473 research outputs found
Star Captures by Quasar Accretion Disks: A Possible Explanation of the M-sigma Relation
A new theory of quasars is presented in which the matter of thin accretion
disks around black holes is supplied by stars that plunge through the disk.
Stars in the central part of the host galaxy are randomly perturbed to highly
radial orbits, and as they repeatedly cross the disk they lose orbital energy
by drag, eventually merging into the disk. Requiring the rate of stellar mass
capture to equal the mass accretion rate into the black hole, a relation
between the black hole mass and the stellar velocity dispersion is predicted of
the form M_{BH} \propto sigma_*^{30/7}. The normalization depends on various
uncertain parameters such as the disk viscosity, but is consistent with
observation for reasonable assumptions. We show that a seed central black hole
in a newly formed stellar system can grow at the Eddington rate up to this
predicted mass via stellar captures by the accretion disk. Once this mass is
reached, star captures are insufficient to maintain an Eddington accretion
rate, and the quasar may naturally turn off as the accretion switches to a
low-efficiency advection mode. The model provides a mechanism to deliver mass
to the accretion disk at small radius, probably solving the problem of
gravitational instability to star formation in the disk at large radius. We
note that the matter from stars that is incorporated to the disk has an average
specific angular momentum that is very small or opposite to that of the disk,
and discuss how a rotating disk may be maintained as it captures this matter if
a small fraction of the accreted mass comes from stellar winds that form a disk
extending to larger radius. We propose several observational tests and
consequences of this theory.Comment: submitted to Ap
The spatial relation between the event horizon and trapping horizon
The relation between event horizons and trapping horizons is investigated in
a number of different situations with emphasis on their role in thermodynamics.
A notion of constant change is introduced that in certain situations allows the
location of the event horizon to be found locally. When the black hole is
accreting matter the difference in area between the two different horizons can
be many orders of magnitude larger than the Planck area. When the black hole is
evaporating the difference is small on the Planck scale. A model is introduced
that shows how trapping horizons can be expected to appear outside the event
horizon before the black hole starts to evaporate. Finally a modified
definition is introduced to invariantly define the location of the trapping
horizon under a conformal transformation. In this case the trapping horizon is
not always a marginally outer trapped surface.Comment: 16 pages, 1 figur
Real-time multi-directional flow MRI using model-based reconstructions of undersampled radial FLASH – A feasibility study
The purpose of this work was to develop an acquisition and reconstruction technique for two- and three-directional (2d and 3d) phase-contrast flow MRI in real time. A previous real-time MRI technique for one-directional (1d) through-plane flow was extended to 2d and 3d flow MRI by introducing in-plane flow sensitivity. The method employs highly undersampled radial FLASH sequences with sequential acquisitions of two or three flow-encoding datasets and one flow-compensated dataset. Echo times are minimized by merging the waveforms of flow-encoding and radial imaging gradients. For each velocity direction individually, model-based reconstructions by regularized nonlinear inversion jointly estimate an anatomical image, a set of coil sensitivities and a phase-contrast velocity map directly. The reconstructions take advantage of a dynamic phase reference obtained by interpolating consecutive flow-compensated acquisitions. Validations include pulsatile flow phantoms as well as in vivo studies of the human aorta at 3 T. The proposed method offers cross-sectional 2d and 3d flow MRI of the human aortic arch at 53 and 67 ms resolution, respectively, without ECG synchronization and during free breathing. The in-plane resolution was 1.5 × 1.5 mm2 and the slice thickness 6 mm. In conclusion, real-time multi-directional flow MRI offers new opportunities to study complex human blood flow without the risk of combining differential phase (i.e., velocity) information from multiple heartbeats as for ECG-gated data. The method would benefit from a further reduction of acquisition time and accelerated computing to allow for extended clinical trials
