553 research outputs found
SGARFACE: A Novel Detector For Microsecond Gamma Ray Bursts
The Short GAmma Ray Front Air Cherenkov Experiment (SGARFACE) is operated at
the Whipple Observatory utilizing the Whipple 10m gamma-ray telescope. SGARFACE
is sensitive to gamma-ray bursts of more than 100MeV with durations from 100ns
to 35us and provides a fluence sensitivity as low as 0.8 gamma-rays per m^2
above 200MeV (0.05 gamma-rays per m^2 above 2GeV) and allows to record the
burst time structure.Comment: 29 pages, 14 figures, accepted for publication in Astroparticle
Physic
The Near Infrared Background: Interplanetary Dust or Primordial Stars?
The intensity of the diffuse ~ 1 - 4 micron sky emission from which solar
system and Galactic foregrounds have been subtracted is in excess of that
expected from energy released by galaxies and stars that formed during the z <
5 redshift interval (Arendt & Dwek 2003, Matsumoto et al. 2005). The spectral
signature of this excess near-infrared background light (NIRBL) component is
almost identical to that of reflected sunlight from the interplanetary dust
cloud, and could therefore be the result of the incomplete subtraction of this
foreground emission component from the diffuse sky maps. Alternatively, this
emission component could be extragalactic. Its spectral signature is consistent
with that of redshifted continuum and recombination line emission from HII
regions formed by the first generation of very massive stars. In this paper we
analyze the implications of this spectral component for the formation rate of
these Population III stars, the redshift interval during which they formed, the
reionization of the universe and evolution of collapsed halo masses. We find
that to reproduce the intensity and spectral shape of the NIRBL requires a peak
star formation rate that is higher by about a factor of 4 to 10 compared to
those derived from hierarchical models. Furthermore, an extragalactic origin
for the NIRBL leads to physically unrealistic absorption-corrected spectra of
distant TeV blazars. All these results suggest that Pop III stars contribute
only a fraction of the NIRBL intensity with zodiacal light, star forming
galaxies, and/or non-nuclear sources giving rise to the remaining fraction.Comment: 28 pages including 7 embedded figures. Submitted to Ap
Optimization with gradient-boosted trees and risk control
Decision trees effectively represent the sparse, high dimensional and noisy nature of chemical data from experiments. Having learned a function from this data, we may want to thereafter optimize the function, e.g., picking the best chemical process catalyst. In this way, we may repurpose legacy predictive models. This work studies a large-scale, industrially-relevant mixed-integer quadratic optimization problem involving: (i) gradient-boosted pre-trained regression trees modeling catalyst behavior, (ii) penalty functions mitigating risk, and (iii) penalties enforcing composition constraints. We develop heuristic methods and an exact, branch-and-bound algorithm leveraging structural properties of gradient-boosted trees and penalty functions. We numerically test our methods on an industrial instance
Is There an Imprint of Primordial Stars in the Tev Gamma-Ray Spectrum of Blazars?
The 1 - 5 micron diffuse sky emission from which local foreground emission
from the solar system and the Galaxy have been subtracted exceeds the
brightness that can be attributed to normal star forming galaxies. The nature
of this excess near-infrared background light (NIRBL) is still controversial.
On one hand, it has been interpreted as a distinct spectral feature created by
the redshifted emission from primordial (Population III) stars that have formed
at redshifts > 8. On the other hand, the NIRBL spectrum is almost identical to
that of the zodiacal cloud, raising the possibility that it is of local origin.
