1,291 research outputs found
A new super-soft X-ray source in the Small Magellanic Cloud: Discovery of the first Be/white dwarf system in the SMC?
The Small Magellanic Cloud (SMC) hosts a large number of Be/X-ray binaries,
however no Be/white dwarf system is known so far, although population synthesis
calculations predict that they might be more frequent than Be/neutron star
systems. XMMUJ010147.5-715550 was found as a new faint super-soft X-ray source
(SSS) with a likely Be star optical counterpart. We investigate the nature of
this system and search for further high-absorbed candidates in the SMC. We
analysed the XMM-Newton X-ray spectrum and light curve, optical photometry, and
the I-band OGLE III light curve. The X-ray spectrum is well represented by
black-body and white dwarf atmosphere models with highly model-dependent
temperature between 20 and 100 eV. The likely optical counterpart AzV 281
showed low near infrared emission during X-ray activity, followed by a
brightening in the I-band afterwards. We find further candidates for
high-absorbed SSSs with a blue star as counterpart. We discuss
XMMUJ010147.5-715550 as the first candidate for a Be/white dwarf binary system
in the SMC.Comment: 6 pages, 4 figures, accepted by A&
The XMM-Newton survey of the Small Magellanic Cloud: The X-ray point-source catalogue
Local-Group galaxies provide access to samples of X-ray source populations of
whole galaxies. The XMM-Newton survey of the Small Magellanic Cloud (SMC)
completely covers the bar and eastern wing with a 5.6 deg^2 area in the
(0.2-12.0) keV band. To characterise the X-ray sources in the SMC field, we
created a catalogue of point sources and sources with moderate extent. Sources
with high extent (>40") have been presented in a companion paper. We searched
for point sources in the EPIC images using sliding-box and maximum-likelihood
techniques and classified the sources using hardness ratios, X-ray variability,
and their multi-wavelength properties. The catalogue comprises 3053 unique
X-ray sources with a median position uncertainty of 1.3" down to a flux limit
for point sources of ~10^-14 erg cm^-2 s^-1 in the (0.2-4.5) keV band,
corresponding to 5x10^33 erg s^-1 for sources in the SMC. We discuss
statistical properties, like the spatial distribution, X-ray colour diagrams,
luminosity functions, and time variability. We identified 49 SMC high-mass
X-ray binaries (HMXB), four super-soft X-ray sources (SSS), 34 foreground
stars, and 72 active galactic nuclei (AGN) behind the SMC. In addition, we
found candidates for SMC HMXBs (45) and faint SSSs (8) as well as AGN (2092)
and galaxy clusters (13). We present the most up-to-date catalogue of the X-ray
source population in the SMC field. In particular, the known population of
X-ray binaries is greatly increased. We find that the bright-end slope of the
luminosity function of Be/X-ray binaries significantly deviates from the
expected universal high-mass X-ray binary luminosity function.Comment: 32 pages, 18 figures, accepted for publication in A&A, catalog will
be available at CD
Dust covering factor, silicate emission and star formation in luminous QSOs
We present Spitzer IRS low resolution, mid-IR spectra of a sample of 25 high
luminosity QSOs at 2<z<3.5. When combined with archival IRS observations of
local, low luminosity type-I AGNs, the sample spans five orders of magnitude in
luminosity. We find that the continuum dust thermal emission at
lambda(rest)=6.7um is correlated with the optical luminosity, following the
non-linear relation L(6.7um) propto L(5100A)^0.82. We also find an anti
correlation between the ratio L(6.7um)/L(5100A) and the [OIII]5007A line
luminosity. These effects are interpreted as a decreasing covering factor of
the circumnuclear dust as a function of luminosity. Such a result is in
agreement with the decreasing fraction of absorbed AGNs as a function of
luminosity recently found in various surveys. We clearly detect the silicate
emission feature in the average spectrum, but also in four individual objects.
These are the Silicate emission in the most luminous objects obtained so far.
When combined with the silicate emission observed in local, low luminosity
type-I AGNs, we find that the silicate emission strength is correlated with
luminosity.
The silicate strength of all type-I AGNs also follows a positive correlation
with the black hole mass and with the accretion rate. The Polycyclic Aromatic
Hydrocarbon (PAH) emission features, expected from starburst activity, are not
detected in the average spectrum of luminous, high-z QSOs. The upper limit
inferred from the average spectrum points to a ratio between PAH luminosity and
QSO optical luminosity significantly lower than observed in lower luminosity
AGNs, implying that the correlation between star formation rate and AGN power
saturates at high luminosities.Comment: accepted for publication in A&A, 17 pages, 9 figure
The deep look onto the hard X-ray sky: The Swift - INTEGRAL X-ray (SIX) survey
The super-massive black-holes in the centers of Active Galactic Nuclei (AGNs)
are surrounded by obscuring matter that can block the nuclear radiation.
