1,936 research outputs found
Selection effects shaping the Gamma Ray Burst redshift distributions
Long Gamma Ray Bursts hold the promise of probing star-formation and metal
enrichment up to very high redshifts. The present GRB samples with redshift
determination are largely incomplete and therefore a careful analysis of
selection effects is mandatory before any conclusion can be drawn from the
observed GRB redshift distribution. We study and compare three well defined
samples of long GRBs detected by Swift, HETE2 and BeppoSAX. We find that Swift
GRBs are slighly fainter and harder than BeppoSAX and HETE2 GRBs, as expected
due to the higher energy range in which Swift GRBs are detected and localized,
compared to BeppoSAX and HETE2. Gas and dust obscuration plays a role in
shaping the GRB samples and the present samples of GRBs with redshift. We argue
that the majority of the bright Swift GRBs without redshift might actually be
z<~2 events therefore the present Swift GRB sample with redshift is biased
against low-z GRBs. On the other hand, the detection of bright UV rest-frame
afterglows from high-z GRBs, and even from those with large X-ray obscuration,
implies a dust amount lower than in nearby GRBs,and/or a different dust
composition. If this is the case, the Swift sample of GRBs with redshifts is
probably a fair sample of the real high-z GRB population. The absence of high-z
GRBs in the BeppoSAX and HETE2 samples of GRBs with redshifts is probably due
to the fact at the time of BeppoSAX and HETE2 follow-up faint afterglows of
high redshift GRBs will have weaken below the spectroscopic capabilities of
even 10m class telescopes. The redshift distribution of a subsample of Swift
GRBs with distributions of peak-fluxes, X-ray obscuration and optical magnitude
at a fixed observing time similar to those of the BeppoSAX and HETE2 samples,
is roughly consistent with BeppoSAX+HETE2 redshift distribution.Comment: 9 pages, back to A&A after referee repor
Shallow decay phase of GRB X-ray afterglows from relativistic wind bubbles
The postburst object of a GRB is likely to be a highly magnetized, rapidly
rotating compact object (e.g., a millisecond magnetar), which could produce an
ultrarelativistic electron-positron-pair wind. The interaction of such a wind
with an outwardly expanding fireball ejected during the burst leads to a
relativistic wind bubble (RWB). We numerically calculate the dynamics and
radiative properties of RWBs and use this model to explain the shallow decay
phase of the early X-ray afterglows observed by Swift. We find that RWBs can
fall into two types: forward-shock-dominated and reverse-shock-dominated
bubbles. Their radiation during a period of seconds is
dominated by the shocked medium and the shocked wind, respectively, based on
different magnetic energy fractions of the shocked materials. For both types,
the resulting light curves always have a shallow decay phase. In addition, we
provide an example fit to the X-ray afterglows of GRB 060813 and GRB 060814 and
show that they could be produced by forward-shock-dominated and
reverse-shock-dominated bubbles, respectively. This implies that, for some
early afterglows (e.g., GRB 060814), the long-lasting reverse shock emission is
strong enough to explain their shallow decay phase.Comment: 5 pages, 4 figures, Accepted for Publication in A&
Confirming the gamma-ray burst spectral-energy correlations in the era of multiple time breaks
We test the spectral-energy correlation including the new bursts detected
(mostly) by Swift with firm measurements of their redshifts and peak energy.
The problem of identifying the jet breaks is discussed in the complex and
multibreak/flaring X-ray light curves observed by Swift. We use the optical
data as the most reliable source for the identification of the jet break, since
the X-ray flux may be produced by a mechanism different from the external
shocks between the fireball and the circumburst medium, which are responsible
for the optical afterglow. We show that the presence of an underlying SN event
in XRF 050416A requires a break to occur in the afterglow optical light curve
at around the expected jet break time. The possible presence of a jet break in
the optical light curve of GRB 050401 is also discussed. We point out that, for
measuring the jet break, it is mandatory that the optical light curve extends
after the epoch where the jet break is expected. The interpretation of the
early optical breaks in GRB 050922C and GRB 060206 as jet breaks is
controversial because they might instead correspond to the flat-to-steep decay
transition common in the early X-ray light curves. All the 16 bursts coming
from Swift are consistent with the E_p-E_gamma and E_p-E_iso-t_jet correlation.
