333 research outputs found
Vrijloopstallen in winterse omstandigheden
Het succes van vrijloopstallen in Nederland hangt erg af van het klimaat. Vooral in koude en vochtige winters is het lastig om de bodem droog en schoon te houden. Studenten van Van Hall Larenstein beschrijven de ervaringen van vier pioniers in Nederland met een vrijloopstal
Vision and refraction in Australian preschool children : measures, prevalence, and associated factors
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The optical light curve of GRB 970228 refined
We present the R and V light curves of the optical counterpart of GRB 970228.
A critical analysis of all the available data is made in light of the results
achieved in the recent GRB Symposium held in Huntsville and by considering the
latest information from the HST images on the underlying nebulosity.Comment: 3 pages, 2 .ps figures, Nuclear Physics style file espcrc2.sty
included. To appear in the proceedings of the conference "The Active X-Ray
Sky: Results from BeppoSAX and Rossi-XTE", Rome, Italy, 21-24 October, 1997.
L. Scarsi, H. Bradt, P. Giommi and F. Fiore editors, Nuc. Phys. B Proc. Supp
SCUBA sub-millimeter observations of gamma-ray bursters: II. GRB 991208, 991216, 000301C, 000630, 000911, 000926
We discuss our ongoing program of Target of Opportunity sub-millimeter
observations of gamma-ray bursts (GRBs) using the Sub-millimetre Common-User
Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT).
Sub-millimeter observations of the early afterglows are of interest because
this is where the emission peaks in some bursts in the days to weeks following
the burst. Of increasing interest is to look for underlying quiescent
sub-millimeter sources that may be dusty star-forming host galaxies. In this
paper, we present observations of GRB 991208, 991216, 000301C, 000630, 000911,
and 000926. For all these bursts, any sub-millimeter emission is consistent
with coming from the afterglow. This means that we did not conclusively detect
quiescent sub-millimeter counterparts to any of the bursts that were studied
from 1997 through 2000. The inferred star formation rates (M > 5 Msun) are
typically < 300 Msun/yr. If GRBs are due to the explosions of high-mass stars,
this may indicate that the relatively small population of extremely luminous
dusty galaxies does not dominate the total star formation in the universe at
early epochs. Instead, the GRBs may be predominantly tracing slightly lower
luminosity galaxies. The optical faintness of some host galaxies is unlikely to
be explained as due to dust absorption in the host.Comment: 9 pages. 0 figures. Astronomy and Astrophysics, in pres
VLT observations of GRB 990510 and its environment
We present BVRI photometry and spectrophotometry of GRB990510 obtained with
the ESO VLT/Antu telescope during the late decline phase. Between days 8 and 29
after the burst, the afterglow faded from R=24.2 to ~26.4. The spectral flux
distribution and the light curve support the interpretation of the afterglow as
synchrotron emission from a jet. The light curve is consistent with the optical
transient alone but an underlying SN with maximum brightness R>27.4 or a galaxy
with R>27.6 (3-sigma upper limits) cannot be ruled out. To a 5-sigma detection
threshold of R=26.1, no galaxy is found within 6'' of the transient. A very
blue V~24.5 extended object which may qualify as a starburst galaxy is located
12'' SE, but at unknown redshift.Comment: 5 pages A&A Latex, accepted for publication in A&A Letter
SCUBA sub-millimeter observations of gamma-ray bursters. I. GRB 970508, 971214, 980326, 980329, 980519, 980703
We discuss the first results of our ongoing program of Target of Opportunity
observations of gamma-ray bursts (GRBs) using the SCUBA instrument on the James
Clerk Maxwell Telescope. We present the results for GRB 970508, 971214, 980326,
980329, 980519, and 980703.
Our most important result to date is the detection of a fading counterpart to
GRB 980329 at 850 microns. Although it proved to be difficult to find the
infrared counterpart to this burst, the sub-millimeter flux was relatively
bright. This indicates that intrinsically the brightness of this counterpart
was very similar to GRB 970508. The radio through sub-millimeter spectrum of
GRB 980329 is well fit by a power law with index alpha = +0.9. However, we
cannot exclude a nu^(1/3) power law attenuated by synchrotron self-absorption.
An alpha = +1 VLA-SCUBA power law spectrum is definitely ruled out for GRB
980703, and possibly also for GRB 980519.
