783 research outputs found
A 5.5-year robotic optical monitoring of Q0957+561: substructure in a non-local cD galaxy
New light curves of the gravitationally lensed double quasar Q0957+561 in the
gr bands during 2008-2010 include densely sampled, sharp intrinsic fluctuations
with unprecedentedly high signal-to-noise ratio. These relatively violent flux
variations allow us to very accurately measure the g-band and r-band time
delays between the two quasar images A and B. Using correlation functions, we
obtain that the two time delays are inconsistent with each other at the 2sigma
level, with the r-band delay exceeding the 417-day delay in the g band by about
3 days. We also studied the long-term evolution of the delay-corrected flux
ratio B/A from our homogeneous two-band monitoring with the Liverpool Robotic
Telescope between 2005 and 2010. This ratio B/A slightly increases in periods
of violent activity, which seems to be correlated with the flux level in these
periods. The presence of the previously reported dense cloud within the cD
lensing galaxy, along the line of sight to the A image, could account for the
observed time delay and flux ratio anomalies.Comment: 8 pages, 6 figures, 4 tables, to appear in Astronomy and Astrophysic
New two-colour light curves of Q0957+561: time delays and the origin of intrinsic variations
We extend the gr-band time coverage of the gravitationally lensed double
quasar Q0957+561. New gr light curves permit us to detect significant intrinsic
fluctuations, to determine new time delays, and thus to gain perspective on the
mechanism of intrinsic variability in Q0957+561. We use new optical frames of
Q0957+561 in the g and r passbands from January 2005 to July 2007. These frames
are part of an ongoing long-term monitoring with the Liverpool robotic
telescope. We also introduce two photometric pipelines that are applied to the
new gr frames of Q0957+561. The transformation pipeline incorporates
zero-point, colour, and inhomogeneity corrections to the instrumental
magnitudes, so final photometry to the 1-2% level is achieved for both quasar
components. The two-colour final records are then used to measure time delays.
The gr light curves of Q0957+561 show several prominent events and gradients,
and some of them (in the g band) lead to a time delay between components of 417
+/- 2 d (1 sigma). We do not find evidence of extrinsic variability in the
light curves of Q0957+561. We also explore the possibility of a delay between a
large event in the g band and the corresponding event in the r band. The gr
cross-correlation reveals a time lag of 4.0 +/- 2.0 d (1 sigma; the g-band
event is leading) that confirms a previous claim of the existence of a delay
between the g and r band in this lensed quasar. The time delays (between quasar
components and between optical bands) from the new records and previous ones in
similar bands indicate that most observed variations in Q0957+561 (amplitudes
of about 100 mmag and timescales of about 100 d) are very probably due to
reverberation within the gas disc around the supermassive black hole.Comment: 13 pages, 9 figures. Accepted for publication in A&
Parameter free Hubble constant from the quadruply lensed quasar SDSS J1004 + 4112
We present a free-form lens model for the multiply lensed quasar in the
galaxy cluster SDSS J. Our lens model makes minimal assumptions
about the distribution of mass in the lens plane. We pay particular attention
to the model uncertainties on the predicted time delay, originating from the
particular configuration of model variables. Taking into account this
uncertainty, we obtain a value of the Hubble constant of km
s Mpc, consistent with independent recent estimates. The
predicted time delay between the central image E and image C (the first to
arrive), is days. Future measurements of will allow to impose a tighter constrain on from this
cluster-QSO system.Comment: 5 pages, 5 figure
Short-timescale Fluctuations in the Difference Light Curves of QSO 0957+561A,B: Microlensing or Noise?
From optical R band data of the double quasar QSO 0957+561A,B, we made two
new difference light curves (about 330 days of overlap between the time-shifted
light curve for the A image and the magnitude-shifted light curve for the B
image). We observed noisy behaviours around the zero line and no
short-timescale events (with a duration of months), where the term event refers
to a prominent feature that may be due to microlensing or another source of
variability. Only one event lasting two weeks and rising - 33 mmag was found.