Depending on the amount of blocked radiation, the flux from the AGN can be too
faint to be detected by currently flying hard X-ray (above 15 keV) missions. At
these energies only ~1% of the intensity of the Cosmic X-ray Background (CXB)
can be resolved into point-like sources that are AGNs. In this work we address
the question of the undetected sources contributing to the CXB with a very
sensitive and new hard X-ray survey: the SIX survey that is obtained with the
new approach of combining the Swift/BAT and INTEGRAL/IBIS X-ray observations.
We merge the observations of both missions. This enhances the exposure time and
reduces systematic uncertainties. As a result we obtain a new survey over a
wide sky area of 6200 deg^2 that is more sensitive than the surveys of
Swift/BAT or INTEGRAL/IBIS alone. Our sample comprises 113 sources: 86 AGNs
(Seyfert-like and blazars), 5 galaxies, 2 clusters of galaxies, 3 Galactic
sources, 3 previously detected unidentified X-ray sources, and 14 unidentified
sources. The scientific outcome from the study of the sample has been properly
addressed to study the evolution of AGNs at redshift below 0.4. We do not find
any evolution using the 1/V_max method. Our sample of faint sources are
suitable targets for the new generation hard X-ray telescopes with focusing
techniques.Comment: ApJS accepte
An XMM-Newton view of Planetary Nebulae in the Small Magellanic Cloud. The X-ray luminous central star of SMP SMC 22
During an X-ray survey of the Small Magellanic Cloud, carried out with the
XMM-Newton satellite, we detected significant soft X-ray emission from the
central star of the high-excitation planetary nebula SMP SMC 22. Its very soft
spectrum is well fit with a non local thermodynamical equilibrium model
atmosphere composed of H, He, C, N, and O, with abundances equal to those
inferred from studies of its nebular lines. The derived effective temperature
of 1.5x10^5 K is in good agreement with that found from the optical/UV data.
The unabsorbed flux in the 0.1-0.5 keV range is about 3x10^{-11} erg cm^-2
s^-1, corresponding to a luminosity of 1.2x10^37 erg/s at the distance of 60
kpc. We also searched for X-ray emission from a large number of SMC planetary
nebulae, confirming the previous detection of SMP SMC 25 with a luminosity of
(0.2-6)x10^35 erg/s (0.1-1 keV). For the remaining objects that were not
detected, we derived flux upper limits corresponding to luminosity values from
several tens to hundreds times smaller than that of SMP SMC 22. The
exceptionally high X-ray luminosity of SMP SMC 22 is probably due to the high
mass of its central star, quickly evolving toward the white dwarf's cooling
branch, and to a small intrinsic absorption in the nebula itself.Comment: Accepted for publication on Astronomy and Astrophysic
Highly absorbed X-ray binaries in the Small Magellanic Cloud
Many of the high mass X-ray binaries (HMXRBs) discovered in recent years in
our Galaxy are characterized by a high absorption, most likely intrinsic to the
system, which hampers their detection at the softest X-ray energies. We have
undertaken a search for highly-absorbed X-ray sources in the Small Magellanic
Cloud (SMC) with a systematic analysis of 62 XMM-Newton SMC observations. We
obtained a sample of 30 sources showing evidence for an equivalent hydrogen
column density larger than 3x10^23 cm^-2. Five of these sources are clearly
identified as HMXRBs: four were already known (including three X-ray pulsars)
and one, XMM J005605.8-720012, reported here for the first time. For the
latter, we present optical spectroscopy confirming the association with a Be
star in the SMC. The other sources in our sample have optical counterparts
fainter than magnitude ~16 in the V band, and many of them have possible NIR
counterparts consistent with highly reddened early type stars in the SMC. While
their number is broadly consistent with the expected population of background
highly-absorbed active galactic nuclei, a few of them could be HMXRBs in which
an early type companion is severely reddened by local material.Comment: 10 pages, 4 figures, 4 tables. Accepted for publication by Astronomy
& Astrophysic
Mid-infrared spectroscopy of infrared-luminous galaxies at z~0.5-3
We present results on low-resolution mid-infrared (MIR) spectra of 70
infrared-luminous galaxies obtained with the Infrared Spectrograph (IRS)
onboard Spitzer. We selected sources from the European Large Area Infrared
Survey (ELAIS) with S15 > 0.8 mJy and photometric or spectroscopic z > 1. About
half of the sample are QSOs in the optical, while the remaining sources are
galaxies, comprising both obscured AGN and starbursts. We classify the spectra
using well-known infrared diagnostics, as well as a new one that we propose,
into three types of source: those dominated by an unobscured AGN (QSOs),
obscured AGN, and starburst-dominated sources. Starbursts concentrate at z ~
0.6-1.0 favored by the shift of the 7.7-micron PAH band into the selection 15
micron band, while AGN spread over the 0.5 < z < 3.1 range. Star formation
rates (SFR) are estimated for individual sources from the luminosity of the PAH
features. An estimate of the average PAH luminosity in QSOs and obscured AGN is
obtained from the composite spectrum of all sources with reliable redshifts.
