502 research outputs found
X-ray Spectral and Variability Properties of Low-Mass AGN
We study the X-ray properties of a sample of 14 optically-selected low-mass
AGN whose masses lie within the range 1E5 -2E6 M(solar) with XMM-Newton. Only
six of these low-mass AGN have previously been studied with sufficient quality
X-ray data, thus, we more than double the number of low-mass AGN observed by
XMM-Newton with the addition of our sample. We analyze their X-ray spectral
properties and variability and compare the results to their more massive
counterparts. The presence of a soft X-ray excess is detectable in all five
objects which were not background dominated at 2-3 keV. Combined with previous
studies, this gives a total of 8 low-mass AGN with a soft excess. The low-mass
AGN exhibit rapid, short-term variability (hundreds to thousands of seconds) as
well as long-term variability (months to years). There is a well-known
anti-correlation between black hole mass and variability amplitude (normalized
excess variance). Comparing our sample of low-mass AGN with this relation we
find that all of our sample lie below an extrapolation of the linear relation.
Such a flattening of the relation at low masses (below about 1E6 M(solar)) is
expected if the variability in all AGN follows the same shape power spectrum
with a break frequency that is dependent on mass. Finally, we also found two
objects that show significant absorption in their X-ray spectrum, indicative of
type 2 objects, although they are classified as type 1 AGN based on optical
spectra.Comment: 12 pages, 5 figures, 7 tables, accepted for publication in MNRA
A Single Circumbinary Disk in the HD 98800 Quadruple System
We present sub-arcsecond thermal infrared imaging of HD 98800, a young
quadruple system composed of a pair of low-mass spectroscopic binaries
separated by 0.8'' (38 AU), each with a K-dwarf primary. Images at wavelengths
ranging from 5 to 24.5 microns show unequivocally that the optically fainter
binary, HD 98800B, is the sole source of a comparatively large infrared excess
upon which a silicate emission feature is superposed. The excess is detected
only at wavelengths of 7.9 microns and longer, peaks at 25 microns, and has a
best-fit black-body temperature of 150 K, indicating that most of the dust lies
at distances greater than the orbital separation of the spectroscopic binary.
We estimate the radial extent of the dust with a disk model that approximates
radiation from the spectroscopic binary as a single source of equivalent
luminosity. Given the data, the most-likely values of disk properties in the
ranges considered are R_in = 5.0 +/- 2.5 AU, DeltaR = 13+/-8 AU, lambda_0 =
2(+4/-1.5) microns, gamma = 0+/-2.5, and sigma_total = 16+/-3 AU^2, where R_in
is the inner radius, DeltaR is the radial extent of the disk, lambda_0 is the
effective grain size, gamma is the radial power-law exponent of the optical
depth, tau, and sigma_total is the total cross-section of the grains. The range
of implied disk masses is 0.001--0.1 times that of the moon. These results show
that, for a wide range of possible disk properties, a circumbinary disk is far
more likely than a narrow ring.Comment: 11 page Latex manuscript with 3 postscript figures. Accepted for
publication in Astrophysical Journal Letters. Postscript version of complete
paper also available at
http://www.hep.upenn.edu/PORG/web/papers/koerner00a.p
Multi-Wavelength Coverage of State Transitions in the New Black Hole X-Ray Binary Swift J1910.2-0546
Understanding how black holes accrete and supply feedback to their
environment is one of the outstanding challenges of modern astrophysics. Swift
J1910.2-0546 is a candidate black hole low-mass X-ray binary that was
discovered in 2012 when it entered an accretion outburst. To investigate the
binary configuration and the accretion morphology we monitored the evolution of
the outburst for ~3 months at X-ray, UV, optical (B,V,R,I), and near-infrared
(J,H,K) wavelengths using Swift and SMARTS. The source evolved from a hard to a
soft X-ray spectral state with a relatively cold accretion disk that peaked at
~0.5 keV. A Chandra/HETG spectrum obtained during this soft state did not
reveal signatures of an ionized disk wind. Both the low disk temperature and
the absence of a detectable wind could indicate that the system is viewed at
relatively low inclination. The multi-wavelength light curves revealed two
notable features that appear to be related to X-ray state changes. Firstly, a
prominent flux decrease was observed in all wavebands ~1-2 weeks before the
source entered the soft state. This dip occurred in (0.6-10 keV) X-rays ~6 days
later than at longer wavelengths, which could possibly reflect the viscous time
scale of the disk. Secondly, about two weeks after the source transitioned back
into the hard state, the UV emission significantly increased while the X-rays
steadily decayed. We discuss how these observations may reflect changes in the
accretion flow morphology, perhaps related to the quenching/launch of a jet or
the collapse/recovery of a hot flow.Comment: 8 pages, 5 figures, 1 table. To be published in Ap
The M-sigma and M-L Relations in Galactic Bulges and Determinations of their Intrinsic Scatter
We derive improved versions of the relations between supermassive black hole
mass (M_BH) and host-galaxy bulge velocity dispersion (sigma) and luminosity
(L) (the M-sigma and M-L relations), based on 49 M_BH measurements and 19 upper
limits. Particular attention is paid to recovery of the intrinsic scatter
(epsilon_0) in both relations. We find log(M_BH / M_sun) = alpha + beta *
log(sigma / 200 km/s) with (alpha, beta, epsilon_0) = (8.12 +/- 0.08, 4.24 +/-
0.41, 0.44 +/- 0.06) for all galaxies and (alpha, beta, epsilon_0) = (8.23 +/-
0.08, 3.96 +/- 0.42, 0.31 +/- 0.06) for ellipticals. The results for
ellipticals are consistent with previous studies, but the intrinsic scatter
recovered for spirals is significantly larger. The scatter inferred reinforces
the need for its consideration when calculating local black hole mass function
based on the M-sigma relation, and further implies that there may be
substantial selection bias in studies of the evolution of the M-sigma relation.