We cannot rule out that part of the sub-millimeter flux from GRB 980329 comes
from a dusty star-forming galaxy at high redshift, such as the ones recently
discovered by SCUBA. Any quiescent dust contribution will be much larger at
sub-millimeter than at radio wavelengths. Both a high redshift and large dust
extinction would help explain the reddening of the counterpart to GRB 980329,
and a redshift of z = 5 has been suggested. The large intensity of this burst
might then indicate that beaming is important.Comment: 6 pages, 3 figures, submitted to Astronomy and Astrophysic
Gamma-Ray Bursts as a Probe of the Very High Redshift Universe
We show that, if many GRBs are indeed produced by the collapse of massive
stars, GRBs and their afterglows provide a powerful probe of the very high
redshift (z > 5) universe.Comment: To appear in Proc. of the 5th Huntsville Gamma-Ray Burst Symposium, 5
pages, LaTe
Colors and luminosities of the optical afterglows of the gamma-ray bursts
Results of the study of the color indices and luminosities of 17 optical
afterglows (OAs) of GRBs are presented. We show that the color variations
during the decline of OAs (except for GRB000131) are relatively small during
t-T_0 < 10 days and allow a comparison among them, even for the less densely
sampled OAs. The colors in the observer frame, corrected for the Galactic
reddening, concentrate at (V-R)_0 = 0.40 +- 0.13, (R-I)_0 = 0.46 +- 0.18,
(B-V)_0 = 0.47 +- 0.17. The color evolution of the OAs is negligible although
their brightness declines by several magnitudes during the considered time
interval. Such a strong concentration of the color indices also suggests that
the intrinsic reddening (inside their host galaxies) must be quite similar and
relatively small for all these events. The absolute brightness of OAs in the
observer frame, corrected for the host galaxy, lies within M_(R_0) = -26.5 to
-22.2 for (t-T_0)_rest = 0.25 days. This spread of M_(R_0) is not significantly
influenced by the shifts of lambda, caused by the different redshift z of the
respective OAs. The general decline rate of the OA sample considered here seems
to be independent of the absolute optical brightness of the OA, measured at
some t-T_0 identical for all OAs, and the light curves of all events are almost
parallel, when corrected for the redshift-induced time dilation.Comment: 11 pages, 2 tables, 7 figures; to be published on Astronomy &
Astrophysic
The Host Galaxy and Optical Light Curve of the Gamma-Ray Burst GRB 980703
We present deep HST/STIS and ground-based photometry of the host galaxy of
the gamma-ray burst GRB 980703 taken 17, 551, 710, and 716 days after the
burst. We find that the host is a blue, slightly over-luminous galaxy with
V_gal = 23.00 +/- 0.10, (V-R)_gal = 0.43 +/- 0.13, and a centre that is
approximately 0.2 mag bluer than the outer regions of the galaxy. The galaxy
has a star-formation rate of 8-13 M_sun/yr, assuming no extinction in the host.
We find that the galaxy is best fit by a Sersic R^(1/n) profile with n ~= 1.0
and a half-light radius of 0.13 arcsec (= 0.72/h_100 proper kpc). This
corresponds to an exponential disk with a scale radius of 0.22 arcsec (=
1.21/h_100 proper kpc). Subtracting a fit with elliptical isophotes leaves
large residuals, which suggests that the host galaxy has a somewhat irregular
morphology, but we are unable to connect the location of GRB 980703 with any
special features in the host. The host galaxy appears to be a typical example
of a compact star forming galaxy similar to those found in the Hubble Deep
Field North. The R-band light curve of the optical afterglow associated with
this gamma-ray burst is consistent with a single power-law decay having a slope
of alpha = -1.37 +/- 0.14. Due to the bright underlying host galaxy the late
time properties of the light-curve are very poorly constrained. The decay of
the optical light curve is consistent with a contribution from an underlying
Type Ic supernova like SN1998bw, or a dust echo, but such contributions cannot
be securely established.Comment: 9 pages, 5 figures, LaTeX using A&A Document Class v4.05, to appear
in A&
Circumstellar interaction in type Ibn supernovae and SN 2006jc
I analyse peculiar properties of light curve and continua of enigmatic Ibn
supernovae, including SN 2006jc, and argue in favour of the early strong
circumstellar interaction. This interaction explains the high luminosity and
fast flux rise of SN 1999cq, while the cool dense shell formed in shocked
ejecta can explain the smooth early continuum of SN 2000er and unusual blue
continuum of SN 2006jc. The dust is shown to condense in the cool dense shell
at about day 50. Monte Carlo modelling of the He I 7065 \AA line profile
affected by the dust occultation supports a picture, in which the dust resides
in the fragmented cool dense shell, whereas He I lines originate from
circumstellar clouds shocked and fragmented in the forward shock wave.Comment: 10 pages, 6 figures, MNRAS accepte
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