230 research outputs found
On the variability of quasars: a link between Eddington ratio and optical variability?
Repeat scans by the Sloan Digital Sky Survey (SDSS) of a 278 square degree
stripe along the Celestial equator have yielded an average of over 10
observations each for nearly 8,000 spectroscopically confirmed quasars. Over
2500 of these quasars are in the redshift range such that the CIV emission line
is visible in the SDSS spectrum. Utilising the width of these CIV lines and the
luminosity of the nearby continuum, we estimate black hole masses for these
objects. In an effort to isolate the effects of black hole mass and luminosity
on the photometric variability of our dataset, we create several subsamples by
binning in these two physical parameters. By comparing the ensemble structure
functions of the quasars in these bins, we are able to reproduce the well-known
anticorrelation between luminosity and variability, now showing that this
anticorrelation is independent of the black hole mass. In addition, we find a
correlation between variability and the mass of the central black hole. By
combining these two relations, we identify the Eddington ratio as a possible
driver of quasar variability, most likely due to differences in accretion
efficiency.Comment: 13 pages, 5 figures, Accepted for publication in MNRA
Dependence of the optical/UV variability on the emission line properties and Eddington ratio in active galactic nuclei
The dependence of the long-term optical/UV variability on the spectral and
the fundamental physical parameters for radio-quiet active galactic nuclei
(AGNs) is investigated. The multi-epoch repeated photometric scanning data in
the Stripe-82 region of the Sloan Digital Sky Survey (SDSS) are exploited for
two comparative AGN samples (mostly quasars) selected therein, a broad-line
Seyfert\,1 (BLS1) type sample and a narrow-line Seyfert\,1 (NLS1) type AGN
sample within redshifts 0.3--0.8. Their spectral parameters are derived from
the SDSS spectroscopic data. It is found that on rest-frame timescales of
several years the NLS1-type AGNs show systematically smaller variability
compared to the BLS1-type. In fact, the variability amplitude is found to
correlate, though only moderately, with the Eigenvector\,1 parameters, i.e.,
the smaller the \hb\ linewidth, the weaker the [O\,III] and the stronger the
\feii\ emission, the smaller the variability amplitude is. Moreover, an
interesting inverse correlation is found between the variability and the
Eddington ratio, which is perhaps more fundamental. The previously known
dependence of the variability on luminosity is not significant, and that on
black hole mass---as claimed in recent papers and also present in our
data---fades out when controlling for the Eddington ratio in the correlation
analysis, though these may be partly due to the limited ranges of luminosity
and black hole mass of our samples. Our result strongly supports that an
accretion disk is likely to play a major role in producing the opitcal/UV
variability.Comment: 6 pages, 4 figures, 1 tables, accepted for publication in ApJ Lette
Measuring Lensing Magnification of Quasars by Large Scale Structure using the Variability-Luminosity Relation
We introduce a technique to measure gravitational lensing magnification using
the variability of type I quasars. Quasars' variability amplitudes and
luminosities are tightly correlated, on average. Magnification due to
gravitational lensing increases the quasars' apparent luminosity, while leaving
the variability amplitude unchanged. Therefore, the mean magnification of an
ensemble of quasars can be measured through the mean shift in the
variability-luminosity relation. As a proof of principle, we use this technique
to measure the magnification of quasars spectroscopically identified in the
Sloan Digital Sky Survey, due to gravitational lensing by galaxy clusters in
the SDSS MaxBCG catalog. The Palomar-QUEST Variability Survey, reduced using
the DeepSky pipeline, provides variability data for the sources. We measure the
average quasar magnification as a function of scaled distance (r/R200) from the
nearest cluster; our measurements are consistent with expectations assuming NFW
cluster profiles, particularly after accounting for the known uncertainty in
the clusters' centers. Variability-based lensing measurements are a valuable
complement to shape-based techniques because their systematic errors are very
different, and also because the variability measurements are amenable to
photometric errors of a few percent and to depths seen in current wide-field
surveys. Given the data volume expected from current and upcoming surveys, this
new technique has the potential to be competitive with weak lensing shear
measurements of large scale structure.Comment: Accepted for publication in Ap
Selecting Quasars by their Intrinsic Variability
We present a new and simple technique for selecting extensive, complete and
pure quasar samples, based on their intrinsic variability. We parametrize the
single-band variability by a power-law model for the light-curve structure
function, with amplitude A and power-law index gamma. We show that quasars can
be efficiently separated from other non-variable and variable sources by the
location of the individual sources in the A-gamma plane. We use ~60 epochs of
imaging data, taken over ~5 years, from the SDSS stripe 82 (S82) survey, where
extensive spectroscopy provides a reference sample of quasars, to demonstrate
the power of variability as a quasar classifier in multi-epoch surveys. For
UV-excess selected objects, variability performs just as well as the standard
SDSS color selection, identifying quasars with a completeness of 90% and a
purity of 95%. In the redshift range 2.5<z<3, where color selection is known to
be problematic, variability can select quasars with a completeness of 90% and a
