times for extrasolar planets may help to explain their unusual orbits compared to those of solar giant planets. 相似文献
90.
Orhan Dönmez 《Astrophysics and Space Science》2006,305(2):187-196
In this paper, perturbations of an accretion disk by a star orbiting around a black hole are studied. We report on a numerical experiment, which has been carried out by using a parallel-machine code originally developed by Dönmez (2004). An initially steady state accretion disk near a non-rotating (Schwarzschild) black hole interacts with a “star”, modeled as an initially circular region of increased density. Part of the disk is affected by the interaction. In some cases, a gap develops and shock wave propagates through the disk. We follow the evolution for order of one dynamical period and we show how the non-axisymetric density perturbation further evolves and moves downwards where the material of the disk and the star become eventually accreted onto the central body. When the star perturbs the steady state accretion disk, the disk around the black hole is destroyed by the effect of perturbation. The perturbed accretion disk creates a shock wave during the evolution and it loses angular momentum when the gas hits on the shock waves. Colliding gas with the shock wave is the one of the basic mechanism of emitting the X-rays in the accretion disk. The series of supernovae occurring in the inner disk could entirely destroy the disk in that region which leaves a more massive black hole behind, at the center of galaxies. 相似文献