2,485 research outputs found
Gamma Ray Bursts as cosmological tools
The use of Gamma Ray Bursts as ``standard candles'' has been made possible by
the recent discovery of a very tight correlation between their rest frame
intrinsic properties. This correlation relates the GRB prompt emission peak
spectral energy E_peak to the energy E_gamma corrected for the collimation
angle theta_jet of these sources. The possibility to use GRBs to constrain the
cosmological parameters and to study the nature of Dark Energy are very
promising.Comment: 6 pages, 3 figures, Proceedings of the workshop 'Astrophysical
sources of high energy particles and radiation', Torun - Poland 20-24 June
2005, Ed. T. Bulik, B. Rudak, G. Madejsk
The Epeak-Eiso plane of long Gamma Ray Bursts and selection effects
We study the distribution of long Gamma Ray Bursts in the Ep-Eiso and in the
Ep,obs-Fluence planes through an updated sample of 76 bursts, with measured
redshift and spectral parameters, detected up to September 2007. We confirm the
existence of a strong rest frame correlation Ep ~ Eiso^0.54+-0.01. Contrary to
previous studies, no sign of evolution with redshift of the Ep-Eiso correlation
(either its slope and normalisation) is found. The 76 bursts define a strong
Ep,obs-Fluence correlation in the observer frame (Ep,obs ~ F^0.32+-0.05) with
redshifts evenly distributed along this correlation. We study possible
instrumental selection effects in the observer frame Ep,obs-Fluence plane. In
particular, we concentrate on the minimum peak flux necessary to trigger a
given GRB detector (trigger threshold) and the minimum fluence a burst must
have to determine the value of Ep,obs (spectral analysis threshold). We find
that the latter dominates in the Ep,obs-Fluence plane over the former. Our
analysis shows, however, that these instrumental selection effects do not
dominate for bursts detected before the launch of the Swift satellite, while
the spectral analysis threshold is the dominant truncation effect of the Swift
GRB sample (27 out of 76 events). This suggests that the Ep,obs-Fluence
correlation defined by the pre--Swift sample could be affected by other, still
not understood, selection effects. Besides we caution about the conclusions on
the existence of the Ep,obs-Fluence correlation based on our Swift sample
alone.Comment: To appear in MNRA
Long Gamma-Ray Bursts as standard candles
As soon as it was realized that long GRBs lie at cosmological distances,
attempts have been made to use them as cosmological probes. Besides their use
as lighthouses, a task that presents mainly the technological challenge of a
rapid deep high resolution follow-up, researchers attempted to find the Holy
Grail: a way to create a standard candle from GRB observables. We discuss here
the attempts and the discovery of the Ghirlanda correlation, to date the best
method to standardize the GRB candle. Together with discussing the promises of
this method, we will underline the open issues, the required calibrations and
how to understand them and keep them under control. Even though GRB cosmology
is a field in its infancy, ongoing work and studies will clarify soon if and
how GRBs will be able to keep up to the promises.Comment: To appear in the proceedings of the 16th Annual October Astrophysics
Conference in Maryland "Gamma Ray Bursts in the Swift Era", eds. S. Holt, N.
Gehrels & J. Nouse
Spectral analysis of Swift long GRBs with known redshift
We study the spectral and energetics properties of 47 long-duration gamma-ray
bursts (GRBs) with known redshift, all of them detected by the Swift satellite.
Due to the narrow energy range (15-150 keV) of the Swift-BAT detector, the
spectral fitting is reliable only for fitting models with 2 or 3 parameters. As
high uncertainty and correlation among the errors is expected, a careful
analysis of the errors is necessary. We fit both the power law (PL, 2
parameters) and cut--off power law (CPL, 3 parameters) models to the
time-integrated spectra of the 47 bursts, and present the corresponding
parameters, their uncertainties, and the correlations among the uncertainties.
