156 research outputs found
Ultraviolet variability of quasars: dependence on the accretion rate
We compiled a catalogue of about 4000 SDSS quasars including individual
estimators V for the variability strength, virial black hole masses M, and mass
accretion rates dM/dt from the Davis-Laor scaling relation. We confirm
significant anti-correlations between V and dM/dt, the Eddington ratio, and the
bolometric luminosity L, respectively. A weak, statistically not significant
positive trend is indicated for the dependence of V on M. As a side product, we
find a strong correlation of the radiative efficiency with M and show that this
trend is most likely produced by selection effects in combination with the mass
errors and the use of the scaling relation for dM/dt. The anti-correlations
found for V cannot be explained in such a way. The strongest anti-correlation
is found with dM/dt. However, it is difficult to decide which of the quantities
(L, Eddington ratio, dM/dt) is intrinsically correlated with V and which of the
observed correlations are produced by the relations between these quantities. A
V-dM/dt anti-correlation is qualitatively expected for the strongly
inhomogeneous accretion disks. We argue that several observed variability
properties are not adequately explained by the simple multi-temperature
black-body model of a standard disk and suggest to check whether the strongly
inhomogeneous disk model is capable of reproducing these observations better.Comment: 16 pages, 10 figures; accepted for publication in Astronomy and
Astrophysics; the full catalogue is only available in electronic form at CD
A large sample of Kohonen-selected SDSS quasars with weak emission lines: selection effects and statistical properties
We performed a search for WLQs in the spectroscopic data from the Sloan
Digital Sky Survey Data Release 7 based on Kohonen self-organising maps for
nearly 10^5 quasar spectra. The final sample consists of 365 quasars and
includes in particular a subsample of 46 WLQs with equivalent widths W(MgII) <
11 A and W(CIV) < 4.8 A. We compared various properties of the WLQs with those
of control samples of ordinary quasars. Particular attention was paid to
selection effects. The WLQs have, on average, significantly higher
luminosities, Eddington ratios, and accretion rates. About half of the excess
comes from a selection bias, but an intrinsic excess remains probably caused
primarily by higher accretion rates. The spectral energy distribution shows a
bluer continuum at rest-frame wavelengths > 1500 A. The variability in the
optical and UV is relatively low, even taking the variability-luminosity
anti-correlation into account. The percentage of radio detected quasars and of
core-dominant radio sources is significantly higher than for the control
sample, whereas the mean radio-loudness is lower. The properties of our WLQ
sample can be consistently understood assuming that it consists of a mix of
quasars at the beginning of a stage of increased accretion activity and of
beamed radio-quiet quasars. (Abstract modified to match the arXiv format)Comment: 17 pages, 12 figures; accepted for publication in Astronomy and
Astrophysics; the full catalogue is only available in electronic form at CD
A QSO survey via optical variability and zero proper motion in the M92 field.III. Narrow emission line galaxies
We study a sample of 23 narrow-emission line galaxies (NELGs) which were
selected by their strong variability as QSO candidates in the framework of a
variability-and-proper motion QSO survey on digitised Schmidt plates. In
previous work, we have shown that variability is an efficient method to find
AGNs. The variability properties of the NELGs are however significantly
different from those of the QSOs. The main aim of this paper is to clarify the
nature of this variability and to estimate the fraction of AGN-dominated NELGs
in this sample. New photometric and spectroscopic observations are presented,
along with revised data from the photographic photometry. The originally
measured high variability indices could not be confirmed. The diagnostic
line-ratios of the NELG spectra are consistent with HII region-like spectra. No
AGN could be proved, yet we cannot rule out the existence of faint
low-luminosity AGNs masked by HII regions from intense star formation.Comment: 10 pages, 8 figures, accepted for publication in Astronomy &
Astrophysic
A systematic search for novae in M31 on a large set of digitized archival Schmidt plates
This paper reports on the detection of optical novae in our neighbour galaxy
M31 based on digitized historical Tautenburg Schmidt plates. The accurate
positions of the detected novae lead to a much larger database when searching
for recurrent novae in M31. We conducted a systematic search for novae on 306
digitized Tautenburg Schmidt plates covering a time span of 36 years from 1960
to 1996. From the database of both ~ 300 000 light curves and about one million
detections on only one plate per colour band, nova candidates were efficiently
selected by automated algorithms and subsequently individually inspected by
eye. We report the detection of 84 nova candidates. We found 55 nova candidates
from the automated analysis of the light curves. Among these, 22 were
previously unknown, 12 were known but not identified on Tautenburg Schmidt
plates before, and 21 novae had been previously discovered on Tautenburg
plates. An additional 29 known novae could be confirmed by the detailed
investigation of single detections. One of our newly discovered nova candidates
shows a high position coincidence with a nova detected about 30 years earlier.
