554 research outputs found
The Afterglows of Swift-era Gamma-ray Bursts. I. Comparing pre-Swift and Swift-era Long/Soft (Type II) GRB Optical Afterglows
We have gathered optical photometry data from the literature on a large sample of Swift-era gamma-ray burst (GRB) afterglows including GRBs up to 2009 September, for a total of 76 GRBs, and present an additional three pre-Swift GRBs not included in an earlier sample. Furthermore, we publish 840 additional new photometry data points on a total of 42 GRB afterglows, including large data sets for GRBs 050319, 050408, 050802, 050820A, 050922C, 060418, 080413A, and 080810. We analyzed the light curves of all GRBs in the sample and derived spectral energy distributions for the sample with the best data quality, allowing us to estimate the host-galaxy extinction. We transformed the afterglow light curves into an extinction-corrected z = 1 system and compared their luminosities with a sample of pre-Swift afterglows. The results of a former study, which showed that GRB afterglows clustered and exhibited a bimodal distribution in luminosity space, are weakened by the larger sample. We found that the luminosity distribution of the two afterglow samples (Swift-era and pre-Swift) is very similar, and that a subsample for which we were not able to estimate the extinction, which is fainter than the main sample, can be explained by assuming a moderate amount of line-of-sight host extinction. We derived bolometric isotropic energies for all GRBs in our sample, and found only a tentative correlation between the prompt energy release and the optical afterglow luminosity at 1 day after the GRB in the z = 1 system. A comparative study of the optical luminosities of GRB afterglows with echelle spectra (which show a high number of foreground absorbing systems) and those without, reveals no indication that the former are statistically significantly more luminous. Furthermore, we propose the existence of an upper ceiling on afterglow luminosities and study the luminosity distribution at early times, which was not accessible before the advent of the Swift satellite. Most GRBs feature afterglows that are dominated by the forward shock from early times on. Finally, we present the first indications of a class of long GRBs, which form a bridge between the typical high-luminosity, high-redshift events and nearby low-luminosity events (which are also associated with spectroscopic supernovae) in terms of energetics and observed redshift distribution, indicating a continuous distribution overal
Opening angles, Lorentz factors and confinement of X-ray binary jets
We present a collation of the available data on the opening angles of jets in
X-ray binaries, which in most cases are small (less than 10 degrees). Under the
assumption of no confinement, we calculate the Lorentz factors required to
produce such small opening angles via the transverse relativistic Doppler
effect. The derived Lorentz factors, which are in most cases lower limits, are
found to be large, with a mean greater than 10, comparable to those estimated
for AGN and much higher than the commonly-assumed values for X-ray binaries of
2 to 5. Jet power constraints do not in most cases rule out such high Lorentz
factors. The upper limits on the opening angles show no evidence for smaller
Lorentz factors in the steady jets of Cygnus X-1 and GRS 1915+105. In those
sources in which deceleration has been observed (notably XTE J1550-564 and
Cygnus X-3), some confinement of the jets must be occurring, and we briefly
discuss possible confinement mechanisms. It is however possible that all the
jets could be confined, in which case the requirement for high bulk Lorentz
factors can be relaxed.Comment: 11 pages, 4 figures (2 colour), accepted for publication in MNRA
GRB afterglow light curves in the pre-Swift era - a statistical study
We present the results of a systematic analysis of the world sample of
optical/near-infrared afterglow light curves observed in the pre-Swift era by
the end of 2004. After selecting the best observed 16 afterglows with
well-sampled light curves that can be described by a Beuermann equation, we
explore the parameter space of the light curve parameters and physical
quantities related to them. In addition, we search for correlations between
these parameters and the corresponding gamma-ray data, and we use our data set
to look for a fine structure in the light curves.Comment: accepted for publication in ApJ; Version 2: minor changes, one figure
adde
