3,891 research outputs found
Fast time variations of supernova neutrino fluxes and their detectability
In the delayed explosion scenario of core-collapse supernovae (SNe), the
accretion phase shows pronounced convective overturns and a low-multipole
hydrodynamic instability, the standing accretion shock instability (SASI).
These effects imprint detectable fast time variations on the emerging neutrino
flux. Among existing detectors, IceCube is best suited to this task, providing
an event rate of ~1000 events per ms during the accretion phase for a fiducial
SN distance of 10 kpc, comparable to what could be achieved with a megaton
water Cherenkov detector. If the SASI activity lasts for several hundred ms, a
Fourier component with an amplitude of 1% of the average signal clearly sticks
out from the shot noise. We analyze in detail the output of axially symmetric
hydrodynamical simulations that predict much larger amplitudes up to
frequencies of a few hundred Hz. If these models are roughly representative for
realistic SNe, fast time variations of the neutrino signal are easily
detectable in IceCube or future megaton-class instruments. We also discuss the
information that could be deduced from such a measurement about the physics in
the SN core and the explosion mechanism of the SN.Comment: 14 pages, 11 figures. Final version accepted in PRD. Section on
astrophysical relevance and several references adde
Gravitational Waves from Phase Transitions at the Electroweak Scale and Beyond
If there was a first order phase transition in the early universe, there
should be an associated stochastic background of gravitational waves. In this
paper, we point out that the characteristic frequency of the spectrum due to
phase transitions which took place in the temperature range 100 GeV - 10^7 GeV
is precisely in the window that will be probed by the second generation of
space-based interferometers such as the Big Bang Observer (BBO). Taking into
account the astrophysical foreground, we determine the type of phase
transitions which could be detected either at LISA, LIGO or BBO, in terms of
the amount of supercooling and the duration of the phase transition that are
needed. Those two quantities can be calculated for any given effective scalar
potential describing the phase transition. In particular, the new models of
electroweak symmetry breaking which have been proposed in the last few years
typically have a different Higgs potential from the Standard Model. They could
lead to a gravitational wave signature in the milli-Hertz frequency, which is
precisely the peak sensitivity of LISA. We also show that the signal coming
from phase transitions taking place at T ~ 1-100 TeV could entirely screen the
relic gravitational wave signal expected from standard inflationary models.Comment: 18 pages, 24 figure
Bow Shocks from Neutron Stars: Scaling Laws and HST Observations of the Guitar Nebula
The interaction of high-velocity neutron stars with the interstellar medium
produces bow shock nebulae, where the relativistic neutron star wind is
confined by ram pressure. We present multi-wavelength observations of the
Guitar Nebula, including narrow-band H-alpha imaging with HST/WFPC2, which
resolves the head of the bow shock. The HST observations are used to fit for
the inclination of the pulsar velocity vector to the line of sight, and to
determine the combination of spindown energy loss, velocity, and ambient
density that sets the scale of the bow shock. We find that the velocity vector
is most likely in the plane of the sky. We use the Guitar Nebula and other
observed neutron star bow shocks to test scaling laws for their size and
H-alpha emission, discuss their prevalence, and present criteria for their
detectability in targeted searches. The set of H-alpha bow shocks shows
remarkable consistency, in spite of the expected variation in ambient densities
and orientations. Together, they support the assumption that a pulsar's
spindown energy losses are carried away by a relativistic wind that is
indistinguishable from being isotropic. Comparison of H-alpha bow shocks with
X-ray and nonthermal, radio-synchrotron bow shocks produced by neutron stars
indicates that the overall shape and scaling is consistent with the same
physics. It also appears that nonthermal radio emission and H-alpha emission
are mutually exclusive in the known objects and perhaps in all objects.Comment: 12 pages, 7 figures (3 degraded), submitted to ApJ; minor revisions
and updates in response to referee report. (AASTeX, includes emulateapj5 and
onecolfloat5.
Characterization and Application of Angled Fluorescence Laminar Optical Tomography
Angled fluorescence laminar optical tomography (aFLOT) is a modified fluorescence tomographic imaging technique that targets the mesoscopic scale (millimeter penetration with resolution in the tens of microns). Traditional FLOT uses multiple detectors to measure a range of scattered fluorescence signals to perform 3D reconstructions. This technology however inherently assumes the sample to be scattering. To extend the capability of FLOT to cover the low scattering regime, the oblique illumination and detection was introduced. The angular degree of freedom for the illumination and detection was theoretically and experimentally investigated. It was concluded that aFLOT enhanced resolution 2.5 times and depth selectivity compared to traditional FLOT, and that it enabled the stacking representation, a process that skips the computationally-intensive reconstruction usually needed to render the tomogram. Because stacking is enabled, the necessity of a reconstruction process is retrospectively discussed. aFLOT systems were constructed and applied in tissue engineering. Phantoms and engineered tissue models were successfully imaged. The aFLOT was shown to perform non-invasive in situ imaging in biologically relevant samples with 1mm penetration and 9-400 micron resolution, depending on the scattering of samples. aFLOT illustrates its potential for studying cell-cell or cell-material interactions
Observations of Microwave Continuum Emission from Air Shower Plasmas
We investigate a possible new technique for microwave measurements of
ultra-high energy cosmic ray (UHECR) extensive air showers which relies on
detection of expected continuum radiation in the microwave range, caused by
free-electron collisions with neutrals in the tenuous plasma left after the
passage of the shower. We performed an initial experiment at the AWA (Argonne
Wakefield Accelerator) laboratory in 2003 and measured broadband microwave
emission from air ionized via high energy electrons and photons. A follow-up
experiment at SLAC (Stanford Linear Accelerator Center) in summer of 2004
confirmed the major features of the previous AWA observations with better
precision and made additional measurements relevant to the calorimetric
capabilities of the method. Prompted by these results we built a prototype
detector using satellite television technology, and have made measurements
indicating possible detection of cosmic ray extensive air showers. The method,
if confirmed by experiments now in progress, could provide a high-duty cycle
complement to current nitrogen fluorescence observations of UHECR, which are
limited to dark, clear nights. By contrast, decimeter microwave observations
can be made both night and day, in clear or cloudy weather, or even in the
presence of moderate precipitation.Comment: 15 pages, 13 figure
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