No outlier is found to date. Moreover, the small dispersion of these
correlations, confirmed also by the Swift bursts, strengthens the case of using
GRBs as standard candles.Comment: 12 pages, 1 table, 8 figures. Accepted for publication in A&
The Dark Bursts population in a complete sample of bright Swift Long Gamma-Ray Bursts
We study the properties of the population of optically dark events present in
a carefully selected complete sample of bright Swift long gamma-ray bursts. The
high level of completeness in redshift of our sample (52 objects out of 58)
allow us to establish the existence of a genuine dark population and we are
able to estimate the maximum fraction of dark burst events (~30%) expected for
the whole class of long gamma-ray burst. The redshift distribution of this
population of dark bursts is similar to the one of the whole sample.
Interestingly, the rest-frame X-ray luminosity (and the de-absorbed X-ray flux)
of the sub-class of dark bursts is slightly higher than the average luminosity
of the non-dark events. At the same time the prompt properties do not differ
and the optical flux of dark events is at the lower tail of the optical flux
distribution, corrected for Galactic absorption. All these properties suggest
that dark bursts events generate in much denser environments with respect to
normal bright events. We can therefore exclude the high-z and the low-density
scenarios and conclude that the major cause of the origin of optically dark
events is the dust extinction.Comment: 8 pages, 4 figures, 3 tables. This version matches the published
version in MNRA
Possible changes of state and relevant timescales for a neutron star in LS I +61{\deg}303
The properties of the short, energetic bursts recently observed from the
gamma-ray binary LS I +61{\deg}303, are typical of those showed by high
magnetic field neutron stars, and thus provide a strong indication in favor of
a neutron star being the compact object in the system. Here, we discuss the
transitions among the states accessible to a neutron star in a system like LS I
+61{\deg}303, such as the ejector, propeller and accretor phases, depending on
the NS spin period, magnetic field and rate of mass captured. We show how the
observed bolometric luminosity (>= few x 1E35 erg/s), and its broad-band
spectral distribution, indicate that the compact object is most probably close
to the transition between working as an ejector all along its orbit, and being
powered by the propeller effect when it is close to the orbit periastron, in a
so-called flip-flop state. By assessing the torques acting onto the compact
object in the various states, we follow the spin evolution of the system,
evaluating the time spent by the system in each of them. Even taking into
account the constraint set by the observed gamma-ray luminosity, we found that
the total age of the system is compatible with being ~5-10 kyr, comparable to
the typical spin-down ages of high-field neutron stars. The results obtained
are discussed in the context of the various evolutionary stages expected for a
neutron star with a high mass companion.Comment: 12 pages, 6 figures. Accepted for publication in Ap
Swift captures the spectrally evolving prompt emission of GRB 070616
The origins of Gamma-ray Burst prompt emission are currently not well
understood and in this context long, well-observed events are particularly
important to study. We present the case of GRB 070616, analysing the
exceptionally long-duration multipeaked prompt emission, and later afterglow,
captured by all the instruments on-board Swift and by Suzaku WAM. The high
energy light curve remained generally flat for several hundred seconds before
going into a steep decline. Spectral evolution from hard to soft is clearly
taking place throughout the prompt emission, beginning at 285 s after the
trigger and extending to 1200 s. We track the movement of the spectral peak
energy, whilst observing a softening of the low energy spectral slope. The
steep decline in flux may be caused by a combination of this strong spectral
evolution and the curvature effect. We investigate origins for the spectral
evolution, ruling out a superposition of two power laws and considering instead
an additional component dominant during the late prompt emission. We also
discuss origins for the early optical emission and the physics of the
afterglow. The case of GRB 070616 clearly demonstrates that both broadband
coverage and good time resolution are crucial to pin down the origins of the
complex prompt emission in GRBs.Comment: 13 pages, 11 figures (2 in colour), MNRAS accepte
United classification of cosmic gamma-ray bursts and their counterparts
United classification of gamma-ray bursts and their counterparts is
established on the basis of measured characteristics: photon energy E and
emission duration T. The founded interrelation between the mentioned
characteristics of events consists in that, as the energy increases, the
duration decreases (and vice versa). The given interrelation reflects the
nature of the phenomenon and forms the E-T diagram, which represents a natural
classification of all observed events in the energy range from 10E9 to 10E-6 eV
and in the corresponding interval of durations from about 10E-2 up to 10E8 s.
