29 research outputs found
New aperture photometry of QSO 0957+561; application to time delay and microlensing
We present a re-reduction of archival CCD frames of the doubly imaged quasar
0957+561 using a new photometry code. Aperture photometry with corrections for
both cross contamination between the quasar images and galaxy contamination is
performed on about 2650 R-band images from a five year period (1992-1997). From
the brightness data a time delay of 424.9 +/- 1.2 days is derived using two
different statistical techniques. The amount of gravitational microlensing in
the quasar light curves is briefly investigated, and we find unambiguous
evidence of both long term and short term microlensing. We also note the
unusual circumstance regarding time delay estimates for this gravitational
lens. Estimates by different observers from different data sets or even with
the same data sets give lag estimates differing by typically 8 days, and error
bars of only a day or two. This probably indicates several complexities where
the result of each estimate depends upon the details of the calculation.Comment: 14 pages, 16 figures (several in color
GLITP Optical Monitoring of QSO 0957+561: VR Light Curves and Variability
The GLITP collaboration observed the first gravitational lens system (QSO
0957+561) from 2000 February 3 to 2000 March 31. The daily VR observations were
made with the 2.56-m Nordic Optical Telescope at Roque de los Muchachos
Observatory, La Palma (Spain). We have derived detailed and robust VR light
curves of the two components Q0957+561A and Q0957+561B. In spite of the
excellent sampling rate, we have not found evidence in favor of true daily
variability. With respect to variability on time-scales of several weeks, we
measure VR gradients of about -0.8 mmag/day in Q0957+561A and + 0.3 mmag/day in
Q0957+561B. The gradients are very probably originated in the far source, thus
adopting this reasonable hypothesis (intrinsic variability), we compare them to
the expected gradients during the evolution of a compact supernova remnant at
the redshift of the source quasar. The starburst scenario is roughly consistent
with some former events, but the new gradients do not seem to be caused by
supernova remnant activity.Comment: MNRAS, in press (21 pages, 11 figures
Time delay of SBS 0909+532
The time delays between the components of a lensed quasar are basic tools to
analyze the expansion of the Universe and the structure of the main lens galaxy
halo. In this paper, we focus on the variability and time delay of the double
system SBS 0909+532A,B as well as the time behaviour of the field stars. We use
VR optical observations of SBS 0909+532A,B and the field stars in 2003. The
frames were taken at Calar Alto, Maidanak and Wise observatories, and the VR
light curves of the field stars and quasar components are derived from aperture
and point-spread function fitting methods. We measure the R-band time delay of
the system from the chi-square and dispersion techniques and 1000 synthetic
light curves based on the observed records. One nearby field star (SBS
0909+532c) is found to be variable, and the other two nearby field stars are
non-variable sources. With respect to the quasar components, the R-band records
seem more reliable and are more densely populated than the V-band ones. The
observed R-band fluctuations permit a pre-conditioned measurement of the time
delay. From the chi-square minimization, if we assume that the quasar emission
is observed first in B and afterwards in A (in agreement with basic
observations of the system and the corresponding predictions), we obtain a
delay of - 45 (+ 1)/(- 11) days (95% confidence interval). The dispersion
technique leads to a similar delay range. A by-product of the analysis is the
determination of a totally corrected flux ratio in the R band (corrected by the
time delay and the contamination due to the galaxy light). Our 95% measurement
of this ratio (0.575 +/- 0.014 mag) is in excellent agreement with previous
results from contaminated fluxes at the same time of observation.Comment: 26 pages, 15 figures, Astronomy and Astrophysics (see also
http://www.astro.ulg.ac.be/RPub/Colloques/JENAM/proceedings/proceedings.html
- Quasars Section
Evidence for energy injection and a fine-tuned central engine at optical wavelengths in GRB 070419A
We present a comprehensive multiwavelength temporal and spectral analysis of
the FRED GRB 070419A. The early-time emission in the -ray and X-ray
bands can be explained by a central engine active for at least 250 s, while at
late times the X-ray light curve displays a simple power-law decay. In
