4,160 research outputs found
Magneto-optical Kerr effect in
We have measured the magneto-optical Kerr rotation of ferromagnetic
with x=0.2 and 0.4, as well as of serving as
the non-magnetic reference material. As previously for , we could
identify a feature at 1 in the Kerr response which is related with
electronic transitions involving the localized 4f electron states. The absence
of this feature in the data for confirms the relevance of the
partially occupied 4f states in shaping the magneto-optical features of
-based hexaborides. Disorder by -doping broadens the itinerant charge
carrier contribution to the magneto-optical spectra
Estimating the Parameters of Sgr A*'s Accretion Flow Via Millimeter VLBI
Recent millimeter-VLBI observations of Sagittarius A* (Sgr A*) have, for the
first time, directly probed distances comparable to the horizon scale of a
black hole. This provides unprecedented access to the environment immediately
around the horizon of an accreting black hole. We leverage both existing
spectral and polarization measurements and our present understanding of
accretion theory to produce a suite of generic radiatively inefficient
accretion flow (RIAF) models of Sgr A*, which we then fit to these recent
millimeter-VLBI observations. We find that if the accretion flow onto Sgr A* is
well described by a RIAF model, the orientation and magnitude of the black
hole's spin is constrained to a two-dimensional surface in the spin,
inclination, position angle parameter space. For each of these we find the
likeliest values and their 1-sigma & 2-sigma errors to be a=0(+0.4+0.7),
inclination=50(+10+30)(-10-10) degrees, and position angle=-20(+31+107)(-16-29)
degrees, when the resulting probability distribution is marginalized over the
others. The most probable combination is a=0(+0.2+0.4), inclination=90(-40-50)
degrees and position angle=-14(+7+11)(-7-11) degrees, though the uncertainties
on these are very strongly correlated, and high probability configurations
exist for a variety of inclination angles above 30 degrees and spins below
0.99. Nevertheless, this demonstrates the ability millimeter-VLBI observations,
even with only a few stations, to significantly constrain the properties of Sgr
A*.Comment: 10 pages, 7 figures, accepted by Ap
Limits on the Position Wander of Sgr A*
We present measurements with the VLBA of the variability in the centroid
position of Sgr A* relative to a background quasar at 7-mm wavelength. We find
an average centroid wander of 71 +/- 45 micro-arcsec for time scales between 50
and 100 min and 113 +/- 50 micro-arcsec for timescales between 100 and 200 min,
with no secular trend. These are sufficient to begin constraining the viability
of the hot-spot model for the radio variability of Sgr A*. It is possible to
rule out hot spots with orbital radii above 15GM_SgrA*/c^2 that contribute more
than 30% of the total 7-mm flux. However, closer or less luminous hot spots
remain unconstrained. Since the fractional variability of Sgr A* during our
observations was ~20% on time scales of hours, the hot-spot model for Sgr A*'s
radio variability remains consistent with these limits. Improved monitoring of
Sgr A*'s centroid position has the potential to place significant constraints
upon the existence and morphology of inhomogeneities in a supermassive black
hole accretion flow.Comment: 14 pages, 3 figures submitted to Ap
Theory and design of InGaAsBi mid-infrared semiconductor lasers: type-I quantum wells for emission beyond 3 m on InP substrates
We present a theoretical analysis and optimisation of the properties and
performance of mid-infrared semiconductor lasers based on the dilute bismide
alloy InGaAsBi, grown on conventional (001) InP
substrates. The ability to independently vary the epitaxial strain and emission
wavelength in this quaternary alloy provides significant scope for band
structure engineering. Our calculations demonstrate that structures based on
compressively strained InGaAsBi quantum wells (QWs)
can readily achieve emission wavelengths in the 3 -- 5 m range, and that
these QWs have large type-I band offsets. As such, these structures have the
potential to overcome a number of limitations commonly associated with this
application-rich but technologically challenging wavelength range. By
considering structures having (i) fixed QW thickness and variable strain, and
(ii) fixed strain and variable QW thickness, we quantify key trends in the
properties and performance as functions of the alloy composition, structural
properties, and emission wavelength, and on this basis identify routes towards
the realisation of optimised devices for practical applications. Our analysis
suggests that simple laser structures -- incorporating
InGaAsBi QWs and unstrained ternary
InGaAs barriers -- which are compatible with established