Therefore, this object is likely to be a recurrent nova. Furthermore, we
re-investigated all 41 nova candidates previously found on Tautenburg plates
and confirm all but two. Positions are given for all nova candidates with a
typical accuracy of ~ 0.4 arcsec. We present light curves and finding charts as
online material. The analysis of the plates has shown the wealth of information
still buried in old plate archives. Extrapolating from this survey,
digitization of other historical M31 plate archives (e.g. from the Mount Wilson
or Asiago observatories) for a systematic nova search looks very promising.Comment: 29 pages, 14 figures, 19 tables, accepted for publication in A&A.
Figs 6-14 are reduced in resolution due to the restrictions on space
available on astro-ph; v2: minor grammatical change
FSR 0190: Another old distant Galactic cluster
We are conducting a large programme to classify newly discovered Milky Way star cluster candidates from Froebrich et al. Here we present near-infrared follow-up observations of FSR0190 (α = 20h05m31s.3, δ = 33°34â?²09â?², J2000). The cluster is situated close to the Galactic plane (l = 70°7302, b = +0°9498). It shows a circular shape, and a relatively large number of core helium burning stars - which clearly distinguishes the cluster from the rich field - but no centrally condensed star density profile. We derive an age of more than 7 Gyr, a Galactocentric distance of 10.5 kpc, a distance of 10 kpc from the Sun, and an extinction of AK = 0.8 mag. The estimated mass is at least of the order of 105 Mâ??, and the absolute brightness is MV â?¤ -4.7 mag; both are rather typical properties for Palomar-type globular clusters. © 2007 The Authors. Journal compilation © 2007 RAS
A QSO survey via optical variability and zero proper motion in the M92 field. IV. More QSOs due to improved photometry
We continue the QSO search in the 10 square degrees Schmidt field around M92
based on variability and proper motion (VPM) constraints. We have re-reduced
162 digitised B plates with a time-baseline of more than three decades and have
considerably improved both the photometric accuracy and the star-galaxy
separation at B>19. QSO candidates are selected and marked with one out of
three degrees of priority based on the statistical significance of their
measured variability and zero proper motion. Spectroscopic follow-up
observations of 84 new candidates with B>19 revealed an additional 37 QSOs and
7 Seyfert1s. In particular, all 92 high-priority candidates are
spectroscopically classified now; among them are 70 QSOs and 9 Seyfert1s
(success rate 86%). We expect that 87% (55%) of all QSOs with B<19.0 (19.8) are
contained in this high-priority subsample. For the combined sample of
high-priority and medium-priority objects, a completeness of 89% is estimated
up to B_lim=19.5. The sample of all AGNs detected in the framework of the VPM
search in the M92 field contains now 95 QSOs and 14 Seyfert1s with B<19.9.