Possible Detection of Lyman-alpha Fluorescence from a Damped Lyman Alpha system at Redshift z=2.8
We have detected Lyman-alpha emission from a damped Lyman-alpha system (DLA)
that lies near the bright quasar HS1549+1919. The DLA has the same redshift as
HS1549+1919 and was discovered in the spectrum of a faint QSO that lies 49"
away (380 proper kpc). The emission line's luminosity, double-peaked profile,
and small spatial separation from the DLA suggest that it may be fluorescent
Lyman-alpha emission from gas that is absorbing the nearby QSO's radiation. If
this is the case, our observations show that the DLA has a size of at least
1.5" and that the QSO's luminosity one million years ago was similar to its
luminosity today. A survey for similar systems within 1' of bright QSOs would
put interesting limits on the mean quasar lifetime.Comment: 6.1 pages including 4 figures; accepted for publication in the Ap
Lyman Break Galaxies and the Lyman-alpha Forest
We use hydrodynamic simulations to predict correlations between Lya forest
absorption and galaxies at redshift z~3. The probability distribution function
(PDF) of Lya flux decrements shifts systematically towards higher values in the
vicinity of galaxies, reflecting the overdense environments in which these
galaxies reside. The predicted signal remains strong in spectra smoothed over
50-200 km/s, allowing tests with moderate resolution quasar spectra. The strong
bias of high redshift galaxies towards high density regions imprints a clear
signature on the flux PDF, but the predictions are not sensitive to galaxy
baryon mass or star formation rate, and they are similar for galaxies and for
dark matter halos. The dependence of the flux PDF on galaxy proximity is
sensitive to redshift determination errors, with rms errors of 150-300 km/s
substantially weakening the predicted trends. On larger scales, the mean galaxy
overdensity in a cube of 5 or 10 Mpc/h (comoving) is strongly correlated with
the mean Lya flux decrement on a line of sight through the cube center. The
slope of the correlation is ~3 times steeper for galaxies than for dark matter
as a result of galaxy bias. The predicted large scale correlation is in
qualitative agreement with recently reported observational results. However,
observations also show a drop in absorption in the immediate vicinity of
galaxies, which our models do not predict even if we allow the galaxies or AGNs
within them to be ionizing sources. This decreased absorption could be a
signature of galaxy feedback on the surrounding IGM, perhaps via galactic
winds. Peculiar velocities often allow gas at comoving distances ~1.5 Mpc/h to
produce saturated absorption at the galaxy redshift, so any feedback mechanism
must suppress neutral hydrogen out to these radii to match the data. (Abridged)Comment: 54 pages, 19 figures. Accepted for publication in ApJ. Includes
discussion of simple wind model
ASASSN-15pz: Revealing Significant Photometric Diversity among 2009dc-like, Peculiar SNe Ia
We report comprehensive multi-wavelength observations of a peculiar Type
Ia-like supernova ("SN Ia-pec") ASASSN-15pz. ASASSN-15pz is a spectroscopic
"twin" of SN 2009dc, a so-called "Super-Chandrasekhar-mass" SN, throughout its
evolution, but it has a peak luminosity M_B,peak = -19.69 +/- 0.12 mag that is
\approx 0.6 mag dimmer and comparable to the SN 1991T sub-class of SNe Ia at
the luminous end of the normal width-luminosity relation. The synthesized Ni56
mass of M_Ni56 = 1.13 +/- 0.14 M_sun is also substantially less than that found
for several 2009dc-like SNe. Previous well-studied 2009dc-like SNe have
generally suffered from large and uncertain amounts of host-galaxy extinction,
which is negligible for ASASSN-15pz. Based on the color of ASASSN-15pz, we
estimate a host extinction for SN 2009dc of E(B-V)_host=0.12 mag and confirm
its high luminosity (M_B, peak[2009dc] \approx -20.3 mag). The 2009dc-like SN
population, which represents ~1% of SNe Ia, exhibits a range of peak
luminosities, and do not fit onto the tight width-luminosity relation. Their
optical light curves also show significant diversity of late-time (>~ 50 days)
decline rates. The nebular-phase spectra provide powerful diagnostics to
identify the 2009dc-like events as a distinct class of SNe Ia. We suggest
referring to these sources using the phenomenology-based "2009dc-like SN
Ia-pec" instead of "Super-Chandrasekhar SN Ia," which is based on an uncertain
theoretical interpretation.Comment: 21 pages, 16 figures, accepted for publication in Ap
- …