📝 Original Info
- Title: Modeling Emission from the Supermassive Black Hole in the Galactic Center with GRMHD Simulations
- ArXiv ID: 0904.0118
- Date: 2009-04-01
- Authors: Kyle W. Martin, Siming Liu, Chris Fragile, Cong Yu, Chris L. Fryer
📝 Abstract
Sagittarius A* is a compact radio source at the Galactic center, powered by accretion of fully ionized plasmas into a supermassive black hole. However, the radio emission cannot be produced through the thermal synchrotron process by a gravitationally bounded flow. General relativistic magneto-hydrodynamical(GRMHD) simulations of black hole accretion show that there are strong unbounded outflows along the accretion. With the flow structure around the black hole given by GRMHD simulations, we investigate whether thermal synchrotron emission from these outflows may account for the observed radio emission. We find that simulations producing relatively high values of plasma beta cannot produce the radio flux level without exceeding the X-ray upper limit set by Chandra observations through the bremsstrahlung process. The predicted radio spectrum is also harder than the observed spectrum both for the one temperature thermal model and a simple nonthermal model with a single power-law electron distribution. The electron temperature needs to be lower than the gas temperature near the black hole to reproduce the observed radio spectrum. A more complete modeling of the radiation processes, including the general relativistic effects and transfer of polarized radiation, will give more quantitative constraints on physical processes in Sgr A* with the current multi-wavelength, multi-epoch, and polarimetric observations of this source.
💡 Deep Analysis
Deep Dive into Modeling Emission from the Supermassive Black Hole in the Galactic Center with GRMHD Simulations.
Sagittarius A* is a compact radio source at the Galactic center, powered by accretion of fully ionized plasmas into a supermassive black hole. However, the radio emission cannot be produced through the thermal synchrotron process by a gravitationally bounded flow. General relativistic magneto-hydrodynamical(GRMHD) simulations of black hole accretion show that there are strong unbounded outflows along the accretion. With the flow structure around the black hole given by GRMHD simulations, we investigate whether thermal synchrotron emission from these outflows may account for the observed radio emission. We find that simulations producing relatively high values of plasma beta cannot produce the radio flux level without exceeding the X-ray upper limit set by Chandra observations through the bremsstrahlung process. The predicted radio spectrum is also harder than the observed spectrum both for the one temperature thermal model and a simple nonthermal model with a single power-law electron
📄 Full Content
Sagittarius (Sgr) A*, the compact radio source at the Galactic center, is powered by accretion of a supermassive black hole of ∼ (3 -4) × 10 6 M J (Schödel et al. 2002;Ghez et al. 2004) in the prevailing stellar winds in the Galactic center region (Melia 1992). The bolometric luminosity of Sgr A* is more than 9 orders of magnitude lower than the corresponding Eddington luminosity, suggesting a radiatively inefficient accretion flow (Melia et al. 2001;Yuan et al. 2003). The radiative cooling processes therefore may be ignored while studying the dynamics of the accretion flow near the black hole, which simplifies the dynamical equations and the corresponding numerical algorithms significantly. Several general relativistic magneto-hydrodynamical (GRMHD) codes have been developed over the past few years to study the structure of non-radiative accretion flows near black holes quantitatively (De Villiers et al. 2003;Gammie et al. 2003;Anninos et al. 2005). Given the extensive observations available for this supermassive black hole, it provides perhaps the best opportunity to study the physical processes in the strong gravity near black holes with GRMHD simulations (Falcke et al. 2000;Huang et al. 2008).
The observed linear polarization and variability of the millimeter to near-infrared (NIR) emissions suggest that they are produced by the synchrotron process near the black hole (Aitken et al. 2000;Melia et al. 2000;Genzel et al. 2003;Eckart et al. 2004Eckart et al. , 2006)). The longer wavelength emissions are circularly polarized (Bower et al. 2001) with weak linear polarization observed during frequent outbursts or flares (Zhao et al. 2004;Yusef-Zadeh et al. 2007), which in combination with the continuity of the centimeter to sub-millimeter spectrum suggests a synchrotron origin for the longer wavelength emissions as well (Falcke et al. 1998;An et al. 2005). The analogy of Sgr A* with radio loud AGNs also favors a synchrotron scenario for the radio emission. However, the longer wavelength flux densities are less variable than the millimeter and sub-millimeter flux densities, indicating emissions from relatively larger radii (Falcke & Markoff 2000;Liu & Melia 2001;Herrnstein et al. 2004). Indeed, the millimeter and higher frequency emissions have been explained reasonably well with an accretion torus within ∼ 10 r S of the black hole, where r S is the Schwarzschild radius (Melia et al. 2000;Goldston et al. 2005;Liu et al. 2007;Noble et al. 2007); And high spatial resolution VLBI observations do show that the intrinsic size of the millimeter emission region is within 20 r S in radius (Bower et al. 2004;Shen et al. 2005). While the general relativistic (GR) effects are important for these shorter wavelength emissions originating near the black hole (Falcke et al. 2000;Bromley et al. 2001), the longer wavelength emissions are produced beyond ∼ 10 r S so that the GR effects are insignificant and one may readily use the GRMHD simulations to model the observed emission without using the sophisticated GR ray-tracing radiation transfer codes for the polarized synchrotron emission (Broderick & Loeb 2005;Huang et al. 2008).
High resolution X-ray observations with the Chandra space telescope shows that the quiescent X-ray excess from the direction of Sgr A* is extended and has ion emission lines in the spectrum, suggesting that the emission is mostly produced at large radii near the capture radius of the accretion flow by a plasma of a few keV in temperature (Baganoff et al. 2001(Baganoff et al. , 2003;;Xu et al. 2006). X-ray flares are routinely observed from the direction of Sgr A* (Belanger et al. 2006). Their spectra can be fitted with a featureless power law (Porquet et al. 2003), and they appear to be always accompanied by NIR flares, which are produced near the black hole (Eckart et al. 2004(Eckart et al. , 2006;;Yusef-Zadeh et al. 2006, 2008). The correlation between the NIR and X-ray flares indicates a synchrotron self-Comptonization (SSC) origin for the X-ray flares (Markoff et al. 2001;Liu & Melia 2001), though the X-ray flares may also be produced via the bremsstrahlung process enhanced by instabilities related to the accretion process (Liu & Melia 2002;Tagger & Melia 2006). Recent observations of flares seem to favor a synchrotron process (Liu & Melia 2001;Dodds-Eden et al. 2009).
The quiescent state X-ray emission from the inner accretion flow therefore must be lower than the observed flux level. X-ray emission can be produced through both the SSC and bremsstrahlung processes. The observed high radio flux level and low upper limit of X-ray flux suggest that the radio emission cannot be produced via thermal synchrotron emission in a bounded flow (Liu & Melia 2001). It therefore has to be produced by unbounded outflows and/or by nonthermal populations of relativistic electrons. Given the non-radiative nature of the GRMHD simulations, strong outflows and winds appear to be inevitable, especially for highly sp
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