944 research outputs found
A role for recurrent processing in object completion: neurophysiological, psychophysical and computational"evidence
Recognition of objects from partial information presents a significant
challenge for theories of vision because it requires spatial integration and
extrapolation from prior knowledge. We combined neurophysiological recordings
in human cortex with psychophysical measurements and computational modeling to
investigate the mechanisms involved in object completion. We recorded
intracranial field potentials from 1,699 electrodes in 18 epilepsy patients to
measure the timing and selectivity of responses along human visual cortex to
whole and partial objects. Responses along the ventral visual stream remained
selective despite showing only 9-25% of the object. However, these visually
selective signals emerged ~100 ms later for partial versus whole objects. The
processing delays were particularly pronounced in higher visual areas within
the ventral stream, suggesting the involvement of additional recurrent
processing. In separate psychophysics experiments, disrupting this recurrent
computation with a backward mask at ~75ms significantly impaired recognition of
partial, but not whole, objects. Additionally, computational modeling shows
that the performance of a purely bottom-up architecture is impaired by heavy
occlusion and that this effect can be partially rescued via the incorporation
of top-down connections. These results provide spatiotemporal constraints on
theories of object recognition that involve recurrent processing to recognize
objects from partial information
Sensitivity to Timing and Order in Human Visual Cortex
Visual recognition takes a small fraction of a second and relies on the
cascade of signals along the ventral visual stream. Given the rapid path
through multiple processing steps between photoreceptors and higher visual
areas, information must progress from stage to stage very quickly. This rapid
progression of information suggests that fine temporal details of the neural
response may be important to the how the brain encodes visual signals. We
investigated how changes in the relative timing of incoming visual stimulation
affect the representation of object information by recording intracranial field
potentials along the human ventral visual stream while subjects recognized
objects whose parts were presented with varying asynchrony. Visual responses
along the ventral stream were sensitive to timing differences between parts as
small as 17 ms. In particular, there was a strong dependency on the temporal
order of stimulus presentation, even at short asynchronies. This sensitivity to
the order of stimulus presentation provides evidence that the brain may use
differences in relative timing as a means of representing information.Comment: 10 figures, 1 tabl
Observational Artifacts of NuSTAR: Ghost Rays and Stray Light
The Nuclear Spectroscopic Telescope Array (NuSTAR), launched in June 2012,
flies two conical approximation Wolter-I mirrors at the end of a 10.15m mast.
The optics are coated with multilayers of Pt/C and W/Si that operate from 3--80
keV. Since the optical path is not shrouded, aperture stops are used to limit
the field of view from background and sources outside the field of view.
However, there is still a sliver of sky (~1.0--4.0 degrees) where photons may
bypass the optics altogether and fall directly on the detector array. We term
these photons Stray-light. Additionally, there are also photons that do not
undergo the focused double reflections in the optics and we term these Ghost
Rays. We present detailed analysis and characterization of these two components
and discuss how they impact observations. Finally, we discuss how they could
have been prevented and should be in future observatories.Comment: Published in Journal of Astronomical Telescopes, Instruments, and
Systems. Open Access. http://dx.doi.org/10.1117/1.JATIS.3.4.04400
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
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
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