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Search Results: 1 - 10 of 325412 matches for " S. Markoff "
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From Multiwavelength to Mass Scaling: Accretion and Ejection in Microquasars and AGN
S. Markoff
Physics , 2009, DOI: 10.1007/978-3-540-76937-8_6
Abstract: A solid theoretical understanding of how inflowing, accreting plasma around black holes and other compact objects gives rise to outflowing winds and jets is still lacking, despite decades of observations. The fact that similar processes and morphologies are observed in both X-ray binaries as well as active galactic nuclei has led to suggestions that the underlying physics could scale with black hole mass, which could provide a new handle on the problem. In the last decade, simultaneous broadband campaigns of the fast-varying X-ray binaries particularly in their microquasar state have driven the development of, and in some cases altered, our ideas about the inflow/outflow connection in accreting black holes. Specifically the discovery of correlations between the radio, infrared and X-ray bands has revealed a remarkable connectivity between the various emission regions, and argued for a more holistic approach to tackling questions about accretion. This article reviews the recent major observational and theoretical advances that focus specifically on the relation between the two "sides" of the accretion process in black holes, with an emphasis on how new tools can be derived for comparisons across the mass scale.
Galactic Center: Implications of Recent Chandra Observations for Spherical Accretion Models of Sgr A*
R. F. Coker,S. Markoff
Physics , 2001,
Abstract: At the center of the Milky Way lurks a unique compact nonthermal radio source, Sgr A*. It is thought to be powered by a 2.6 million solar mass black hole that is accreting the stellar winds from the numerous early-type stars that exist in the central parsec. However, until recent high resolution Chandra observations, Sgr A* had never been unequivocably detected at wavelengths shorter than the sub-millimeter. We present a spherical accretion model which is consistent with both the flux and steep spectral shape of the X-ray emission from Sgr A*.
On the Nature of the EGRET Source at the Galactic Center
S. Markoff,F. Melia,I. Sarcevic
Physics , 1997, DOI: 10.1086/310958
Abstract: The recent detection of a gamma-ray flux from the direction of the Galactic center by EGRET on the Compton GRO raises the question of whether this is a point source (possibly coincident with the massive black hole candidate Sgr A*) or a diffuse emitter. Using the latest experimental particle physics data and theoretical models, we examine in detail the gamma-ray spectrum produced by synchrotron, inverse Compton scattering and mesonic decay resulting from the interaction of relativistic protons with hydrogen accreting onto a point-like object. Such a distribution of high-energy baryons may be expected to form within an accretion shock as the inflowing gas becomes supersonic. This scenario is motivated by hydrodynamic studies of Bondi-Hoyle accretion onto Sgr A*, which indicate that many of its radiative characteristics may ultimately be associated with energy liberated as this plasma descends down into the deep potential well. Earlier attempts at analyzing this process concluded that the EGRET data are inconsistent with a massive point-like object. Here, we demonstrate that a more careful treatment of the physics of p-p scattering suggests that a ~10^6 solar mass black hole may be contributing to this high-energy emission.
Modeling the X-ray Contribution of X-ray Binary Jets
S. Markoff,M. Nowak,S. Corbel,R. Fender,H. Falcke
Physics , 2003, DOI: 10.1016/S1387-6473(03)00078-2
Abstract: Astrophysical jets exist in both XRBs and AGN, and seem to share common features, particularly in the radio. While AGN jets are known to emit X-rays, the situation for XRB jets is not so clear. Radio jets have been resolved in several XRBs in the low/hard state, establishing that some form of outflow is routinely present in this state. Interestingly, the flat-to-inverted radio synchrotron emission associated with these outflows strongly correlates with the X-ray emission in several sources, suggesting that the jet plasma plays a role at higher frequencies. In this same state, there is increasing evidence for a turnover in the IR/optical where the flat-to-inverted spectrum seems to connect to an optically thin component extending into the X-rays. We discuss how jet synchrotron emission is likely to contribute to the X-rays, in addition to inverse Compton up-scattering, providing a natural explanation for these correlations and the turnover in the IR/optical band. We present model parameters for fits to several sources, and address some common misconceptions about the jet model.
Relativistic AGN jets I. The delicate interplay between jet structure, cocoon morphology and jet-head propagation
S. Walg,A. Achterberg,S. Markoff,R. Keppens,Z. Meliani
Physics , 2013, DOI: 10.1093/mnras/stt823
Abstract: Current observations have shown that astrophysical jets reveal strong signs of radial structure. They suggest that the inner region of the jet, the jet spine, consists of a low-density, fast-moving gas, while the outer region of the jet consists of a more dense and slower moving gas, called the jet sheath. Moreover, if jets carry angular momentum, the resultant centrifugal forces lead to a radial stratification. Current observations are not able to fully resolve the radial structure, so little is known about its actual profile. We present three AGN jet models in $2.5D$ of which two have been given a radial structure. The first model is a homogeneous jet, the only model that doesn't carry angular momentum; the second model is a spine-sheath jet with an isothermal equation of state; and the third jet model is a (piecewise) isochoric spine-sheath jet, with constant but different densities for jet spine and jet sheath. In this paper, we look at the effects of radial stratification on jet integrity, mixing between the different jet components and global morphology of the jet-head and surrounding cocoon.
