1,885 research outputs found
Electrodynamic Structure of an Outer Gap Accelerator: Location of the Gap and the Gamma-ray Emission from the Crab Pulsar
We investigate a stationary pair production cascade in the outer
magnetosphere of a spinning neutron star. The charge depletion due to global
flows of charged particles, causes a large electric field along the magnetic
field lines. Migratory electrons and/or positrons are accelerated by this field
to radiate curvature gamma-rays, some of which collide with the X-rays to
materialize as pairs in the gap. The replenished charges partially screen the
electric field, which is self-consistently solved together with the
distribution functions of particles and gamma-rays. If no current is injected
at neither of the boundaries of the accelerator, the gap is located around the
conventional null surface, where the local Goldreich-Julian charge density
vanishes. However, we first find that the gap position shifts outwards (or
inwards) when particles are injected at the inner (or outer) boundary. Applying
the theory to the Crab pulsar, we demonstrate that the pulsed TeV flux does not
exceed the observational upper limit for moderate infrared photon density and
that the gap should be located near to or outside of the conventional null
surface so that the observed spectrum of pulsed GeV fluxes may be emitted via a
curvature process. Some implications of the existence of a solution for a super
Goldreich-Julian current are discussed.Comment: 17 pages, 12 figures, submitted to Ap
Large-Scale Image Processing with the ROTSE Pipeline for Follow-Up of Gravitational Wave Events
Electromagnetic (EM) observations of gravitational-wave (GW) sources would
bring unique insights into a source which are not available from either channel
alone. However EM follow-up of GW events presents new challenges. GW events
will have large sky error regions, on the order of 10-100 square degrees, which
can be made up of many disjoint patches. When searching such large areas there
is potential contamination by EM transients unrelated to the GW event.
Furthermore, the characteristics of possible EM counterparts to GW events are
also uncertain. It is therefore desirable to be able to assess the statistical
significance of a candidate EM counterpart, which can only be done by
performing background studies of large data sets. Current image processing
pipelines such as that used by ROTSE are not usually optimised for large-scale
processing. We have automated the ROTSE image analysis, and supplemented it
with a post-processing unit for candidate validation and classification. We
also propose a simple ad hoc statistic for ranking candidates as more likely to
be associated with the GW trigger. We demonstrate the performance of the
automated pipeline and ranking statistic using archival ROTSE data. EM
candidates from a randomly selected set of images are compared to a background
estimated from the analysis of 102 additional sets of archival images. The
pipeline's detection efficiency is computed empirically by re-analysis of the
images after adding simulated optical transients that follow typical light
curves for gamma-ray burst afterglows and kilonovae. We show that the automated
pipeline rejects most background events and is sensitive to simulated
transients to limiting magnitudes consistent with the limiting magnitude of the
images
Status of the Whipple Observatory Cerenkov air shower imaging telescope array
Recently the power of the Cerenkov imaging technique in Very High Energy gamma-ray astronomy was demonstrated by the detection of the Crab nebula at high statistical significance. In order to further develop this technique to allow the detection of weaker or more distant sources a second 10 m class reflector was constructed about 120 m from the original instrument. The addition of the second reflector will allow both a reduction in the energy threshold and an improvement in the rejection of the hadronic background. The design and construction of the second reflector, Gamma Ray Astrophysics New Imaging TElescope (GRANITE) is described
Searching for Needles in Haystacks - Looking for GRB gamma-rays with the Fermi/LAT Detector
Since the launch of the Fermi Gamma-ray Space Telescope on June 11, 2008, 55
gamma-ray bursts (GRBs) have been observed at coordinates that fall within
66^\circ of the Fermi Large Area Telescope (LAT) boresight with precise
localizations provided by the NASA Swift mission or other satellites. Imposing
selection cuts to exclude low Galactic latitudes and high zenith angles reduces
the sample size to 41. Using matched filter techniques, the Fermi/LAT photon
data for these fields have been examined for evidence of bursts that have so
far evaded detection at energies above 100 MeV. Following comparisons with
similar random background fields, two events, GRB 080905A and GRB 091208B,
stand out as excellent candidates for such an identification. After excluding
the six bright bursts previously reported by the LAT team, the remaining 35
events exhibit an excess of LAT "diffuse" photons with a statistical
significance greater than 2 sigma, independent of the matched filter analysis.
