Table 2 - uploaded by Michal Zajacek
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The inner radii of the stellar cusp for different stellar populations M calculated according to Eq. 17.
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The compact and, with about 4.3+-0.3 million solar masses, very massive object located at the center of the Milky Way is currently the very best candidate for a supermassive black hole (SMBH) in our immediate vicinity. If SgrA* is indeed a SMBH it will, in projection onto the sky, have the largest event horizon and will certainly be the first and m...
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Context 1
... to the analysis of Hopman and Alexander (2006) the total number of MS stars within the radius of the gravitational influence r h = 2 pc is N h = 3.4×10 6 . Table 2 summarizes the inner radii of the cusp for different components. The range of power-law exponents Hopman and Alexander (2006) obtained in their dynamical calculation for different stellar populations (see Tab. 2) are included in the range of exponents covered in Tab. 1). ...
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Citations
... We select the dimensionless spin parameter and uniform magnetic field strength in the vicinity of the black hole as a * = 0.5 and B 0 = 30, 100 G [19]. The spin parameter value corresponds to orbiting spot models with variable emission [66]. The magnetic field strength coincides with the emission model of Sgr A* that requires the ordered magnetic field with B 0 = 30 − 100 G and most accretion models in equipartition state have the magnetic field with few hundred gauss [67]. ...
Typical mechanisms to extract energies from a rotating black hole are the Blandford-Znajek process and the Penrose process. The Penrose process requires a special condition that is difficult to occur in common astrophysical situations. However, the magnetic Penrose process (MPP) does not require such a special condition, and can produce ultra-high energy cosmic rays. When neutrons decay near a rotating black hole, the MPP efficiency of the produced proton is maximized. The supermassive black hole in Sagittarius A* (Sgr A*) is likely to have a radiatively inefficient accretion flow that is hot enough to produce neutrons by nuclear reactions, which can be subsequently accelerated to high-energy by the MPP. We calculate the production rate of accelerated protons from the Sgr A* to estimated the gamma-ray flux at Earth produced by these accelerated protons and the flux of the accelerated protons themselves transported from Sgr A* to Earth. We find that these very high-energy gamma rays ($E_{\gamma}\gtrsim10\,\mathrm{TeV}$) amount to a significant fraction of the flux of the gamma-ray from the HESS J1745-290 and the central molecular zone around $100\,\mathrm{TeV}$. The accelerated proton flux, when the dimensionless spin parameter $a_{*}=0.5$ and the magnetic field strength in the vicinity of the black hole $B_{0}=100\,\mathrm{G}$, is about $1.6-4.1\%$ of the cosmic ray proton flux from KASCADE experiment at about $1\,\mathrm{PeV}$. Due to the finite decay time of neutrons which need to be transported from the accretion flow to the acceleration zone, our acceleration model can operate only around black holes with mass not much greater than $\sim10^8\,M_\odot$.
... For a strongly relativistic system-such as an accretion disk in the environment of a rapidly rotating supermassive object or a supermassive black hole, see Refs. [45][46][47]-the conditions (in covariant description) of adiabaticity and incompressibility remain the same, while the equation of fluid motion (in spacetime with a metric defined by the square of the four-interval ds 2 = m ik dq i dq k in the equatorial plane of a rotating black hole when the coordinate q 2 ≡ θ = π/2) has the following form in the leading approximation (these details of our analysis are not listed in this paper): ...
Thermo-vortices (bright spots, blobs, swirls) in cosmic fluids (planetary atmospheres, or even black hole accretion disks) are sometimes observed as clustered into quasi-symmetrical quasi-stationary groups but conceptualized in models as autonomous items. We demonstrate—using the (analytical) Sharp Boundaries Evolution Method and a generic model of a thermo-vorticial field in a rotating “thin” fluid layer in a spacetime that may be curved or flat—that these thermo-vortices may be not independent but represent interlinked parts of a single, coherent, multi-petal macro-structure. This alternative conceptualization may influence the designs of numerical models and image-reconstruction methods.