contrast, the observed behaviour in the optical band is complex (from 10 up
to 10 s). We investigate the light curve behaviour in the context of the
standard forward/reverse shock model; associating the peak in the optical light
curve at 450 s with the fireball deceleration time results in a Lorenz
factor at this time. In contrast, the shallow optical
decay between 450 and 1500 s remains problematic, requiring a reverse shock
component whose typical frequency is above the optical band at the optical peak
time for it to be explained within the standard model. This predicts an
increasing flux density for the forward shock component until t 4
10 s, inconsistent with the observed decay of the optical emission
from t 10 s. A highly magnetized fireball is also ruled out due to
unrealistic microphysic parameters and predicted light curve behaviour that is
not observed. We conclude that a long-lived central engine with a finely tuned
energy injection rate and a sudden cessation of the injection is required to
create the observed light curves - consistent with the same conditions that are
invoked to explain the plateau phase of canonical X-ray light curves of GRBs.Comment: 9 pages, 10 figures, accepted for publication in MNRA
A deep search for the host galaxies of GRBs with no detected optical afterglow
Gamma-Ray Bursts can provide information about star formation at high
redshifts. Even in the absence of a optical/near-infrared/radio afterglow, the
high detection rate of X-ray afterglows by swift/XRT and its localization
precision of 2-3 arcsec facilitates the identification and study of GRB host
galaxies. We focus on the search for the host galaxies of a sample of 17 bursts
with XRT error circles but no detected long-wavelength afterglow. Three of
these events can also be classified as truly dark bursts: the observed upper
limit on the optical flux of the afterglow was less than expected based on the
X-ray flux. Our study is based on deep R and K-band observations performed with
ESO/VLT instruments, supported by GROND and NEWFIRM. To be conservative, we
searched for host galaxies in an area with a radius twice the 90% swift/XRT
error circle. For 15 of the 17 bursts we find at least one galaxy inside the
doubled XRT error circle. In seven cases we discover extremely red objects in
the error circles. The most remarkable case is the host of GRB 080207 which as
a colour of R-K~4.7 mag (AB), one of the reddest galaxies ever associated with
a GRB. As a by-product of our study we identify the optical afterglow of GRB
070517A. Optically dim afterglows result from cosmological Lyman drop out and
dust extinction, but the former process is only equired for a minority of cases
(<1/3). Extinction by dust in the host galaxies might explain all other events.
Thereby, a seemingly non-negligible fraction of these hosts are globally
dust-enshrouded, extremely red galaxies. This suggests that bursts with
optically dim afterglows trace a subpopulation of massive starburst galaxies,
which are markedly different from the main body of the GRB host galaxy
population, namely the blue, subluminous, compact galaxies.Comment: 29 pages, 31 figures, accepted for publication in A&
The optically unbiased GRB host (TOUGH) survey. VI. Radio observations at z<1 and consistency with typical star-forming galaxies
The objective of this paper is to determine the level of obscured star
formation activity and dust attenuation in a sample of gamma-ray burst (GRB)
hosts; and to test the hypothesis that GRB hosts have properties consistent
with those of the general star-forming galaxy populations. We present a radio
continuum survey of all z<1 GRB hosts in The Optically Unbiased GRB Host
(TOUGH) sample supplemented with radio data for all (mostly pre-Swift) GRB-SN
hosts discovered before October 2006. We present new radio data for 22 objects
and have obtained a detection for three of them (GRB 980425, 021211, 031203;
none in the TOUGH sample), increasing the number of radio-detected GRB hosts
from two to five. The star formation rate (SFR) for the GRB 021211 host of ~825
Mo yr^-1, the highest ever reported for a GRB host, places it in the category
of ultraluminous infrared galaxies. We found that at least 63% of GRB hosts
have SFR 500 Mo yr^-1. For the
undetected hosts the mean radio flux (<35 uJy 3sigma) corresponds to an average
SFR < 15 Mo yr^-1. Moreover, ~88% of the z<1 GRB hosts have ultraviolet dust
attenuation A_UV < 6.7 mag (visual attenuation A_V < 3 mag). Hence we did not
find evidence for large dust obscuration in a majority of GRB hosts. Finally,