The CPL model is reliable only for 29 bursts for which we estimate the nuf_nu
peak energy Epk. For these GRBs, we calculate the energy fluence and the rest-
frame isotropic-equivalent radiated energy, Eiso, as well as the propagated
uncertainties and correlations among them. We explore the distribution of our
homogeneous sample of GRBs on the rest-frame diagram E'pk vs Eiso. We confirm a
significant correlation between these two quantities (the "Amati" relation) and
we verify that, within the uncertainty limits, no outliers are present. We also
fit the spectra to a Band model with the high energy power law index frozen to
-2.3, obtaining a rather good agreement with the "Amati" relation of non-Swift
GRBs.Comment: 16 pages. To appear in MNRAS. Minor changes were introduced in this
last versio
Cosmological constraints with GRBs: homogeneous medium vs wind density profile
We present the constraints on the cosmological parameters obtained with the
-- correlation found with the most recent sample of
19 GRBs with spectroscopically measured redshift and well determined prompt
emission spectral and afterglow parameters. We compare our results obtained in
the two possible uniform jet scenarios, i.e. assuming a homogeneous density
profile (HM) or a wind density profile (WM) for the circumburst medium. Better
constraints on and are obtained with the
(tighter) -- correlation derived in the wind density
scenario. We explore the improvements to the constraints of the cosmological
parameters that could be reached with a large sample, 150 GRBs, in the
future. We study the possibility to calibrate the slope of these correlations.
Our optimization analysis suggests that GRBs with redshift
can be used to calibrate the -- with
a precision better than 1%. The same precision is expected for the same number
of bursts with . This result suggests that we do not
necessarily need a large sample of low z GRBs for calibrating the slope of
these correlations.Comment: 7 pages, 7 figures, submitted to A&
Novel substrates for Helium adsorption: Graphane and Graphene-Fluoride
The discovery of fullerenes has stimulated extensive exploration of the
resulting behavior of adsorbed films. Our study addresses the planar substrates
graphene-fluoride (GF) and graphane (GH) in comparison to graphene. We present
initial results concerning the potential energy, energy bands and low density
behavior of 4He and 3He films on such different surfaces. For example, while
graphene presents an adsorption potential that is qualitatively similar to that
on graphite, GF and GH yield potentials with different symmetry, a number of
adsorption sites double that on graphene/graphite and a larger corrugation for
the adatom. In the case of GF, the lowest energy band width is similar to that
on graphite but the He atom has a significantly larger effective mass and the
adsorption energy is about three time that on graphite. Implications concerning
the monolayer phase diagram of 4He are explored with the exact path integral
ground state method. A commensurate ordered state similar to the sqrt{3} x
sqrt{3} R30^o state on graphite is found the be unstable both on GF and on GH.
The ground states of submonolayer 4He on both GF and GH are superfluids with a
Bose Einstein condensate fraction of about 10%.Comment: 6 pages, 3 figures, LT26 proceedings, accepted for publication in
Journal of Physics: Conference Serie
Insetos galhadores da pitangueira e do araçazeiro no Sul do Rio Grande do Sul.
bitstream/item/78795/1/documento-337.pd
Re-born fireballs in Gamma-Ray Bursts
We consider the interaction between a relativistic fireball and material assumed to be still located just outside the progenitor star. Only a small fraction of the expected mass is sufficient to decelerate the fireball efficiently, leading to dissipation of most of its kinetic energy. Since the scattering optical depths are still large at distances comparable to the progenitor radius, the dissipated energy is trapped in the system, accelerating it to relativistic velocities. The process resembles the birth of another fireball at radii R similar to 10(11) cm, not far from the transparency radius, and with starting bulk Lorentz factors Gamma(c) similar to 10. As seen in the observer frame, this `re- generated' fireball appears collimated within an angle theta = 1/Gamma(c). If the central engine works intermittently, the funnel can, at least partially, refill and the process can repeat itself. We discuss how this idea can help to solve some open issues of the more conventional internal shock scenario for interpreting gamma-ray burst propertie
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