1,076 research outputs found
Intrinsic Parameters of GRB990123 from Its Prompt Optical Flash and Afterglow
We have constrained the intrinsic parameters, such as the magnetic energy
density fraction (), the electron energy density fraction
(), the initial Lorentz factor () and the Lorentz factor
of the reverse external shock (), of GRB990123, in terms of the
afterglow information (forward shock model) and the optical flash information
(reverse shock model). Our result shows: 1) the inferred values of
and are consistent with the suggestion that they may be universal
parameters, comparing to those inferred for GRB970508; 2) the reverse external
shock may have become relativistic before it passed through the ejecta shell.
Other instrinsic parameters of GRB990123, such as energy contained in the
forward shock and the ambient density are also determined and discussed
in this paper.Comment: 5 pages, MN LaTeX style, a few changes made according to referee's
suggestions, references up dated, MNRAS accepte
GRANAT/WATCH catalogue of cosmic gamma-ray bursts: December 1989 to September 1994
We present the catalogue of gamma-ray bursts observed with the WATCH all-sky
monitor on board the GRANAT satellite during the period December 1989 to
September 1994. The cosmic origin of 95 bursts comprising the catalogue is
confirmed either by their localization with WATCH or by their detection with
other GRB experiments. For each burst its time history and information on its
intensity in the two energy ranges 8-20 keV and 20-60 keV are presented. Most
events show hardening of the energy spectrum near the burst peak. In part of
the bursts an X-ray precursor or a tail is seen at 8-20 keV. We have determined
the celestial positions of the sources of 47 bursts. Their localization regions
(at 3-sigma confidence level) are equivalent in area to circles with radii
ranging from 0.2 to 1.6 deg. The burst sources appear isotropically distributed
on the sky on large angular scales.Comment: 18 pages (including 3 tables and 7 figures), LaTeX, l-aa style.
Accepted by Astronomy and Astrophysics Suppl. Serie
Gamma-ray bursts: afterglows from cylindrical jets
Nearly all previous discussion on beaming effects in GRBs have assumed a
conical geometry. However, more and more observations on relativistic jets in
radio galaxies, active galactic nuclei, and "microquasars" in the Galaxy have
shown that many of these outflows are not conical, but cylindrical, i.e., they
maintain constant cross sections at large scales. Thus it is necessary to
discuss the possibility that GRBs may be due to highly collimated cylindrical
jets, not conical ones. Here we study the dynamical evolution of cylindrical
jets and discuss their afterglows. Both analytical and numerical results are
presented. It is shown that when the lateral expansion is not taken into
account, a cylindrical jet typically remains to be highly relativistic for \sim
10^8 - 10^9 s. During this relativistic phase, the optical afterglow decays as
\propto t^{-p/2} at first, where p is the index characterizing the power-law
energy distribution of electrons. Then the light curve steepens to be \propto
t^{-(p+1)/2} due to cooling of electrons. After entering the non-relativistic
phase (i.e., t > 10^{11} s), the afterglow is \propto t^{-(5p-4)/6}. But if the
cylindrical jet expands laterally at co-moving sound speed, then the decay
becomes \propto t^{-p} and \propto t^{-(15p-21)/10} - t^{-(15p-20)/10} in the
ultra-relativistic and non-relativistic phase respectively. Note that in both
cases, the light curve turns flatter after the relativistic-Newtonian
transition point, which differs markedly from the behaviour of a conical jet.
It is suggested that some GRBs with afterglows decaying as t^{-1.1} - t^{-1.3}
may be due to cylindrical jets, not necessarily isotropic fireballs.Comment: 19 pages(use mn.sty), 9 eps figures, MNRAS accepte
On the Optical Light Curves of Afterglows from Jetted Gamma-ray Burst Ejecta: Effects of Parameters
Due to some refinements in the dynamics, we can follow the overall evolution
of a realistic jet numerically till its bulk velocity being as small as . We find no obvious break in the optical light curve during
the relativistic phase itself. However, an obvious break does exist at the
transition from the relativistic phase to the non-relativistic phase, which
typically occurs at time --- s (i.e., 10 --- 30 d).