Blazars can, in principle, offer a simple test for the nature and origin of the
NIRBL. Very high energy gamma-ray photons emitted by these objects are
attenuated on route to earth by photon-photon interactions with the
extragalactic background light (EBL). This paper examines whether the
extragalactic nature of the NIRBL can be determined from the analysis of the
TeV spectra of blazars. (Abridged)Comment: 16 pages, 8 figures, accepted for publication in the ApJ on July 15,
200
Gamma-Hadron Separation Methods for the VERITAS Array of Four Imaging Atmospheric Cherenkov Telescopes
Ground-based arrays of imaging atmospheric Cherenkov telescopes have emerged
as the most sensitive gamma-ray detectors in the energy range of about 100 GeV
and above. The strengths of these arrays are a very large effective collection
area on the order of 100,000 square meter, combined with excellent single
photon angular and energy resolutions. The sensitivity of such detectors is
limited by statistical fluctuations in the number of Cosmic Ray initiated air
showers that resemble gamma-ray air showers in many ways. In this paper, we
study the performance of simple event reconstruction methods when applied to
simulated data of the Very Energetic Radiation Imaging Telescope Array System
(VERITAS) experiment. We review methods for reconstructing the arrival
direction and the energy of the primary photons, and examine means to improve
on their performance. For a software threshold energy of 300 GeV (100 GeV), the
methods achieve point source angular and energy resolutions of sigma[63%]= 0.1
degree (0.2 degree) and sigma[68%]= 15% (22%), respectively. The main emphasis
of the paper is the discussion of gamma-hadron separation methods for the
VERITAS experiment. We find that the information from several methods can be
combined based on a likelihood ratio approach and the resulting algorithm
achieves a gamma-hadron suppression with a quality factor that is substantially
higher than that achieved with the standard methods used so far.Comment: Astroparticle Physics, in press, 22 pages, 10 figure
BeppoSAX Observations of the TeV Blazar Mkn 421
The blazar Mkn 421 has been observed, as part of the AO1 Core Program, five
times from 2 to 7 May 1997. In the LECS+MECS energy band the spectrum shows
convex curvature, well represented by a broken power--law. Flux variability
(more than a factor 2) has been detected over the entire 0.1--10 keV range,
accompanying which the spectrum steepens with the decrease in intensity. Mkn
421 has also been detected with the PDS instrument. Our preliminary analysis
indicates that the PDS spectrum lies significantly above the extrapolation from
the MECS, suggesting a contribution from a flatter high energy component.Comment: 4 pages, 4 Postscript figures, uses espcrc2.sty and psfig.sty (both
included). To appear in "The Active X-ray Sky: Results from BeppoSAX and
Rossi-XTE", Rome, Italy, 21-24 October, 1997. Eds.: L. Scarsi, H. Bradt, P.
Giommi and F. Fior
New Limits to the Infrared Background: Bounds on Radiative Neutrino Decay and on Contributions of Very Massive Objects to the Dark Matter Problem
From considering the effect of Îł-Îł interactions on recently observed TeV gamma-ray spectra, improved limits are set to the density of extragalactic infrared photons which are robust and essentially model independent. The resulting limits are more than an order of magnitude more restrictive than direct observations in the 0.025â0.3 eV regime. These limits are used to improve constraints on radiative neutrino decay in the mass range above 0.05 eV and to rule out very massive objects as providing the dark matter needed to explain galaxy rotation curves. Lower bounds on the maximum distance which TeV gamma rays may probe are also derived
The Flux Variability of Markarian 501 in Very High Energy Gamma Rays
The BL Lacertae object Markarian 501 was identified as a source of gamma-ray
emission at the Whipple Observatory in March 1995. Here we present a flux
variability analysis on several times-scales of the 233 hour data set
accumulated over 213 nights (from March 1995 to July 1998) with the Whipple
Observatory 10 m atmospheric Cherenkov imaging telescope. In 1995, with the
exception of a single night, the flux from Markarian 501 was constant on daily
and monthly time-scales and had an average flux of only 10% that of the Crab
Nebula, making it the weakest VHE source detected to date. In 1996, the average
flux was approximately twice the 1995 flux and showed significant
month-to-month variability. No significant day-scale variations were detected.
The average gamma-ray flux above ~350 GeV in the 1997 observing season rose to
1.4 times that of the Crab Nebula -- 14 times the 1995 discovery level --
allowing a search for variability on time-scales shorter than one day.
Significant hour-scale variability was present in the 1997 data, with the
shortest, observed on MJD 50607, having a doubling time of ~2 hours. In 1998
the average emission level decreased considerably from that of 1997 (to ~20% of
the Crab Nebula flux) but two significant flaring events were observed. Thus,
the emission from Markarian 501 shows large amplitude and rapid flux
variability at very high energies as does Markarian 421. It also shows large
mean flux level variations on year-to-year time-scales, behaviour which has not
been seen from Markarian 421 so far.Comment: 19 pages, 3 figures, to appear in ApJ, June 20, 1999, Vol. 518 #
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