... These sources are also detected in the X-ray domain and are listed in the XMM-Newton X-ray source catalog (4XMMDR11). The sample position on the sky to the Galactic plane and the Galactic center (Sgr A * ; Eckart et al. 2017) is shown in Figure 1, which is color coded using the decadic logarithm of the (B − V ) color index -E log B V according to Schlegel et al. (1998). ...
In Khadka et al., a sample of X-ray-detected reverberation-mapped quasars was presented and applied for the comparison of cosmological constraints inferred using two well-established relations in active galactic nuclei—the X-ray/UV luminosity ( L X – L UV ) relation and the broad-line region radius–luminosity ( R – L ) relation. L X – L UV and R – L luminosity distances to the same quasars exhibit a distribution of their differences that is generally asymmetric and positively shifted for the six cosmological models we consider. We demonstrate that this behavior can be interpreted qualitatively as arising as a result of the dust extinction of UV/X-ray quasar emission. We show that the extinction always contributes to the nonzero difference between L X – L UV -based and R – L -based luminosity distances and we derive a linear relationship between the X-ray/UV color index E X −UV and the luminosity-distance difference, which also depends on the value of the L X – L UV relation slope. Taking into account the median and the peak values of the luminosity-distance difference distributions, the average X-ray/UV color index falls in the range of E ¯ X − UV = 0.03 – 0.28 mag for the current sample of 58 sources. This amount of extinction is typical for the majority of quasars and can be attributed to the circumnuclear and interstellar media of host galaxies. After applying the standard hard X-ray and far-UV extinction cuts, heavily extincted sources are removed but overall the shift toward positive values persists. The effect of extinction on luminosity distances is more pronounced for the L X – L UV relation since the extinction of UV and X-ray emissions both contribute.
... There are many proofs to indicate that most of galaxies host a supermassive black hole (SMBH) at the central part of these sources [for example see: [14], [15], [16]and reference therein]. This huge mass is responsible for keeping the galaxy hold together by very powerful gravitational force with a roughly mass of 10 7 Msun -10 9 Msun [17], [18] and even more . ...
A Spectroscopic study has been focused in this article to study one of the main types of active galaxies which are quasars, and to be more precise this research focuses on studying the correlation between the main engine of Quasi-Stellar Objects (QSO), the central black hole mass (SMBH) and other physical properties (e.g. the star formation rate (SFR)). Twelve objects have been randomly selected for “The Half Million Quasars (HMQ) Catalogue” published in 2015 and the data collected from Salon Digital Sky survey (SDSS) Dr. 16. The redshift range of these galaxies were between (0.05 – 0.17). The results show a clear linear proportionality between the SMBH and the SFR, as well as direct proportional between the luminosity at V-band and the central black hole masses of the QSO. Furthermore, the calculation exhibits the ration between the SMBH and it hosts bulge approximately equal to 10-3.
... In the case of Sgr A ⋆ , the presence of a jet is still to be demonstrated (e.g., Falcke 1999;Li et al. 2013;Yusef-Zadeh et al. 2020;Fragione & Loeb 2020;Cecil et al. 2021). For Sgr A*, asymmetric X-ray flares seem to be associated with nearly edge-on orbits (Mossoux et al. 2015;Eckart et al. 2017;Karssen et al. 2017), which could be the sign of the accretion-flow precession or the disk warping due to the Bardeen-Petterson effect. Dexter & Fragile (2013) modelled tilted thick accretion flows around Sgr A* and obtained a good agreement with the near-infrared and the mm-domain observations in terms of the variability and the spectral evolution. ...