The break is affected by many parameters, such as the electron energy fraction
, the magnetic energy fraction , the initial half
opening angle , and the medium number density . Increase of any of
them to a large enough value will make the break disappear. Although the break
itself is parameter-dependent, afterglows from jetted GRB remnants are
uniformly characterized by a quick decay during the non-relativistic phase,
with power law timing index . This is quite different from
that of isotropic fireballs, and may be of fundamental importance for
determining the degree of beaming in -ray bursts observationally.Comment: 8 pages, 10 embedded eps figures, MNRAS in press, using epsfig.sty
and mn.st
Failed gamma-ray bursts and orphan afterglows
It is believed that orphan afterglow searches can help to measure the beaming
angle in gamma-ray bursts (GRBs). Great expectations have been put on this
method. We point out that the method is in fact not as simple as we originally
expected. Due to the baryon-rich environment that is common to almost all
popular progenitor models, there should be many failed gamma-ray bursts, i.e.,
fireballs with Lorentz factor much less than 100 -- 1000, but still much larger
than unity. In fact, the number of failed gamma-ray bursts may even be much
larger than that of successful bursts. Owing to the existence of these failed
gamma-ray bursts, there should be many orphan afterglows even if GRBs are due
to isotropic fireballs, then the simple discovery of orphan afterglows never
means that GRBs be collimated. Unfortunately, to distinguish a failed-GRB
orphan and a jetted but off-axis GRB orphan is not an easy task. The major
problem is that the trigger time is unknown. Some possible solutions to the
problem are suggested.Comment: 6 pages, 4 eps figures, a short paragraph added at the end of the
main text, a slightly different version will appear in MNRAS 200
X-Ray Flares and Oscillations from the Black Hole Candidate X-Ray Transient XTE J1650-500 at Low Luminosity
We report on X-ray observations made with the Rossi X-ray Timing Explorer of
the black hole candidate (BHC) transient XTE J1650-500 at the end of its first,
and currently only, outburst. By monitoring the source at low luminosities over
several months, we found 6 bright ~100 second X-ray flares and long time scale
oscillations of the X-ray flux. The oscillations are aperiodic with a
characteristic time scale of 14.2 days and an order of magnitude variation in
the 2.8-20 keV flux. The oscillations may be related to optical
"mini-outbursts" that have been observed at the ends of outbursts for other
short orbital period BHC transients. The X-ray flares have durations between 62
and 215 seconds and peak fluxes that are 5-24 times higher than the persistent
flux. The flares have non-thermal energy spectra and occur when the persistent
luminosity is near 3E34 (d/4 kpc)^2 erg/s (2.8-20 keV). The rise time for the
brightest flare demonstrates that physical models for BHC systems must be able
to account for the situation where the X-ray flux increases by a factor of up
to 24 on a time scale of seconds. We discuss the flares in the context of
observations and theory of Galactic BHCs and compare the flares to those
detected from Sgr A*, the super-massive black hole at the Galactic center. We
also compare the flares to X-ray bursts that are seen in neutron star systems.
While some of the flare light curves are similar to those of neutron star
bursts, the flares have non-thermal energy spectra in contrast to the blackbody
spectra exhibited in bursts. This indicates that X-ray bursts should not be
taken as evidence that a given system contains a neutron star unless the
presence of a blackbody component in the burst spectrum can be demonstrated.Comment: 9 pages, accepted by Ap
Los atentados del 11 de marzo de 2004: Análisis de la comunicación política y sus efectos en la opinión pública.
El 11 de marzo de 2004 un ataque terrorista en varias líneas de trenes de la red de Cercanías de Madrid conmocionaba a la opinión pública. El Gobierno atribuyó a ETA la autoría, suponiendo que era una respuesta al Pacto Antiterrorista que se había firmado en 2000. El 14 de marzo, el Partido Socialista ganaba las elecciones, a pesar de los sondeos electorales, que daban la victoria al Partido Popular. En este trabajo se analiza el proceso comunicativo que tuvo lugar entre los días 11, 12 y 13 de marzo y se comprueba el efecto que dicho proceso provocó en la opinión pública, plasmado en las urnas
Tuberculosis vaccine candidates based on mycobacterial cell envelope components
Even after decades searching for a new and more effective vaccine against tuberculosis, the scientific community is still pursuing this goal due to the complexity of its causative agent, Mycobacterium tuberculosis (Mtb). Mtb is a microorganism with a robust variety of survival mechanisms that allow it to remain in the host for years. The structure and nature of the Mtb envelope play a leading role in its resistance and survival. Mtb has a perfect machinery that allows it to modulate the immune response in its favor and to adapt to the host's environmental conditions in order to remain alive until the moment to reactivate its normal growing state. Mtb cell envelope protein, carbohydrate and lipid components have been the subject of interest for developing new vaccines because most of them are responsible for the pathogenicity and virulence of the bacteria. Many indirect evidences, mainly derived from the use of monoclonal antibodies, support the potential protective role of Mtb envelope components. Subunit and DNA vaccines, lipid extracts, liposomes and membrane vesicle formulations are some examples of technologies used, with encouraging results, to evaluate the potential of these antigens in the protective response against Mtb.Fil: Sarmiento, M.E.. Universiti Sains Malaysia; MalasiaFil: Alvarez, N.. Public Health Research Institute; Estados UnidosFil: Chin, K.L.. Universiti Sains Malaysia; MalasiaFil: Bigi, Fabiana. Instituto Nacional de Tecnología Agropecuaria. Centro de Investigación en Ciencias Veterinarias y Agronómicas. Instituto de Agrobiotecnología y Biología Molecular. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Agrobiotecnología y Biología Molecular; ArgentinaFil: Tirado, Y.. No especifíca;Fil: García, M.A.. No especifíca;Fil: Anis, F.Z.. Universiti Sains Malaysia; MalasiaFil: Norazmi, M.N.. Universiti Sains Malaysia; MalasiaFil: Acosta, A.. Universiti Sains Malaysia; Malasi
- …