4,201 research outputs found
Photometric Variability in the Faint Sky Variability Survey
The Faint Sky Variability Survey (FSVS) is aimed at finding photometric
and/or astrometric variable objects between 16th and 24th mag on time-scales
between tens of minutes and years with photometric precisions ranging from 3
millimag to 0.2 mag. An area of 23 deg, located at mid and high Galactic
latitudes, was covered using the Wide Field Camera (WFC) on the 2.5-m Isaac
Newton Telescope (INT) on La Palma. Here we present some preliminary results on
the variability of sources in the FSVS.Comment: 4 pages, 3 figures, to appear in 14th European Workshop on White
Dwarfs, ASP Conference Series, eds. D. Koester, S. Moehle
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
Time-dependent excitation and ionization modelling of absorption-line variability due to GRB 080310
We model the time-variable absorption of FeII, FeIII, SiII, CII and CrII
detected in UVES spectra of GRB 080310, with the afterglow radiation exciting
and ionizing the interstellar medium in the host galaxy at a redshift of
z=2.42743. To estimate the rest-frame afterglow brightness as a function of
time, we use a combination of the optical VRI photometry obtained by the
RAPTOR-T telescope array -- which are presented in this paper -- and Swift's
X-Ray Telescope observations. Excitation alone, which has been successfully
applied for a handful of other GRBs, fails to describe the observed
column-density evolution in the case of GRB 080310. Inclusion of ionization is
required to explain the column-density decrease of all observed FeII levels
(including the ground state 6D9/2) and increase of the FeIII 7S3 level. The
large population of ions in this latter level (up to 10% of all FeIII) can only
be explained through ionization of FeII, whereby a large fraction of the
ionized FeII ions -- we calculate 31% using the Flexible Atomic (FAC) and Cowan
codes -- initially populate the 7S3 level of FeIII rather than the ground
state. This channel for producing a significant FeIII 7S3 level population may
be relevant for other objects in which absorption lines from this level -- the
UV34 triplet -- are observed, such as BAL quasars and Eta Carinae. This
provides conclusive evidence for time-variable ionization in the circumburst
medium, which to date has not been convincingly detected. However, the best-fit
distance of the neutral absorbing cloud to the GRB is 200--400 pc, i.e. similar
to GRB-absorber distance estimates for GRBs without any evidence for
ionization. We find that the presence of time-varying ionization in GRB 080310
is likely due to a combination of the super-solar iron abundance ([Fe/H]=+0.2)
and the low HI column density (log N(HI)=18.7). [abridged]Comment: 14 pages, 6 figures; accepted for publication in A&A (on August 8,
2012
The discovery of polarization in the afterglow of GRB 990510 with the ESO Very Large Telescope
Following a BeppoSAX alert (Piro 1999a) and the discovery of the OT at SAAO
(Vreeswijk et al. 1999a), we observed GRB 990510 with the FORS instrument on
ESO's VLT Unit 1 (`Antu'). The burst is unremarkable in gamma rays, but in
optical is the first one to show good evidence for jet-like outflow (Stanek et
al. 1999, Harrison et al. 1999). We report the detection of significant linear
polarization in the afterglow: it is (1.6 +/- 0.2)% 0.86 days after trigger,
and after 1.81 days is consistent with that same value, but much more
uncertain. The polarization angle is constant on a time scale of hours, and may
be constant over one day. We conclude that the polarization is intrinsic to the
source and due to the synchrotron nature of the emission, and discuss the
random and ordered field geometries that may be responsible for it.Comment: submitted to ApJ Lett., 5 pages including 2 figures, uses
emulateapj.st
Spike-Train Responses of a Pair of Hodgkin-Huxley Neurons with Time-Delayed Couplings
Model calculations have been performed on the spike-train response of a pair
of Hodgkin-Huxley (HH) neurons coupled by recurrent excitatory-excitatory
couplings with time delay. The coupled, excitable HH neurons are assumed to
receive the two kinds of spike-train inputs: the transient input consisting of
impulses for the finite duration (: integer) and the sequential input
with the constant interspike interval (ISI). The distribution of the output ISI
shows a rich of variety depending on the coupling strength and the
time delay. The comparison is made between the dependence of the output ISI for
the transient inputs and that for the sequential inputs.Comment: 19 pages, 4 figure
The Supreme Court and the American Civil Liberties Union\u27s Colorblind Protection of Cross Burning in First Amendment Jurisprudence Legitimates White Supremacy
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GRB990712: First Indication of Polarization Variability in a Gamma-ray Burst Afterglow
We report the detection of significant polarization in the optical afterglow
of GRB990712 on three instances 0.44, 0.70 and 1.45 days after the gamma-ray
burst, with (P, theta) being (2.9% +- 0.4%, 121.1 degr +- 3.5 degr), (1.2% +-
0.4%, 116.2 degr +- 10.1 degr) and (2.2% +- 0.7%, 139.2 degr +- 10.4 degr)
respectively. The polarization is intrinsic to the afterglow. The degree of
polarization is not constant, and smallest at the second measurement. The
polarization angle does not vary significantly during these observations. We
find that none of the existing models predict such polarization variations at
constant polarization angle, and suggest ways in which these models might be
modified to accommodate the observed behavior of this afterglow.Comment: 10 pages including 6 figures, accepted by ApJ. Uses aastex 5.
Short timescale variability in the Faint Sky Variability Survey
We present the V band variability analysis of the point sources in the Faint
Sky Variability Survey on time scales from 24 minutes to tens of days. We find
that about one percent of the point sources down to V = 24 are variables. We
discuss the variability detection probabilities for each field depending on
field sampling, amplitude and timescale of the variability. The combination of
colour and variability information allows us to explore the fraction of
variable sources for different spectral types. We find that about 50 percent of
the variables show variability timescales shorter than 6 hours. The total
number of variables is dominated by main sequence sources. The distribution of
variables with spectral type is fairly constant along the main sequence, with 1
per cent of the sources being variable, except at the blue end of the main
sequence, between spectral types F0--F5, where the fraction of variable sources
increases to about 2 percent. For bluer sources, above the main sequence, this
percentage increases to about 3.5. We find that the combination of the sampling
and the number of observations allows us to determine the variability
timescales and amplitudes for a maximum of 40 percent of the variables found.
About a third of the total number of short timescale variables found in the
survey were not detected in either B or/and I. These show a similar variability
timescale distribution to that found for the variables detected in all three
bands.Comment: 14 pages, 12 figures and 4 tables. Accepted for publication in MNRA
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