28 research outputs found
Probing the Two-Temperature Paradigm: Observational Tests for the Basic Assumptions in ADAFs
We calculate the flux and spectrum of synchrotron radiation produced by high
energy electrons and positrons (\epm) in an advection dominated accretion flow
(ADAF) around a black hole. The \epm are from the decay of charged pions which
are created through proton-proton collisions. We consider both a thermal and
power-law energy distribution of protons, and show that the resulting \epm
synchrotron emission produces a characteristic spectrum between radio and X-ray
frequencies. While previous signatures of the hot protons were only possible at
gamma-ray energies, via the production of gamma-rays through neutral pion
decay, the present results provide a more observationally tractable way of
probing the proton energy distribution and the two temperature structure in
these accretion flows. We discuss a number of strong observational predictions
from these systems, as well as the recent results of Mahadevan (1998) which
appear to confirm the two temperature structure in ADAFs. We show that the
results provide support for both a power-law and thermal distribution of
protons, with at least a third of the viscous energy going into the power-law.Comment: Submitted to MNRAS. 27 pages including 5 figure
Harmony in Electrons: Cyclotron and Synchrotron Emission by Thermal Electrons in a Magnetic Field
We present a complete solution to the cyclotron-synchrotron radiation due to
an isotropic distribution of electrons moving in a magnetic field. We make no
approximations in the calculations other than artificially broadening the
harmonics by a small amount in order to facilitate the numerics. In contrast to
previous calculations, we sum over all relevant harmonics and integrate over
all particle and observer angles relative to the magnetic field. We present
emission spectra for electron temperatures K, K,
K to K, and provide simple fitting formulae
which give a fairly accurate representation of the detailed results. For
K, the spectrum is represented well by the asymptotic
synchrotron formula, which is obtained by assuming that the radiating electrons
have Lorentz factors large compared to unity. We give an improved fitting
formula also for this asymptotic case.Comment: 17 pages, 6 Postscript figures, tarred and gzipped. Accepted to The
Astrophysical Journal. Also available at this
http://cfa-www.harvard.edu/~rohan
Are Particles in Advection-Dominated Accretion Flows Thermal?
We investigate the form of the momentum distribution function for protons and
electrons in an advection-dominated accretion flow (ADAF). We show that for all
accretion rates, Coulomb collisions are too inefficient to thermalize the
protons. The proton distribution function is therefore determined by the
viscous heating mechanism, which is unknown. The electrons, however, can
exchange energy quite efficiently through Coulomb collisions and the emission
and absorption of synchrotron photons. We find that for accretion rates greater
than \sim 10^{-3} of the Eddington accretion rate, the electrons have a thermal
distribution throughout the accretion flow. For lower accretion rates, the
electron distribution function is determined by the electron's source of
heating, which is primarily adiabatic compression. Using the principle of
adiabatic invariance, we show that an adiabatically compressed collisionless
gas maintains a thermal distribution until the particle energies become
relativistic. We derive a new, non-thermal, distribution function which arises
for relativistic energies and provide analytic formulae for the synchrotron
radiation from this distribution. Finally, we discuss its implications for the
emission spectra from ADAFs.Comment: 29 pages (Latex), 3 Figures. Submitted to Ap
Scaling Laws for Advection Dominated Flows: Applications to Low Luminosity Galactic Nuclei
We present analytical scaling laws for self-similar advection dominated
flows. The spectra from these systems range from 10 - 10 Hz, and
are determined by considering cooling of electrons through synchrotron,
bremsstrahlung, and Compton processes. We show that the spectra can be quite
accurately reproduced without detailed numerical calculations, and that there
is a strong testable correlation between the radio and X-ray fluxes from these
systems. We describe how different regions of the spectrum scale with the mass
of the accreting black hole, , the accretion rate of the gas, , and
the equilibrium temperature of the electrons, . We show that the universal
radio spectral index of 1/3 observed in most elliptical galaxies (Slee et al.
1994) is a natural consequence of self-absorbed synchrotron radiation from
these flows. We also give expressions for the total luminosity of these flows,
and the critical accretion rate, , above which the advection
solutions cease to exist. We find that for most cases of interest the
equilibrium electron temperature is fairly insensitive to , , and
parameters in the model. We apply these results to low luminosity black holes
in galactic nuclei. We show that the problem posed by Fabian & Canizares (1988)
of whether bright elliptical galaxies host dead quasars is resolved, as pointed
out recently by Fabian & Rees (1995), by considering advection-dominated flows.Comment: 30 pages, 5 postscript files. Accepted to ApJ. Also available
http://cfa-www.harvard.edu/~rohan/publications.htm
Gamma-ray Emission From Advection-Dominated Accretion Flows Around Black Holes: Application to the Galactic Center
We calculate the flux and spectrum of \gamma-rays emitted by a
two-temperature advection-dominated accretion flow (ADAF) around a black hole.
The \gamma-rays are from the decay of neutral pions produced through
proton-proton collisions. We discuss both thermal and power-law distributions
of proton energies and show that the \gamma-ray spectra in the two cases are
very different. We apply the calculations to the \gamma-ray source, 2EG
J1746-2852, detected by EGRET from the direction of the Galactic Center. We
show that the flux and spectrum of this source are consistent with emission
from an ADAF around the supermassive accreting black hole Sgr A^* if the proton
distribution is a power-law. The model uses accretion parameters within the
range made likely by other considerations. If this model is correct, it
provides evidence for the presence of a two temperature plasma in Sgr A^*, and
predicts \gamma-ray fluxes from other accreting black holes which could be
observed with more sensitive detectors.Comment: 19 pages (Latex), 4 Figures. ApJ 486. Revised Tables and Figure
Advection-Dominated Accretion Model of Sagittarius A*: Evidence for a Black Hole at the Galactic Center
Sgr A* at the Galactic Center is a puzzling source. It has a mass
M=(2.5+/-0.4) x 10^6 solar masses which makes it an excellent black hole
candidate. Observations of stellar winds and other gas flows in its vicinity
suggest a mass accretion rate approximately few x 10^{-6} solar masses per
year. However, such an accretion rate would imply a luminosity > 10^{40} erg/s
if the radiative efficiency is the usual 10 percent, whereas observations
indicate a bolometric luminosity <10^{37} erg/s. The spectrum of Sgr A* is
unusual, with emission extending over many decades of wavelength. We present a
model of Sgr A* which is based on a two-temperature optically-thin
advection-dominated accretion flow. The model is consistent with the estimated
mass and accretion rate, and fits the observed fluxes in the cm/mm and X-ray
bands as well as upper limits in the sub-mm and infrared bands; the fit is less
good in the radio below 86 GHz and in gamma-rays above 100 MeV. The very low
luminosity of Sgr A* is explained naturally in the model by means of advection.
Most of the viscously dissipated energy is advected into the central mass by
the accreting gas, and therefore the radiative efficiency is extremely low,
approximately 5 x 10^{-6}. A critical element of the model is the presence of
an event horizon at the center which swallows the advected energy. The success
of the model could thus be viewed as confirmation that Sgr A* is a black hole.Comment: 41 pages (Latex) including 6 Figures and 2 Tables. Final Revised
Version changes to text, tables and figures. ApJ, 492, in pres