1,224 research outputs found
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
Does Low Frequency X-ray QPO Behavior in GRS 1915+105 Influence Subsequent X-ray and Infrared Evolution?
Using observations with the Rossi X-ray Timing Explorer, we examine the
behavior of 2-10 Hz quasi-periodic oscillations (QPOs) during spectrally-hard
dips in the x-ray light curve of GRS 1915+105 that are accompanied by infrared
flares. Of the twelve light-curves examined, nine are beta-class and three are
alpha-class following the scheme of Belloni et al. (2000). In most cases, the
QPO frequency is most strongly correlated to the power law flux, which
partially contradicts some earlier claims that the strongest correlation is
between QPO frequency and blackbody flux. Seven beta-class curves are highly
correlated to blackbody features. In several cases, the QPO evolution appears
to decouple from the spectral evolution. We find that beta-class light-curves
with strong correlations can be distinguished from those without by their
``trigger spike'' morphology. We also show that the origin and strength of the
subsequent infrared flare may be causally linked to the variations in QPO
frequency evolution and not solely tied to the onset of soft x-ray flaring
behavior. We divide the twelve alpha- and beta-class light-curves into three
groups based on the evolution of the QPO, the morphology of the trigger spike,
and the infrared flare strength. An apparent crossover case leads us to
conclude that these groups are not unique modes but represent part of a
continuum of accretion behaviors. We believe the QPO behavior at the initiation
of the hard dip can ultimately be used to determine the terminating x-ray
behavior, and the following infrared flaring behavior.Comment: 29 pages, 9 figures, to be published in Ap
La desinstitucionalización y la crisis de las instituciones
.Hablar de instituciones se ha convertido en hablar de crisis de las instituciones. En este sentido, la noción de desinstitucionalización ocupa un lugar relevante en la reflexión institucional. Por este motivo, en este artÃculo se lleva a cabo una revisión del concepto de institución y de las principales crÃticas que ha recibido como paso previo al análisis más detallado del fenómeno desinstitucionalizador en la práctica psiquiátrica. Tal análisis lleva a la elaboración de una propuesta alternativa que cuestiona la pertinencia de seguir hablando de instituciones asà como de procesos desinstitucionalizadores. Concretamente, se plantea que las especiales caracterÃsticas de algunas prácticas psiquiátricas contemporáneas se ajustan mejor a la noción de ex-titución tomada de la obra de Serres
INTEGRAL observation of 3EG J1736-2908
The possible identification by INTEGRAL of the EGRET source 3EG J1736-2908
with the active galactic nucleus GRS 1734-292 is discussed. The latter was
discovered in 1990 and later identified with a Seyfert 1 galaxy. At the time of
the compilation of the 3rd EGRET Catalog, it was not considered as a possible
counterpart of the source 3EG J1736-2908, which remained unidentified. A
detailed multiwavelength study of the EGRET error circle is presented, by
including archival radio, soft- and hard-X observations, suggesting that GRS
1734-292 could be a likely counterpart of 3EG J1736-2908, even though this
poses very interesting questions about the production mechanisms of gamma-rays
with energies greater than 100 MeV.Comment: 6 pages, 3 figures. Accepted for publication on A&A Main Journa
GRB010222: afterglow emission from a rapidly decelerating shock
The GRB010222 optical and near-infrared (NIR) afterglow was monitored at the
TNG and other Italian telescopes starting ~1 day after the high-energy prompt
event. The optical BVR light curves, which are the best sampled, are
continuously steepening, and can be described by two power laws, f(t) =
t^(-alpha), of indices alpha_1 ~ 0.7 and alpha_2 ~ 1.3 before and after a break
occurring at about 0.5 days after the GRB start time, respectively. This model
accounts well also for the flux in the U, I and J bands, which are less well
monitored. The temporal break appears to be achromatic. The two K-band points
are not consistent with the above behaviour, and rather suggest a constant
trend. A low-resolution optical spectrum has also been taken with TNG. In the
optical spectrum we found three absorption systems at different redshifts
(0.927, 1.155 and 1.475), the highest of which represents a lower limit to, and
probably coincides with, the redshift of this GRB. The broad-band optical
spectral energy distributions do not appear to vary with time, consistently
with the achromatic behaviour of the light curves. We compare our measurements
with different afterglow evolution scenarios and we find that they favor a
transition from relativistic to non-relativistic conditions in the shock
propagation.Comment: Accepted by Astronomy and Astrophysics; modified according to
referee's comments. Two figures added, U-band photometry corrected,
hydrodynamic description of the afterglow revised, host galaxy absorption
considered, references adde
An optical study of the GRB 970111 field beginning 19 hours after the Gamma-Ray Burst
We present the results of the monitoring of the GRB 970111 field that started
19 hours after the event. This observation represents the fastest ground-based
follow-up performed for GRB 970111 in all wavelengths. As soon as the detection
of the possible GRB 970111 X-ray afterglow was reported by Feroci et al. (1998)
we reanalyzed the optical data collected for the GRB 970111 field. Although we
detect small magnitude variability in some objects, no convincing optical
counterpart is found inside the WFC error box. Any change in brightness 19
hours after the GRB is less than 0.2 mag for objects with B < 21 and R < 20.8.
The bluest object found in the field is coincident with 1SAXJ1528.8+1937.
Spectroscopic observations revealed that this object is a Seyfert-1 galaxy with
redshift z=0.657, which we propose as the optical counterpart of the X-ray
source.
Further observations allowed to perform multicolour photometry for objects in
the GRB 970111 error box. The colour-colour diagrams do not show any object
with unusual colours. We applied a photometric classification method to the
objects inside the GRB error box, that can distinguish stars from galaxies and
estimate redshifts. We were able to estimate photometric redshifts in the range
0.2 < z < 1.4 for several galaxies in this field and we did not find any
conspicuous unusual object.
We note that GRB 970111 and GRB 980329 could belong to the same class of
GRBs, which may be related to nearby sources (z ~1) in which high intrinsic
absorption leads to faint optical afterglows.Comment: 10 pages with 11 encapsulated PostScript figures included. Uses
Astronomy & Astrophysics LaTeX macros. Accepted for publication in Astronomy
& Astrophysic
Afterglow upper limits for four short duration, hard spectrum gamma-ray bursts
We present interplanetary network localization, spectral, and time history
information for four short-duration, hard spectrum gamma-ray bursts, GRB000607,
001025B, 001204, and 010119. All of these events were followed up with
sensitive radio and optical observations (the first and only such bursts to be
followed up in the radio to date), but no detections were made, demonstrating
that the short bursts do not have anomalously intense afterglows. We discuss
the upper limits, and show that the lack of observable counterparts is
consistent both with the hypothesis that the afterglow behavior of the short
bursts is like that of the long duration bursts, many of which similarly have
no detectable afterglows, as well as with the hypothesis that the short bursts
have no detectable afterglows at all. Small number statistics do not allow a
clear choice between these alternatives, but given the present detection rates
of various missions, we show that progress can be expected in the near future.Comment: 19 pages, 4 figures; Revised version, accepted by the Astrophysical
Journa
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