4,757 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
The host galaxy of GRB010222: The strongest damped Lyman-alpha system known
Analysis of the absorption lines in the afterglow spectrum of the gamma-ray
burst GRB010222 indicates that its host galaxy (at a redshift of z=1.476) is
the strongest damped Lyman-alpha (DLA) system known, having a very low
metallicity and modest dust content. This conclusion is based on the detection
of the red wing of Lyman-alpha plus a comparison of the equivalent widths of
ultraviolet Mg I, Mg II, and Fe II lines with those in other DLAs. The column
density of H I, deduced from a fit to the wing of Lyman-alpha, is (5 +/- 2)
10^22 cm^-2. The ratio of the column densities of Zn and Cr lines suggests that
the dust content in our line of sight through the galaxy is low. This could be
due to either dust destruction by the ultraviolet emission of the afterglow or
to an initial dust composition different to that of the diffuse interstellar
material, or a combination of both.Comment: Submitted to MNRAS 12 page
New search strategy for high z intervening absorbers: GRB021004, a pilot study
We present near-infrared narrow- and broad-band imaging of the field of
GRB021004, performed with ISAAC on the UT1 of the ESO Very Large Telescope. The
narrow-band filters were chosen to match prominent emission lines at the
redshift of the absorption-line systems found against the early-time afterglow
of GRB021004: [OIII] at z=1.38 and Halpha at z=1.60, respectively. For the
z=1.38 system we find an emission-line source at an impact parameter of 16",
which is somewhat larger than the typical impact parameters of a sample of MgII
absorbers at redshifts around unity. Assuming that this tentative
redshift-identification is correct, the star formation rate of the galaxy is 13
+- 2 Msun/year. Our study reaches star-formation rate limits (5 sigma) of 5.7
Msun/year at z=1.38, and 7.7 Msun/year at z=1.60. These limits correspond to a
depth of roughly 0.13 L*. Any galaxy counterpart of the absorbers nearer to the
line of sight either has to be fainter than this limit or not be an
emission-line source.Comment: 4 pages, 3 figures, accepted for publication in A&A letter
SCUBA observations of the host galaxies of four dark gamma-ray bursts
We present the results of a search for submillimetre-luminous host galaxies
of optically dark gamma-ray bursts (GRBs) using the Submillimetre Common-User
Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT). We made
photometry measurements of the 850-micron flux at the location of four `dark
bursts', which are those with no detected optical afterglow despite rapid deep
searches, and which may therefore be within galaxies containing substantial
amounts of dust. We were unable to detect any individual source significantly.
Our results are consistent with predictions for the host galaxy population as a
whole, rather than for a subset of dusty hosts. This indicates that optically
dark GRBs are not especially associated with very submillimetre-luminous
galaxies and so cannot be used as reliable indicators of dust-enshrouded
massive star-formation activity. Further observations are required to establish
the relationship between the wider GRB host galaxy population and SCUBA
galaxies.Comment: 6 pages. Accepted for publication in MNRA
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
Dust-to-metal ratios in damped Lyman-alpha absorbers: Fresh clues to the origins of dust and optical extinction towards gamma-ray bursts
Motivated by the anomalous dust-to-metal ratios derived in the literature for
gamma-ray burst (GRB) damped Lyman-alpha absorbers (DLAs), we measure these
ratios using the dust-depletion pattern observed in UV/optical afterglow
spectra associated with the ISM at the GRB host-galaxy redshifts. Our sample
consists of 20 GRB absorbers and a comparison sample of 72 QSO-DLAs with
redshift 1.2 < z < 4.0 and down to Z = 0.002 Z_Sol metallicities. The
dust-to-metal ratio in QSO- and GRB-DLAs increases both with metallicity and
metal column density, spanning ~10--110% of the Galactic value and pointing to
a non universal dust-to-metal ratio. The low values of dust-to-metal ratio
suggest that low-metallicity systems have lower dust fractions than typical
spiral galaxies and perhaps that the dust in these systems is produced
inefficiently, i.e. by grain growth in the low-metallicity regime with
negligible contribution from supernovae (SNe) and asymptotic giant branch (AGB)
stars. On the other hand, some GRB- and QSO-DLAs show high dust-to-metal ratio
values out to z ~ 4, requiring rapid dust production, such as in SN ejecta, but
also in AGB winds and via grain growth for the highest metallicity systems.
GRB-DLAs overall follow the dust-to-metal-ratio properties of QSO-DLAs, GRBs
probing up to larger column and volume densities. For comparison, the
dust-to-metal ratio that we derive for the SMC and LMC are ~82--100% and ~98%
of the Galactic value, respectively. The literature dust-to-metal ratio of the
low-metallicity galaxy I Zw 18 (< 37%) is consistent with the distribution that
we find. The dust extinction Av increases steeply with the column density of
iron in dust, N(Fe)dust, calculated from relative metal abundances, confirming
that dust extinction is mostly occurring in the host galaxy ISM. Most GRB-DLAs
display log N(Fe)dust > 14.7, above which several QSO-DLAs reveal H2
(abridged).Comment: 14 pages, 9 figures. A&A, in pres
The Host Galaxy of GRB 990712
We present a comprehensive study of the z=0.43 host galaxy of GRB 990712,
involving ground-based photometry, spectroscopy, and HST imaging. The
broad-band UBVRIJHKs photometry is used to determine the global spectral energy
distribution (SED) of the host galaxy. Comparison with that of known galaxy
types shows that the host is similar to a moderately reddened starburst galaxy
with a young stellar population. The estimated internal extinction in the host
is Av=0.15+/-0.1 and the star-formation rate (SFR) from the UV continuum is
1.3+/-0.3 M_sun/yr, (not corrected for the effects of extinction). Other galaxy
template spectra than starbursts failed to reproduce the observed SED. We also
present VLT spectra leading to the detection of Halpha from the GRB host
galaxy. A SFR of 2.8+/-0.7 M_sun/yr, is inferred from the Halpha line flux, and
the presence of a young stellar population is supported by a large equivalent
width. Images from HST/STIS show that the host has two separate knots, which
could be two distinct star-forming regions.Comment: 11 pages, accepted for publication in 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 star-formation rate in the host of GRB 990712
We have observed the host galaxy of GRB 990712 at 1.4 GHz with the Australia
Telescope Compact Array, to obtain an estimate of its total star-formation
rate. We do not detect a source at the position of the host. The 2 sigma upper
limit of 70 microJy implies that the total star-formation rate is lower than
100 Msun/yr, using conservative values for the spectral index and cosmological
parameters. This upper limit is in stark contrast with recent reports of
radio/submillimeter-determined star-formation rates of roughly 500 Msun/yr for
two other GRB host galaxies. Our observations present the deepest
radio-determined star-formation rate limit on a GRB host galaxy yet, and show
that also from the unobscured radio point-of-view, not every GRB host galaxy is
a vigorous starburst.Comment: A&A Letters, in press, 5 pages; a high-resolution color gif version
of the paper figure is also supplie
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