we found that the distributions of SFRs and A_UV of GRB hosts are consistent
with those of Lyman break galaxies, Halpha emitters at similar redshifts and of
galaxies from cosmological simulations. The similarity of the GRB population
with other star-forming galaxies is consistent with the hypothesis that GRBs, a
least at z<1, trace a large fraction of all star formation, and are therefore
less biased indicators than once thought.Comment: Accepted by ApJ. 13 pages, 4 figures, 2 tables. V3: minor changes to
match the published versio
The Host Galaxies of Swift Dark Gamma-Ray Bursts: Observational Constraints on Highly Obscured and Very High-Redshift GRBs
In this work we present the first results of our imaging campaign at Keck
Observatory to identify the host galaxies of "dark" gamma-ray bursts (GRBs),
events with no detected optical afterglow or with detected optical flux
significantly fainter than expected from the observed X-ray afterglow. We find
that out of a uniform sample of 29 Swift bursts rapidly observed by the Palomar
60-inch telescope through March 2008 (14 of which we classify as dark), all
events have either a detected optical afterglow, a probable optical host-galaxy
detection, or both. Our results constrain the fraction of Swift GRBs coming
from very high redshift (z > 7), such as the recent GRB 090423, to between
0.2-7 percent at 80% confidence. In contrast, a significant fraction of the
sample requires large extinction columns (host-frame A_V > 1 mag, with several
events showing A_V > 2-6 mag), identifying dust extinction as the dominant
cause of the dark GRB phenomenon. We infer that a significant fraction of GRBs
(and, by association, of high-mass star formation) occurs in highly obscured
regions. However, the host galaxies of dark GRBs seem to have normal optical
colors, suggesting that the source of obscuring dust is local to the vicinity
of the GRB progenitor or highly unevenly distributed within the host galaxy.Comment: 21 pages, 10 figures. Published in A
The afterglows of Swift-era gamma-ray bursts. I. Comparing pre-Swift and Swift-era long/soft (type II) GRB optical afterglows
We have gathered optical photometry data from the literature on a large sample of Swift-era gamma-ray burst (GRB) afterglows including GRBs up to 2009 September, for a total of 76 GRBs, and present an additional three pre-Swift GRBs not included in an earlier sample. Furthermore, we publish 840 additional new photometry data points on a total of 42 GRB afterglows, including large data sets for GRBs 050319, 050408, 050802, 050820A, 050922C, 060418, 080413A, and 080810. We analyzed the light curves of all GRBs in the sample and derived spectral energy distributions for the sample with the best data quality, allowing us to estimate the host-galaxy extinction. We transformed the afterglow light curves into an extinction-corrected z = 1 system and compared their luminosities with a sample of pre-Swift afterglows. The results of a former study, which showed that GRB afterglows clustered and exhibited a bimodal distribution in luminosity space, are weakened by the larger sample. We found that the luminosity distribution of the two afterglow samples (Swift-era and pre-Swift) is very similar, and that a subsample for which we were not able to estimate the extinction, which is fainter than the main sample, can be explained by assuming a moderate amount of line-of-sight host extinction. We derived bolometric isotropic energies for all GRBs in our sample, and found only a tentative correlation between the prompt energy release and the optical afterglow luminosity at 1 day after the GRB in the z = 1 system. A comparative study of the optical luminosities of GRB afterglows with echelle spectra (which show a high number of foreground absorbing systems) and those without, reveals no indication that the former are statistically significantly more luminous. Furthermore, we propose the existence of an upper ceiling on afterglow luminosities and study the luminosity distribution at early times, which was not accessible before the advent of the Swift satellite. Most GRBs feature afterglows that are dominated by the forward shock from early times on. Finally, we present the first indications of a class of long GRBs, which form a bridge between the typical highluminosity, high-redshift events and nearby low-luminosity events (which are also associated with spectroscopic supernovae) in terms of energetics and observed redshift distribution, indicating a continuous distribution overall. © 2010. The American Astronomical Society. All rights reserved. Printed in the U.S.A