14 research outputs found
Hot-spot model for accretion disc variability as random process
Theory of random processes provides an attractive mathematical tool to
describe the fluctuating signal from accreting sources, such as active galactic
nuclei and Galactic black holes observed in X-rays. These objects exhibit
featureless variability on different timescales, probably originating from an
accretion disc. We study the basic features of the power spectra in terms of a
general framework, which permits semi-analytical determination of the power
spectral density (PSD) of the resulting light curve. We consider the expected
signal generated by an ensemble of spots randomly created on the accretion disc
surface. Spot generation is governed by Poisson or by Hawkes processes. We
include general relativity effects shaping the signal on its propagation to a
distant observer. We analyse the PSD of a spotted disc light curve and show the
accuracy of our semi-analytical approach by comparing the obtained PSD with the
results of Monte Carlo simulations. The asymptotic slopes of PSD are 0 at low
frequencies and they drop to -2 at high frequencies, usually with a single
frequency break. More complex two-peak solutions also occur. The amplitude of
the peaks and their frequency difference depend on the inherent timescales of
the model. At intermediate frequencies, the intrinsic PSD is influenced by the
individual light curve profile as well as by the type of the underlying
process. However, even in cases when two Lorentzians seem to dominate the PSD,
it does not necessarily imply that two single oscillation mechanisms operate
simultaneously. Instead, it may well be the manifestation of the avalanche
mechanism. The main advantage of our approach is an insight in the model
functioning and the fast evaluation of the PSD.Comment: 18 pages, 9 figures; Astronomy & Astrophysics accepte
Tidal effects on small bodies by massive black holes
The compact radio source Sagittarius A (Sgr A) at the centre of our Galaxy
harbours a supermassive black hole, whose mass has been measured from stellar
orbital motions. Sgr A is therefore the nearest laboratory where super-massive
black hole astrophysics can be tested, and the environment of black holes can
be investigated. Since it is not an active galactic nucleus, it also offers the
possibility of observing the capture of small objects that may orbit the
central black hole. We study the effects of the strong gravitational field of
the black hole on small objects, such as a comet or an asteroid. We also
explore the idea that the flares detected in Sgr A might be produced by the
final accretion of single, dense objects with mass of the order of 10^20 g, and
that their timing is not a characteristic of the sources, but rather of the
space-time of the central galactic black hole in which they are moving. We find
that tidal effects are strong enough to melt the solid object, and present
calculations of the temporal evolution of the light curve of infalling objects
as a function of various parameters. Our modelling of tidal disruption suggests
that during tidal squeezing, the conditions for synchrotron radiation can be
met. We show that the light curve of a flare can be deduced from dynamical
properties of geodesic orbits around black holes and that it depends only
weakly on the physical properties of the source.Comment: 10 pages, 14 figures, A&A accepte
Line emission from optically thick relativistic accretion tori
We calculate line emission from relativistic accretion tori around Kerr black
holes and investigate how the line profiles depend on the viewing inclination,
spin of the central black hole, parameters describing the shape of the tori,
and spatial distribution of line emissivity on the torus surface. We also
compare the lines with those from thin accretion disks. Our calculations show
that lines from tori and lines from thin disks share several common features.
In particular, at low and moderate viewing inclination angles they both have
asymmetric double-peaked profiles with a tall, sharp blue peak and a shorter
red peak which has an extensive red wing. At high viewing inclination angles
they both have very broad, asymmetric lines which can be roughly considered
single-peaked. Torus and disk lines may show very different red and blue line
wings, but the differences are due to the models for relativistic tori and
disks having differing inner boundary radii. Self-eclipse and lensing play some
role in shaping the torus lines, but they are effective only at high
inclination angles. If inner and outer radii of an accretion torus are the same
as those of an accretion disk, their line profiles show substantial differences
only when inclination angles are close to 90 degrees, and those differences
manifest mostly at the central regions of the lines instead of the wings.Comment: 14 pages, 17 figures. Accepted to A&
Unveiling the broad band X-ray continuum and iron line complex in Mkr 841
Mkr 841 is a bright Seyfert 1 galaxy known to harbor a strong soft excess and
a variable K iron line. It has been observed during 3 different periods
by XMM for a total cumulated exposure time of 108 ks. We present in this
paper a broad band spectral analysis of the complete EPIC-pn data sets. We were
able to test two different models for the soft excess, a relativistically
blurred photoionized reflection (\r model) and a relativistically smeared
ionized absorption (\a model). The continuum is modeled by a simple cut-off
power law and we also add a neutral reflection. These observations reveal the
extreme and puzzling spectral and temporal behaviors of the soft excess and
iron line. The 0.5-3 keV soft X-ray flux decreases by a factor 3 between 2001
and 2005 and the line shape appears to be a mixture of broad and narrow
components. We succeed in describing this complex broad-band 0.5-10 keV
spectral variability using either \r or \a to fit the soft excess. Both models
give statistically equivalent results even including simultaneous BeppoSAX data
