2,344 research outputs found
Early observations of the Afterglow of GRB000301c
We report multiband observations of the Optical Transient (OT) associated
with GRB000301c carried out between 2--4 March 2000 using the 2.34-m Vainu
Bappu Telescope (VBT) at Kavalur, India. When combined with other reported
data, the initial decline in the R-band magnitude with log (), the time
since the burst is fit with a slope = -0.70 0.07 which
steepens after about 6.0 days to a slope of = -2.44 0.29.
This change in slope does not occur smoothly but there is an indication for a
bimodal distribution. The available measurements of the evolution of (B--R)
color do not show any discernible evolution in the first 12 days.Comment: 14 pages, 2 postscript figures, Accepted for Publication in ApJ
Letter
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 Variation of Integrated Star IMFs among Galaxies
The integrated galaxial initial mass function (IGIMF) is the relevant
distribution function containing the information on the distribution of stellar
remnants, the number of supernovae and the chemical enrichment history of a
galaxy. Since most stars form in embedded star clusters with different masses
the IGIMF becomes an integral of the assumed (universal or invariant) stellar
IMF over the embedded star-cluster mass function (ECMF). For a range of
reasonable assumptions about the IMF and the ECMF we find the IGIMF to be
steeper (containing fewer massive stars per star) than the stellar IMF, but
below a few Msol it is invariant and identical to the stellar IMF for all
galaxies. However, the steepening sensitively depends on the form of the ECMF
in the low-mass regime. Furthermore, observations indicate a relation between
the star formation rate of a galaxy and the most massive young stellar cluster
in it. The assumption that this cluster mass marks the upper end of a
young-cluster mass function leads to a connection of the star formation rate
and the slope of the IGIMF above a few Msol. The IGIMF varies with the star
formation history of a galaxy. Notably, large variations of the IGIMF are
evident for dE, dIrr and LSB galaxies with a small to modest stellar mass. We
find that for any galaxy the number of supernovae per star (NSNS) is suppressed
relative to that expected for a Salpeter IMF. Dwarf galaxies have a smaller
NSNS compared to massive galaxies. For dwarf galaxies the NSNS varies
substantially depending on the galaxy assembly history and the assumptions made
about the low-mass end of the ECMF. The findings presented here may be of some
consequence for the cosmological evolution of the number of supernovae per
low-mass star and the chemical enrichment of galaxies of different mass.Comment: 27 pages, accepted for publication by Ap
Physical Parameters for the Afterglows of GRB 980703, 990123, 990510, and 991216 Determined from Modeling of Multi-Frequency Data
We model the radio, optical, and X-ray emission for the afterglows of GRB
980703, 990123, 990510, and 991216, within the framework of relativistic jets,
to determine their physical parameters. The models that yield acceptable fits
to the data have jet energies mostly between 10^{50} to 10^{51} erg and initial
opening angles between 1 deg and 4 deg. The external medium density is
uncertain by at least one order of magnitude in each case, being around
10^{-3}/cm^3 for GRB 980703 and 990123, ~0.1/cm^3 for GRB 990510, and ~3/cm^3
for GRB 991216. If the jets are uniform (i.e. there are no angular gradients of
the energy per solid angle) then the 20 keV -- 1 MeV radiative efficiency
during the GRB phase must have been at least 2-3% for GRB 990510, 20% for GRB
990123, and 30% for GRB 991216.Comment: accepted for publication by the ApJ, vol. 554. 11 pages, color
figures. Last figures replaced with probability distributions of model
parameter
Continued Radio Monitoring of the Gamma Ray Burst 991208
We present radio observations of the afterglow of the bright gamma-ray burst
GRB 991208 at frequencies of 1.4, 4.9 and 8.5 GHz, taken between two weeks and
300 days after the burst. The well-sampled radio light curve at 8.5 GHz shows
that the peak flux density peaked about 10 days after the burst and decayed
thereafter as a power-law t^-1.07. This decay rate is more shallow than the
optical afterglow with t^-2.2, which was measured during the first week. These
late-time data are combined with extensive optical, millimeter and centimeter
measurements and fitted to the standard relativistic blast wave model. In
agreement with previous findings, we find that an isotropic explosion in a
constant density or wind-blown medium cannot explain these broadband data
without modifying the assumption of a single power-law slope for the electron
energy distribution. A jet-like expansion provides a reasonable fit to the
data. In this case, the flatter radio light curve compared to the optical may
be due to emission from an underlying host galaxy, or due to the blastwave
making a transition to non-relativistic expansion. The model that best
represents the data is a free-form model in which it is assumed that the
broadband emission originates from a synchrotron spectrum, while the
time-evolution of the break frequencies and peak flux density are solved for
explicitly. Although the decay indices for most of the synchrotron parameters
are similar to the jet model, the evolution of the cooling break is unusually
rapid, and therefore requires some non-standard evolution in the shock.
