47 research outputs found
An accretion disc model for quasar optical variability
Some different correlations between optical-UV variability and other quasar
properties, such as luminosity, black hole mass and rest-frame wavelength, were
discovered. The positive correlation between optical-UV variability amplitude
and black hole mass was first found by Wold et al., and this was confirmed by
Wilhite et al. We suggest that the accretion disk model can explain these
correlations, provided the optical-UV variability is triggered by the change of
accretion rate. The disk temperature of accretion discs decreases with
increasing black hole mass, which leads to systematical spectral shape
difference with black hole mass even if the black hole is accreting at the same
rate m_dot (m_dot = M_dot / M_dotEdd). The observed positive correlation
between optical-UV variability and black hole mass can be well reproduced by
our model calculations, if the mean accretion rate m_dot0 ~ 0.1 with variation
of m_delta ~ 0.4 - 0.5 m_dot0. We also found that the observed correlations of
optical-UV variability amplitude with luminosity or rest-frame wavelength can
be qualitatively explained by this accretion disc model.Comment: 4 pages, 4 figures, accepted for publication in MNRAS Letter
Interacting coronae of two T Tauri stars: first observational evidence for solar-like helmet streamers
Context {The young binary system V773 Tau A exhibits a persistent radio
flaring activity that gradually increases from a level of a few mJy at
apoastron to more than 100 mJy at periastron. Interbinary collisions between
very large (> 15 R) magnetic structures anchored on the two rotating stars of
the system have been proposed to be the origin of these periodic radio flares.
Magnetic structures extended over tens of stellar radii, that can also account
for the observed fast decay of the radio flares, seem to correspond to the
typical solar semi-open quite extended magnetic configurations called helmet
streamers.} Aims {We aim to find direct observational evidence for the
postulated, solar-like, coronal topologies.} Methods {We performed
seven-consecutive-day VLBI observations at 8.4 GHz using an array consisting of
the VLBA and the 100-m Effelsberg telescope.} Results {Two distintive
structures appear in the radio images here presented. They happen to be
associated with the primary and secondary stars of the V773 Tau A system. In
one image (Fig.2-B) the two features are extended up to 18 R each and are
nearly parallel revealing the presence of two interacting helmet streamers. One
image (Fig.2-E) taken a few hours after a flare monitored by the 100-m
Effelsberg telescope shows one elongated fading structure substantially rotated
with respect to those seen in the B run. The same decay scenario is seen in
Fig.2-G for the helmet streamer associated with the other star.} Conclusions
{This is the very first direct evidence revealing that even if the flare origin
is magnetic reconnection due to interbinary collision, both stars independently
emit in the radio range with structures of their own. These structures are
helmet streamers, observed for the first time in stars other than the Sun.}Comment: 7 pages, 3 figures, A&A in pres
X-ray absorption by Broad Line Region Clouds in Mrk 766
We present a new analysis of a 9-day long XMM-Newton monitoring of the Narrow
Line Seyfert 1 galaxy Mrk 766. We show that the strong changes in spectral
shape which occurred during this observation can be interpreted as due to Broad
Line Region clouds crossing the line of sight to the X-ray source. Within the
occultation scenario, the spectral and temporal analysis of the eclipses
provides precise estimates of the geometrical structure, location and physical
properties of the absorbing clouds. In particular, we show that these clouds
have cores with column densities of at least a few 10^23 cm^-2 and velocities
in the plane of the sky of the order of thousands km/s. The three different
eclipses monitored by XMM-Newton suggest a broad range in cloud velocities (by
a factor ~4-5). Moreover, two iron absorption lines clearly associated with
each eclipse suggest the presence of highly ionized gas around the obscuring
clouds, and an outflow component of the velocity spanning from 3,000 to 15,000
km/sComment: 10 pages, 7 figures. Accepted for publication in MNRA
"Comets" orbiting a black hole
We use a long (300 ksec), continuous Suzaku X-ray observation of the active
nucleus in NGC1365 to investigate the structure of the circumnuclear BLR clouds
through their occultation of the X-ray source. The variations of the absorbing
column density and of the covering factor indicate that the clouds surrounding
the black hole are far from having a spherical geometry (as sometimes assumed),
instead they have a strongly elongated and cometary shape, with a dense head
(n=10^11 cm^-3) and an expanding, dissolving tail. We infer that the cometary
tails must be longer than a few times 10^13 cm and their opening angle must be
smaller than a few degrees. We suggest that the cometary shape may be a common
feature of BLR clouds in general, but which has been difficult to recognize
observationally so far. The cometary shape may originate from shocks and
hydrodynamical instabilities generated by the supersonic motion of the BLR
clouds into the intracloud medium. As a consequence of the mass loss into their
