163 research outputs found
The Ep,i - Eiso correlation in GRBs: updated observational status, re-analysis and main implications
The correlation between the cosmological rest-frame nuFnu spectrum peak
energy, Ep,i, and the isotropic equivalent radiated energy, Eiso, discovered by
Amati et al. in 2002 and confirmed/extended by subsequent osbervations, is one
of the most intriguing and debated observational evidences in Gamma-Ray Bursts
(GRB) astrophysics. In this paper I provide an update and a re-analysis of the
Ep,i - Eiso correlation basing on an updated sample consisting of 41 long
GRBs/XRFs with firm estimates of z and observed peak energy, Ep,obs, 12 GRBs
with uncertain valeus of z and/or Ep,obs, 2 short GRBs with firm estimates of z
and Ep,obs and the peculiar sub-energetic events GRB980425/SN1998bw and
GRB031203/SN2003lw. In addition to standard correlation analysis and power-law
fitting, the data analysis here reported includes a modelization which accounts
for sample variance. All 53 classical long GRBs and XRFs, including 11 Swift
events with published spectral parameters and fluences, have Ep,i and Eiso
values, or upper/lower limits, consistent with the correlation, which shows a
chance probability as low as ~7x10{-15}, a slope of ~0.57 (~0.5 when fitting by
accounting for sample variance) and an extra-Poissonian logarithmic dispersion
of ~0.15, it extends over ~5 orders of magnitude in Eiso and ~3 orders of
magnitude in Ep,i and holds from the closer to the higher z GRBs. I also
discuss the main implications of the updated Ep,i - Eiso correlation for the
models of the physics and geometry of GRB emission, its use for discriminating
different classes and as a pseudo-z estimator, and the tests of possible
selection effects with GRBs of unknown redshift.Comment: 15 pages, 5 figures, accepted for publication in MNRAS, main journa
Cosmology with gamma-ray bursts: II Cosmography challenges and cosmological scenarios for the accelerated Universe
Context. Explaining the accelerated expansion of the Universe is one of the
fundamental challenges in physics today. Cosmography provides information about
the evolution of the universe derived from measured distances, assuming only
that the space time ge- ometry is described by the
Friedman-Lemaitre-Robertson-Walker metric, and adopting an approach that
effectively uses only Taylor expansions of basic observables. Aims. We perform
a high-redshift analysis to constrain the cosmographic expansion up to the
fifth order. It is based on the Union2 type Ia supernovae data set, the
gamma-ray burst Hubble diagram, a data set of 28 independent measurements of
the Hubble param- eter, baryon acoustic oscillations measurements from galaxy
clustering and the Lyman-{\alpha} forest in the SDSS-III Baryon Oscillation
Spectroscopic Survey (BOSS), and some Gaussian priors on h and {\Omega}M .
Methods. We performed a statistical analysis and explored the probability
distributions of the cosmographic parameters. By building up their regions of
confidence, we maximized our likelihood function using the Markov chain Monte
Carlo method. Results. Our high-redshift analysis confirms that the expansion
of the Universe currently accelerates; the estimation of the jerk parameter
indicates a possible deviation from the standard {\Lambda}CDM cosmological
model. Moreover, we investigate implications of our results for the
reconstruction of the dark energy equation of state (EOS) by comparing the
standard technique of cosmography with an alternative approach based on
generalized Pad\'e approximations of the same observables. Because these
expansions converge better, is possible to improve the constraints on the
cosmographic parameters and also on the dark matter EOS. Conclusions. The
estimation of the jerk and the DE parameters indicates at 1{\sigma} a possible
deviation from the {\Lambda}CDM cosmological model.Comment: 10 pages, 7 figures, accepted for publication in A &
An upscattering spectral formation model for the prompt emission of Gamma-Ray Bursts
We propose a model for the spectral formation of Gamma Ray Burst (GRB) prompt
emission, where the phenomenological Band's function is usually applied to
describe the GRB prompt emission. We suggest that the GRB prompt emission is
mainly a result of two upscattering processes. The first process is the
Comptonization of relatively cold soft photons of the star off electrons of a
hot shell of plasma of temperature T_e of the order of 10^{9} K (or kT_e~100
keV) that moves sub-relativistically with the bulk velocity V_b substantially
less than the speed of light c. In this phase, the Comptonization parameter Y
is high and the interaction between a blackbody-like soft seed photon
population and hot electrons leads to formation of a saturated Comptonization
spectrum modified by the sub-relativistic bulk outflow. The second process is
an upscattering of the previously Comptonized spectrum by the plasma outflow
once it becomes relativistic. This process gives rise to the high-energy
power-law component above the peak in the EF(E)-diagram where F(E) is the
