738 research outputs found
Spatially resolved spectroscopy of the globular cluster RZ 2109 and the nature of its black hole
We present optical HST/STIS spectroscopy of RZ 2109, a globular cluster in
the elliptical galaxy NGC 4472. This globular cluster is notable for hosting an
ultraluminous X-ray source as well as associated strong and broad [OIII] 4959,
5007 emission. We show that the HST/STIS spectroscopy spatially resolves the
[OIII] emission in RZ 2109. While we are unable to make a precise determination
of the morphology of the emission line nebula, the best fitting models all
require that the [OIII] 5007 emission has a half light radius in the range 3-7
pc. The extended nature of the [OIII] 5007 emission is inconsistent with
published models that invoke an intermediate mass black hole origin. It is also
inconsistent with the ionization of ejecta from a nova in the cluster. The
spatial scale of the nebula could be produced via the photoionization of a
strong wind driven from a stellar mass black hole accreting at roughly its
Eddington rate.Comment: 7 pages, 4 figures - accepted for publication in Ap
Broadband X-ray Imaging and Spectroscopy of the Crab Nebula and Pulsar with NuSTAR
We present broadband (3 -- 78 keV) NuSTAR X-ray imaging and spectroscopy of
the Crab nebula and pulsar. We show that while the phase-averaged and spatially
integrated nebula + pulsar spectrum is a power-law in this energy band,
spatially resolved spectroscopy of the nebula finds a break at 9 keV in
the spectral photon index of the torus structure with a steepening
characterized by . We also confirm a previously reported
steepening in the pulsed spectrum, and quantify it with a broken power-law with
break energy at 12 keV and . We present spectral
maps of the inner 100\as\ of the remnant and measure the size of the nebula as
a function of energy in seven bands. These results find that the rate of
shrinkage with energy of the torus size can be fitted by a power-law with an
index of , consistent with the predictions of Kennel
and Coroniti (1984). The change in size is more rapid in the NW direction,
coinciding with the counter-jet where we find the index to be a factor of two
larger. NuSTAR observed the Crab during the latter part of a -ray
flare, but found no increase in flux in the 3 - 78 keV energy band
High-Energy X-ray Imaging of the Pulsar Wind Nebula MSH~15-52: Constraints on Particle Acceleration and Transport
We present the first images of the pulsar wind nebula (PWN) MSH 15-52 in the
hard X-ray band (>8 keV), as measured with the Nuclear Spectroscopic Telescope
Array (NuSTAR). Overall, the morphology of the PWN as measured by NuSTAR in the
3-7 keV band is similar to that seen in Chandra high-resolution imaging.
However, the spatial extent decreases with energy, which we attribute to
synchrotron energy losses as the particles move away from the shock. The
hard-band maps show a relative deficit of counts in the northern region towards
the RCW 89 thermal remnant, with significant asymmetry. We find that the
integrated PWN spectra measured with NuSTAR and Chandra suggest that there is a
spectral break at 6 keV which may be explained by a break in the
synchrotron-emitting electron distribution at ~200 TeV and/or imperfect cross
calibration. We also measure spatially resolved spectra, showing that the
spectrum of the PWN softens away from the central pulsar B1509-58, and that
there exists a roughly sinusoidal variation of spectral hardness in the
azimuthal direction. We discuss the results using particle flow models. We find
non-monotonic structure in the variation with distance of spectral hardness
within 50" of the pulsar moving in the jet direction, which may imply particle
and magnetic-field compression by magnetic hoop stress as previously suggested
for this source. We also present 2-D maps of spectral parameters and find an
interesting shell-like structure in the NH map. We discuss possible origins of
the shell-like structure and their implications.Comment: 15 pages, 9 figures, accepted for publication in Ap
The Hard X-Ray View of the Young Supernova Remnant G1.9+0.3
NuSTAR observed G1.9+0.3, the youngest known supernova remnant in the Milky
Way, for 350 ks and detected emission up to 30 keV. The remnant's X-ray
morphology does not change significantly across the energy range from 3 to 20
keV. A combined fit between NuSTAR and CHANDRA shows that the spectrum steepens
with energy. The spectral shape can be well fitted with synchrotron emission
from a power-law electron energy distribution with an exponential cutoff with
no additional features. It can also be described by a purely phenomenological
model such as a broken power-law or a power-law with an exponential cutoff,
though these descriptions lack physical motivation. Using a fixed radio flux at
1 GHz of 1.17 Jy for the synchrotron model, we get a column density of N = cm, a spectral index of
, and a roll-off frequency of Hz. This can be explained by particle
acceleration, to a maximum energy set by the finite remnant age, in a magnetic
field of about 10 G, for which our roll-off implies a maximum energy of
about 100 TeV for both electrons and ions. Much higher magnetic-field strengths
would produce an electron spectrum that was cut off by radiative losses, giving
a much higher roll-off frequency that is independent of magnetic-field
strength. In this case, ions could be accelerated to much higher energies. A
search for Ti emission in the 67.9 keV line results in an upper limit of
assuming a line width of 4.0 keV (1 sigma).Comment: 9 pages, 6 figures, accepted Ap
A Spatially Resolved Study of the Synchrotron Emission and Titanium in Tycho's Supernova Remnant with NuSTAR
We report results from deep observations (~750 ks) of Tycho's supernova
remnant (SNR) with NuSTAR. Using these data, we produce narrow-band images over
several energy bands to identify the regions producing the hardest X-rays and
to search for radioactive decay line emission from 44Ti. We find that the
