2,066 research outputs found
GRB Afterglows from Anisotropic Jets
Some progenitor models of gamma-ray bursts (GRBs) (e.g., collapsars) may
produce anisotropic jets in which the energy per unit solid angle is a
power-law function of the angle (). We calculate light
curves and spectra for GRB afterglows when such jets expand either in the
interstellar medium or in the wind medium. In particular, we take into account
two kinds of wind: one () possibly from a typical red
supergiant star and another () possibly from a Wolf-Rayet
star. We find that in each type of medium, one break appears in the late-time
afterglow light curve for small but becomes weaker and smoother as
increases. When , the break seems to disappear but the afterglow decays
rapidly. Thus, one expects that the emission from expanding, highly anisotropic
jets provides a plausible explanation for some rapidly fading afteglows whose
light curves have no break. We also present good fits to the optical afterglow
light curve of GRB 991208. Finally, we argue that this burst might arise from a
highly anisotropic jet expanding in the wind () from a red
supergiant to interpret the observed radio-to-optical-band afterglow data
(spectrum and light curve).Comment: 12 pages + 10 figures, accepted by Ap
The Near Infrared and Multiwavelength Afterglow of GRB 000301c
We present near-infrared observations of the counterpart of GRB 000301c. The
K' filter (2.1 micron) light curve shows a well-sampled break in the decay
slope at t=3.5 days post-burst. The early time slope is very shallow (~ -0.1),
while the late time slope is steep (-2.2). Comparison with the optical (R band)
light curve shows marginally significant differences, especially in the early
time decay slope (which is steeper in the optical) and the break time (which
occurs later in the optical). This is contrary to the general expectation that
light curve breaks should either be achromatic (e.g., for breaks due to
collimation effects) or should occur later at longer wavelengths (for most
other breaks). The observed color variations might be intrinsic to the
afterglow, or might indicate systematic errors of > 0.08 magnitude in all
fluxes. Even if the break is achromatic, we argue that its sharpness poses
difficulties for explanations that depend on collimated ejecta. The R light
curve shows further signs of fairly rapid variability (a bump, steep drop, and
plateau) that are not apparent in the K' light curve. In addition, by combining
the IR-optical-UV data with millimeter and radio fluxes, we are able to
constrain the locations of the self-absorption break and cooling break and to
infer the location of the spectral peak at t=3 days: f_nu = 3.4 mJy at nu=1e12
Hz. Using the multiwavelength spectral energy distribution, we are able to
constrain the blast wave energy, which was E > 3e53 erg if the explosion was
isotropic. This implies a maximum gamma ray production efficiency of ~ 0.15 for
GRB 000301C.Comment: Accepted to The Astrophysical Journal. 24 pages, 4 figures, 3 tables;
uses AASTeX 5 macros. This version includes a new figure (R-K' color vs.
time), a better sampled R band light curve, and more extensive discussion of
the optical data and error analysi
On the determination of age and mass functions of stars in young open star clusters from the analysis of their luminosity functions
Based on the CCD observations of remote young open clusters NGC 2383, NGC
2384, NGC 4103, NGC 4755, NGC 7510 and Hogg 15, we constructed their observed
luminosity functions (LFs). The observed LFs are corrected for field star
contamination determined with the help of galactic star count model. In the
case of Hogg 15 and NGC 2383 we also considered the additional contamination
from neighbouring clusters NGC 4609 and NGC 2384 respectively. These
corrections provided the realistic pattern of cluster LF in the vicinity of the
MS turn on point and at fainter magnitudes, revealed the so called H-feature
arising due to transition of the Pre-MS phase to MS, which is dependent on the
cluster age. The theoretical LFs were constructed representing a cluster
population model with continuous star formation for a short time scale and a
power law Initial Mass Function (IMF) and these were fitted to the observed LF.
As a result we are able to determine for each cluster a set of parameters,
describing cluster population (the age, duration of star formation, IMF slope
and percentage of field star contamination). It was found that in spite of the
non-monotonic behaviour of observed LFs, cluster IMFs can be described as the
power law functions with slopes similar to Salpeter's value. The present MS
turn on cluster ages are several times lower than those derived from the
fitting of theoretical isochrones to the turn off region of the upper Main
Sequences.Comment: 17 pages, 5 figures, To appear in MNRA
Energy Injection in GRB Afterglow Models
We extend the standard fireball model, widely used to interpret gamma-ray
burst (GRB) afterglow light curves, to include energy injections, and apply the
model to the afterglow light curves of GRB 990510, GRB 000301C and GRB 010222.
We show that discrete energy injections can cause temporal variations in the
optical light curves and present fits to the light curves of GRB 000301C as an
example. A continuous injection may be required to interpret other bursts such
as GRB 010222. The extended model accounts reasonably well for the observations
in all bands ranging from X-rays to radio wavelengths. In some cases, the radio
light curves indicate that additional model ingredients may be needed.Comment: Accepted for publication in the Astrophysical Journa
A Jet Model With a Hard Electron Distribution for the Afterglow of GRB 000301c
The parameters of the GRB 000301c afterglow are determined within the usual
framework of synchrotron emission from relativistic ejecta, through fits to the
available radio-to-optical data. It is found that 1) the jet energy after the
GRB phase is ~3x10^{50} erg, 2) the initial opening angle of the jet is ~13
deg, 3) the medium that decelerates the afterglow has a density ~25/cm^3, 4)
the power-law distribution of the shock-energized electrons is hard, with an
index around 1.5, and 5) the cooling frequency was located below the optical
domain. Furthermore we find that the collimation of ejecta alone cannot explain
the sharp, large magnitude break observed in the R-band emission of this
afterglow at few days, and that a high energy break in the electron
distribution, corresponding to an electron energy close to equipartition,
together with the lateral spreading of the jet accommodate better this break.