purity of 96%. This is a factor of 5-10 times more pure than existing
color-selection of quasars in this redshift range. Selecting objects from a
broad griz color box without u-band information, variability selection in S82
can afford completeness and purity of 92%, despite a factor of 30 more
contaminants than quasars in the color-selected feeder sample. This confirms
that the fraction of quasars hidden in the 'stellar locus' of color-space is
small. To test variability selection in the context of Pan-STARRS 1 (PS1) we
created mock PS1 data by down-sampling the S82 data to just 6 epochs over 3
years. Even with this much sparser time sampling, variability is an
encouragingly efficient classifier. For instance, a 92% pure and 44% complete
quasar candidate sample is attainable from the above -selected catalog.Comment: 16 pages, 9 color figures and 5 tables - v3: Equations corrected and
text updated (see Erratum for details of corrections). Erratum:
http://adsabs.harvard.edu/abs/2010ApJ...721.1941S Original Paper:
http://adsabs.harvard.edu/abs/2010ApJ...714.1194
A Description of Quasar Variability Measured Using Repeated SDSS and POSS Imaging
We provide a quantitative description and statistical interpretation of the
optical continuum variability of quasars. The Sloan Digital Sky Survey (SDSS)
has obtained repeated imaging in five UV-to-IR photometric bands for 33,881
spectroscopically confirmed quasars. About 10,000 quasars have an average of 60
observations in each band obtained over a decade along Stripe 82 (S82), whereas
the remaining ~25,000 have 2-3 observations due to scan overlaps. The observed
time lags span the range from a day to almost 10 years, and constrain quasar
variability at rest-frame time lags of up to 4 years, and at rest-frame
wavelengths from 1000A to 6000A. We publicly release a user-friendly catalog of
quasars from the SDSS Data Release 7 that have been observed at least twice in
SDSS or once in both SDSS and the Palomar Observatory Sky Survey, and we use it
to analyze the ensemble properties of quasar variability. Based on a damped
random walk (DRW) model defined by a characteristic time scale and an
asymptotic variability amplitude that scale with the luminosity, black hole
mass, and rest wavelength for individual quasars calibrated in S82, we can
fully explain the ensemble variability statistics of the non-S82 quasars such
as the exponential distribution of large magnitude changes. All available data
are consistent with the DRW model as a viable description of the optical
continuum variability of quasars on time scales of ~5-2000 days in the rest
frame. We use these models to predict the incidence of quasar contamination in
transient surveys such as those from PTF and LSST.Comment: 33 pages, 19 figures, replaced with accepted version. Catalog is
available at http://www.astro.washington.edu/users/ivezic/macleod/qso_dr7
X-ray Emission from Optically Selected Radio-Intermediate and Radio-Loud Quasars
We present the results of an investigation into the X-ray properties of
radio-intermediate and radio-loud quasars (RIQs and RLQs, respectively). We
combine large, modern optical (e.g., SDSS) and radio (e.g., FIRST) surveys with
archival X-ray data from Chandra, XMM-Newton, and ROSAT to generate an
optically selected sample that includes 188 RIQs and 603 RLQs. This sample is
constructed independently of X-ray properties but has a high X-ray detection
rate (85%); it provides broad and dense coverage of the l-z plane, including at
high redshifts (22% of objects have z=2-5), and it extends to high
radio-loudness values (33% of objects have R*=3-5, using logarithmic units). We
measure the "excess" X-ray luminosity of RIQs and RLQs relative to radio-quiet
quasars (RQQs) as a function of radio loudness and luminosity, and parameterize
the X-ray luminosity of RIQs and RLQs both as a function of optical/UV
luminosity and also as a joint function of optical/UV and radio luminosity.
RIQs are only modestly X-ray bright relative to RQQs; it is only at high values
of radio-loudness (R*>3.5) and radio luminosity that RLQs become strongly X-ray
bright. We find no evidence for evolution in the X-ray properties of RIQs and
RLQs with redshift (implying jet-linked IC/CMB emission does not contribute
substantially to the nuclear X-ray continuum). Finally, we consider a model in
which the nuclear X-ray emission contains both disk/corona-linked and
jet-linked components and demonstrate that the X-ray jet-linked emission is
likely beamed but to a lesser degree than applies to the radio jet. This model
is used to investigate the increasing dominance of jet-linked X-ray emission at
low inclinations.Comment: Accepted to ApJ; 42 pages, 21 figures, 10 tables; version with
full-res figures at http://www.astro.lsa.umich.edu/~mbrendan/rlqx.htm
Ensemble Variability of Near-Infrared-Selected Active Galactic Nuclei
We present the properties of the ensemble variability for nearly 5000
near-infrared (NIR) AGNs selected from the catalog of Quasars and Active
Galactic Nuclei (13th Ed.) and the SDSS-DR7 quasar catalog. From 2MASS, DENIS,
and UKIDSS/LAS point source catalogs, we extract 2MASS-DENIS and 2MASS-UKIDSS
counterparts for cataloged AGNs by catalog cross-identification. We further
select variable AGNs based on an optimal criterion for selecting the variable
sources. The sample objects are divided into subsets according to whether NIR
light originates by optical or NIR emission in the rest frame; and we examine
the correlations of the ensemble variability with the rest-frame wavelength,
redshift, luminosity, and rest-frame time lag. In addition, we also examine the
correlations of variability amplitude with optical variability, radio
intensity, and radio-to-optical flux ratio. The rest-frame optical variability
of our samples shows known negative correlations with luminosity and positive
correlations with rest-frame time lag (i.e., the structure function, SF).