The estimated mean SFR in the QSOs is 50-100 Mo yr^-1, but the implied FIR
luminosity is 3-10 times lower than that obtained from stacking analysis of the
FIR photometry, suggesting destruction of the PAH carriers by energetic photons
from the AGN. The SFR estimated in obscured AGN is 2-3 times higher than in
QSOs of similar MIR luminosity. This discrepancy might not be due to luminosity
effects or selection bias alone, but could instead indicate a connection
between obscuration and star formation. However, the observed correlation
between silicate absorption and the slope of the near- to mid-infrared spectrum
is compatible with the obscuration of the AGN emission in these sources being
produced in a dust torus.Comment: 32 pages, 24 figures, 15 tables, accepted for publication in MNRA
Galectin-4 Controls Intestinal Inflammation by Selective Regulation of Peripheral and Mucosal T Cell Apoptosis and Cell Cycle
Galectin-4 is a carbohydrate-binding protein belonging to the galectin family. Here we provide novel evidence that galectin-4 is selectively expressed and secreted by intestinal epithelial cells and binds potently to activated peripheral and mucosal lamina propria T-cells at the CD3 epitope. The carbohydrate-dependent binding of galectin-4 at the CD3 epitope is fully functional and inhibited T cell activation, cycling and expansion. Galectin-4 induced apoptosis of activated peripheral and mucosal lamina propria T cells via calpain-, but not caspase-dependent, pathways. Providing further evidence for its important role in regulating T cell function, galectin-4 blockade by antisense oligonucleotides reduced TNF-alpha inhibitor induced T cell death. Furthermore, in T cells, galectin-4 reduced pro-inflammatory cytokine secretion including IL-17. In a model of experimental colitis, galectin-4 ameliorated mucosal inflammation, induced apoptosis of mucosal T-cells and decreased the secretion of pro-inflammatory cytokines. Our results show that galectin-4 plays a unique role in the intestine and assign a novel role of this protein in controlling intestinal inflammation by a selective induction of T cell apoptosis and cell cycle restriction. Conclusively, after defining its biological role, we propose Galectin-4 is a novel anti-inflammatory agent that could be therapeutically effective in diseases with a disturbed T cell expansion and apoptosis such as inflammatory bowel disease
Mid-Infrared Properties of the Swift Burst Alert Telescope Active Galactic Nuclei Sample of the Local Universe. I. Emission-Line Diagnostics
We compare mid-infrared emission-line properties, from high-resolution
Spitzer spectra of a hard X-ray (14 -- 195 keV) selected sample of nearby (z <
0.05) AGN detected by the Burst Alert Telescope (BAT) aboard Swift. The
luminosity distribution for the mid-infrared emission-lines, [O IV] 25.89
micron, [Ne II] 12.81 micron, [Ne III] 15.56 micron and [Ne V] 14.32/24.32
micron, and hard X-ray continuum show no differences between Seyfert 1 and
Seyfert 2 populations, however six newly discovered BAT AGNs are under-luminous
in [O IV], most likely the result of dust extinction in the host galaxy. The
overall tightness of the mid-infrared correlations and BAT fluxes and
luminosities suggests that the emission lines primarily arise in gas ionized by
the AGN. We also compare the mid-infrared emission-lines in the BAT AGNs with
those from published studies of ULIRGs, PG QSOs, star-forming galaxies and
LINERs. We find that the BAT AGN sample fall into a distinctive region when
comparing the [Ne III]/[Ne II] and the [O IV]/[Ne III] ratios. These line
ratios are lower in sources that have been previously classified in the
mid-infrared/optical as AGN than those found for the BAT AGN, suggesting that,
in our X-ray selected sample, the AGN represents the main contribution to the
observed line emission. These ratios represent a new emission line diagnostic
for distinguishing between AGN and star forming galaxies.Comment: 54 pages, 9 Figures. Accepted for publication in The Astrophysical
Journal
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