2,246 research outputs found
Evolution of supermassive stars as a pathway to black hole formation
Supermassive stars, with masses greater than a million solar masses, are
possible progenitors of supermassive black holes in galactic nuclei. Because of
their short nuclear burning timescales, such objects can be formed only when
matter is able to accumulate at a rate exceeding ~ 1 solar mass/yr. Here we
revisit the structure and evolution of rotationally-stabilized supermassive
stars, taking into account their continuous accumulation of mass and their
thermal relaxation. We show that the outer layers of supermassive stars are not
thermally relaxed during much of the star's main sequence lifetime. As a
result, they do not resemble n=3 polytropes, as assumed in previous literature,
but rather consist of convective (polytropic) cores surrounded by convectively
stable envelopes that contain most of the mass. We compute the structures of
these envelopes, in which the specific entropy is proportional to the enclosed
mass M(R) to the 2/3-power. By matching the envelope solutions to convective
cores, we calculate the core mass as a function of time. We estimate the
initial black hole masses formed as a result of core-collapse, and their
subsequent growth via accretion from the bloated envelopes ("quasistars") that
result. The seed black holes formed in this way could have typical masses in
the range ~ 10^4-10^5 solar masses, considerably larger than the remnants
thought to be left by the demise of Population III stars. Supermassive black
holes therefore could have been seeded during an epoch of rapid infall
considerably later than the era of Pop III star formation.Comment: 10 pages, 5 figures, to appear in Monthly Notices of the Royal
Astronomical Societ
Reconnection-Powered Linear Accelerator and Gamma-Ray Flares in the Crab Nebula
The recent discovery of day-long gamma-ray flares in the Crab Nebula,
presumed to be synchrotron emission by PeV (10^{15} eV) electrons in milligauss
magnetic fields, presents a strong challenge to particle acceleration models.
The observed photon energies exceed the upper limit (~100 MeV) obtained by
balancing the acceleration rate and synchrotron radiation losses under standard
conditions where the electric field is smaller than the magnetic field. We
argue that a linear electric accelerator, operating at magnetic reconnection
sites, is able to circumvent this difficulty. Sufficiently energetic electrons
have gyroradii so large that their motion is insensitive to small-scale
turbulent structures in the reconnection layer and is controlled only by
large-scale fields. We show that such particles are guided into the
reconnection layer by the reversing magnetic field as they are accelerated by
the reconnection electric field. As these electrons become confined within the
current sheet, they experience a decreasing perpendicular magnetic field that
may drop below the accelerating electric field. This enables them to reach
higher energies before suffering radiation losses and hence to emit synchrotron
radiation in excess of the 100 MeV limit, providing a natural resolution to the
Crab gamma-ray flare paradox.Comment: 14 pages including 4 figure
Extreme AGN variability: evidence of magnetically elevated accretion?
Rapid, large amplitude variability at optical to X-ray wavelengths is now
seen in an increasing number of Seyfert galaxies and luminous quasars. The
variations imply a global change in accretion power, but are too rapid to be
communicated by inflow through a standard thin accretion disc. Such discs are
long known to have difficulty explaining the observed optical/UV emission from
active galactic nuclei. Here we show that alternative models developed to
explain these observations have larger scale heights and shorter inflow times.
Accretion discs supported by magnetic pressure in particular are geometrically
thick at all luminosities, with inflow times as short as the observed few year
timescales in extreme variability events to date. Future time-resolved,
multi-wavelength observations can distinguish between inflow through a
geometrically thick disc as proposed here, and alternative scenarios of extreme
reprocessing of a central source or instability-driven limit cycles.Comment: 5 pages, 2 figures, submitted to MNRAS letter
Force-feeding Black Holes
We propose that the growth of supermassive black holes is associated mainly
with brief episodes of highly super-Eddington infall of gas ("hyperaccretion").
This gas is not swallowed in real time, but forms an envelope of matter around
the black hole that can be swallowed gradually, over a much longer timescale.
However, only a small fraction of the black hole mass can be stored in the
envelope at any one time. We argue that any infalling matter above a few per
cent of the hole's mass is ejected as a result of the plunge in opacity at
temperatures below a few thousand degrees K, corresponding to the Hayashi
track. The speed of ejection of this matter, compared to the velocity
dispersion (sigma) of the host galaxy's core, determines whether the ejected
matter is lost forever or returns eventually to rejoin the envelope, from which
it can be ultimately accreted. The threshold between matter recycling and
permanent loss defines a relationship between the maximum black hole mass and
sigma that resembles the empirical M_BH-sigma relation.Comment: 5 pages, no figures, accepted for publication in The Astrophysical
Journal Letter
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