However, no well-known negative correlation exists between the rest wavelength
and optical variability. This inconsistency might be due to a biased sampling
of high-redshift AGNs. NIR variability in the rest frame is anticorrelated with
the rest wavelength, which is consistent with previous suggestions. However,
correlations of NIR variability with luminosity and rest-frame time lag are the
opposite of these correlations of the optical variability; that is, the NIR
variability is positively correlated with luminosity but negatively correlated
with the rest-frame time lag. Because these trends are qualitatively consistent
with the properties of radio-loud quasars reported by some previous studies,
most of our sample objects are probably radio-loud quasars. Finally, we also
discuss the negative correlations seen in the NIR SFs.Comment: 13 pages, 10 figures, Accepted for publication in Ap
Long-Term Optical Continuum Color Variability of Nearby Active Galactic Nuclei
We examine whether the spectral energy distribution of optical continuum
emission of active galactic nuclei (AGNs) changes during flux variation, based
on accurate and frequent monitoring observations of 11 nearby Seyfert galaxies
and QSOs carried out in the B, V, and I bands for seven years by the MAGNUM
telescope. The multi-epoch flux data in any two different bands obtained on the
same night show a very tight linear flux to flux relationship for all target
AGNs. The flux of the host galaxy within the photometric aperture is carefully
estimated by surface brightness fitting to available high-resolution HST images
and MAGNUM images. The flux of narrow emission lines in the photometric bands
is also estimated from available spectroscopic data. We find that the
non-variable component of the host galaxy plus narrow emission lines for all
target AGNs is located on the fainter extension of the linear regression line
of multi-epoch flux data in the flux to flux diagram. This result strongly
indicates that the spectral shape of AGN continuum emission in the optical
region does not systematically change during flux variation. The trend of
spectral hardening that optical continuum emission becomes bluer as it becomes
brighter, which has been reported by many studies, is therefore interpreted as
the domination of the variable component of the nearly constant spectral shape
of an AGN as it brightens over the non-variable component of the host galaxy
plus narrow lines, which is usually redder than AGN continuum emission.Comment: 47 pages, 29 figures, AASTeX, Accepted for publication in Ap
A Candidate Sub-Parsec Supermassive Binary Black Hole System
We identify SDSS J153636.22+044127.0, a QSO discovered in the Sloan Digital
Sky Survey, as a promising candidate for a binary black hole system. This QSO
has two broad-line emission systems separated by 3500 km/sec. The redder system
at z=0.3889 also has a typical set of narrow forbidden lines. The bluer system
(z=0.3727) shows only broad Balmer lines and UV Fe II emission, making it
highly unusual in its lack of narrow lines. A third system, which includes only
unresolved absorption lines, is seen at a redshift, z=0.3878, intermediate
between the two emission-line systems. While the observational signatures of
binary nuclear black holes remain unclear, J1536+0441 is unique among all QSOs
known in having two broad-line regions, indicative of two separate black holes
presently accreting gas. The interpretation of this as a bound binary system of
two black holes having masses of 10^8.9 and 10^7.3 solar masses, yields a
separation of ~ 0.1 parsec and an orbital period of ~100 years. The separation
implies that the two black holes are orbiting within a single narrow-line
region, consistent with the characteristics of the spectrum. This object was
identified as an extreme outlier of a Karhunen-Loeve Transform of 17,500 z <
0.7 QSO spectra from the SDSS. The probability of the spectrum resulting from a
chance superposition of two QSOs with similar redshifts is estimated at
2X10^-7, leading to the expectation of 0.003 such objects in the sample
studied; however, even in this case, the spectrum of the lower redshift QSO
remains highly unusual.Comment: 8 pages, 2 figures, Nature in pres
Evolution of the Lyman-alpha Halos around High-Redshift Radio Galaxies
We have obtained the first constraints on extended Ly-alpha emission at z ~ 1
in a sample of five radio galaxies. We detect Ly-alpha emission from four of
the five galaxies. The Ly-alpha luminosities range from 0.1 - 4 times 10^43
erg/s and are much smaller than those observed for halos around higher redshift
radio galaxies. If the z ~ 1 radio galaxies are the descendents the z >~ 2
radio galaxies, then their Ly-alpha luminosities evolve strongly with redshift
as ~(1+z)^5. There do not appear to be strong correlations between other
parameters, such as radio power, suggesting that this observed evolution is
real and not an observational artifact or secondary correlation. We speculate
that this evolution of luminous halos may be due to gas depletion (as gas
cools, settles, and forms stars) accompanied by an overall rise in the mean gas
temperature and a decrease in specific star-formation rate in and around these
massive galaxies.Comment: 5 pages, 4 figures, published in ApJ Letters, 694, L31-35 March 20
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