969 research outputs found
GRB 060218 and the outliers with respect to the Ep-Eiso correlation
GRB 031203 and GRB 980425 are the two outliers with respect to the Ep-Eiso
correlation of long GRBs. Recently Swift discovered a nearby extremely long GRB
060218 associated with a SN event. The spectral properties of this bursts are
striking: on the one hand its broad band SED presents both thermal and
non-thermal components which can be interpreted as due to the emission from the
hot cocoon surrounding the GRB jet and as standard synchrotron self absorbed
emission in the GRB prompt phase, respectively; on the other hand it is its
long duration and its hard--to--soft spectral evolution which make this
underluminous burst consistent with the Ep-Eiso correlation of long GRBs. By
comparing the available spectral informations on the two major outliers we
suggests that they might be twins of 060218 and, therefore, only apparent
outliers with respect to the Ep_Eiso correlation. This interpretation also
suggests that it is of primary importance the study the broad band spectra of
GRBs in order to monitor their spectral evolution throughout their complete
duration.Comment: To appear in the conference proceeding of the IV workshop on "Science
with the new generation of high energy Gamma-Ray Experiment", 20-22 June
2006, Isola d'Elb
Did we observe the supernova shock breakout in GRB 060218?
If the early optical data of GRB 060218 up to 1e5 s are interpreted as the
black-body flux associated with the supernova shock breakout, we can derive
lower limits to the bolometric luminosity and energetics of this black-body
component. These limits are more severe for the very early data that imply
energetics of order of 1e51 erg. These values, puzzlingly large, are rather
independent of the assumed time profile of the emitting surface, provided that
the corresponding radius does not increase superluminally. Another concern is
the luminosity of the black-body component observed in the X-rays, that is
large and appears to be produced by an approximately constant temperature and a
surface area increasing only slowly in time. Although it has been suggested
that the long X-ray black-body duration is consistent with the supernova shock
breakout if anisotropy is assumed, the nearly constant emitting surface
requires some fine tuning, allowing and suggesting an alternative
interpretation, i.e. emission from late dissipation of the fireball bulk
kinetic energy. This in turn requires a small value of the bulk Lorentz factor.Comment: 5 pages, 2 figures, revised version, MNRAS Letters, in pres
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&
Constraining the location of the emitting region in Fermi blazars through rapid gamma-ray variability
We consider the 1.5 years Fermi Large Area Telescope light curves (E > 100
MeV) of the flat spectrum radio quasars 3C 454.3 and PKS 1510-089, which show
high activity in this period of time. We characterise the duty cycle of the
source by comparing the time spent by the sources at different flux levels. We
consider in detail the light curves covering periods of extreme flux. The large
number of high-energy photons collected by LAT in these events allows us to
find evidence of variability on timescales of few hours. We discuss the
implications of significant variability on such short timescales, that
challenge the scenario recently advanced in which the bulk of the gamma-ray
luminosity is produced in regions of the jet at large distances (tens of
parsec) from the black hole.Comment: 5 pages, 5 figures, accpted for publication in MNRAS Letters
Evidence for anisotropy in the distribution of short-lived gamma-ray bursts
Measurements of the two-point angular correlation function w(\theta) for 407
short gamma-ray bursts collected in the Current BATSE Catalogue reveal a ~2
\sigma deviation from isotropy on angular scales \theta ~ 2-4 degrees. Such an
anisotropy is not observed in the distribution of long gamma-ray bursts and
hints to the presence of repeated bursts for up to ~13% of the sources under
exam. However, the available data cannot exclude the signal as due to the
presence of large-scale structure. Under this assumption, the amplitude of the
observed w(\theta) is compatible with those derived for different populations
of galaxies up to redshifts ~0.5, result that suggests short gamma-ray bursts
to be relatively local sources.Comment: 5 pages, 4 figures, submitted to MNRA
Gamma Ray Bursts: new rulers to measure the Universe
The best measure of the Universe should be done using a standard "ruler" at
any redshift. Type Ia Supernovae (SN Ia) probe the universe up to z1.5,
while the Cosmic Microwave Background (CMB) primary anisotropies concern
basically 1000. Apparently, Gamma--Ray Bursts (GRBs) are all but
standard candles. However, their emission is collimated and the
collimation--corrected energy correlates tightly with the frequency at which
most of the radiation of the prompt is emitted, as found by Ghirlanda et al.
(2004). Through this correlation we can infer the burst energy accurately
enough to probe the intermediate redshift () Universe. Using the best
known 15 GRBs we find very encouraging results that emphasize the cosmological
GRB role. A combined fit with SN Ia yields and
. Assuming in addition a flat Universe, the
parameters are constrained to be and
. GRBs accomplish the role of "missing link"
between SN Ia and CMB primary anisotropies. They can provide a new insight on
the cosmic effects of dark energy, complementary to the one supplied by CMB
secondary anisotropies through the Integrated Sachs Wolfe effect. The
unexpected Standard Candle cosmological role of GRBs motivates the most
optimistic hopes for what can be obtained when the GRB-dedicated satellite,
Swift, will be launched.Comment: 11 pages, 4 color figures, ApJ Letters (vol. 613) in pres
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