The Cosmic Ray Precursor of Relativistic Collisionless Shocks: A Missing Link in Gamma-Ray Burst Afterglows
Collisionless shocks are commonly argued to be the sites of cosmic ray (CR)
acceleration. We study the influence of CRs on weakly magnetized relativistic
collisionless shocks and apply our results to external shocks in gamma-ray
burst (GRB) afterglows. The common view is that the transverse Weibel
instability (TWI) generates a small-scale magnetic field that facilitates
collisional coupling and thermalization in the shock transition. The TWI field
is expected to decay rapidly, over a finite number of proton plasma skin depths
from the transition. However, the synchrotron emission in GRB afterglows
suggests that a strong and persistent magnetic field is present in the plasma
that crosses the shock; the origin of this field is a key open question. Here
we suggest that the common picture involving TWI demands revision. Namely, the
CRs drive turbulence in the shock upstream on scales much larger than the skin
depth. This turbulence generates a large-scale magnetic field that quenches TWI
and produces a magnetized shock. The new field efficiently confines CRs and
enhances the acceleration efficiency. The CRs modify the shocks in GRB
afterglows at least while they remain relativistic. The origin of the magnetic
field that gives rise to the synchrotron emission is plausibly in the CR-driven
turbulence. We do not expect ultrahigh energy cosmic ray production in external
GRB shocks.Comment: 6 pages, 1 figur
Inhomogeneity in the Supernova Remnant Distribution as the Origin of the PAMELA Anomaly
Recent measurements of the positron/electron ratio in the cosmic ray (CR)
flux exhibits an apparent anomaly, whereby this ratio increases between 10 and
100 GeV. We show that inhomogeneity of CR sources on a scale of order a kpc,
can naturally explain this anomaly. If the nearest major CR source is about a
kpc away, then low energy electrons ( GeV) can easily reach us. At
higher energies ( GeV), the source electrons cool via synchrotron
and inverse-Compton before reaching Earth. Pairs formed in the local vicinity
through the proton/ISM interactions can reach Earth also at high energies, thus
increasing the positron/electron ratio. A natural origin of source
inhomogeneity is the strong concentration of supernovae in the galactic spiral
arms. Assuming supernova remnants (SNRs) as the sole primary source of CRs, and
taking into account their concentration near the galactic spiral arms, we
consistently recover the observed positron fraction between 1 and 100 GeV.
ATIC's electron excess at GeV is explained, in this picture, as the
contribution of a few known nearby SNRs. The apparent coincident similarity
between the cooling time of electrons at 10 GeV (where the positron/electron
ratio upturn), Myr, and the CRs protons cosmogenic age at the same
energy is predicted by this model
Polarization in the prompt emission of gamma-ray bursts and their afterglows
Synchrotron is considered the dominant emission mechanism in the production
of gamma-ray burst photons in the prompt as well as in the afterglow phase.
Polarization is a characteristic feature of synchrotron and its study can
reveal a wealth of information on the properties of the magnetic field and of
the energy distribution in gamma-ray burst jets. In this paper I will review
the theory and observations of gamma-ray bursts polarization. While the theory
is well established, observations have prove difficult to perform, due to the
weakness of the signal. The discriminating power of polarization observations,
however, cannot be overestimated.Comment: 16 pages, 9 figures, accepted for publication in the New Journal of
Physics focus issue on Gamma Ray Burst
Detection of a very bright optical flare from a gamma-ray burst at redshift 6.29
In this letter we discuss the flux and the behavior of the bright optical
flare emission detected by the 25 cm TAROT robotic telescope during the prompt
high-energy emission and the early afterglow. We combine our data with
simultaneous observations performed in X-rays and we analyze the broad-band
spectrum. These observations lead us to emphasize the similarity of GRB 050904
with GRB 990123, a remarkable gamma-ray burst whose optical emission reached
9th magnitude. While GRB 990123 was, until now, considered as a unique event,
this observation suggests the existence of a population of GRBs which have very
large isotropic equivalent energies and extremely bright optical counterparts.
