2,053 research outputs found
A Closer Look at a Gamma-Ray Burst
A study of gamma rays produced when stars collapse or collide reveals details
of the explosion mechanism, particularly the role of magnetic fields.Comment: 2 pages, 1 figure; Science perspective to Mundell et al. 2007,
Science, 315, 182
Strongly Polarized Optical Afterglows of Gamma-Ray Bursts
The optical afterglows of the gamma ray bursts can be strongly polarized, in
principle up to tens of percents, if: (i) the afterglow is synchrotron
radiation from an ultra-relativistic blast, (ii) the blast is beamed during the
afterglow phase, i.e. the shock propagates within a narrow jet, (iii) we
observe at the right time from the right viewing angle, (iv) magnetic fields
parallel and perpendicular to the jet have different proper strengths.Comment: 5 pages, submitted to Ap
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
Dust Echos from Gamma Ray Bursts
The deviation from the power-law decline of the optical flux observed in GRB
970228 and GRB 980326 has been used recently to argue in favor of the
connection between GRBs and supernovae. We consider an alternative explanation
for this phenomenon, based on the scattering of a prompt optical burst by 0.1
solar masses of dust located beyond its sublimation radius 0.1-1 pc from the
burst. In both cases, the optical energy observed at the time of the first
detection of the afterglow suffices to produce an echo after 20-30 days, as
observed. Prompt optical monitoring of future bursts and multiband photometry
of the afterglows will enable quantitative tests of simple models of dust
reprocessing and a prediction of the source redshift.Comment: 4 pages including 3 postscript figures, LaTeX (emulateapj.sty;
newapa.sty for bibliography definitions); accepted for publication in ApJ
Optical and infrared polarimetry of the transient LMXB Cen X-4 in quiescence
We present the first optical and infrared polarimetric study of the low mass
transient X-ray binary Cen X-4 during its quiescent phase. This work is aimed
to search for an intrinsic linear polarisation component in the system emitted
radiation that might be due, e.g., to synchrotron emission from a compact jet,
or to Thomson scattering with free electrons in an accretion disc. Multiband
(BVRI) optical polarimetric observations were obtained during two nights in
2008 at the ESO La Silla 3.6 m telescope (EFOSC2) in polarimetric mode. These
observations cover about the 30% of the 15.1 hours orbital period. J-band
observations were obtained in 2007 with the NICS (TNG) instrument at La Palma,
for a totality of 1 hour observation. We obtained 3-sigma upper limits to the
polarisation degree in all the optical bands, with the most constraining one
being in the I-band (P<0.5%). No phase-correlated variability has been noticed
in all the filters. The J-band observations provided a 6% upper limit on the
polarisation level. The constraining upper limits to the polarisation in the
optical allowed us to evaluate the contribution of the possible emission of a
relativistic particles jet to the total system radiation to be less then the
10%. This is in agreement with the observation of a spectral energy
distribution typical of a single black body of a K-spectral type main sequence
star irradiated from the compact object. Due to the low S/N ratio it was not
possible to investigate the possible dependency of the polarisation degree from
the wavelength, that could be suggestive of polarisation induced by Thomson
scattering of radiation with free electrons in the outer part of the accretion
disc. Observations with higher S/N ratio are required to examine in depth this
hypothesis, searching for significant phase-correlated variability.Comment: 7 pages, 9 figures, accepted for publication in section 7. Stellar
structure and evolution of Astronomy and Astrophysic
On the detection of very high redshift Gamma Ray Bursts with Swift
We compute the probability to detect long Gamma Ray Bursts (GRBs) at z>5 with
Swift, assuming that GRBs form preferentially in low-metallicity environments.
The model fits well both the observed BATSE and Swift GRB differential peak
flux distribution and is consistent with the number of z>2.5 detections in the
2-year Swift data. We find that the probability to observe a burst at z>5
becomes larger than 10% for photon fluxes P<1 ph s^{-1} cm^{-2}, consistent
with the number of confirmed detections. The corresponding fraction of z>5
bursts in the Swift catalog is ~10%-30% depending on the adopted metallicity
threshold for GRB formation. We propose to use the computed probability as a
tool to identify high redshift GRBs. By jointly considering promptly-available
information provided by Swift and model results, we can select reliable z>5
candidates in a few hours from the BAT detection. We test the procedure against
last year Swift data: only three bursts match all our requirements, two being
confirmed at z>5. Other three possible candidates are picked up by slightly
relaxing the adopted criteria. No low-z interloper is found among the six
candidates.Comment: 5 pages, 2 figures, MNRAS in pres
Theory of "Jitter" Radiation from Small-Scale Random Magnetic Fields and Prompt Emission from Gamma-Ray Burst Shocks
Abridged.-- We demonstrate that the radiation emitted by ultrarelativistic
electrons in highly nonuniform, small-scale magnetic fields is different from
synchrotron radiation if the electron's transverse deflections in these fields
are much smaller than the beaming angle. A quantitative analytical theory of
this radiation, which we refer to as jitter radiation, is developed. It is
shown that the emergent spectrum is determined by statistical properties of the
magnetic field. As an example,we then use the model of a magnetic field in
internal shocks of GRBs. The spectral power distribution of radiation produced
by the power-law electrons is well described by a sharply broken power-law with
indices 1 and -(p-1)/2 and the jitter break frequency is independent of the
field strength but depends on the electron density in the ejecta. Since
large-scale fields may also be present in the ejecta, we construct a
two-component, jitter+synchrotron spectral model of the prompt -ray
emission. Quite surprisingly, this model seems to be readily capable of
explaining several properties of time-resolved spectra of some GRBs, such as
(i) the violation of the constraint on the low-energy spectral index called the
synchrotron ``line of death'', (ii) the sharp spectral break at the peak
frequency, inconsistent with the broad synchrotron bump, (iii) the evidence for
two spectral sub-components, and (iv) possible existence of emission features
called ``GRB lines''. We believe these facts strongly support both the
existence of small-scale magnetic fields and the proposed radiation mechanism
from GRB shocks. As an example, we use the composite model to analyze GRB
910503 which has two spectral peaks.Comment: 12 pages (emulateapj), 11 figures (EPS), ApJ, accepted. For related
work, see http://cfa-www.harvard.edu/~mmedved
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