tail, we infer that the BLR clouds probably have a lifetime of only a few
months, implying that they must be continuously replenished. We also find a
large, puzzling discrepancy (two orders of magnitude) between the mass of the
BLR inferred from the properties of the absorbing clouds and the mass of the
BLR inferred from photoionization models; we discuss the possible solutions to
this discrepancy.Comment: Accepted for publication in A&A. 11 pages, 9 figure
Spectral Variability of Quasars in the Sloan Digital Sky Survey. I: Wavelength Dependence
Sloan Digital Sky Survey (SDSS) repeat spectroscopic observations have
resulted in multiple-epoch spectroscopy for roughly 2500 quasars observed more
than 50 days apart. From this sample, we identify 315 quasars that have varied
significantly between observations. We create an ensemble difference spectrum
(bright phase minus faint phase) covering rest-frame wavelengths from 1000 to
6000 Angstroms. This average difference spectrum is bluer than the average
single-epoch quasar spectrum; a power-law fit to the difference spectrum yields
a spectral index alpha_lambda = -2.00, compared to an index of alpha_lambda =
-1.35 for the single-epoch spectrum. The strongest emission lines vary only 30%
as much as the continuum. Due to the lack of variability of the lines, measured
photometric color is not always bluer in brighter phases, but depends on
redshift and the filters used. Lastly, the difference spectrum is bluer than
the ensemble quasar spectrum only for lambda_rest < 2500 Angstroms, indicating
that the variability cannot result from a simple scaling of the average quasar
spectrum.Comment: 47 pages, 14 figures, 3 tables, accepted for publication in Ap
Stellar Envelopes as Sources of Broad Line Region Emission: New Possibilities Allowed
In Active Galactic Nuclei (AGNs) the presence of a star cluster around the
central black hole can have several effects on the dynamics and the emission of
the global system. In this paper we analyze the interaction of stellar
atmospheres with a wind outflowing from the central region of the AGN nucleus.
Even a small mass loss from stars, as well as possible star collisions, can
give a non-negligible contribution in feeding matter into the AGN nuclear wind.
Moreover, stellar mass loss can produce envelopes surrounding stars that turn
out to be suitable for reproducing the observed emission from the Broad Line
Region (BLR). In this framework, the envelope can be confined by the bow shock
arising from the interaction between the expanding stellar atmosphere and the
AGN nuclear wind.Comment: 21 pages, Latex, accepted for publication in A&
Synchrotron emission from the T Tauri binary system V773 Tau A
The pre-main sequence binary system V773 Tau A shows remarkable flaring
activity around periastron passage. Here, we present the observation of such a
flare at a wavelength of 3 mm (90 GHz) performed with the Plateau de Bure
Interferometer. We examine different possible causes for the energy losses
responsible for the e-folding time of 2.3 hours of that flare. We exclude
synchrotron, collisional, and inverse Compton losses because they are not
consistent with observational constraints, and we propose that the fading of
the emission is due to the leakage of electrons themselves at each reflection
between the two mirror points of the magnetic structure partially trapping
them. The magnetic structure compatible with both our leakage model and
previous observations is that of a helmet streamer that, as in the solar case,
can occur at the top of the X-ray-emitting, stellar-sized coronal loops of one
of the stars. The streamer may extend up to 20 R and interact with the corona
of the other star at periastron passage, causing recurring flares. The inferred
magnetic field strength at the two mirror points of the helmet streamer is in
the range 0.12 - 125 G, and the corresponding Lorentz factor, gamma, of the
partially trapped electrons is in the range 20 < gamma < 632. We therefore rule
out that the emission could be of gyro-synchrotron nature: the derived high
Lorentz factor proves that the nature of the emission at 90 GHz from this
pre-main binary system is synchrotron radiation.
Based on observations carried out with the IRAM Plateau de Bure
Interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN
(Spain).Comment: 8 pages, 5 figures, A&A in pres
Quasar Spectral Slope Variability in the Optical Band
We performed a new analysis of B and R light curves of a sample of PG
quasars. We confirm the variability-redshift correlation and its explanation in
terms of spectral variability, coupled with the increase of rest-frame
observing frequency for quasars at high redshift. The analysis of the
instantaneous spectral slope for the whole quasar samples indicates both an
inter-QSO and intra-QSO variability-luminosity correlation. Numerical
simulations show that the latter correlation cannot be entirely due to the
addition of the host galaxy emission to a nuclear spectrum of variable
luminosity but constant shape, implying a spectral variability of the nuclear
component. Changes of accretion rate are also insufficient to explain the
amount of spectral variation, while hot spots possibly caused by local disk
instabilities can explain the observations.Comment: 20 pages, 6 figures, to appear in Ap.J., January 200