171 research outputs found
Compton Thick AGN in the 70 Month Swift-BAT All-Sky Hard X-ray Survey: a Bayesian approach
The 70-month Swift/BAT catalogue provides a sensitive view of the
extragalactic X-ray sky at hard energies (>10 keV) containing about 800 Active
Galactic Nuclei. We explore its content in heavily obscured, Compton-thick AGN
by combining the BAT (14-195 keV) with the lower energy XRT (0.3-10 keV) data.
We apply a Bayesian methodology using Markov chains to estimate the exact
probability distribution of the column density for each source. We find 53
possible Compton-thick sources (with probability 3 to 100%) translating to a
~7% fraction of the AGN in our sample. We derive the first parametric
luminosity function of Compton-thick AGN. The unabsorbed luminosity function
can be represented by a double power-law with a break at in the 20-40 keV band.Comment: 13 pages, 9 figure
On the Lx-L6micron ratio as a diagnostic for Compton-thick AGN
As the mid-IR luminosity represents a good isotropic proxy of the AGN power,
a low X-ray to mid-IR luminosity ratio is often claimed to be a reliable
indicator for selecting Compton-thick (CT) AGN. We assess the efficiency of
this diagnostic by examining the 12mu IRAS AGN sample for which high
signal-to-noise XMM observations have been recently become available. We find
that the vast majority (10/11) of the AGN that have been classified as CT on
the basis the X-ray spectroscopy by Brightman & Nandra present a low Lx/L6
luminosity ratio, i.e. lower than a few percent of the average AGN ratio which
is typical of reflection-dominated CT sources. At low Lx/L6 ratios we also find
a comparable number of AGN, most of which are heavily absorbed but not CT. This
implies that although most Compton-thick AGN present low Lx/L6 ratios, at least
in the local, Universe, the opposite is not necessarily true. Next, we extend
our analysis to higher redshifts. We perform the same analysis in the CDFS
where excellent quality chandra (4 Ms) and xmm (3 Ms) X-ray spectra are
available. We derive accurate X-ray luminosities for chandra sources using
X-ray spectral fits, as well as 6mu luminosities from SED fits. We find 8 AGN
with low Lx/L6 ratios in total, after excluding one source where the 6mu
emission primarily comes from star-formation. One of these sources has been
already demonstrated to host a CT nucleus, while for another one at a redshift
of z=1.22 we argue it is most likely CT on the basis of its combined chandra
and xmm spectrum. We find a large number of non CT contaminant with low Lx/L6
ratios. The above suggest that a low Lx/L6 ratio alone cannot ascertain the
presence of a CT AGN, albeit the majority of low Lx/L6 AGN are heavily
obscured. The two most reliable CT AGN in the high redshift Universe have high
Lx/L6 ratios, showing that this method cannot provide complete CT AGN samples.Comment: 11 pages, to appear to A&
Searching for X-ray luminous 'normal' galaxies in 2dfGRS
We cross-correlated the Chandra XASSIST and XMM-Newton Serendipitous Source
Catalogues with the 2 degree Field Galaxy Redshift Survey (2dfGRS) database.
Our aim was to identify the most X-ray luminous (L_X > 10^42 erg s^-1) examples
of galaxies in the local Universe whose X-ray emission is dominated by stellar
processes rather than AGN activity ('normal' galaxies) as well as to test the
empirical criterion log(f_X/f_O) < -2 for separating AGN from NGs. With
XMM-Newton (Chandra) we covered an area of ~8.2 (~5.8)deg^2 down to a flux
limit of ~10^-15 (~1.6 x 10^-15) erg cm^-2 s^-1 and found 18 (20) 2dfGRS
galaxies. Using emission-line intensity ratios, we classified 6 2dfGRS spectra
as star-forming, H II nuclei, and 2 spectra as possible H II nuclei. The rest
of the objects are absorption-line galaxies and AGN, including 3 possible
LINERs. No luminous 'normal' galaxies have been found but out of 19 'normal'
galaxies in this sample 5 H II and 3 absorption-line galaxies have log(f_X/f_O)
> -2. We performed a similar search in two nearby-galaxy samples from the
literature. All 44 galaxies in the Zezas (2001) sample have log(f_X/f_O) < -2
and L_X < 10^42 erg s^-1. In the Fabbiano et al. (1992) sample, out of a total
of 170 'normal' galaxies, we found 16 galaxies with log(f_X/f_O) >-2, the
majority of which are massive ellipticals. Three of these have L_X > 10^42 erg
s^-1 .Comment: 8 pages, 2 figures. Accepted for publication in Astronomy &
Astrophysic
The XMM deep survey in the CDF-S. X. X-ray variability of bright sources
We aim to study the variability properties of bright hard X-ray selected
Active Galactic Nuclei (AGN) with redshift between 0.3 and 1.6 detected in the
Chandra Deep Field South (XMM-CDFS) by a long XMM observation. Taking advantage
of the good count statistics in the XMM CDFS we search for flux and spectral
variability using the hardness ratio techniques. We also investigated spectral
variability of different spectral components. The spectra were merged in six
epochs (defined as adjacent observations) and in high and low flux states to
understand whether the flux transitions are accompanied by spectral changes.