Although the VPM QSOs were selected by completely different criteria, their
properties do not significantly differ from those of QSOs found by more
traditional optical survey techniques. In particular, the spectra and the
optical broad band colours do not provide any hints on a substantial population
of red QSOs up to the present survey limit.Comment: 13 pages, 12 figures, accepted for publication in Astronomy and
Astrophysic
A comparison of compact, presumably young with extended, evolved radio active galactic nuclei
Radio sources with peaked spectra (peaked spectrum sources, PSS) and compact
symmetric objects (CSO) are powerful, compact, and presumably young AGNs and
therefore particularly suitable to study aspects of the AGN-host connection. We
use a statistical approach to compare a PSS-CSO sample with a matching
comparison sample of extended sources (ECS). We find significant differences
between the two samples. In particular, we find that the ECS sample has a
higher proportion of passive galaxies with a lower star formation activity.
This applies to both sub-samples of QSOs or radio galaxies as well as to the
entire sample. The star formation rates of the PSS-CSO host galaxies are
typically in the range 0 to 5 M_sun/yr and the stellar masses are in the range
3x10^11 to 10^12 M_sun. Secondly, in agreement with previous results, we find a
remarkably high proportion of PSS-CSO host galaxies with merger signatures. The
merger fraction of the PSS-CSO sample is 0.61, which is significantly higher
than that of the comparison sample (0.15). We suggest that this difference can
be explained by assuming that the majority of the PSSs and CSOs cannot evolve
to extended radio sources and are therefore not represented in our comparison
sample.Comment: 39 pages, accepted by A&
High-Resolution NIR Observations of the Circumstellar Disk System in the Bok Globule CB 26
We report on results of near-infrared and optical observations of the mm disk
embedded in the Bok globule CB 26 (Launhardt & Sargent 2001). The near-infrared
images show a bipolar reflection nebula with a central extinction lane which
coincides with the mm disk. Imaging polarimetry of this object yielded a
polarization pattern which is typical for a young stellar object surrounded by
a large circumstellar disk and an envelope, seen almost edge-on. The strong
linear polarization in the bipolar lobes is caused by single scattering at dust
grains and allowed to locate the illuminating source which coincides with the
center of the mm disk. The spectral energy distribution of the YSO embedded in
CB 26 resembles that of a ClassI source with a luminosity of 0.5 L_sun.Using
the pre-main-sequence evolutionary tracks and the stellar mass inferred from
the rotation curve of the disk, we derive an age of the system of <10^6 yr.
H_alpha and [SII] narrow-band imaging as well as optical spectroscopy revealed
an Herbig-Haro object 6.15 arcmin northwest of CB 26 YSO 1, perfectly aligned
with the symmetry axis of the bipolar nebula. This Herbig-Haro object (HH 494)
indicates ongoing accretion and outflow activity in CB 26 YSO 1. Its excitation
characteristics indicate that the Herbig-Haro flow is propagating into a
low-density environment. We suggest that CB 26 YSO 1 represents the transition
stage between embedded protostellar accretion disks and more evolved
protoplanetary disks around T Tauri stars in an undisturbed environment.Comment: 21 pages, 6 figures (reduced resolution), ApJ accepte
VPMS J1342+2840 - an unusual quasar from the variability and proper motion survey
We report the discovery of the highly peculiar, radio-loud quasar VPMS
J1342+2840 (z ~ 1.3) from the variability and proper motion survey. We present
spectroscopic, imaging and photometric observations. The unusual spectrum shows
a strong depression of the continuum over a wide wavelength range in the blue
part without the typical structures of broad absorption line (BAL) troughs. The
image of the quasar is unresolved and there is no evidence for a foreground
object on the line of sight. The broad-band spectral energy distribution is not
consistent with obvious dust reddening with the standard SMC extinction curve.
The downturn of the continuum flux of VPMS J1342+2840 at short wavelengths can
be caused by dust reddening only if the reddening curve is steeper then the SMC
curve in the ultraviolet and is very flat at longer wavelengths. Alternatively,
the dominant spectral features can be explained by low-ionization BALs forming
unusually wide, overlapping absorption troughs.Comment: 4 pages, 3 figures, accepted for publication in A&A Letter
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