The combined study of the flaring of Active Galactic Nuclei (AGN) at radio wavelengths and pc-scale jet kinematics with Very Long Baseline Interferometry (VLBI) has led to the view that i) the observed flares are associated with ejections of synchrotron blobs from the core, and ii) most of the flaring would follow a one-to-one correlation with the component ejection. Recent results have provided mounting evidence that the quasi-regular component injections into the relativistic jet may not be the only cause of the flux variability. We propose that AGN flux variability and jet morphology changes can both be of deterministic nature, i.e. having a geometric/kinetic origin linked to the time-variable Doppler beaming of the jet emission as its direction changes due to precession (and nutation). The physics of the underlying jet leads to shocks, instabilities, or to ejections of plasmoids. The appearance (morphology, flux, etc.) of the jet can, however, be strongly affected and modulated by precession. We demonstrate this modulating power of precession for OJ 287. For the first time, we show that the spectral state of the Spectral Energy Distribution (SED) can be directly related to the jet's precession phase. We model the SED evolution and reproduce the precession parameters. Further, we apply our precession model to eleven prominent AGN. We show that for OJ 287 precession seems to dominate the long-term variability ($\gtrsim 1\,{\rm yr}$) of the AGN flux, SED spectral state, and jet morphology, while stochastic processes affect the variability on short timescales ($\lesssim 0.2\,{\rm yr}$).
... Recently, Eckart et al. (2017) have argued that "[super-massive black holes] are philosophically interesting entities given that they are only observable by indirect means." Eckart et al. do not define what they mean by "indirect" here, but we can draw on a precise characterization of directness due to . ...
... 14 Assuming Shapere's notion of direct vs indirect observability, what side do black holes fall on? Eckart et al. (2017) point at the nature of evidence concerning black holes to justify their claims. If an astronomical source is a black hole, there can be no emission from the black hole itself; an isolated classical black hole, after all, is a perfect absorber. ...
... The black hole on its own is, then, an in principle unobservable entity. This is in line with , invoked by Eckart et al. (2017), who notes that "[a] black hole is as theoretical an entity as could be. Moreover, it is in principle unobservable. ...
This chapter examines some of the philosophical literature on idealizations in science and the epistemic challenges idealizations potentially pose for astrophysical methodology, particularly its use of computer simulations. We begin by surveying philosophical literature on idealization connected to (1) kinds of idealizations deployed in science, (2) the aims of idealization in science, and (3) various strategies for de-idealization. Using collisional ring galaxy simulations as a case study, we examine how these three themes play out in the context of astrophysical computer simulations. Ultimately, we argue that deploying de-idealization strategies is central to bolstering epistemic confidence in simulations in astrophysics. We conclude with some remarks on the role of idealization in the context of astrophysical computer simulations more generally.
... Recently, Eckart et al. (2017) have argued that "[super-massive black holes] are philosophically interesting entities given that they are only observable by indirect means." Eckart et al. do not define what they mean by "indirect" here, but we can draw on a precise characterization of directness due to . ...
... 14 Assuming Shapere's notion of direct vs indirect observability, what side do black holes fall on? Eckart et al. (2017) point at the nature of evidence concerning black holes to justify their claims. If an astronomical source is a black hole, there can be no emission from the black hole itself; an isolated classical black hole, after all, is a perfect absorber. ...
... The black hole on its own is, then, an in principle unobservable entity. This is in line with , invoked by Eckart et al. (2017), who notes that "[a] black hole is as theoretical an entity as could be. Moreover, it is in principle unobservable. ...
It is generally accepted that science sometimes involves reasoning with analogies. Often, this simply means that analogies between disparate objects of study might be used as heuristics to guide theory development. Contemporary black hole physics, however, deploys analogical reasoning in a way that seems to overreach this traditional heuristic role. In this chapter, I describe two distinct pieces of analogical reasoning that are quite central to the contemporary study of black holes. The first underpins arguments for the existence of astrophysical Hawking radiation, and the second underpins arguments for black holes being ‘genuinely’ thermodynamical in nature. I argue that while these are distinct analogical arguments, they depend on one another in an interesting way: the success of the second analogical argument presupposes the success of the first. This induces a tension for those who wish to take black hole thermodynamics seriously, but who are sceptical of the evidence provided for astrophysical Hawking radiation by the results of analogue gravity. I consider three ways to resolve this tension, and show that each fails.
... Recently, Eckart et al. (2017) have argued that "[super-massive black holes] are philosophically interesting entities given that they are only observable by indirect means." Eckart et al. do not define what they mean by "indirect" here, but we can draw on a precise characterization of directness due to . ...