The flux variability is significant in all the sources investigated. The
hardness ratios in general are not as variable as the fluxes. Only one source
displays a variable HR, anti-correlated with the flux (source 337). The
spectral analysis in the available epochs confirms the steeper when brighter
trend consistent with Comptonisation models only in this source. Finding this
trend in one out of seven unabsorbed sources is consistent, within the
statistical limits, with the 15 % of unabsorbed AGN in previous deep surveys.
No significant variability in the column densities, nor in the Compton
reflection component, has been detected across the epochs considered. The high
and low states display in general different normalisations but consistent
spectral properties. X-ray flux fluctuations are ubiquitous in AGN. In general,
the significant flux variations are not associated with a spectral variability:
photon index and column densities are not significantly variable in nine out of
the ten AGN over long timescales (from 3 to 6.5 years). The photon index
variability is found only in one source (which is steeper when brighter) out of
seven unabsorbed AGN. These results are consistent with previous deep samples.Comment: 14 pages, 11 figures. Accepted in A&
Research on differences in AIDS reports from Mainland China and Taiwan —From the Perspective of Health Communication
本文以两岸报纸对艾滋病相关议题的报道为研究对象,使用内容分析与文本分析相结合的方法对《人民日报》与《联合报》2006年至2012年期间的艾滋病报道进行对比剖析,探究这两家媒体在艾滋病议题的建构上有何异同,以及从传媒生态的角度探析其成因。 研究发现,两份报纸艾滋病相关议题报道具有多方面的共同点,即报道最多的年份均在2006年;“世界艾滋病日”前后报道比较集中;立足本土化;报道覆盖了艾滋病病毒的三大传播途径;消息来源以官方为主;涉及传播途径的报道不够科学。两份报纸艾滋病相关议题报道的不同点有:《联合报》艾滋病报道更趋“常态化”,报道数量相对稳定,而《人民日报》仍将艾滋病及其相关事件看作是社会的异...This research is designed to explore the differences between AIDS reports from mainland China and Taiwan media and to analyze the reason from the perspective of media ecology. Applying content analysis and text analysis, the study compares the AIDS reports from People’s Daily (PD) to those from United Daily News (UDN) during the period of 2006 to 2012. The results of this study propose the follow...学位:新闻与传播硕士院系专业:新闻传播学院_传播学学号:3192010115285
The XMM spectral catalog of SDSS optically selected Seyfert 2 galaxies
We present an X-ray spectroscopic study of optically selected (SDSS) Seyfert
2 (Sy2) galaxies. The goal is to study the obscuration of Sy2 galaxies beyond
the local universe, using good quality X-ray spectra in combination with high
S/N optical spectra for their robust classification. We analyzed all available
XMM-Newton archival observations of narrow emission line galaxies that meet the
above criteria in the redshift range 0.05<z<0.35. We initially selected narrow
line AGN using the SDSS optical spectra and the BPT classification diagram. We
further modeled and removed the stellar continuum, and we analyzed the residual
emission line spectrum to exclude any possible intermediate-type Seyferts. Our
final catalog comprises 31 Sy2 galaxies with median redshift z~0.1. X-ray
spectroscopy is performed using the available X-ray spectra from the 3XMM and
the XMMFITCAT catalogs. Implementing various indicators of obscuration, we find
seven (~23%) Compton-thick AGN. The X-ray spectroscopic Compton-thick
classification agrees with other commonly used diagnostics, such as the X-ray
to mid-IR luminosity ratio and the X-ray to [OIII] luminosity ratio. Most
importantly, we find four (~13%) unobscured Sy2 galaxies, at odds with the
simplest unification model. Their accretion rates are significantly lower than
the rest of our Sy2 sample, in agreement with previous studies that predict the
absence of the broad line region below a certain Eddington ratio threshold.Comment: 12 pages, 6 figures, accepted for publication in A&
Searching for highly obscured AGN in the XMM-Newton serendipitous source catalog
The majority of active galactic nuclei (AGN) are obscured by large amounts of
absorbing material that makes them invisible at many wavelengths. X-rays, given
their penetrating power, provide the most secure way for finding these AGN. The
XMM-Newton serendipitous source catalog is the largest catalog of X-ray sources
ever produced; it contains about half a million detections. These sources are
mostly AGN. We have derived X-ray spectral fits for very many 3XMM-DR4 sources
( 114 000 observations, corresponding to 77 000 unique
sources), which contain more than 50 source photons per detector. Here, we use
a subsample of 1000 AGN in the footprint of the SDSS area (covering
120 deg) with available spectroscopic redshifts. We searched for highly
obscured AGN by applying an automated selection technique based on X-ray
spectral analysis that is capable of efficiently selecting AGN. The selection
is based on the presence of either a) flat rest-frame spectra; b) flat observed
spectra; c) an absorption turnover, indicative of a high rest-frame column
density; or d) an Fe K line with an equivalent width > 500 eV. We found
81 highly obscured candidate sources. Subsequent detailed manual spectral fits
revealed that 28 of them are heavily absorbed by column densities higher than
10 cm. Of these 28 AGN, 15 are candidate Compton-thick AGN on the
basis of either a high column density, consistent within the 90% confidence
level with N 10 cm, or a large equivalent width
(>500 eV) of the Fe K line. Another six are associated with
near-Compton-thick AGN with column densities of 510
cm. A combination of selection criteria a) and c) for low-quality
spectra, and a) and d) for medium- to high-quality spectra, pinpoint highly
absorbed AGN with an efficiency of 80%.Comment: 18 pages, 10 figures, accepted for publication in A&
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