3,924 research outputs found
Dynamics of adaptive agents with asymmetric information
We apply path-integral techniques to study the dynamics of agent-based models
with asymmetric information structures. In particular, we devise a batch
version of a model proposed originally by Berg et al. [Quant. Fin. 1 (2001)
203], and convert the coupled multi-agent processes into an effective-agent
problem from which the dynamical order parameters in ergodic regimes can be
derived self-consistently together with the corresponding phase structure. Our
dynamical study complements and extends the available static theory. Results
are confirmed by numerical simulations.Comment: minor revision of text, accepted by JSTA
Dust and dark Gamma-Ray Bursts: mutual implications
In a cosmological context dust has been always poorly understood. That is
true also for the statistic of GRBs so that we started a program to understand
its role both in relation to GRBs and in function of z. This paper presents a
composite model in this direction. The model considers a rather generic
distribution of dust in a spiral galaxy and considers the effect of changing
some of the parameters characterizing the dust grains, size in particular. We
first simulated 500 GRBs distributed as the host galaxy mass distribution,
using as model the Milky Way. If we consider dust with the same properties as
that we observe in the Milky Way, we find that due to absorption we miss about
10% of the afterglows assuming we observe the event within about 1 hour or even
within 100s. In our second set of simulations we placed GRBs randomly inside
giants molecular clouds, considering different kinds of dust inside and outside
the host cloud and the effect of dust sublimation caused by the GRB inside the
clouds. In this case absorption is mainly due to the host cloud and the
physical properties of dust play a strong role. Computations from this model
agree with the hypothesis of host galaxies with extinction curve similar to
that of the Small Magellanic Cloud, whereas the host cloud could be also
characterized by dust with larger grains. To confirm our findings we need a set
of homogeneous infrared observations. The use of coming dedicated infrared
telescopes, like REM, will provide a wealth of cases of new afterglow
observations.Comment: 16 pages, 8 figures, accepted by A&
Continuous optical monitoring during the prompt emission of GRB 060111B
We present the time-resolved optical emission of GRB 060111B during its
prompt phase, measured with the TAROT robotic observatory. This is the first
time that the optical emission from a gamma-ray burst has been continuously
monitored with a temporal resolution of a few seconds during the prompt
gamma-ray phase. The temporal evolution of the prompt optical emission at the
level of several seconds is used to provide a clue to the origin of this
emission. The optical emission was found to decay steadily from our first
measure, 28s after the trigger, in contrast to the gamma-ray emission, which
exhibits strong variability at the same time. This behaviour strongly suggests
that the optical emission is due to the reverse shock
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
Experimental study of Pomeron
A Pomeron phenomenon remains a mystery. A short review of the experimental
situation in diffractive physics and an account of some spectacular
manifestations of the Pomeron are given.Comment: 17 pages, 7 Figs, LATEX. Talk given at the conference "From the
smallest to largest distances", ITEP, Moscow, 24-26 May 2001. Changes: Fig.2
replace
The Host Galaxy of GRB 990123
We present deep images of the field of gamma-ray burst (GRB) 990123 obtained
in a broad-band UV/visible bandpass with the Hubble Space Telescope, and deep
near-infrared images obtained with the Keck-I 10-m telescope. Both the HST and
Keck images show that the optical transient (OT) is clearly offset by 0.6
arcsec from an extended object, presumably the host galaxy. This galaxy is the
most likely source of the metallic-line absorption at z = 1.6004 seen in the
spectrum of the OT. With magnitudes V_{C} ~ 24.6 +/- 0.2 and K = 21.65 +/- 0.30
mag this corresponds to an L ~ 0.7 L_* galaxy, assuming that it is located at z
= 1.6. The estimated unobscured star formation rate is SFR ~ 6 M_sun/yr, which
is not unusually high for normal galaxies at comparable redshifts. The strength
of the observed metallic absorption lines is suggestive of a relatively high
metallicity of the gas, and thus of a chemically evolved system which may be
associated with a massive galaxy. It is also indicative of a high column
density of the gas, typical of damped Ly-alpha systems at high redshifts. We
conclude that this is the host galaxy of GRB 990123. No other obvious galaxies
are detected within the same projected radius from the OT. There is thus no
evidence for strong gravitational lensing magnification of this burst, and some
alternative explanation for its remarkable energetics may be required. The
observed offset of the OT from the center of its apparent host galaxy, 5.5 +/-
0.9 proper kpc (projected) in the galaxy's rest-frame, both refutes the
possibility that GRBs are related to galactic nuclear activity and supports
models of GRBs which involve the death and/or merger of massive stars. Further,
the HST image suggests an intimate connection of GRB 990123 and a star-forming
region.Comment: Updated references. 12 pages including 3 Postscript figures. Camera-
ready reproductions of the figures can be found at
http://astro.caltech.edu/~jsb/GRB/grb990123.htm
ROTSE All Sky Surveys for Variable Stars I: Test Fields
The ROTSE-I experiment has generated CCD photometry for the entire Northern
sky in two epochs nightly since March 1998. These sky patrol data are a
powerful resource for studies of astrophysical transients. As a demonstration
project, we present first results of a search for periodic variable stars
derived from ROTSE-I observations. Variable identification, period
determination, and type classification are conducted via automatic algorithms.
In a set of nine ROTSE-I sky patrol fields covering about 2000 square degrees
we identify 1781 periodic variable stars with mean magnitudes between m_v=10.0
and m_v=15.5. About 90% of these objects are newly identified as variable.
Examples of many familiar types are presented. All classifications for this
study have been manually confirmed. The selection criteria for this analysis
have been conservatively defined, and are known to be biased against some
variable classes. This preliminary study includes only 5.6% of the total
ROTSE-I sky coverage, suggesting that the full ROTSE-I variable catalog will
include more than 32,000 periodic variable stars.Comment: Accepted for publication in AJ 4/00. LaTeX manuscript. (28 pages, 11
postscript figures and 1 gif
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