... 14 Assuming Shapere's notion of direct vs indirect observability, what side do black holes fall on? Eckart et al. (2017) point at the nature of evidence concerning black holes to justify their claims. If an astronomical source is a black hole, there can be no emission from the black hole itself; an isolated classical black hole, after all, is a perfect absorber. ...
... The black hole on its own is, then, an in principle unobservable entity. This is in line with , invoked by Eckart et al. (2017), who notes that "[a] black hole is as theoretical an entity as could be. Moreover, it is in principle unobservable. ...
We discuss three philosophically interesting epistemic peculiarities of black hole astrophysics: (1) issues concerning whether and in what sense black holes do exist; (2) how to best approach multiplicity of available definitions of black holes; (3) short (i.e., accessible within an individual human lifespan) dynamical timescales present in many of the recent, as well as prospective, observations involving black holes. In each case we argue that the prospects for our epistemic situation are optimistic.
... Recently, Eckart et al. (2017) have argued that "[super-massive black holes] are philosophically interesting entities given that they are only observable by indirect means." Eckart et al. do not define what they mean by "indirect" here, but we can draw on a precise characterization of directness due to . ...
... 14 Assuming Shapere's notion of direct vs indirect observability, what side do black holes fall on? Eckart et al. (2017) point at the nature of evidence concerning black holes to justify their claims. If an astronomical source is a black hole, there can be no emission from the black hole itself; an isolated classical black hole, after all, is a perfect absorber. ...
... The black hole on its own is, then, an in principle unobservable entity. This is in line with , invoked by Eckart et al. (2017), who notes that "[a] black hole is as theoretical an entity as could be. Moreover, it is in principle unobservable. ...
Next-generation observational surveys in astronomy provide empirical data with increasingly high resolution and precision. After presenting the basic methods of population synthesis (via Conroy C, Ann Rev Astronom Astrophys 51:393–455, 2013; Maraston C, Mon Not Royal Astronom Soc 362:799–825, 2005), this paper argues for several related conclusions. The increased precision of the new methods requires the development of improved theoretical resources and models to provide the richest interpretation of the new data (as argued by Maraston C, Strömbäck G, Monthly Not Royal Astronom Soc 418:2785–2811, 2011). The measurement of physical variables and parameters in population synthesis is best understood using a model-based account along the lines of (Tal E, The epistemology of measurement: a model-based approach. Dissertation, The University of Toronto, 2012) and (Parker WS, Br J Philos Sci 68:273–304, 2017). Finally, in the case of population synthesis, improved empirical data does not dispense with the need for theoretical reasoning in post-data analysis. In fact, the high-resolution data used in next-generation population synthesis demands ever richer theories and models, a process that results in hybrid enrichment of theoretical and observational methods and results.
... Recently, Eckart et al. (2017) have argued that "[super-massive black holes] are philosophically interesting entities given that they are only observable by indirect means." Eckart et al. do not define what they mean by "indirect" here, but we can draw on a precise characterization of directness due to . ...
... 14 Assuming Shapere's notion of direct vs indirect observability, what side do black holes fall on? Eckart et al. (2017) point at the nature of evidence concerning black holes to justify their claims. If an astronomical source is a black hole, there can be no emission from the black hole itself; an isolated classical black hole, after all, is a perfect absorber. ...
... The black hole on its own is, then, an in principle unobservable entity. This is in line with , invoked by Eckart et al. (2017), who notes that "[a] black hole is as theoretical an entity as could be. Moreover, it is in principle unobservable. ...
A theoretical astrophysicist discusses the principles and rules-of-thumb underlying the construction of models and simulations from the perspective of an active practitioner, where it is emphasised that they are designed to address specific scientific questions. That models are valid only within a restricted space of parameters and degenerate combinations of parameter values produce the same observable outcome are features, and not bugs, of competent practice that fit naturally within a Bayesian framework of inference. Idealisations within a model or simulation are strongly tied to the questions they are designed to address and the precision at which they are confronted by data. If the practitioner visualises a hierarchy of models of varying sophistication (which is standard practice in astrophysics and climate science), then de-idealisation becomes an irrelevant concept. Opportunities for future collaborations between astrophysicists and philosophers of science are suggested.