We estimate the M-L relationship as log(M_BH / M_sun) = alpha + beta * log(L_V
/ 10^11 L_sun,V) of (alpha, beta, epsilon_0) = (8.95 +/- 0.11, 1.11 +/- 0.18,
0.38 +/- 0.09); using only early-type galaxies. These results appear to be
insensitive to a wide range of assumptions about the measurement errors and the
distribution of intrinsic scatter. We show that culling the sample according to
the resolution of the black hole's sphere of influence biases the relations to
larger mean masses, larger slopes, and incorrect intrinsic residuals.Comment: 27 pages, 18 figures, 7 tables, ApJ accepte
Numerical evolution of multiple black holes with accurate initial data
We present numerical evolutions of three equal-mass black holes using the
moving puncture approach. We calculate puncture initial data for three black
holes solving the constraint equations by means of a high-order multigrid
elliptic solver. Using these initial data, we show the results for three black
hole evolutions with sixth-order waveform convergence. We compare results
obtained with the BAM and AMSS-NCKU codes with previous results. The
approximate analytic solution to the Hamiltonian constraint used in previous
simulations of three black holes leads to different dynamics and waveforms. We
present some numerical experiments showing the evolution of four black holes
and the resulting gravitational waveform.Comment: Published in PR
On the Role of the Accretion Disk in Black Hole Disk-Jet Connections
Models of jet production in black hole systems suggest that the properties of
the accretion disk - such as its mass accretion rate, inner radius, and
emergent magnetic field - should drive and modulate the production of
relativistic jets. Stellar-mass black holes in the "low/hard" state are an
excellent laboratory in which to study disk-jet connections, but few
coordinated observations are made using spectrometers that can incisively probe
the inner disk. We report on a series of 20 Suzaku observations of Cygnus X-1
made in the jet-producing low/hard state. Contemporaneous radio monitoring was
done using the Arcminute MicroKelvin Array radio telescope. Two important and
simple results are obtained: (1) the jet (as traced by radio flux) does not
appear to be modulated by changes in the inner radius of the accretion disk;
and (2) the jet is sensitive to disk properties, including its flux,
temperature, and ionization. Some more complex results may reveal aspects of a
coupled disk-corona-jet system. A positive correlation between the reflected
X-ray flux and radio flux may represent specific support for a plasma ejection
model of the corona, wherein the base of a jet produces hard X-ray emission.
Within the framework of the plasma ejection model, the spectra suggest a jet
base with v/c ~ 0.3, or the escape velocity for a vertical height of z ~ 20
GM/c^2 above the black hole. The detailed results of X-ray disk continuum and
reflection modeling also suggest a height of z ~ 20 GM/c^2 for hard X-ray
production above a black hole, with a spin in the range 0.6 < a < 0.99. This
height agrees with X-ray time lags recently found in Cygnus X-1. The overall
picture that emerges from this study is broadly consistent with some
jet-focused models for black hole spectral energy distributions in which a
relativistic plasma is accelerated at z = 10-100 GM/c^2.Comment: Accepted for publication in Ap
Wear and Friction Behavior of Metal Impregnated Microporous Carbon Composites
Metal-matrix composites have been prepared by pressure-infiltration casting of copper-base alloy melts into microporous carbon preforms. The carbon preforms contained varying proportions of amorphous carbon and graphite. Load dependence of the wear and friction behavior of the composite pins has been examined under ambient conditions against cast-iron plates, using a pin-on-plate reciprocating wear tester. The wear resistance of the composite is significantly improved, as compared with the base alloy. Contrary to the normally expected behavior, the addition of graphite to the amorphous carbon does not reduce the friction coefficient, especially at high loads. The wear and friction behavior of the composites is very sensitive to the size and distribution of the microstructural constituents
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