Reconciling the spectrum of Sagittarius A* with a two-temperature plasma model

Mahadevan, Rohan

doi:10.1038/24204

Cited by 5 publications

(6 citation statements)

References 8 publications

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“…where α is the dimensionless Shakura-Sunyaev viscosity parameter, and R mean = (R max + R min )/2 = 0.15 arcsec. This is consistent with the upper limit on Ṁ from the lack of Faraday depolarization of Sgr A* [7], but lower than Ṁ predicted in T ≥ 10 7 K gas at this radius in most radiatively inefficient accretion flow models [16,31,32,33].…”

supporting

confidence: 88%

mentioning

confidence: 99%

“…X-ray measurements of gas density and temperature at the outer edge of the accretion flow, together with the assumptions of spherical adiabatic and constant in time accretion [13], can be used to estimate the mass supply to the black hole as ṀBondi ∼ 3×10 −6 M yr −1 [4,5,6]. If the radiative efficiency were ∼ 10% [14,15], the luminosity would be roughly 30,000 times the bolometric luminosity of ∼ 3 × 10 36 erg s −1 [16]. Extensive theoretical efforts aimed at resolving this discrepancy [8,9,10] have concluded that the accretion proceeds via radiatively inefficient accretion flow (RIAF) in a geometrically thick torus [17,18], in which the electrons remain cooler than the protons, allowing the observed luminosity to be produced by a much higher accretion rate than in a thin disk (see Supplementary Material), while also explaining the mm to γ−ray spectrum [19].…”

mentioning

confidence: 99%

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A cool accretion disk around the Galactic Centre black hole

Murchikova

Phinney

Pancoast

et al. 2019

Nature

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A supermassive black hole Sagittarius A* (SgrA*) with the mass M SgrA * 4 × 10 6 M resides at the centre of our galaxy [1,2]. A large reservoir of hot (10 7 K) and cooler (10 2 − 10 4 K) gas surrounds it within few pc [3]. Building up such a massive black hole within the ∼ 10 10 year lifetime of our galaxy would require a mean accretion rate of ∼ 4 × 10 −4 M yr −1 . At present, X-ray observations constrain the rate of hot gas accretion at the Bondi radius (10 5 R Sch = 0.04 pc at 8 kpc) tȯ M Bondi ∼ 3 × 10 −6 M yr −1 [4,5,6], and polarization measurements [7] constrain it near the event horizon toṀ horizon ∼ 10 −8 M yr −1 . A range of models was developed to describe the accretion gas onto an underfed black hole [8,9,10]. However, the exact physics still remains to be understood. One challenge with the radiation inefficient accretion flows (RIAFs) is that even if one understands the dynamics there is no accepted prescription for associating emissivity (and absorption) with the flow. The other issue is the lack of model-independent probes of accretion flow at intermediate radii (between few and ∼ 10 5 R s ), i.e. the constraints that do not assume a model of accretion flow as an input parameter. Here we report a detection and imaging of the 10 4 K ionized gas disk within 2×10 4 R Sch in a millimetre hydrogen recombination line H30α : n = 31 → 30 at 231.9 GHz [11,12] using the Atacama Large Millimeter/submillimeter Array (ALMA). The emission was detected with a double-peaked line profile spanning full width of 2, 200 km s −1 with the approaching and the receding components straddling Sgr A*, each offset from it by 0.11 arcsec = 0.004 pc. The red-shifted side is displaced to the north-east, while the blue-shifted side is displaced to the south-west. The limit on the total mass of ionized gas estimated from the emission is 10 −4 − 10 −5 M at a mean hydrogen density 10 5 − 10 6 cm −3 , depending upon whether or not we assume the presence of a uniform density disk or an ensemble of orbiting clouds, and the amplification factor of the mm radiation due to the strong background source which is Sgr A* continuum.Black hole accretion and feedback are crucial to understanding the evolution of galaxies, the origin of relativistic jets, and to study physics near black hole horizons. Sgr A* is our nearest supermassive black hole. As such it offers an excellent opportunity to study accretion and outflow processes close to a black hole. X-ray measurements of gas density and temperature at the outer edge of the accretion flow, together with the assumptions of spherical adiabatic and constant in time accretion [13], can be used to estimate the mass supply to the black hole aṡ M Bondi ∼ 3×10 −6 M yr −1 [4,5,6]. If the radiative efficiency were ∼ 10% [14,15], the luminosity would be roughly 30,000 times the bolometric luminosity of ∼ 3 × 10 36 erg s −1 [16]. Extensive

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supporting

confidence: 88%

mentioning

confidence: 99%

mentioning

confidence: 99%

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A cool accretion disk around the Galactic Centre black hole

Murchikova

Phinney

Pancoast

et al. 2019

Nature

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“…Another example is metallic conductors subjected to fast laser heating (Qiu and Tien 1993), which mainly heats the electrons, which slowly transmit their energy to the lattice, or hot electrons in semiconductor devices, which do not thermalize with the lattice. In contrast, systems under strong gravitational forces, as in accretion disks around black holes, proton temperature may be much higher than electron temperature (Mahadevan et al 1997, Mahadevan 1998. Still another example is radiation and matter: the exchange between matter and radiation is fast in ionized matter but it is slow when matter is neutral.…”

