108 research outputs found
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
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Contribution of Underlying Connective Tissue Cells to Taste Buds in Mouse Tongue and Soft Palate
Taste buds, the sensory organs for taste, have been described as arising solely from the surrounding epithelium, which is in distinction from other sensory receptors that are known to originate from neural precursors, i.e., neural ectoderm that includes neural crest (NC). Our previous study suggested a potential contribution of NC derived cells to early immature fungiform taste buds in late embryonic (E18.5) and young postnatal (P1-10) mice. In the present study we demonstrated the contribution of the underlying connective tissue (CT) to mature taste buds in mouse tongue and soft palate. Three independent mouse models were used for fate mapping of NC and NC derived connective tissue cells: (1) P0-Cre/R26-tdTomato (RFP) to label NC, NC derived Schwann cells and derivatives; (2) Dermo1-Cre/RFP to label mesenchymal cells and derivatives; and (3) Vimentin-CreER/mGFP to label Vimentin-expressing CT cells and derivatives upon tamoxifen treatment. Both P0-Cre/RFP and Dermo1-Cre/RFP labeled cells were abundant in mature taste buds in lingual taste papillae and soft palate, but not in the surrounding epithelial cells. Concurrently, labeled cells were extensively distributed in the underlying CT. RFP signals were seen in the majority of taste buds and all three types (I, II, III) of differentiated taste bud cells, with the neuronal-like type III cells labeled at a greater proportion. Further, Vimentin-CreER labeled cells were found in the taste buds of 3-month-old mice whereas Vimentin immunoreactivity was only seen in the CT. Taken together, our data demonstrate a previously unrecognized origin of taste bud cells from the underlying CT, a conceptually new finding in our knowledge of taste bud cell derivation, i.e., from both the surrounding epithelium and the underlying CT that is primarily derived from NC
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
NuSTAR observations of X-ray bursts from the magnetar 1E 1048.1-5937
We report the detection of eight bright X-ray bursts from the 6.5-s magnetar
1E 1048.1-5937, during a 2013 July observation campaign with the Nuclear
Spectroscopic Telescope Array (NuSTAR). We study the morphological and spectral
properties of these bursts and their evolution with time. The bursts resulted
in count rate increases by orders of magnitude, sometimes limited by the
detector dead time, and showed blackbody spectra with kT=6-8 keV in the T90
duration of 1-4 s, similar to earlier bursts detected from the source. We find
that the spectra during the tail of the bursts can be modeled with an absorbed
blackbody with temperature decreasing with flux. The bursts flux decays
followed a power-law of index 0.8-0.9. In the burst tail spectra, we detect a
~13 keV emission feature, similar to those reported in previous bursts from
this source as well as from other magnetars observed with the Rossi X-ray
Timing Explorer (RXTE). We explore possible origins of the spectral feature
such as proton cyclotron emission, which implies a magnetic field strength of
B~2X10^15 G in the emission region. However, the consistency of the energy of
the feature in different objects requires further explanation.Comment: 10 pages, 6 figures, accepted for publication in Ap
NuSTAR Detection Of A Cyclotron Line In The Supergiant Fast X-ray Transient IGR J17544-2619
We present NuSTAR spectral and timing studies of the Supergiant Fast X-ray
Transient (SFXT) IGR J17544-2619. The spectrum is well-described by a ~1 keV
blackbody and a hard continuum component, as expected from an accreting X-ray
pulsar. We detect a cyclotron line at 17 keV, confirming that the compact
object in IGR J17544-2619 is indeed a neutron star. This is the first
measurement of the magnetic field in a SFXT. The inferred magnetic field
strength, B = (1.45 +/- 0.03) * 10^12 G * (1+z) is typical of neutron stars in
X-ray binaries, and rules out a magnetar nature for the compact object. We do
not find any significant pulsations in the source on time scales of 1-2000 s.Comment: MNRAS Accepted. 8 pages, 4 figures, 3 table
The Broadband XMM-Newton and NuSTAR X-ray Spectra of Two Ultraluminous X-ray Sources in the Galaxy IC 342
We present results for two Ultraluminous X-ray Sources (ULXs), IC 342 X-1 and
IC 342 X-2, using two epochs of XMM-Newton and NuSTAR observations separated by
