1,049 research outputs found
Quasar optical variability: searching for interband time delays
Aims. The main purpose of this paper is to study time delays between the
light variations in different wavebands for a sample of quasars. Measuring a
reliable time delay for a large number of quasars may help constraint the
models of their central engines. The standard accretion disk irradiation model
predicts a delay of the longer wavelengths behind the shorter ones, a delay
that depends on the fundamental quasar parameters. Since the black hole masses
and the accretion rates are approximately known for the sample we use, one can
compare the observed time delays with the expected ones.
Methods. We applied the interpolation cross-correlation function (ICCF)
method to the Giveon et al. sample of 42 quasars, monitored in two (B and R)
colors, to find the time lags represented by the ICCF peaks. Different tests
were performed to assess the influence of photometric errors, sampling, etc.,
on the final result.
Results. We found that most of the objects show a delay in the red light
curve behind the blue one (a positive lag), which on average for the sample is
about +4 days (+3 for the median), although the scatter is significant. These
results are broadly consistent with the reprocessing model, especially for the
well-sampled objects. The normalized time-lag deviations do not seem to
correlate significantly with other quasar properties, including optical, radio,
or X-ray measurables. On the other hand, many objects show a clear negative
lag, which, if real, may have important consequences for the variability
models.Comment: 5 pages, 4 figures, accepted in A&
Assessing statistical significance of periodogram peaks
The least-squares (or Lomb-Scargle) periodogram is a powerful tool which is
used routinely in many branches of astronomy to search for periodicities in
observational data. The problem of assessing statistical significance of
candidate periodicities for different periodograms is considered. Based on
results in extreme value theory, improved analytic estimations of false alarm
probabilities are given. They include an upper limit to the false alarm
probability (or a lower limit to the significance). These estimations are
tested numerically in order to establish regions of their practical
applicability.Comment: 7 pages, 6 figures, 1 table; To be published in MNRA
The Lick Planet Search: Detectability and Mass Thresholds
We analyse 11 years of precise radial velocities for 76 solar type stars from
the Lick survey. Eight stars in this sample have previously reported
planetary-mass companions, all with mass (m sin i) less than 8 Jupiter masses
(MJ). For the stars without a detected companion, we place upper limits on
possible companion mass. For most stars, we can exclude companions with m sin i
> 0.7 MJ (a/AU)^1/2 for orbital radii a < 5 AU.
We use our results to interpret the observed masses and orbital radii of
planetary-mass companions. For example, we show that the finite duration of the
observations makes detection of Jupiter mass companions more and more difficult
for orbital radii beyond 3 AU. Thus it is possible that the majority of solar
type stars harbor Jupiter-mass companions much like our own, and if so these
companions should be detectable in a few years.
To search for periodicities, we adopt a "floating-mean" periodogram, which
improves on the traditional Lomb-Scargle periodogram by accounting for
statistical fluctuations in the mean of a sampled sinusoid. We discuss in
detail the normalization of the periodogram, an issue which has been of some
debate in the literature.Comment: To appear in the Astrophysical Journal (50 pages, LaTeX, including 11
figures
On the orbital period of the cataclysmic variable RZ Leonis
In this research note we present a time-resolved study of the Balmer emission
lines of RZ Leo. From the analysis of the radial velocities we find an orbital
period of 0.07651(26) d. This is in excellent agreement with the
photometrically determined periods in quiescence and during the early stages of
superoutburst. A comparison of the recently determined superhump period gives
an excess of ~0.03, which is a typical value for an SU UMa star of this period.Comment: 3 pages, 6 figures, A&A, accepte
The existence of warm and optically thick dissipative coronae above accretion disks
In the past years, several observations of AGN and X-ray binaries have
suggested the existence of a warm T around 0.5-1 keV and optically thick, \tau
~ 10-20, corona covering the inner parts of the accretion disk. These
properties are directly derived from spectral fitting in UV to soft-X-rays
using Comptonization models. However, whether such a medium can be both in
radiative and hydrostatic equilibrium with an accretion disk is still
uncertain. We investigate the properties of such warm, optically thick coronae
and put constraints on their existence. We solve the radiative transfer
equation for grey atmosphere analytically in a pure scattering medium,
including local dissipation as an additional heating term in the warm corona.
The temperature profile of the warm corona is calculated assuming it is cooled
by Compton scattering, with the underlying dissipative disk providing photons
to the corona. Our analytic calculations show that a dissipative thick,
(\tau_{cor} ~ 10-12) corona on the top of a standard accretion disk can reach
temperatures of the order of 0.5-1 keV in its upper layers provided that the
disk is passive. But, in absence of strong magnetic fields, the requirement of
a Compton cooled corona in hydrostatic equilibrium in the vertical direction
sets an upper limit on the Thomson optical depth \tau_{cor} < 5 . We show this
value cannot be exceeded independently of the accretion disk parameters.
However, magnetic pressure can extend this result to larger optical depths.
Namely, a dissipative corona might have an optical depth up to ~ 20 when the
magnetic pressure is 100 times higher that the gas pressure. The observation of
warm coronae with Thomson depth larger than ~ 5 puts tights constraints on the
physics of the accretion disk/corona systems and requires either strong
magnetic fields or vertical outflows to stabilize the system.Comment: 9 pages 6 figure, submitted to A&A, comments are welcom
X-ray Variability of AGN and the Flare Model
Short-term variability of X-ray continuum spectra has been reported for
several Active Galactic Nuclei. Significant X-ray flux variations are observed
within time scales down to 10^3-10^5 seconds. We discuss short variability time
scales in the frame of the X-ray flare model, which assumes the release of a
large hard X-ray flux above a small portion of the accretion disk. The
resulting observed X-ray spectrum is composed of the primary radiation and of a
reprocessed Compton reflection component that we model with numerical radiative
transfer simulations. The incident hard X-rays of the flare will heat up the
atmosphere of the accretion disk and hence induce thermal expansion.
Eventually, the flare source will be surrounded by an optically thick medium,
which should modify the observed spectra.Comment: 4 pages, 3 figures, accepted proceedings for a talk at the conference
"AGN variability from the X-rays to the radio", June 2004, Crimean
Observator
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