536 research outputs found
Hypervelocity stars and the environment of Sgr A*
Hypervelocity stars (HVSs) are a natural consequence of the presence of a
massive nuclear black hole (Sgr A*) in the Galactic Center. Here we use the
Brown et al. sample of unbound and bound HVSs together with numerical
simulations of the propagation of HVSs in the Milky Way halo to constrain three
plausible ejection mechanisms: 1) the scattering of stars bound to Sgr A* by an
inspiraling intermediate-mass black hole (IMBH); 2) the disruption of stellar
binaries in the tidal field of Sgr A*; and 3) the two-body scattering of stars
off a cluster of stellar-mass black holes orbiting Sgr A*. We compare the
predicted radial and velocity distributions of HVSs with the limited-statistics
dataset currently available, and show that the IMBH model appears to produce a
spectrum of ejection velocities that is too flat. Future astrometric and deep
wide-field surveys of HVSs should shed unambiguous light on the stellar
ejection mechanism and probe the Milky Way potential on scales as large as 200
kpc.Comment: 5 pages, 5 figures, accepted for publication in MNRAS letter
X-rays from Radio-Galaxies: BeppoSAX Observations
We briefly review BeppoSAX observations of X-ray bright radio-galaxies. Their
X-ray spectra are quite varied, and perhaps surprisingly, any similarity
between radio-loud AGN and Seyfert galaxies is the exception rather than the
rule. When detected, reprocessing features (iron line and reflection) are
generally weak, suggesting two possible scenarios: either: (1) non-thermal
(jet?) radiation dilutes the X-ray emission from the disk in radio-loud
objects, or (2) the solid angle subtended by the X-ray reprocessing material is
smaller in radio-loud than in radio-quiet AGN due to different characteristics
of the accretion disk itself.Comment: 6 pages, to appear in `Life Cycles of Radio Galaxies', ed. J. Biretta
et al., New Astronomy Review
The effects of a comptonizing corona on the appearance of the reflection components in accreting black hole spectra
We discuss the effects of a comptonizing corona on the appearance of the
reflection components, and in particular of the reflection hump, in the X-rays
spectra of accreting black holes. Indeed, in the framework of a thermal corona
model, we expect that part (or even all, depending on the coronal covering
factor) of the reflection features should cross the hot plasma, and thus suffer
Compton scattering, before being observed. We have studied in detail the
dependence of these effects on the physical (i.e. temperature and optical
depth) and geometrical (i.e. inclination angle) parameters of the corona,
concentrating on the slab geometry . Due to the smoothing and shifting towards
high energies of the comptonized reflection hump, the main effects on the
emerging spectra appear above 100 keV. We have also investigated the importance
of such effects on the interpretation of the results obtained with the standard
fitting procedures. We found that fitting Comptonization models, taking into
account comptonized reflection, by the usual cut-off power law + uncomptonized
reflection model, may lead to an underestimation of the reflection
normalization and an overestimation of the high energy cut-off. We discuss and
illustrate the importance of these effects by analysing recent observational
results as those of the galaxy NGC 4258. We also find that the comptonizing
corona can produce and/or emphasize correlations between the reflection
features characteristics (like the iron line equivalent width or the covering
fraction) and the X-ray spectral index similar to those recently reported in
the literature. We also underline the importance of these effects when dealing
with accurate spectral fitting of the X-ray background.Comment: 11 pages, 14 figures accepted for publication in MNRAS. Version
printable on US 8.5x11 pape
X-ray Dust Scattering at Small Angles: The Complete Halo around GX13+1
The exquisite angular resolution available with Chandra should allow
precision measurements of faint diffuse emission surrounding bright sources,
such as the X-ray scattering halos created by interstellar dust. However, the
ACIS CCDs suffer from pileup when observing bright sources, and this creates
difficulties when trying to extract the scattered halo near the source. The
initial study of the X-ray halo around GX13+1 using only the ACIS-I detector
done by Smith, Edgar & Shafer (2002) suffered from a lack of sensitivity within
50'' of the source, limiting what conclusions could be drawn.
To address this problem, observations of GX13+1 were obtained with the
Chandra HRC-I and simultaneously with the RXTE PCA. Combined with the existing
ACIS-I data, this allowed measurements of the X-ray halo between 2-1000''.