Microlensing by a star in an intervening galaxy produces only a flattening of
the afterglow emission and cannot explain the mild brightening exhibited by the
optical emission of 000301c at ~4 days.Comment: to be published in the ApJ, v.556. Figures changed, one added. Other
alterations are mino
Emission from Bow Shocks of Beamed Gamma-Ray Bursts
Beamed gamma-ray burst (GRB) sources produce a bow shock in their gaseous
environment. The emitted flux from this bow shock may dominate over the direct
emission from the jet for lines of sight which are outside the angular radius
of the jet emission, theta. The event rate for these lines of sight is
increased by a factor of 260*(theta/5_degrees)^{-2}. For typical GRB
parameters, we find that the bow shock emission from a jet with half-angle of
about 5 degrees is visible out to tens of Mpc in the radio and hundreds of Mpc
in the X-rays. If GRBs are linked to supernovae, studies of peculiar supernovae
in the local universe should reveal this non-thermal bow shock emission for
weeks to months following the explosion.Comment: ApJ, submitted, 15 pages, 3 figure
Afterglow Emission from Highly Collimated Jets with Flat Electron Spectra: Application to the GRB 010222 Case?
We derive light curves of the afterglow emission from highly collimated jets
if the power-law index () of the electron energy distribution is above 1 but
below 2. We find (1) below the characteristic synchrotron frequency, the light
curve index depends generally on . (2) As long as the jet expansion is
spherical, the light curve index above the characteristic frequency increases
slowly as the spectral index of the emission increases. (3) Once the jet enters
the spreading phase, the high-frequency emission flux decays as rather than . All these results differ from those
in the case of . We compare our analytical results with the observations
on the GRB 010222 afterglow, and conclude that the jet model may be unable to
explain the observed data. Thus, a more promising explanation for this
afterglow seems to be the expansion of a relativistic fireball or a mildly
collimated jet in a dense medium.Comment: shortened version accepted for publication in ApJ Letter
GRB Energetics and the GRB Hubble Diagram: Promises and Limitations
We present a complete sample of 29 GRBs for which it has been possible to
determine temporal breaks (or limits) from their afterglow light curves. We
interpret these breaks within the framework of the uniform conical jet model,
incorporating realistic estimates of the ambient density and propagating error
estimates on the measured quantities. In agreement with our previous analysis
of a smaller sample, the derived jet opening angles of those 16 bursts with
redshifts result in a narrow clustering of geometrically-corrected gamma-ray
energies about E_gamma = 1.33e51 erg; the burst-to-burst variance about this
value is a factor of 2.2. Despite this rather small scatter, we demonstrate in
a series of GRB Hubble diagrams, that the current sample cannot place
meaningful constraints upon the fundamental parameters of the Universe. Indeed
for GRBs to ever be useful in cosmographic measurements we argue the necessity
of two directions. First, GRB Hubble diagrams should be based upon fundamental
physical quantities such as energy, rather than empirically-derived and
physically ill-understood distance indicators. Second, a more homogeneous set
should be constructed by culling sub-classes from the larger sample. These
sub-classes, though now first recognizable by deviant energies, ultimately must
be identifiable by properties other than those directly related to energy. We
identify a new sub-class of GRBs (``f-GRBs'') which appear both underluminous
by factors of at least 10 and exhibit a rapid fading at early times. About
10-20% of observed long-duration bursts appear to be f-GRBs.Comment: Accepted to the Astrophysical Journal (20 May 2003). 19 pages, 3
Postscript figure
Floral resources of Karnataka: a geographic perspective
We compiled the data on the floral resources of Karnataka from diverse published sources and analysed the geographic patterns of distribution of floral diversity. Our database shows that Karnataka harbours 4758 species from 1408 genera and 178 families and accounts for about 27 per cent of the country's floral diversity. We computed the 'endemicity value' of different districts based on the number of endemic species (those restricted to a maximum of five districts) harboured by them and found that the most species-rich districts (viz. Uttara Kannada, Dakshina Kannada, Mysore, Hassan, Udupi and Kodagu) were also characterized by high values of endemicity while the species-poor districts had low values of endemicity. However, the relation between the species richness and endemicity of the districts was not linear; the species richness increases abruptly at lower levels of endemicity but plateaus off later at high levels of endemicity. Based on the number of species packed into the families, all the 27 districts segregated distinctly into three clusters that geographically correspond with the three major agro-climatic zones of the state. Our analysis showed that though the districts along the Western Ghats are florally rich, those along the dry tracts also harbour certain unique elements of the flora; thus these dry zone districts appear to be as important as those along the Western Ghats in conserving the floral resources
Wind Interaction Models for the Afterglows of GRB 991208 and GRB 000301C
The simplest model of the afterglows of the gamma-ray bursts (GRBs) envisions
a spherical blast wave with a power-law distribution of electron energy above
some cutoff running into a constant density medium. A refinement involves a
narrow jet, often invoked to explain the steep decline and/or steepening of
light curves observed in some afterglows. The constant (ambient) density jet
model has been applied to GRBs 991208 and 000301C, based to a large extent on
radio observations. We show that, for these two sources, a spherical wind model
(with an density ambient medium) can fit the radio data as well as the
jet model. %%We demonstrate that the The relatively steep decline and the
fairly abrupt steepening of the R-band light curves of, respectively, GRB
991208 and GRB 000301C can be accounted for with a non-standard, broken
power-law distribution of electron energy. Our model predicts a slower late
decline for the radio flux than does the jet model.Comment: 17 pages, submitted to Ap
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