1,874 research outputs found
Broad band time-resolved E_{p,i}--L_{iso} correlation in GRBs
We report results of a systematic study of the broad band (2--2000 keV) time
resolved prompt emission spectra of a sample of gamma-ray bursts (GRBs)
detected with both Wide Field Cameras on board the \sax\ satellite and the
\batse\ experiment on board CGRO. In this first paper, we study the
time-resolved dependence of the intrinsic peak energy of the
spectrum on the corresponding isotropic bolometric luminosity .
The -- relation or the equivalent relation between
and the bolometric released energy , derived using the time
averaged spectra of long GRBs with known redshift, is well established, but its
physical origin is still a subject of discussion. In addition, some authors
maintain that these relations are the result of instrumental selection effects.
We find that not only a relation between the measured peak energy and the
corresponding energy flux, but also a strong versus
correlation are found within each burst and merging together the time resolved
data points from different GRBs. We do not expect significant instrumental
selection effects that can affect the obtained results, apart from the fact
that the GRBs in our sample are sufficiently bright to perform a time-resolved
spectroscopy and that they have known redshift. If the fundamental physical
process that gives rise to the GRB phenomenon does not depend on its
brightness, we conclude that the found versus
correlation within each GRB is intrinsic to the emission process, and that the
correlations discovered by Amati et al. and Yonetoku et al. are likely not the
result of selection effects. We also discuss the properties of the correlations
found.Comment: 27 pages,4 tables, 7 figure, accepted for publication in The
Astrophysical Journa
Numerical study of the random field Ising model at zero and positive temperature
In this paper the three dimensional random field Ising model is studied at
both zero temperature and positive temperature. Critical exponents are
extracted at zero temperature by finite size scaling analysis of large
discontinuities in the bond energy. The heat capacity exponent is
found to be near zero. The ground states are determined for a range of external
field and disorder strength near the zero temperature critical point and the
scaling of ground state tilings of the field-disorder plane is discussed. At
positive temperature the specific heat and the susceptibility are obtained
using the Wang-Landau algorithm. It is found that sharp peaks are present in
these physical quantities for some realizations of systems sized and
larger. These sharp peaks result from flipping large domains and correspond to
large discontinuities in ground state bond energies. Finally, zero temperature
and positive temperature spin configurations near the critical line are found
to be highly correlated suggesting a strong version of the zero temperature
fixed point hypothesis.Comment: 11 pages, 14 figure
Expected performances of a Laue lens made with bent crystals
In the context of the LAUE project devoted to build a Laue lens prototype for
focusing celestial hard X-/soft gamma-rays, a Laue lens made of bent crystal
tiles, with 20 m focal length, is simulated. The focusing energy passband is
assumed to be 90--600 keV. The distortion of the image produced by the lens on
the focal plane, due to effects of crystal tile misalignment and radial
distortion of the crystal curvature, is investigated. The corresponding
effective area of the lens, its point spread function and sensitivity are
calculated and compared with those exhibited by a nominal Laue lens with no
misalignment and/or distortion. Such analysis is crucial to estimate the
optical properties of a real lens, in which the investigated shortcomings could
be present.Comment: 20 pages, 14 figure
Probing phase coexistence and stabilization of the spin-ordered ferrimagnetic state by Calcium addition in the YBa_{1-x}Ca_{x}Co_{2}O_{5.5} layered cobaltites using neutron diffraction
In this article we study the effects of a partial substitution of Ba with the
smaller cation Ca in the layered cobaltites YBaCo_2O_{5+\delta} for \delta
\approx 0.5. Neutron thermodiffractograms are reported for the compounds
YBa_{0.95}Ca_{0.05}Co_2O_{5.5} (x_{Ca}=0.05) and YBa_{0.90}Ca_{0.10}Co_2O_{5.5}
(x_{Ca}=0.10) in the temperature range 20 K \leq T \leq 300 K, as well as high
resolution neutron diffraction experiments at selected temperatures for the
samples x_{Ca}=0.05, x_{Ca}=0.10 and the parent compound x_{Ca}=0. We have
found the magnetic properties to be strongly affected by the cationic
substitution. Although the "122" perovskite structure seems unaffected by Ca
addition, the magnetic arrangements of Co ions are drastically modified: the
antiferromagnetic (AFM) long-range order is destroyed, and a ferrimagnetic
phase with spin state order is stabilized below T \sim 290 K. For the sample
with x_{Ca}=0.05 a fraction of AFM phase coexists with the ferrimagnetic one
below T \sim 190 K, whereas for x_{Ca}=0.10 the AFM order is completely lost.
The systematic refinement of the whole series has allowed for a better
understanding of the observed low-temperature diffraction patterns of the
parent compound, YBaCo_2O_{5.5}, which had not yet been clarified. A two-phase
scenario is proposed for the x_{Ca}=0 compound which is compatible with the
phase coexistence observed in the x_{Ca}=0.05 sample
Physical parameters of GRB 970508 and GRB 971214 from their afterglow synchrotron emission
We have calculated synchrotron spectra of relativistic blast waves, and find
predicted characteristic frequencies that are more than an order of magnitude
different from previous calculations. For the case of an adiabatically
expanding blast wave, which is applicable to observed gamma-ray burst (GRB)
afterglows at late times, we give expressions to infer the physical properties
of the afterglow from the measured spectral features.