energy flux. The latter process can be described by a convolution of the
Comptonized spectrum with a broken-power-law Green function. Possible physical
scenarios for this second upscattering process are discussed. In the framework
of our model, we give an interpretation of the Amati relation between the
intrinsic spectral peak photon energy and radiated energy or luminosity, and we
propose a possible explanation of the GRB temporal variability.Comment: 27 pages, 8 figures, accepted for publication in the Astrophysical
Journa
Cosmology with gamma-ray bursts: I. The Hubble diagram through the calibrated - correlation
Gamma-ray bursts are the most energetic explosions in the Universe. They are
detectable up to very high redshifts, therefore can be used to study the
expansion rate of the Universe and to investigate the observational properties
of dark energy, provided that empirical correlations between spectral and
intensity properties are appropriately calibrated. We used the type Ia
supernova luminosity distances to calibrate the correlation between the peak
photon energy, , and the isotropic equivalent radiated energy, in GRBs. With this correlation, we tested the reliability of applying
GRBs to measure cosmological parameters and to obtain indications on the basic
properties and evolution of dark energy. Using 162 GRBs with measured redshifts
and spectra, we applied a local regression technique to calibrate the - correlation against the type Ia SN data to build a calibrated GRB
Hubble diagram. We tested the possible redshift dependence of the correlation
and its effect on the Hubble diagram. Finally, we used the GRB Hubble diagram
to investigate the dark energy EOS. For this, we focused on the so-called
Chevalier-Polarski-Linder (CPL) parametrization of the dark energy EOS and
implemented the Markov chain Monte Carlo (MCMC) method to efficiently sample
the space of cosmological parameters. Our analysis shows once more that the
- correlation has no significant redshift dependence.
Therefore the high-redshift GRBs can be used as a cosmological tool to
determine the basic cosmological parameters and to test different models of
dark energy in the redshift region (), which is unexplored by the
SNIa and baryonic acoustic oscillations data. Our updated calibrated Hubble
diagram of GRBs provides some marginal indication (at level) of an
evolving dark energy EOS.Comment: 12 pages, 11 figure
Update on the GRB universal scaling E-E-E with ten years of data
From a comprehensive statistical analysis of X-ray light-curves of
gamma-ray bursts (GRBs) collected from December 2004 to the end of 2010, we
found a three-parameter correlation between the isotropic energy emitted in the
rest frame 1-10 keV energy band during the prompt emission
(E), the rest frame peak of the prompt emission energy
spectrum (E), and the X-ray energy emitted in the rest frame 0.3-30
keV observed energy band (E), computed excluding the
contribution of the flares. In this paper, we update this correlation with the
data collected until June 2014, expanding the sample size with 35% more
objects, where the number of short GRBs doubled. With this larger sample we
confirm the existence of a universal correlation that connects the prompt and
afterglow properties of long and short GRBs. We show that this correlation does
not depend on the X-ray light-curve morphology and that further analysis is
necessary to firmly exclude possible biases derived by redshift measurements.
In addition we discuss about the behavior of the peculiar objects as ultra-long
GRBs and we propose the existence of an intermediate group between long and
short GRBs. Interestingly, two GRBs with uncertain classification fall into
this category. Finally, we discuss the physics underlying this correlation, in
the contest of the efficiency of conversion of the prompt -ray emission
energy into the kinetic energy of the afterglow, the photosferic model, and the
cannonball model.Comment: 11 pages, 5 figures, accepted for publication in MNRA
INVESTIGATING THE Ep, i –Eiso CORRELATION
The correlation between the spectral peak photon energy, Ep, and the radiated energy or luminosity (i.e., the “Amati relation” and other correlations derived from it) is one of the central and most debated topics in GRB astrophysics, with implications for physics and the geometry of prompt emission, the identification and understanding of various classes of GRBs (short/long, XRFs,sub-energetic), and GRB cosmology. Fermi is exceptionally suited to provide, also in conjunction with Swift observations, a significant step forward in this field of research. Indeed, one of the main goals of Fermi/GBM is to make accurate measurements of Ep, by exploiting its unprecedented broad energy band from ~8 keV to ~30MeV; in addition, for a small fraction of GRBs, the LAT can extend the spectral measurements up to the GeV energy range, thus allowing a reliable estimate of the bolometric radiated energy/luminosity. We provide a review, an update and a discussion of the impact of Fermi observations in the investigation, understanding and testing of the Ep,i –Eiso (“Amati”) relation