7 days. We observe little spectral or flux variability above 1 keV
between epochs, with unabsorbed 0.3--30 keV luminosities being
erg s for IC 342 X-1 and
erg s for IC 342 X-2, so that both were
observed in a similar, luminous state. Both sources have a high absorbing
column in excess of the Galactic value. Neither source has a spectrum
consistent with a black hole binary in low/hard state, and both ULXs exhibit
strong curvature in their broadband X-ray spectra. This curvature rules out
models that invoke a simple reflection-dominated spectrum with a broadened iron
line and no cutoff in the illuminating power-law continuum. X-ray spectrum of
IC 342 X-1 can be characterized by a soft disk-like black body component at low
energies and a cool, optically thick Comptonization continuum at high energies,
but unique physical interpretation of the spectral components remains
challenging. The broadband spectrum of IC 342 X-2 can be fit by either a hot
(3.8 keV) accretion disk, or a Comptonized continuum with no indication of a
seed photon population. Although the seed photon component may be masked by
soft excess emission unlikely to be associated with the binary system, combined
with the high absorption column, it is more plausible that the broadband X-ray
emission arises from a simple thin blackbody disk component. Secure
identification of the origin of the spectral components in these sources will
likely require broadband spectral variability studies.Comment: 12 pages, 11 figures, 5 Tables, Accepted for publication in The
Astrophysical Journa
The smooth cyclotron line in Her X-1 as seen with NuSTAR
Her X-1, one of the brightest and best studied X-ray binaries, shows a
cyclotron resonant scattering feature (CRSF) near 37 keV. This makes it an
ideal target for detailed study with the Nuclear Spectroscopic Telescope Array
(NuSTAR), taking advantage of its excellent hard X-ray spectral resolution. We
observed Her X-1 three times, coordinated with Suzaku, during one of the high
flux intervals of its 35d super-orbital period. This paper focuses on the shape
and evolution of the hard X-ray spectrum. The broad-band spectra can be fitted
with a powerlaw with a high-energy cutoff, an iron line, and a CRSF. We find
that the CRSF has a very smooth and symmetric shape, in all observations and at
all pulse-phases. We compare the residuals of a line with a Gaussian optical
depth profile to a Lorentzian optical depth profile and find no significant
differences, strongly constraining the very smooth shape of the line. Even
though the line energy changes dramatically with pulse phase, we find that its
smooth shape does not. Additionally, our data show that the continuum is only
changing marginally between the three observations. These changes can be
explained with varying amounts of Thomson scattering in the hot corona of the
accretion disk. The average, luminosity-corrected CRSF energy is lower than in
past observations and follows a secular decline. The excellent data quality of
NuSTAR provides the best constraint on the CRSF energy to date.Comment: 13 pages, 13 figures, accepted for publication in Ap
NuSTAR Observations of the Magnetar 1E 2259+586
We report on new broad band spectral and temporal observations of the
magnetar 1E 2259+586, which is located in the supernova remnant CTB 109. Our
data were obtained simultaneously with the Nuclear Spectroscopic Telescope
Array (NuSTAR) and Swift, and cover the energy range from 0.5-79 keV. We
present pulse profiles in various energy bands and compare them to previous
RXTE results. The NuSTAR data show pulsations above 20 keV for the first time
and we report evidence that one of the pulses in the double-peaked pulse
profile shifts position with energy. The pulsed fraction of the magnetar is
shown to increase strongly with energy. Our spectral analysis reveals that the
soft X-ray spectrum is well characterized by an absorbed double-blackbody or
blackbody plus power-law model in agreement with previous reports. Our new hard
X-ray data, however, suggests that an additional component, such as a
power-law, is needed to describe the NuSTAR and Swift spectrum. We also fit the
data with the recently developed coronal outflow model by Beloborodov for hard
X-ray emission from magnetars. The outflow from a ring on the magnetar surface
is statistically preferred over outflow from a polar cap.Comment: 37 pages, 9 figures, corresponding author, [email protected]
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