(abridged)Comment: ApJ, in pres
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
Modeling a high mass turn down in the stellar initial mass function
Statistical sampling from the stellar initial mass function (IMF) for all
star-forming regions in the Galaxy would lead to the prediction of ~1000 Msun
stars unless there is a rapid turn-down in the IMF beyond several hundred solar
masses. Such a turndown is not necessary for dense clusters because the number
of stars sampled is always too small. Here we explore several mechanisms for an
upper mass cutoff, including an exponential decline of the star formation
probability after a turbulent crossing time. The results are in good agreement
with the observed IMF over the entire stellar mass range, and they give a
gradual turn down compared to the Salpeter function above ~100 Msun for normal
thermal Jeans mass, M_J. The upper mass turn down should scale with M_J in
different environments. A problem with the models is that they cannot give both
the observed power-law IMF out to the high-mass sampling limit in dense
clusters, as well as the observed lack of supermassive stars in whole galaxy
disks. Either there is a sharper upper-mass cutoff in the IMF, perhaps from
self-limitation, or the IMF is different for dense clusters than for the
majority of star formation that occurs at lower density. Dense clusters seem to
have an overabundance of massive stars relative to the average IMF in a galaxy.Comment: 19 pages, 2 figures, Astrophysical Journal, Vol 539, August 10, 200
CCD Photometry of Galactic Globular Clusters. IV. The NGC 1851 RR Lyraes
The variable star population of the galactic globular cluster NGC 1851
(C0512-400) has been studied by CCD photometry, from observations made in the
B, V, and I bands during 1993-4. Light curves are presented for 29 variables,
seven of which are new discoveries. The behavior of the RR lyraes in the
period-temperature diagram appears normal when compared to clusters which
bracket the NGC 1851 metallicity. Reddening and metallicity are re-evaluated,
with no compelling evidence to change from accepted values. Photometry for
stars within an annulus with radii 80 and 260 arcsec agrees to better than 0.02
mag in all colors with extensive earlier photometry, to at least V = 18.5.
Instability strip boundary positions for several clusters shows a trend for the
red boundary to move to redder colors as the metallicity increases.Comment: 29 pages, 9 figures, accepted by A.
Jet break time -- flux density relationship and constraints on physical parameters of GRB afterglows
We derive a relation between the flux density at the light-curve
break of a gamma-ray burst (GRB) afterglow and the break time . The
break is due to the transition from the spherical-like to jet-like evolution of
the afterglow, when the Lorentz factor of the jet equals the inverse of the
initial half-opening angle, i.e., . We show that this
relation indeed behaves as among GRBs for the
slow-cooling case, where is the power-law index of electron distribution. A
statistical analysis of the optical jet breaks of nine GRBs gives , which is consistent with the shock acceleration theory. The value of
derived in this way is different from the observed temporal index
() of the late-time light curve after ,
which suffers several uncertainties from the unclear hydrodynamics of the
sideways expansion and exhibits a large dispersion. Our results not only
confirm that the remnants of GRBs are standard candles, but also provide the
first evidence that the physical parameters of relativistic shocks are
universal, with the favored values and .Comment: 11 pages including 3 color figure
Fossil biomass preserved as graphitic carbon in a late paleoproterozoic banded iron formation metamorphosed at more than 550°C
Metamorphism is thought to destroy microfossils, partly through devolatilization and graphitization of biogenic organic matter. However, the extent to which there is a loss of molecular, elemental and isotope signatures from biomass during high-temperature metamorphism is not clearly established. We report on graphitic structures inside and coating apatite grains from the c. 1850 Ma Michigamme silicate banded iron formation from Michigan, metamorphosed above 550°C. Traces of N, S, O, H, Ca and Fe are preserved in this graphitic carbon and X-ray spectra show traces of aliphatic groups. Graphitic carbon has an expanded lattice around 3.6 Å, forms microscopic concentrically-layered and radiating polygonal flakes and has homogeneous δ13C values around −22‰, identical to bulk analyses. Graphitic carbon inside apatite is associated with nanometre-size ammoniated phyllosilicate. Precursors of these metamorphic minerals and graphitic carbon originated from ferruginous clayrich sediments with biomass. We conclude that graphite coatings and inclusions in apatite grains indicate fluid remobilization during amphibolite-facies metamorphism of precursor biomass. This new evidence fills in observational gaps of metamorphosed biomass into graphite and supports the existence of biosignatures in the highly metamorphosed iron formation from the Eoarchean Akilia Association, which dates from the beginning of the sedimentary rock record
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