epitaxial growth, provide a route to realising InP-based mid-infrared diode
lasers.Comment: Submitted versio
Using Millimeter VLBI to Constrain RIAF Models of Sagittarius A*
The recent detection of Sagittarius A* at lambda = 1.3 mm on a baseline from
Hawaii to Arizona demonstrates that millimeter wavelength very long baseline
interferometry (VLBI) can now spatially resolve emission from the innermost
accretion flow of the Galactic center region. Here, we investigate the ability
of future millimeter VLBI arrays to constrain the spin and inclination of the
putative black hole and the orientation of the accretion disk major axis within
the context of radiatively inefficient accretion flow (RIAF) models. We examine
the range of baseline visibility and closure amplitudes predicted by RIAF
models to identify critical telescopes for determining the spin, inclination,
and disk orientation of the Sgr A* black hole and accretion disk system. We
find that baseline lengths near 3 gigalambda have the greatest power to
distinguish amongst RIAF model parameters, and that it will be important to
include new telescopes that will form north-south baselines with a range of
lengths. If a RIAF model describes the emission from Sgr A*, it is likely that
the orientation of the accretion disk can be determined with the addition of a
Chilean telescope to the array. Some likely disk orientations predict
detectable fluxes on baselines between the continental United States and even a
single 10-12 m dish in Chile. The extra information provided from closure
amplitudes by a four-antenna array enhances the ability of VLBI to discriminate
amongst model parameters.Comment: Accepted for publication in ApJ
The Event Horizon of M87
The 6 billion solar mass supermassive black hole at the center of the giant
elliptical galaxy M87 powers a relativistic jet. Observations at millimeter
wavelengths with the Event Horizon Telescope have localized the emission from
the base of this jet to angular scales comparable to the putative black hole
horizon. The jet might be powered directly by an accretion disk or by
electromagnetic extraction of the rotational energy of the black hole. However,
even the latter mechanism requires a confining thick accretion disk to maintain
the required magnetic flux near the black hole. Therefore, regardless of the
jet mechanism, the observed jet power in M87 implies a certain minimum mass
accretion rate. If the central compact object in M87 were not a black hole but
had a surface, this accretion would result in considerable thermal
near-infrared and optical emission from the surface. Current flux limits on the
nucleus of M87 strongly constrain any such surface emission. This rules out the
presence of a surface and thereby provides indirect evidence for an event
horizon.Comment: 9 pages, 2 figures, submitted to Ap
Event-Horizon-Telescope Evidence for Alignment of the Black Hole in the Center of the Milky Way with the Inner Stellar Disk
Observations of the black hole in the center of the Milky Way with the Event
Horizon Telescope at 1.3 mm have revealed a size of the emitting region that is
smaller than the size of the black-hole shadow. This can be reconciled with the
spectral properties of the source, if the accretion flow is seen at a
relatively high inclination (50-60 degrees). Such an inclination makes the
angular momentum of the flow, and perhaps of the black hole, nearly aligned
with the angular momenta of the orbits of stars that lie within 3 arcsec from
the black hole. We discuss the implications of such an alignment for the
properties of the black hole and of its accretion flow. We argue that future
Event-Horizon-Telescope observations will not only refine the inclination of
Sgr A* but also measure precisely its orientation on the plane of the sky.Comment: To appear in the Astrophysical Journa
Strong deflection limit of black hole gravitational lensing with arbitrary source distances
The gravitational field of supermassive black holes is able to strongly bend
light rays emitted by nearby sources. When the deflection angle exceeds ,
gravitational lensing can be analytically approximated by the so-called strong
deflection limit. In this paper we remove the conventional assumption of
sources very far from the black hole, considering the distance of the source as
an additional parameter in the lensing problem to be treated exactly. We find
expressions for critical curves, caustics and all lensing observables valid for
any position of the source up to the horizon. After analyzing the spherically
symmetric case we focus on the Kerr black hole, for which we present an
analytical 3-dimensional description of the higher order caustic tubes.Comment: 20 pages, 8 figures, appendix added. In press on Physical Review
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