37 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
Anomalous Fluctuations in Observations of Q0957+561 A,B: Smoking Gun of a Cosmic String?
We report the detection of anomalous brightness fluctuations in the multiple
image Q0957+561 A,B gravitational lens system, and consider whether such
anomalies have a plausible interpretation within the framework of cosmic string
theory. We study a simple model of gravitational lensing by an asymmetrical
rotating string. An explicit form of the lens equation is obtained and
approximate relations for magnification are derived. We show that such a model
with typical parameters of the GUT string can quantitatively reproduce the
observed pattern of brightness fluctuations. On the other hand, explanation
involving a binary star system as an alternative cause requires an unacceptably
large massive object at a small distance. We also discuss possible
observational manifestations of cosmic strings within our lens model.Comment: Published in Astronomy and Astrophysics. 7 pages, 6 figure
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
Long duration radio transients lacking optical counterparts are possibly Galactic Neutron Stars
(abridged) Recently, a new class of radio transients in the 5-GHz band was
detected by Bower et al. We present new deep near-Infrared (IR) observations of
the field containing these transients, and find no counterparts down to a
limiting magnitude of K=20.4 mag. We argue that the bright (>1 Jy) radio
transients recently reported by Kida et al. are consistent with being
additional examples of the Bower et al. transients. We refer to these groups of
events as "long-duration radio transients". The main characteristics of this
population are: time scales longer than 30 minute but shorter than several
days; rate, ~10^3 deg^-2 yr^-1; progenitors sky surface density of >60 deg^-2
(95% C.L.) at Galactic latitude ~40 deg; 1.4-5 GHz spectral slopes, f_\nu ~
\nu^alpha, with alpha>0; and most notably the lack of any counterparts in
quiescence in any wavelength. We rule out an association with many types of
objects. Galactic brown-dwarfs or some sort of exotic explosions remain
plausible options. We argue that an attractive progenitor candidate for these
radio transients is the class of Galactic isolated old neutron stars (NS). We
confront this hypothesis with Monte-Carlo simulations of the space distribution
of old NSs, and find satisfactory agreement for the large areal density.
Furthermore, the lack of quiescent counterparts is explained quite naturally.
In this framework we find: the mean distance to events in the Bower et al.
sample is of order kpc; the typical distance to the Kida et al. transients are
constrained to be between 30 pc and 900 pc (95% C.L.); these events should
repeat with a time scale of order several months; and sub-mJy level bursts
should exhibit Galactic latitude dependence. We discuss possible mechanisms
giving rise to the observed radio emission.Comment: Submitted to ApJ, 17 pages, 10 figure
How accurate are the time delay estimates in gravitational lensing?
We present a novel approach to estimate the time delay between light curves
of multiple images in a gravitationally lensed system, based on Kernel methods
in the context of machine learning. We perform various experiments with
artificially generated irregularly-sampled data sets to study the effect of the
various levels of noise and the presence of gaps of various size in the
monitoring data. We compare the performance of our method with various other
popular methods of estimating the time delay and conclude, from experiments
with artificial data, that our method is least vulnerable to missing data and
irregular sampling, within reasonable bounds of Gaussian noise. Thereafter, we
use our method to determine the time delays between the two images of quasar
Q0957+561 from radio monitoring data at 4 cm and 6 cm, and conclude that if
only the observations at epochs common to both wavelengths are used, the time
delay gives consistent estimates, which can be combined to yield 408\pm 12
days. The full 6 cm dataset, which covers a longer monitoring period, yields a
value which is 10% larger, but this can be attributed to differences in
sampling and missing data.Comment: 14 pages, 12 figures; accepted for publication in Astronomy &
Astrophysic
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&