We show that enough data exist for GRB970508 to compute unambiguously the
ambient density, n=0.03/cm**3, and the blast wave energy per unit solid angle,
E=3E52 erg/4pi sr. We also compute the energy density in electrons and magnetic
field. We find that they are 12% and 9%, respectively, of the nucleon energy
density and thus confirm for the first time that both are close to but below
equipartition.
For GRB971214, we discuss the break found in its spectrum by Ramaprakash et
al. (1998). It can be interpreted either as the peak frequency or as the
cooling frequency; both interpretations have some problems, but on balance the
break is more likely to be the cooling frequency. Even when we assume this, our
ignorance of the self-absorption frequency and presence or absence of beaming
make it impossible to constrain the physical parameters of GRB971214 very well.Comment: very strongly revised analysis of GRB971214 and discussion, submitted
to ApJ, 11 pages LaTeX, 4 figures, uses emulateapj.sty (included
GRB 970228 Revisited: Evidence for a Supernova in the Light Curve and La te Spectral Energy Distribution of the Afterglow
At the time of its discovery, the optical and X-ray afterglow of GRB 970228
appeared to be a ringing endorsement of the previously untried relativistic
fireball model of gamma-ray burst (GRB) afterglows, but now that nearly a dozen
optical afterglows to GRBs have been observed, the wavering light curve and
reddening spectrum of this afterglow make it perhaps the most difficult of the
observed afterglows to reconcile with the fireball model. In this Letter, we
argue that this afterglow's unusual temporal and spectral properties can be
attributed to a supernova that overtook the light curve nearly two weeks after
the GRB. This is the strongest case yet for a GRB/supernova connection. It
strengthens the case that a supernova also dominated the late afterglow of GRB
980326, and the case that GRB 980425 is related to SN 1998bw.Comment: Accepted to The Astrophysical Journal (Letters), 14 pages, LaTe
Evidence for a long duration component in the prompt emission of short Gamma-Ray Bursts detected with BeppoSAX
A statistical study on the light curves of all the short Gamma-Ray Bursts
detected with the Gamma Ray Burst Monitor (GRBM) aboard BeppoSAX is reported.
Evidence for a very weak and long duration component associated with these
events in the two 1 s counters of the GRBM (40-700 keV and >100 keV) is found.
It starts a few tens of seconds before the burst and continues for about 30 s
after the burst. The overall hardness of this component is comparable with that
of the event itself. The detection of a signal before the onset time and the
similar hardness are consistent with an interpretation of the long duration
component in terms of prompt emission associated with short GRBs.Comment: 12 pages, 6 figures, accepted for publication in ApJ
Intrinsic spectra and energetics of BeppoSAX Gamma-Ray Bursts with known redshifts
We present the main results of a study of spectral and energetics properties
of twelve gamma-ray bursts (GRBs) with redshift estimates. All GRBs in our
sample were detected by BeppoSAX in a broad energy range (2-700 keV). From the
redshift estimates and the good-quality BeppoSAX time-integrated spectra we
deduce the main properties of GRBs in their cosmological rest frames. All
spectra in our sample are satisfactorily represented by the Band model with no
significant soft X-ray excesses or spectral absorptions. We find a positive
correlation between the estimated total (isotropic) energies in the 1-10000 keV
energy range (E_rad) and redshifts z. Interestingly, more luminous GRBs are
characterized also by larger peak energies E_p of their EF(E) spectra.
Furthermore, more distant GRBs appear to be systematically harder in the X-ray
band compared to GRBs with lower redshifts. We discuss how selection and data
truncation effects could bias our results and give possible explanations for
the correlations that we found.Comment: 10 pages, 5 figures, accepted for publication in Astronomy &
Astrophysic
Preliminary investigation into the geometric calibration of the Micasense RedEdge-M multispectral camera
Multispectral cameras, in the past the prerogative of Remote Sensing (RS) applications via satellites and manned aircraft, are becoming increasingly used in photogrammetric applications. Moreover, the ubiquitous use of Unmanned Aerial Vehicles (UAVs) has created a need for the miniaturisation of sensors, which has contributed to the availability of a wide range of relatively low-cost and lightweight cameras. Therefore, small multispectral cameras mounted on UAVs provide an effective and low-cost solution when it comes to acquiring airborne radiometric data.With the growing interest for such sensors to perform photogrammetric tasks, camera calibration remains an essential step in order to obtain reliable and geometrically accurate information.This paper will investigate the camera calibration parameters between the five bands of the MicaSense RedEdge-M sensor from laboratory trials. The results of the camera calibration will be obtained from the use, primarily, of Australis software and a calibration frame within the Nottingham Geospatial Institute. The variations of the parameters demonstrate the need for distortion correction separately within each band before using the images for photogrammetry
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