682 research outputs found
A changing-look AGN to be probed by X-ray polarimetry
Active galactic nuclei (AGN) produce the highest intrinsic luminosities in
the Universe from within a compact region. The central engine is thought to be
powered by accretion onto a supermassive black hole. A fraction of this huge
release of energy influences the evolution of the host galaxy, and in
particular, star formation. Thus, AGN are key astronomical sources not only
because they play an important role in the evolution of the Universe, but also
because they constitute a laboratory for extreme physics. However, these
objects are under the resolution limit of current telescopes. Polarimetry is a
unique technique capable of providing us with information on physical AGN
structures. The incoming new era of X-ray polarimetry will give us the
opportunity to explore the geometry and physical processes taking place in the
innermost regions of the accretion disc. Here we exploit this future powerful
tool in the particular case of changing-look AGN, which are key for
understanding the complexity of AGN physics.Comment: 9 pages, 1 figures, published by Galaxies under the special issue
"The Bright Future of Astronomical X-ray Polarimetry
The Cotton, Simon-Mars and Cotton-York Tensors in Stationary Spacetimes
The Cotton-York and Simon-Mars tensors in stationary vacuum spacetimes are
studied in the language of the congruence approach pioneered by Hawking and
Ellis. Their relationships with the Papapetrou field defined by the stationary
Killing congruence and with a recent characterization of the Kerr spacetime in
terms of the alignment between of the principal null directions of the Weyl
tensor with those of the Papapetrou field are also investigated in this more
transparent language.Comment: 14 pages latex(2e) iopart style, no figure
A Chandra view of the clumpy reflector at the heart of the Circinus galaxy
We present a spectral and imaging analysis of the X-ray reflecting structure
at the heart of the Circinus galaxy, investigating the innermost regions
surrounding the central black hole. By studying an archival 200 ks Chandra
ACIS-S observation, we are able to image the extended clumpy structure
responsible for both cold reflection of the primary radiation and neutral iron
Ka line emission. We measure an excess of the equivalent width of the iron Ka
line which follows an axisymmetric geometry around the nucleus on a hundred pc
scale. Spectra extracted from different regions confirm a scenario in which the
dominant mechanism is the reflection of the nuclear radiation from
Compton-thick gas. Significant differences in the equivalent width of the iron
Ka emission line (up to a factor of 2) are found. It is argued that these
differences are due to different scattering angles with respect to the line of
sight rather than to different iron abundances.Comment: 6 pages, 4 figures, accepted for publication on MNRA
IRAS 13197-1627 has them all: Compton-thin absorption, photo-ionized gas, thermal plasmas, and a broad Fe line
We report results from the XMM-Newton observation of IRAS 13197-1627, a
luminous IR galaxy with a Seyfert 1.8 nucleus. The hard X-ray spectrum is steep
and is absorbed by Compton-thin neutral gas. We detect an Fe emission line at
6.4 keV, consistent with transmission through the absorber. The most striking
result of our spectral analysis is the detection of a dominant X-ray reflection
component and broad Fe line from the inner accretion disc. The
reflection-dominated hard X-ray spectrum is confirmed by the strong Compton
hump seen in a previous BeppoSAX observation and could be the sign that most of
the primary X-rays are radiated from a compact corona (or e.g. base of the jet)
within a few gravitational radii from the black hole. We also detect a
relatively strong absorption line at 6.81 keV which, if interpreted as Fe xxv
resonant absorption intrinsic to the source, implies an outflow with velocity
of about 5000 km/s. In the soft energy band, the high-resolution RGS and the
CCD-resolution data show the presence of both photo-ionized gas and thermal
plasma emission, the latter being most likely associated with a recent
starburst of 15-20 solar masses per year.Comment: accepted for publication in MNRA
The absorption-dominated model for the X-ray spectra of type I active galaxies: MCG-6-30-15
MCG-6-30-15 is the archetypal example of a type I active galaxy showing broad
"red-wing" emission in its X-ray spectrum at energies below the 6.4 keV Fe
K-alpha emission line and a continuum excess above 20 keV. Miller et al. (2008)
showed that these spectral features could be caused by clumpy absorbing
material, but Reynolds et al. (2009) have argued that the observed Fe K-alpha
line luminosity is inconsistent with this explanation unless the global
covering factor of the absorber(s) is very low. However, the Reynolds et al.
calculation effectively considers the only source of opacity to be the Fe K
bound-free transition and neglects the opacity at the line energy: correction
to realistic opacity decreases the predicted line flux by a large factor. We
also discuss the interpretation of the covering factor and the possible effect
of occultation by the accretion disk. Finally, we consider a model for
MCG-6-30-15 dominated by clumpy absorption, which is consistent with global
covering factor 0.45, although models that include the effects of Compton
scattering are required to reach a full understanding. Variations in covering
fraction may dominate the observed X-ray spectral variability.Comment: Accepted for publication in MNRAS letter
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The effects of a revision technique on urban fifth grade students\u27 writing mechanics of the writing process.