up to 200 keV. Both models are consistent with the presence of remote
reflection characterized by a constant narrow component in the data. However
they differ in the presence of a broad line component present in \r but not
needed in \a. This study also reveals the sporadic presence of relativistically
redshifted narrow iron lines.Comment: Accepted in A&A. 17 pages and 21 figure
Leaving the ISCO: the inner edge of a black-hole accretion disk at various luminosities
The "radiation inner edge" of an accretion disk is defined as the inner
boundary of the region from which most of the luminosity emerges. Similarly,
the "reflection edge" is the smallest radius capable of producing a significant
X-ray reflection of the fluorescent iron line. For black hole accretion disks
with very sub-Eddington luminosities these and all other "inner edges" locate
at ISCO. Thus, in this case, one may rightly consider ISCO as the unique inner
edge of the black hole accretion disk. However, even for moderate luminosities,
there is no such unique inner edge as differently defined edges locate at
different places. Several of them are significantly closer to the black hole
than ISCO. The differences grow with the increasing luminosity. For nearly
Eddington luminosities, they are so huge that the notion of the inner edge
losses all practical significance.Comment: 12 pages, 15 figures, submitted to A&
Near infrared flares of Sagittarius A*: Importance of near infrared polarimetry
We report on the results of new simulations of near-infrared (NIR)
observations of the Sagittarius A* (Sgr A*) counterpart associated with the
super-massive black hole at the Galactic Center. The observations have been
carried out using the NACO adaptive optics (AO) instrument at the European
Southern Observatory's Very Large Telescope and CIAO NIR camera on the Subaru
telescope (13 June 2004, 30 July 2005, 1 June 2006, 15 May 2007, 17 May 2007
and 28 May 2008). We used a model of synchrotron emission from relativistic
electrons in the inner parts of an accretion disk. The relativistic simulations
have been carried out using the Karas-Yaqoob (KY) ray-tracing code. We probe
the existence of a correlation between the modulations of the observed flux
density light curves and changes in polarimetric data. Furthermore, we confirm
that the same correlation is also predicted by the hot spot model. Correlations
between intensity and polarimetric parameters of the observed light curves as
well as a comparison of predicted and observed light curve features through a
pattern recognition algorithm result in the detection of a signature of
orbiting matter under the influence of strong gravity. This pattern is detected
statistically significant against randomly polarized red noise. Expected
results from future observations of VLT interferometry like GRAVITY experiment
are also discussed.Comment: 26 pages, 38 figures, accepted for publication by A&
Coordinated NIR/mm observations of flare emission from Sagittarius A*
We report on a successful, simultaneous observation and modelling of the
millimeter (mm) to near-infrared (NIR) flare emission of the Sgr A* counterpart
associated with the supermassive black hole at the Galactic centre (GC). We
present a mm/sub-mm light curve of Sgr A* with one of the highest quality
continuous time coverages and study and model the physical processes giving
rise to the variable emission of Sgr A*.Comment: 14 pages, 16 figure
The extreme luminosity states of Sagittarius A*
We discuss mm-wavelength radio, 2.2-11.8um NIR and 2-10 keV X-ray light
curves of the super massive black hole (SMBH) counterpart of Sagittarius A*
(SgrA*) near its lowest and highest observed luminosity states. The luminosity
during the low state can be interpreted as synchrotron emission from a
continuous or even spotted accretion disk. For the high luminosity state SSC
emission from THz peaked source components can fully account for the flux
density variations observed in the NIR and X-ray domain. We conclude that at
near-infrared wavelengths the SSC mechanism is responsible for all emission
from the lowest to the brightest flare from SgrA*. For the bright flare event
of 4 April 2007 that was covered from the radio to the X-ray domain, the SSC
model combined with adiabatic expansion can explain the related peak
luminosities and different widths of the flare profiles obtained in the NIR and
X-ray regime as well as the non detection in the radio domain.Comment: 18 pages, 13 figures, accepted by A&
Simultaneous NIR/sub-mm observation of flare emission from SgrA*
We report on a successful, simultaneous observation and modeling of the
sub-millimeter to near-infrared flare emission of the Sgr A* counterpart
associated with the super-massive black hole at the Galactic center. Our
modeling is based on simultaneous observations that have been carried out on 03
June, 2008 using the NACO adaptive optics (AO) instrument at the ESO VLT and
the LABOCA bolometer at the APEX telescope. Inspection and modeling of the
light curves show that the sub-mm follows the NIR emission with a delay of
1.5+/-0.5 hours. We explain the flare emission delay by an adiabatic expansion
of the source components.Comment: 12 pages, 9 figures, 3 tables, in press with A&
XMM-Newton observation of the bright Seyfert 1 galaxy, MCG+8-11-11
We report on the XMM-Newton observation of the bright Seyfert 1 galaxy,
MCG+8-11-11. Data from the EPIC/p-n camera, the Reflection Gratings
Spectrometers (RGS) and the Optical Monitor (OM) have been analyzed. The p-n
spectrum is well fitted by a power law, a spectrally unresolved Fe Kalpha line,
a Compton reflection component (whose large value, when compared to the iron
line equivalent width, suggests iron underabundance), and absorption by warm
material. Absorption lines are apparent in the RGS spectra, but their
identification is uncertain and would require large matter velocities. The UV
fluxes measured by the OM are well above the extrapolation of the X-ray
spectrum, indicating the presence of a UV bump.Comment: 7 pages. Accepted for publication in Astronomy & Astrophysic