Assessing the X-ray Contribution from Jets in X-ray Binaries
S. Markoff,M. Nowak,S. Corbel,R. Fender,H. Falcke
Physics , 2002,
Abstract: Astrophysical jets exist from the stellar scale up to AGN, and seem to share common features particularly in the radio. But while AGN jets are known to emit X-rays, the situation for XRB jets is not so clear. Radio jets have been resolved in several XRBs in the low/hard state, and it seems likely that some form of outflow is present whenever this state is achieved. Interestingly, the flat-to-inverted radio synchrotron emission associated with these outflows strongly correlates with the X-ray emission in several sources, suggesting that the jet plasma plays a role at higher frequencies. In this same state, there is also increasing evidence for a turnover in the IR/optical where the flat-to-inverted spectrum seems to connect to an optically thin component extending into the X-rays. We discuss how jet synchrotron emission is likely to contribute to the X-rays, in addition to inverse Compton up-scattering, providing a natural explanation for these correlations and the turnover in the IR/optical band.
Exploring the Role of Jets in the Radio/X-ray Correlations of GX339-4
S. Markoff,M. Nowak,S. Corbel,R. Fender,H. Falcke
Physics , 2002, DOI: 10.1051/0004-6361:20021497
Abstract: The Galactic black hole candidate X-ray binary GX339-4 spends most of its time in the low/hard state, making it an ideal candidate for modeling the assumedly low accretion phase. The radio emission correlates very tightly with the X-rays over more than two orders of magnitude in X-ray flux density, suggesting that the jet plasma also plays a role at higher frequencies. We compare the predictions of our jet model, with and without acceleration, to thirteen broadband simultaneous or quasi-simultaneous spectra over this changing flux history. In addition, we consider a simple standard thin disk which transitions to an optically thin accretion flow, in order to account for the assumedly thermal optical data seen in some observations. A solution without acceleration cannot describe the data without unrealistic energy requirements. But because of the low disk luminosity, and possibly the assumed disk geometry, acceleration in the jet is limited only by synchrotron cooling and can extend easily into the X-rays. We present a model which can account for all the broadband spectra included here, by changing only two parameters in the jet model: the input power and the location of the first acceleration zone. However, the model is most sensitive to changes in the jet power, the varying of which can also account for the slope of the observed radio/X-ray correlation analytically. At the highest low/hard state luminosities, the synchrotron self-Compton emission from the jet could be detectable with missions such as {\em GLAST}, providing a way to test the extent of the synchrotron contribution. We conclude that jet synchrotron is a possible way to explain the broadband features and this correlation, and discuss ways of incorporating this component into the ``standard'' corona picture.
PERSPECTIVE: Is Contentious Politics Relevant in Liberal Democracy?
Anton L. Allahar,Dianto Bachriadi,John Markoff,David S. Meyer
Kasarinlan : Philippine Journal of Third World Studies , 2008,
Abstract:
A Self-Consistent Model for the Broad-band Spectrum of Sgr A East at the Galactic Center
F. Melia,M. Fatuzzo,F. Yusef-Zadeh,S. Markoff
Physics , 1998, DOI: 10.1086/311713
Abstract: Sgr A East is a very prominent elongated shell structure surrounding (though off-centered from) the Galactic nucleus. Its energetics (~4x10^52 ergs), based on the power required to carve out the radio synchrotron remnant within the surrounding dense molecular cloud, appear to be extreme compared to the total energy (~10^51 ergs) released in a typical supernova (SN) explosion. Yet it shares several characteristics in common with SN remnants (SNRs), the most significant of which is the ~0.1-10 GeV gamma-ray spectrum measured by EGRET, if we associate the Galactic center source 2EGJ1746-2852 with this nonthermal shell. We here show that the highest-energy component in Sgr A East's spectrum, like that of SNRs, can be fitted with the gamma-rays produced in neutral pion decays. Further, we demonstrate in a self-consistent manner that the leptons released in the associated charged pion decays produce an e+e- distribution that can mimic a power-law with index ~3, like that inferred from the VLA data for this source. These relativistic electrons and positrons also radiate by bremsstrahlung, and inverse Compton scattering with the intense IR and UV fields from the nucleus. We show that the overall emissivity calculated in this way may account for Sgr A East's broadband spectrum ranging from GHz frequencies all the way to TeV energies, where Whipple has thus far set an upper limit to the flux corresponding to a 2.5-sigma signal above the noise.
Radio / X-ray correlation in the low/hard state of GX 339--4
S. Corbel,M. A. Nowak,R. P. Fender,A. K. Tzioumis,S. Markoff
Physics , 2003, DOI: 10.1051/0004-6361:20030090
Abstract: We present the results of a long-term study of the black hole candidate GX 339-4 using simultaneous radio (from the Australia Telescope Compact Array) and X-ray (from the Rossi X-ray Timing Explorer and BeppoSAX) observations performed between 1997 and 2000. We find strong evidence for a correlation between these two emission regimes that extends over more than three decades in X-ray flux, down to the quiescence level of GX 339-4. This is the strongest evidence to date for such strong coupling between radio and X-ray emission. We discuss these results in light of a jet model that can explain the radio/X-ray correlation. This could indicate that a significant fraction of the X-ray flux that is observed in the low-hard state of black hole candidates may be due to optically thin synchrotron emission from the compact jet.
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