After a decade dominated by studies and rhetoric about school reform, a national report card released on January, 1990, indicated that children showed no improvement in writing. The purpose of this research was to determine if by using the Cumulative Writing Folder Program, a mandated Program, with the additional use of sentence combining as a revision strategy urban fifth graders would improve their writing in the six areas of topic development, organization, supporting details, sentence structure, word choice, and mechanics. This study fit in with the existing knowledge and research in the field. It focused on the writing habits of fifth grade students and examined a program that improved their writing skills. This study used concrete strategies in a well-defined writing program to improve revision processes for students which added to the current research in this area. John Collins\u27 Cumulative Writing Folder Program was incorporated in the design of the study. Both the experimental and control groups utilized the Cumulative Writing Folder. The experimental group used sentence combining as a revision strategy. The teacher instructed this group on the techniques of sentence combining and instructed them to use this strategy to revise their writing samples. In September and June the two groups produced writing samples which were analytically scored by independent scorers. An analysis of the comparison of the pre and post scores of the experimental group with the control group in the six variables was given. Results showed that the overall writing performance of the experimental group showed improvement at a significant level. In the variable of topic development, there was a level of significance. The five variables which showed no level of significance were organization, mechanics, supporting details, sentence structure, and word choice. The study suggested that a well defined writing program with the revision strategy of sentence combining did provide overall improvement in the quality of writing over the course of the school year. The study further suggested that more research and subsequent solutions to the problem of the inferior quality of writing at the elementary level needed addressing. It indicated clear directions for further study
Revealing the X-ray source in IRAS 13224-3809 through flux-dependent reverberation lags
IRAS 13224-3809 was observed in 2011 for 500 ks with the XMM-Newton
observatory. We detect highly significant X-ray lags between soft (0.3 - 1 keV)
and hard (1.2 - 5 keV) energies. The hard band lags the soft at low frequencies
(i.e. hard lag), while the opposite (i.e. soft lag) is observed at high
frequencies. In this paper, we study the lag during flaring and quiescent
periods. We find that the frequency and absolute amplitude of the soft lag is
different during high-flux and low-flux periods. During the low flux intervals,
the soft lag is detected at higher frequencies and with smaller amplitude.
Assuming that the soft lag is associated with the light travel time between
primary and reprocessed emission, this behaviour suggests that the X-ray source
is more compact during low-flux intervals, and irradiates smaller radii of the
accretion disc (likely because of light bending effects). We continue with an
investigation of the lag dependence on energy, and find that isolating the
low-flux periods reveals a strong lag signature at the Fe K line energy,
similar to results found using 1.3 Ms of data on another well known Narrow-Line
Seyfert I galaxy, 1H0707-495.Comment: 6 pages, 8 figures, accepted for publication in MNRA
Relativistic reflection in the average X-ray spectrum of AGN in the V\'eron-Cetty & V\'eron catalogue
The X-ray spectra of active galactic nuclei (AGN) unveil properties of matter
around the super massive black hole (SMBH). We investigate the X-ray spectra of
AGN focusing on Compton reflection and fluorescence, important processes of
interaction between primary radiation and circum-nuclear material. Unresolved
emission lines (most notably the Fe line) in the X-ray spectra of AGN indicate
that this material is located far away from the SMBH. Contributions from the
inner accretion disk, affected by relativistic effects, have also been detected
in several cases.
We studied the average X-ray spectrum of a sample of 263 X-ray unabsorbed AGN
that yield 419023 counts in the 2-12 keV rest-frame band distributed among 388
XMM-Newton spectra. We fitted the average spectrum using a (basically)
unabsorbed power law (primary radiation). From second model that represents the
interaction of the primary radiation with matter located far away from the
SMBH, we found that it was very significantly detected. Finally, we added a
contribution from interaction with neutral material in the accretion disk close
to the central SMBH, which is therefore smeared by relativistic effects, which
improved the fit at 6 sigma. The reflection factors are 0.65 for the accretion
disk and 0.25 for the torus. Replacing the neutral disk-reflection with
low-ionisation disk reflection, also relativistically smeared, fits the data
equally well, suggesting that we do not find evidence for a significant
ionisation of the accretion disk.
We detect distant neutral reflection in the average spectrum of unabsorbed
AGN with z=0.8. Adding the disk-reflection component associated with a
relativistic Fe line improves the data description at 6 sigma confidence level,
suggesting that both reflection components are present. The disk-reflection
component accounts for about 70 % of the total reflected flux.Comment: Accepted by A&A. 10 pages, 7 figure
Weighing the black holes in ultraluminous X-ray sources through timing
We describe a new method to estimate the mass of black holes in Ultraluminous
X-ray Sources (ULXs). The method is based on the recently discovered
``variability plane'', populated by Galactic stellar-mass black-hole candidates
(BHCs) and supermassive active galactic nuclei (AGNs), in the parameter space
defined by the black-hole mass, accretion rate and characteristic frequency. We
apply this method to the two ULXs from which low-frequency quasi-periodic
oscillations have been discovered, M82 X-1 and NGC 5408 X-1. For both sources
we obtain a black-hole mass in the range 100~1300 Msun, thus providing evidence
for these two sources to host an intermediate-mass black hole.Comment: 5 pages, 2 figures, Accepted by MNRA
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