After accounting for the total number of photons in the well-localized fields
and including estimates of detection efficiency, one concludes that somewhere
in the range of 11% to 19% of all GRBs within the LAT field of view illuminate
the detector with two or more energetic photons. These are the most stringent
estimates of the high energy photon content of GRBs to date. The two new events
associated with high energy photon emission have similar ratios of high to low
energy fluences as observed previously. This separates them from bursts with
similar low energy fluences by a factor of ten, suggesting a distinct class of
events rather than a smooth continuum.Comment: 8 pages, 4 figures, 2 tables, accepted for publication in ApJ, minor
revision
Locating very high energy gamma ray sources with arc minute accuracy
The angular accuracy of gamma-ray detectors is intrinsically limited by the physical processes involved in photon detection. Although a number of point-like sources were detected by the COS-B satellite, only two were unambiguously identified by time signature with counterparts at longer wavelengths. By taking advantage of the extended longitudinal structure of Very High Energy gamma-ray showers, measurements in the TeV energy range can pinpoint source coordinates to arc minute accuracy. This was demonstrated using Cerenkov air shower imaging techniques. With two telescopes in coincidence, the individual event circular probable error will be 0.13 deg. The half-cone angle of the field of view is effectively 1 deg
Multiwavelength Observations of Markarian 421 During a TeV/X-Ray Flare
A Te V flare from the BL Lac object Mrk 421 was detected in May of 1994 by the Whipple Observatory air Cherenkov experiment during which the flux above 250 GeV increased by nearly an order of magnitude over a 2-day period. Contemporaneous observations by ASCA showed the X-ray flux to be in a very high state. We present these results, combined with the first ever simultaneous or nearly simultaneous observations at Ge V gamma-ray, UV, IR, mm, and radio energies for this nearest BL Lac object. While the GeV gamma-ray flux increased slightly, there is little evidence for variability comparable to that seen at TeV and X-ray energies. Other wavelengths show even less variability. This provides important constraints on the emission mechanisms at work. We present the multiwavelength spectrum of this gamma-ray blazar for both quiescent and flaring states and discuss the data in terms of current models of blazar emission
Exclusive diffractive processes and the quark substructure of mesons
Exclusive diffractive processes on the nucleon are investigated within a
model in which the quark-nucleon interaction is mediated by Pomeron exchange
and the quark substructure of mesons is described within a framework based on
the Dyson-Schwinger equations of QCD. The model quark-nucleon interaction has
four parameters which are completely determined by high-energy and elastic scattering data. The model is then used to predict vector-meson
electroproduction observables. The obtained - and -meson
electroproduction cross sections are in excellent agreement with experimental
data. The predicted dependence of -meson electroproduction also
agrees with experimental data. It is shown that confined-quark dynamics play a
central role in determining the behavior of the diffractive, vector-meson
electroproduction cross section. In particular, the onset of the asymptotic
behavior of the cross section is determined by a momentum scale that is
set by the current-quark masses of the quark and antiquark inside the vector
meson. This is the origin of the striking differences between the
dependence of -, - and -meson electroproduction cross
sections observed in recent experiments.Comment: 53 pages, 23 figures, revtex and epsfig. Minor additions to tex
Observations of the Optical Counterpart to XTE J1118+480 During Outburst by the ROTSE-I Telescope
The X-ray nova XTE J1118+480 exhibited two outbursts in the early part of
2000. As detected by the Rossi X-ray Timing Explorer (RXTE), the first outburst
began in early January and the second began in early March. Routine imaging of
the northern sky by the Robotic Optical Transient Search Experiment (ROTSE)
shows the optical counterpart to XTE J1118+480 during both outbursts. These
data include over 60 epochs from January to June 2000. A search of the ROTSE
data archives reveal no previous optical outbursts of this source in selected
data between April 1998 and January 2000. While the X-ray to optical flux ratio
of XTE J1118+480 was low during both outbursts, we suggest that they were full
X-ray novae and not mini-outbursts based on comparison with similar sources.
The ROTSE measurements taken during the March 2000 outburst also indicate a
rapid rise in the optical flux that preceded the X-ray emission measured by the
RXTE by approximately 10 days. Using these results, we estimate a pre-outburst
accretion disk inner truncation radius of 1.2 x 10^4 Schwarzschild radii.Comment: 9 pages, 1 table, 2 figure
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