The luminosity of these GRBs is such that they are easily detectable through
the entire universe. Since we can detect them to very high redshift even with
small aperture telescopes like TAROT, they will constitute powerful tools for
the exploration of the high-redshift Universe and might be used to probe the
first generation of stars.Comment: 9 pages, 3 figures. Accepted in ApJ
Pure and loaded fireballs in SGR giant flares
On December 27, 2004, a giant flare from SGR 180620 was detected on earth.
Its thermal spectrum and temperature suggest that the flare resulted from an
energy release of about erg/sec close to the surface of a neutron
star in the form of radiation and/or pairs. This plasma expanded under its own
pressure producing a fireball and the observed gamma-rays escaped once the
fireball became optically thin. The giant flare was followed by a bright radio
afterglow, with an observable extended size, implying an energetic relativistic
outflow. We revisit here the evolution of relativistic fireballs and we
calculate the Lorentz factor and energy remaining in relativistic outflow once
the radiation escapes. We show that pairs that arise naturally in a pure
pairs-radiation fireball do not carry enough energy to account for the observed
afterglow. We consider various alternatives and we show that if the
relativistic outflow that causes the afterglow is related directly to the
prompt flare, then the initial fireball must be loaded by baryons or Poynting
flux. While we focus on parameters applicable to the giant flare and the radio
afterglow of SGR 180620 the calculations presented here might be also
applicable to GRBs
Implications of the -ray Polarization of GRB 021206
We compare two possible scenarios for the producing of high level of
polarization within the prompt emission of a GRB: synchrotron emission from a
relativistic jet with a uniform (in space and time) magnetic field and
synchrotron emission from a jet with a random magnetic field in the plane of
the shock. Somewhat surprisingly we find that both scenarios can produce a
comparable level of polarization (% for the uniform field and % for a random field). Uniform time independent field most naturally
arises by expansion of the field from the compact object. It requires a
G field at the source and a transport of the field as . It {\it does not} imply Poynting flux domination of the energy of the
wind. There is a serious difficulty however, within this scenario, accounting
for particle acceleration (which requires random magnetic fields) both for
Poynting flux and non-Poynting flux domination. Significant polarization can
also arise from a random field provided that the observer is located within
orientation from a narrow () jet. While most
jets are wider, the jet of GRB 021206 from which strong polarization was
recently observed, was most likely very narrow. GRB 021206 is among the
strongest bursts ever. Adopting the energy-angle relation we find an estimated
angle of rad or even smaller. Thus, for this particular burst the
required geometry is not unusual. We conclude that the RHESSI observations
suggest that the prompt emission results from synchrotron radiation. However,
in view of the comparable levels of polarizations predicted by both the random
field and the homogeneous field scenarios these observations are insufficient
to rule out or confirm either one.Comment: 14 pages, 4 figure
No visible optical variability from a relativistic blast wave encountering a wind-termination shock
Gamma-ray burst afterglow flares and rebrightenings of the optical and X-ray
light curve have been attributed to both late time inner engine activity and
density changes in the medium surrounding the burster. To test the latter, we
study the encounter between the relativistic blast wave from a gamma-ray
burster and a stellar wind termination shock. The blast wave is simulated using
a high performance adaptive mesh relativistic hydrodynamics code, AMRVAC, and
the synchrotron emission is analyzed in detail with a separate radiation code.
We find no bump in the resulting light curve, not even for very high density
jumps. Furthermore, by analyzing the contributions from the different shock
wave regions we are able to establish that it is essential to resolve the blast
wave structure in order to make qualitatively correct predictions on the
observed output and that the contribution from the reverse shock region will
not stand out, even when the magnetic field is increased in this region by
repeated shocks. This study resolves a controversy in recent literature.Comment: 4 figures, submitted to MNRAS letter
- âŠ