hardest (>10 keV) X-rays are concentrated in the southwest of Tycho, where
recent Chandra observations have revealed high emissivity "stripes" associated
with particles accelerated to the knee of the cosmic-ray spectrum. We do not
find evidence of 44Ti, and we set limits on its presence and distribution
within the SNR. These limits correspond to a upper-limit 44Ti mass of M44 <
2.4x10^-4 M_sun for a distance of 2.3 kpc. We perform spatially resolved
spectroscopic analysis of sixty-six regions across Tycho. We map the best-fit
rolloff frequency of the hard X-ray spectra, and we compare these results to
measurements of the shock expansion and ambient density. We find that the
highest energy electrons are accelerated at the lowest densities and in the
fastest shocks, with a steep dependence of the roll-off frequency with shock
velocity. Such a dependence is predicted by models where the maximum energy of
accelerated electrons is limited by the age of the SNR rather than by
synchrotron losses, but this scenario requires far lower magnetic field
strengths than those derived from observations in Tycho. One way to reconcile
these discrepant findings is through shock obliquity effects, and future
observational work is necessary to explore the role of obliquity in the
particle acceleration process.Comment: 12 pages, 12 figures, ApJ in pres
A single codon insertion in PICALM is associated with development of familial subvalvular aortic stenosis in Newfoundland dogs
Familial subvalvular aortic stenosis (SAS) is one of the most common congenital heart defects in dogs and is an inherited defect of Newfoundlands, golden retrievers and human children. Although SAS is known to be inherited, specific genes involved in Newfoundlands with SAS have not been defined. We hypothesized that SAS in Newfoundlands is inherited in an autosomal dominant pattern and caused by a single genetic variant. We studied 93 prospectively recruited Newfoundland dogs, and 180 control dogs of 30 breeds. By providing cardiac screening evaluations for Newfoundlands we conducted a pedigree evaluation, genome-wide association study and RNA sequence analysis to identify a proposed pattern of inheritance and genetic loci associated with the development of SAS. We identified a three-nucleotide exonic insertion in phosphatidylinositol-binding clathrin assembly protein (PICALM) that is associated with the development of SAS in Newfoundlands. Pedigree evaluation best supported an autosomal dominant pattern of inheritance and provided evidence that equivocally affected individuals may pass on SAS in their progeny. Immunohistochemistry demonstrated the presence of PICALM in the canine myocardium and area of the subvalvular ridge. Additionally, small molecule inhibition of clathrin-mediated endocytosis resulted in developmental abnormalities within the outflow tract (OFT) of Xenopus laevis embryos. The ability to test for presence of this PICALM insertion may impact dog-breeding decisions and facilitate reduction of SAS disease prevalence in Newfoundland dogs. Understanding the role of PICALM in OFT development may aid in future molecular and genetic investigations into other congenital heart defects of various species
Locating the most energetic electrons in Cassiopeia A
We present deep (2.4 Ms) observations of the Cassiopeia A supernova
remnant with {\it NuSTAR}, which operates in the 3--79 keV bandpass and is the
first instrument capable of spatially resolving the remnant above 15 keV. We
find that the emission is not entirely dominated by the forward shock nor by a
smooth "bright ring" at the reverse shock. Instead we find that the 15 keV
emission is dominated by knots near the center of the remnant and dimmer
filaments near the remnant's outer rim. These regions are fit with unbroken
power-laws in the 15--50 keV bandpass, though the central knots have a steeper
() spectrum than the outer filaments ().
We argue this difference implies that the central knots are located in the 3-D
interior of the remnant rather than at the outer rim of the remnant and seen in
the center due to projection effects. The morphology of 15 keV emission does
not follow that of the radio emission nor that of the low energy (12 keV)
X-rays, leaving the origin of the 15 keV emission as an open mystery. Even
at the forward shock front we find less steepening of the spectrum than
expected from an exponentially cut off electron distribution with a single
cutoff energy. Finally, we find that the GeV emission is not associated with
the bright features in the {\it NuSTAR} band while the TeV emission may be,
suggesting that both hadronic and leptonic emission mechanisms may be at work.Comment: 12 pages, 11 figures, accepted for publication in Ap
THEORY OF PHASE-LOCKING IN SMALL JOSEPHSON JUNCTION CELLS
Within the RSJ model, we performed a theoretical analysis of phase-locking in
elementary strongly coupled Josephson junction cells. For this purpose, we
developed a systematic method allowing the investigation of phase-locking in
cells with small but non-vanishing loop inductance.The voltages across the
junctions are found to be locked with very small phase difference for almost
all values of external flux. However, the general behavior of phase-locking is
found to be just contrary to that according to weak coupling. In case of strong
coupling there is nearly no influence of external magnetic flux on the phases,
but the locking-frequency becomes flux-dependent. The influence of parameter
splitting is considered as well as the effect of small capacitive shunting of
the junctions. Strongly coupled cells show synchronization even for large
parameter splitting. Finally, a study of the behavior under external microwave
radiation shows that the frequency locking-range becomes strongly
flux-dependent, whereas the locking frequency itself turns out to be
flux-independent.Comment: 26 pages, REVTEX, 9 PS figures appended in uuencoded form at the end,
submitted to Phys. Rev. B
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