Measured constraints on the possible microlensing variability can be used to
obtain information on the granularity of the dark matter in the main lensing
galaxy and the size of the source. In addition, one can also test the ability
of the observational noise to cause the rms averages and the local features of
the difference signals. We focused on this last issue. The combined
photometries were related to a process consisting of an intrinsic signal plus a
Gaussian observational noise. The intrinsic signal has been assumed to be
either a smooth function (polynomial) or a smooth function plus a stationary
noise process or a correlated stationary process. Using these three pictures
without microlensing, we derived some models totally consistent with the
observations. We finally discussed the sensitivity of our telescope (at Teide
Observatory) to several classes of microlensing variability.Comment: MNRAS, in press (LaTeX, 14 pages, 22 eps figures
The Abundance of SiC2 in Carbon Star Envelopes: Evidence that SiC2 is a gas-phase precursor of SiC dust
Silicon carbide dust is ubiquitous in circumstellar envelopes around C-rich
AGB stars. However, the main gas-phase precursors leading to the formation of
SiC dust have not yet been identified. The most obvious candidates among the
molecules containing an Si--C bond detected in C-rich AGB stars are SiC2, SiC,
and Si2C. We aim to study how widespread and abundant SiC2, SiC, and Si2C are
in envelopes around C-rich AGB stars and whether or not these species play an
active role as gas-phase precursors of silicon carbide dust in the ejecta of
carbon stars. We carried out sensitive observations with the IRAM 30m telescope
of a sample of 25 C-rich AGB stars to search for emission lines of SiC2, SiC,
and Si2C in the 2 mm band. We performed non-LTE excitation and radiative
transfer calculations based on the LVG method to model the observed lines of
SiC2 and to derive SiC2 fractional abundances in the observed envelopes. We
detect SiC2 in most of the sources, SiC in about half of them, and do not
detect Si2C in any source, at the exception of IRC +10216. Most of these
detections are reported for the first time in this work. We find a positive
correlation between the SiC and SiC2 line emission, which suggests that both
species are chemically linked, the SiC radical probably being the
photodissociation product of SiC2 in the external layer of the envelope. We
find a clear trend in which the denser the envelope, the less abundant SiC2 is.
The observed trend is interpreted as an evidence of efficient incorporation of
SiC2 onto dust grains, a process which is favored at high densities owing to
the higher rate at which collisions between particles take place. The observed
behavior of a decline in the SiC2 abundance with increasing density strongly
suggests that SiC2 is an important gas-phase precursor of SiC dust in envelopes
around carbon stars.Comment: Published in A&A. 16 pages and 10 figure
Determination of the properties of the central engine in microlensed QSOs
We study a recently observed gravitational microlensing peak in the V-band
light curve of Q2237+0305A using a relatively simple, but highly consistent
with the data (the best-fit reduced \chi^2 is very close to 1), physical model.
The source quasar is assumed to be a Newtonian geometrically-thin and
optically-thick accretion disk. The disk has an arbitrary orientation, and both
blackbody and greybody emission spectra are considered. When the
electron-photon scattering plays a role, the greybody spectrum will be a
simplified version of the exact one. In our model the microlensing variability
result from the source crossing a caustic straight line. The main goal is to
estimate the black hole mass and the mass accretion rate in QSO 2237+0305 as
well as to discuss the power and the weakness of the technique, some possible
improvements, and the future prospects from multifrequency monitoring of new
microlensing peaks. We also put into perspective the new methodology and the
results on the central engine in QSO 2237+0305. From the fitted microlensing
parameters and reasonable dynamical/cosmological constraints, it is concluded
that QSO 2237+0305 harbours a central massive black hole: 10^7 M_Sun < M < 6
10^8 M_Sun. While the information about the central dark mass is very
interesting, the mass accretion rate is not so well constrained. The typical
values of the disk luminosity/Eddington luminosity ratio are in the
(1-20)*\epsilon range, where \epsilon \leq 1 is the emissivity relative to a
blackbody and the highest L/L_{Edd} ratio corresponds to the largest deflector
motion. Therefore, in order to verify L/L_{Edd} \leq 1, a relatively small
projected peculiar motion of the lens galaxy and a greybody emission seem to be
favored.Comment: Astron. Astrophys., in press (LaTeX, 18 pages, 4 eps figures
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