After considering a range of dust models, each assumed to be smoothly
distributed with or without a dense cloud along the line of sight, the results
show that there is no evidence in this data for a dense cloud near the source,
as suggested by Xiang et al. (2005). Finally, although no model leads to
formally acceptable results, the Weingartner & Draine (2001) and nearly all of
the composite grain models from Zubko, Dwek & Arendt (2004) give poor fits.Comment: 8 pages, 6 figures, accepted for publication in Ap
Growing massive black holes through super-critical accretion of stellar-mass seeds
The rapid assembly of the massive black holes that power the luminous quasars
observed at remains a puzzle. Various direct collapse models have
been proposed to head-start black hole growth from initial seeds with masses
, which can then reach a billion solar mass while
accreting at the Eddington limit. Here we propose an alternative scenario based
on radiatively inefficient super-critical accretion of stellar-mass holes
embedded in the gaseous circum-nuclear discs (CNDs) expected to exist in the
cores of high redshift galaxies. Our sub-pc resolution hydrodynamical
simulations show that stellar-mass holes orbiting within the central 100 pc of
the CND bind to very high density gas clumps that arise from the fragmentation
of the surrounding gas. Owing to the large reservoir of dense cold gas
available, a stellar-mass black hole allowed to grow at super-Eddington rates
according to the "slim disc" solution can increase its mass by 3 orders of
magnitudes within a few million years. These findings are supported by
simulations run with two different hydro codes, RAMSES based on the Adaptive
Mesh Refinement technique and GIZMO based on a new Lagrangian Godunov-type
method, and with similar, but not identical, sub-grid recipes for star
formation, supernova feedback, black hole accretion and feedback. The low
radiative efficiency of super-critical accretion flows are instrumental to the
rapid mass growth of our black holes, as they imply modest radiative heating of
the surrounding nuclear environment.Comment: 12 pages, 8 figures, 2 tables. Accepted for publication in MNRA
Interaction of massive black hole binaries with their stellar environment: II. Loss-cone depletion and binary orbital decay
We study the long-term evolution of massive black hole binaries (MBHBs) at
the centers of galaxies using detailed scattering experiments to solve the full
three-body problem. Ambient stars drawn from a isotropic Maxwellian
distribution unbound to the binary are ejected by the gravitational slingshot.
We construct a minimal, hybrid model for the depletion of the loss cone and the
orbital decay of the binary, and show that secondary slingshots - stars
returning on small impact parameter orbits to have a second super-elastic
scattering with the MBHB - may considerably help the shrinking of the pair in
the case of large binary mass ratios. In the absence of loss-cone refilling by
two-body relaxation or other processes, the mass ejected before the stalling of
a MBHB is half the binary reduced mass. About 50% of the ejected stars are
expelled ejected in a "burst" lasting ~1E4 yrs M_6^1/4, where M_6 is the binary
mass in units of 1E6 Msun. The loss cone is completely emptied in a few bulge
crossing timescales, 1E7 yrs M_6^1/4. Even in the absence of two-body
relaxation or gas dynamical processes, unequal mass and/or eccentric binaries
with M_6 >0.1 can shrink to the gravitational wave emission regime in less than
a Hubble time, and are therefore "safe" targets for the planned Laser
Interferometer Space Antenna (LISA).Comment: Minor revision. 10 pages, 7 figures, ApJ in pres
Low-frequency gravitational radiation from coalescing massive black hole binaries in hierarchical cosmologies
We compute the expected gravitational wave signal from coalescing massive
black hole (MBH) binaries at the center of galaxies in a hierarchical structure
formation scenario in which seed holes of intermediate mass form far up in the
dark halo merger tree. The merger history of DM halos and MBHs is followed from
z=20 to the present in a LCDM cosmology. MBHs get incorporated through halo
mergers into larger and larger structures, sink to the center owing to
dynamical friction against the DM background, accrete cold material in the
merger remnant, and form MBH binary systems. Stellar dynamical interactions
cause the hardening of the binary at large separations, while gravitational
wave emission takes over at small radii and leads to the final coalescence of
the pair. The integrated emission from inspiraling MBH binaries results in a
gravitational wave background (GWB). The characteristic strain spectrum has the
standard h_c(f)\propto f^{-2/3} behavior only in the range 1E-9<f<1E-6 Hz. At
lower frequencies the orbital decay of MBH binaries is driven by the ejection
of background stars, and h_c(f) \propto f. At higher frequencies, f>1E-6 Hz,
the strain amplitude is shaped by the convolution of last stable circular orbit
emission. We discuss the observability of inspiraling MBH binaries by the
planned LISA. Over a 3-year observing period LISA should resolve this GWB into
discrete sources, detecting ~60 (~250) individual events above a S/N=5 (S/N=1)
confidence level. (Abridged)Comment: 11 pages, 8 figues. Revised version accepted to be published in ApJ
Discussion on number counts corrected and expande
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