Section: Two-temperature Systemsmentioning

confidence: 99%

Temperature in non-equilibrium states: a review of open problems and current proposals

2003

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The conceptual problems arising in the definition and measurement of temperature in nonequilibrium states are discussed in this paper in situations where the local-equilibrium hypothesis is no longer satisfactory. This is a necessary and urgent discussion because of the increasing interest in thermodynamic theories beyond local equilibrium, in computer simulations, in non-linear statistical mechanics, in new experiments, and in technological applications of nanoscale systems and material sciences. First, we briefly review the concept of temperature from the perspectives of equilibrium thermodynamics and statistical mechanics. Afterwards, we explore which of the equilibrium concepts may be extrapolated beyond local equilibrium and which of them should be modified, then we review several attempts to define temperature in non-equilibrium situations from macroscopic and microscopic bases. A wide review of proposals is offered on effective non-equilibrium temperatures and their application to ideal and real gases, electromagnetic radiation, nuclear collisions, granular systems, glasses, sheared fluids, amorphous semiconductors and turbulent fluids. The consistency between the different relativistic transformation laws for temperature is discussed in the new light gained from this perspective. A wide bibliography is provided in order to foster further research in this field.

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“…, less efficient thermalization may lead to a hybrid thermal-nonthermal energy distribution. Observationally, it is indeed known that non-thermal electrons are needed to reproduce the quiescent low-frequency radio emission in Sgr A* [93][94][95] and other lowluminosity AGNs [96]. Theoretically, the direct heating of electrons (i.e., not via Coulomb collisions with ions) is proposed via several mechanisms, such as magnetic reconnection [97][98][99][100][101], MHD turbulence [98,[102][103][104][105], or dissipation of pressure anisotropy in a collisionless plasma [106].…”

Section: Advection-dominated Accretion Flowmentioning

confidence: 99%

Very High-Energy Emission from the Direct Vicinity of Rapidly Rotating Black Holes

Hirotani

2018

Galaxies

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When a black hole accretes plasmas at very low accretion rate, an advection-dominated accretion flow (ADAF) is formed. In an ADAF, relativistic electrons emit soft gamma-rays via Bremsstrahlung. Some MeV photons collide with each other to materialize as electron-positron pairs in the magnetosphere. Such pairs efficiently screen the electric field along the magnetic field lines, when the accretion rate is typically greater than 0.03-0.3% of the Eddington rate. However, when the accretion rate becomes smaller than this value, the number density of the created pairs becomes less than the rotationally induced Goldreich-Julian density. In such a charge-starved magnetosphere, an electric field arises along the magnetic field lines to accelerate charged leptons into ultra-relativistic energies, leading to an efficient TeV emission via an inverse-Compton (IC) process, spending a portion of the extracted hole's rotational energy. In this review, we summarize the stationary lepton accelerator models in black hole magnetospheres. We apply the model to super-massive black holes and demonstrate that nearby low-luminosity active galactic nuclei are capable of emitting detectable gamma-rays between 0.1 and 30 TeV with the Cherenkov Telescope Array. plasma accretion toward the rotation axis, the magnetic energy density dominates the plasmas' rest-mass energy density in these polar funnels.Within such a nearly vacuum, polar funnel, electron-positron pairs are supplied via the collisions of MeV photons emitted from the equatorial, accreting region. For example, when the mass accretion rate is typically less than 1% of the Eddington rate, the accreting plasmas form an advection-dominated accretion flow (ADAF), emitting radio to infrared photons via the synchrotron process and MeV photons via free-free and inverse-Compton (IC) processes [28,29]. Particularly, when the accretion rate becomes much less than the Eddington rate, the ADAF MeV photons can no longer sustain a force-free magnetosphere, which inevitably leads to the appearance of an electric field, E‖, along the magnetic field lines in the polar funnel. In such a vacuum gap, we can expect that the BZ power may be partially dissipated as particle acceleration and emission near the central engine, in the same manner as in pulsar outer gap (OG) model. In what follows, we summarize this vacuum gap model in BH magnetospheres. The Pulsar Outer Gap ModelThe Large Area Telescope (LAT) aboard the Fermi space gamma-ray observatory has detected pulsed signals in high-energy (HE) (0.1 GeV-10 GeV) gamma-rays from more than 200 rotation-powered pulsars [30]. Among them, 20 pulsars exhibit pulsed signals above 10 GeV, including 10 pulsars up to 25 GeV and other 2 pulsars above 50 GeV. Moreover, more than 99% of the LAT-detected young and millisecond pulsars exhibit phase-averaged spectra that are consistent with a pure-exponential or a sub-exponential cut off above the cutoff energies at a few GeV. What is more, 30% of these young pulsars show sub-exponential cut off, a slower decay than t...

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Reconciling the spectrum of Sagittarius A* with a two-temperature plasma model

Cited by 5 publications

References 8 publications

A cool accretion disk around the Galactic Centre black hole

A cool accretion disk around the Galactic Centre black hole

Temperature in non-equilibrium states: a review of open problems and current proposals

Very High-Energy Emission from the Direct Vicinity of Rapidly Rotating Black Holes

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