The proposed classification results in the consequences, which are principal
for the theory and practical study of the phenomenon.Comment: Keywords Gamma rays: burst
Diversity of multiwavelength emission bumps in the GRB 100219A afterglow
Context. Multi-wavelength observations of gamma-ray burst (GRB) afterglows
provide important information about the activity of their central engines and
their environments. In particular, the short timescale variability, such as
bumps and/or rebrightening features visible in the multi-wavelength light
curves, is still poorly understood.
Aims. We analyze the multi-wavelength observations of the GRB100219A
afterglow at redshift 4.7. In particular, we attempt to identify the physical
origin of the late achromatic flares/bumps detected in the X-ray and optical
bands.
Methods. We present ground-based optical photometric data and Swift X-ray
observations on GRB100219A. We analyzed the temporal behavior of the X-ray and
optical light curves, as well as the X-ray spectra.
Results. The early flares in the X-ray and optical light curves peak
simultaneously at about 1000 s after the burst trigger, while late achromatic
bumps in the X-ray and optical bands appear at about 20000 s after the burst
trigger. These are uncommon features in the afterglow phenomenology.
Considering the temporal and spectral properties, we argue that both optical
and X-ray emissions come from the same mechanism. The late flares/bumps may be
produced by late internal shocks from long-lasting activity of the central
engine. An off-axis origin for a structured jet model is also discussed to
interpret the bump shapes. The early optical bump can be interpreted as the
afterglow onset, while the early X-ray flare could be caused by the internal
activity. GRB 100219A exploded in a dense environment as revealed by the strong
attenuation of X-ray emission and the optical-to-X-ray spectral energy
distribution.Comment: A&A accepte
GRB 060313: A New Paradigm for Short-Hard Bursts?
We report the simultaneous observations of the prompt emission in the
gamma-ray and hard X-ray bands by the Swift-BAT and the KONUS-Wind instruments
of the short-hard burst, GRB 060313. The observations reveal multiple peaks in
both the gamma-ray and hard X-ray bands suggesting a highly variable outflow
from the central explosion. We also describe the early-time observations of the
X-ray and UV/Optical afterglows by the Swift XRT and UVOT instruments. The
combination of the X-ray and UV/Optical observations provide the most
comprehensive lightcurves to date of a short-hard burst at such an early epoch.
The afterglows exhibit complex structure with different decay indices and
flaring. This behavior can be explained by the combination of a structured jet,
radiative loss of energy, and decreasing microphysics parameters occurring in a
circum-burst medium with densities varying by a factor of approximately two on
a length scale of 10^17 cm. These density variations are normally associated
with the environment of a massive star and inhomogeneities in its windy medium.
However, the mean density of the observed medium (n approximately 10^−4
cm^3) is much less than that expected for a massive star. Although the collapse
of a massive star as the origin of GRB 060313 is unlikely, the merger of a
compact binary also poses problems for explaining the behavior of this burst.
Two possible suggestions for explaining this scenario are: some short bursts
may arise from a mechanism that does not invoke the conventional compact binary
model, or soft late-time central engine activity is producing UV/optical but no
X-ray flaring.Comment: 28 pages, 6 figures. Accepted for publication in ApJ. Clarifications
made and typos correcte
On the consistency of the peculiar GRBs 060218 and 060614 with the Ep,i - Eiso correlation
We analyze and discuss the position of GRB 060218 and GRB 060614 in the Ep,i
- Eiso plane. GRB 060218 is important because of its similarity with GRB
980425, the proto-type event of the GRB-SN connection. While GRB 980425 is an
outlier of the Ep,i - Eiso correlation, we find that GRB 060218 is fully
consistent with it. This evidence, combined with the `chromatic' behavior of
the afterglow light curves, is at odds with the hypothesis that GRB 060218 was
a `standard' GRB seen off-axis and supports the existence of a class of truly
sub--energetic GRBs. GRB 060614 is a peculiar event not accompanied by a bright
Supernova. Based on published spectral information, we find that also this
event is consistent with the Ep,i - Eiso correlation. We discuss the
implications of our results for the rate of sub--energetic GRBs, the GRB/SN
connection and the properties of the newly discovered sub-class of long GRBs
not associated with bright Supernovae. We have included in our analysis other
recent GRBs with clear evidence (or clear no evidence) of associated SNe.Comment: 9 pages, 4 figures, FINAL REVISED VERSION (added Figure 4 and short
GRBs in Figure 1; minor changes and style corrections applied; references
updated), submitted to A&A on November 25, 200
- …