14,841 research outputs found
A neural basis for percept stabilization binocular rivalry
When the same visual input has conflicting interpretations, conscious perception can alternate spontaneously between each competing percept. Surprisingly, such bistable perception can be stabilized by intermittent stimulus removal, suggesting the existence of perceptual "memory" across interruptions in stimulation. The neural basis of such a process remains Unknown. Here, we studied binocular rivalry, one type of bistable perception, in two linked experiments in human participants. First, we showed, in a behavioral experiment using binocular rivalry between face and grating stimuli, that the stabilizing effect of stimulus removal was specific to perceptual alternations evoked by rivalry, and did not occur following physical alternations in the absence of rivalry. We then used functional magnetic resonance imaging to measure brain activity in a variable delay period Of Stimulus removal. Activity in the fusiform face area during the delay period following removal of rivalrous Stimuli was greater following face than grating perception, whereas such a difference was absent during removal of non-rivalrous Stimuli. Moreover, activity in areas of fronto-parietal regions during the delay period correlated with the degree to which individual participants tended to experience percept stabilization. Our findings Suggest that percept-related activity in specialized extrastriate visual areas help to stabilize perception during perceptual conflict, and that high-level mechanisms may determine the influence of such signals on conscious perception
The Edge of a Gamma Ray Burst Afterglow
We discuss the formation of spectral features in the decelerating ejecta of
gamma-ray bursts, including the possible effect of inhomogeneities. These
should lead to blueshifted and broadened absorption edges and resonant
features, especially from H and He. An external neutral ISM could produce
detectable H and He, as well as Fe X-ray absorption edges and lines. Hypernova
scenarios may be diagnosed by Fe K- and H Ly- emission lines.Comment: M.N.R.A.S., accepted July 16 1998; submitted June 4 1998; latex, 11
page
Gamma-ray bursts as X-ray depth-gauges of the Universe
We discuss the X-ray flux of gamma-ray burst afterglows at redshifts in the
range 3-30, including the effects of the intergalactic He II absorption. We
point out that strong X-ray lines may form locally in burst afterglows starting
minutes after the trigger. This can provide distinctive X-ray distance
indicators out to the redshifts where the first generation of massive stars
form.Comment: ApJ(Lett) in press 5/31/03; subm. 5/7/0
Collapsar Jets, Bubbles and Fe Lines
In the collapsar scenario, gamma ray bursts are caused by relativistic jets
expelled along the rotation axis of a collapsing stellar core. We discuss how
the structure and time-dependence of such jets depends on the stellar envelope
and central engine properties, assuming a steady jet injection. It takes a few
seconds for the jet to bore its way through the stellar core; most of the
energy output during that period goes into a cocoon of relativistic plasma
surrounding the jet. This material subsequently forms a bubble of magnetized
plasma that takes several hours to expand, subrelativistically, through the
envelope of a high-mass supergiant. Jet break-through and a conventional burst
would be expected not only in He stars but possibly also in blue supergiants.
Shock waves and magnetic dissipation in the escaping bubble can contribute a
non thermal UV/X-ray afterglow, and also excite Fe line emission from thermal
gas, in addition to the standard jet deceleration power-law afterglow.Comment: Ap.J. Letters, accepted 6/20/01, first subm 4/24/01; aaspp4, 9 pages,
no figures; minor revision
Delayed Gev Emission from Cosmological Gamma-Ray Bursts : Impact of a Relativistic Wind on External Matter
Sudden collapse of a compact object, or coalescence of a compact binary, can
generate an unsteady relativistic wind that lasts for a few seconds. The wind
is likely to carry a high magnetic field; and its Lorentz factor depends on the
extent to which it is 'loaded' with baryons. If the Lorentz factor is , internal dissipation and shocks in this wind produce a non-thermal
gamma-ray burst, detectable in the range 0.1\MeV \siml E_\gamma \siml
0.1-1\GeV out to cosmological distances. The cooled wind ejecta would
subsequently be decelerated by the external medium. The resultant blast wave
and reverse shock can then give rise to a second burst component, mainly
detectable in the GeV range, with a time delay relative to the MeV burst
ranging from minutes to hours.Comment: 5 pages, plain Te
Gamma-Ray Bursts: Multiwaveband Spectral Predictions for Blast Wave Models
In almost any scenario for 'cosmological' gamma-ray bursts (and in many
models where they originate in our own Galaxy), the initial energy density is
so large that the resulting relativistic plasma expands with
producing a blast wave ahead of it and a reverse shock moving into the ejecta,
as it ploughs into the external medium. We evaluate the radiation expected from
these shocks,for both cosmological and galactic bursts, for various assumptions
about the strength of the magnetic field and the particle acceleration
mechanisms in the shocks. The spectra are evaluated over the whole range from
the IR to GeV, and are compared with the variety of spectral behavior
reported by BATSE, and with the X-ray and optical constraints. For bursts of
duration \simg 1\s acceptable -ray spectra and ratios
are readily obtained for 'cosmological' models. Blast waves in galactic models
can produce bursts of similar gamma-ray fluence and duration, but they violate
the X-ray paucity constraint, except for the shorter bursts (\siml 1\s). We
discuss the prospects for using O/UV and X-ray observations to discriminate
among alternative models.Comment: 7 pages with one figure (figure in uuencoded compressed postscript
file),te
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