INVESTIGATING THE Ep, i –Eiso CORRELATION
The correlation between the spectral peak photon energy, Ep, and the radiated energy or luminosity (i.e., the “Amati relation” and other correlations derived from it) is one of the central and most debated topics in GRB astrophysics, with implications for physics and the geometry of prompt emission, the identification and understanding of various classes of GRBs (short/long, XRFs,sub-energetic), and GRB cosmology. Fermi is exceptionally suited to provide, also in conjunction with Swift observations, a significant step forward in this field of research. Indeed, one of the main goals of Fermi/GBM is to make accurate measurements of Ep, by exploiting its unprecedented broad energy band from ~8 keV to ~30MeV; in addition, for a small fraction of GRBs, the LAT can extend the spectral measurements up to the GeV energy range, thus allowing a reliable estimate of the bolometric radiated energy/luminosity. We provide a review, an update and a discussion of the impact of Fermi observations in the investigation, understanding and testing of the Ep,i –Eiso (“Amati”) relation
New measurements of from gamma-ray bursts
Context: Data from cosmic microwave background radiation (CMB), baryon
acoustic oscillations (BAO), and supernovae Ia (SNe-Ia) support a constant dark
energy equation of state with . Measuring the evolution of
along the redshift is one of the most demanding challenges for observational
cosmology. Aims: We discuss the existence of a close relation for GRBs, named
Combo-relation, based on characteristic parameters of GRB phenomenology such as
the prompt intrinsic peak energy , the X-ray afterglow, the initial
luminosity of the shallow phase , the rest-frame duration of the
shallow phase, and the index of the late power-law decay . We use it
to measure and the evolution of the dark energy equation of state.
We also propose a new calibration method for the same relation, which reduces
the dependence on SNe Ia systematics. Methods: We have selected a sample of
GRBs with 1) a measured redshift ; 2) a determined intrinsic prompt peak
energy , and 3) a good coverage (0.3-10) keV afterglow light curves.
The fitting technique of the rest.frame (0.3-10) keV luminosity light curves
represents the core of the Combo-relation. We separate the early steep decay,
considered a part of the prompt emission, from the X-ray afterglow additional
component. Data with the largest positive residual, identified as flares, are
automatically eliminated until the p-value of the fit becomes greater than 0.3.
Results: We strongly minimize the dependency of the Combo-GRB calibration on
SNe Ia. We also measure a small extra-Poissonian scatter of the Combo-relation,
which allows us to infer from GRBs alone
(1) for the CDM cosmological model, and , for the flat-Universe
variable equation of state case.Comment: 10 pages, 9 figures, 3 tables. Accepted for publication in A&A.
Truncated abstract tex
Time delays between Fermi LAT and GBM light curves of GRBs
Most Gamma-Ray Bursts (GRBs) detected by the Fermi Gamma-ray Space Telescope
exhibit a delay of up to about 10 seconds between the trigger time of the hard
X-ray signal as measured by the Fermi GBM and the onset of the MeV-GeV
counterpart detected by the LAT. This delay may hint at important physics,
whether it is due to the intrinsic variability of the inner engine or it is
related to quantum dispersion effects in the velocity of light propagation from
the sources to the observer. It is critical to have a proper assessment of how
these time delays affect the overall properties of the light curves. We
cross-correlated the 5 brightest GRBs of the 1st Fermi LAT Catalog by means of
the continuous correlation function (CCF) and of the Discrete Correlation
Function (DCF). A maximum in the DCF suggests the presence of a time lag
between the curves, whose value and uncertainty are estimated through a
Gaussian fitting of the DCF profile and light curve simulation via a Monte
Carlo approach. The cross-correlation of the observed LAT and GBM light curves
yields time lags that are mostly similar to those reported in the literature,
but they are formally consistent with zero. The cross-correlation of the
simulated light curves yields smaller errors on the time lags and more than one
time lag for GRBs 090902B and 090926A; for all 5 GRBs, the time lags are
significantly different from zero and consistent with those reported in the
literature, when only the secondary maxima are considered for those two GRBs.
The DCF method evidences the presence of time lags between the LAT and GBM
light curves and underlines their complexity. While this suggests that the
delays should be ascribed to intrinsic physical mechanisms, more sensitivity
and larger statistics are needed to assess whether time lags are universally
present in the early GRB emission and which dynamical time scales they trace.Comment: 9 pages, 3 figures, accepted for publication in Astronomy &
Astrophysic
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