441 research outputs found
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&
C IV BAL disappearance in a large SDSS QSO sample
Broad absorption lines (BALs) in the spectra of quasi-stellar objects (QSOs)
originate from outflowing winds along our line of sight; winds are thought to
originate from the inner regions of the QSO accretion disk, close to the
central supermassive black hole (SMBH). Winds likely play a role in galaxy
evolution and aid the accretion mechanism onto the SMBH. BAL equivalent widths
can change on typical timescales from months to years; such variability is
generally attributed to changes in the covering factor and/or in the ionization
level of the gas. We investigate BAL variability, focusing on BAL
disappearance. We analyze multi-epoch spectra of more than 1500 QSOs -the
largest sample ever used for such a study- observed by different programs from
the Sloan Digital Sky Survey-I/II/III (SDSS), and search for disappearing C IV
BALs. The spectra rest-frame time baseline ranges from 0.28 to 4.9 yr; the
source redshifts range from 1.68 to 4.27. We detect 73 disappearing BALs in the
spectra of 67 sources. This corresponds to 3.9% of disappearing BALs, and 5.1%
of our BAL QSOs exhibit at least one disappearing BAL. We estimate the average
lifetime of a BAL along our line of sight (~ 80-100 yr), which appears
consistent with the accretion disk orbital time at distances where winds are
thought to originate. We inspect properties of the disappearing BALs and
compare them to the properties of our main sample. We also investigate the
existence of a correlation in the variability of multiple troughs in the same
spectrum, and find it persistent at large velocity offsets between BAL pairs,
suggesting that a mechanism extending on a global scale is necessary to explain
the phenomenon. We select a more reliable sample of disappearing BALs following
Filiz Ak et al. (2012), where a subset of our sample was analyzed, and compare
the findings from the two works, obtaining generally consistent results.Comment: 22 pages, 9 figures. Accepted for publication in A&
The connection between star formation and supermassive Black Hole activity in the local Universe
We present a study of the active galactic nucleus (AGN) activity in the local
Universe (z < 0.33) and its correlation with the host galaxy properties,
derived from a Sloan Digital Sky Survey (SDSS DR8) sample with spectroscopic
star-formation rate (SFR) and stellar mass ()
determination. To quantify the level of AGN activity we used X-ray information
from the XMM-Newton Serendipitous Source Catalogue (3XMM DR8). Applying
multiwavelength AGN selection criteria (optical BPT-diagrams, X-ray/optical
ratio etc) we found that 24% of the detected sources are efficiently-accreting
AGN with moderate-to-high X-ray luminosity, which are twice as likely to be
hosted by star-forming galaxies than by quiescent ones. The distribution of the
specific Black Hole accretion rate (sBHAR, ) shows
that nuclear activity in local, non-AGN dominated galaxies peaks at very low
accretion rates () in all
stellar mass ranges. However, we observe systematically larger values of sBHAR
for galaxies with active star-formation than for quiescent ones, as well as an
increase of the mean with SFR for both star-forming
and quiescent galaxies. These findings confirm the decreased level of AGN
activity with cosmic time and are consistent with a scenario where both
star-formation and AGN activity are fuelled by a common gas reservoir.Comment: 20 pages, 20 figures, accepted for publication in MNRA
Linking the X-ray and infrared properties of star-forming galaxies at z < 1.5
We present the most complete study to date of the X-ray emission from star formation in high-redshift (median z = 0.7; z −3 in both hard and soft X-ray bands. From the sources which are star formation dominated, only a small fraction are individually X-ray detected and for the bulk of the sample we calculate average X-ray luminosities through stacking. We find an average soft X-ray to infrared ratio of log ?L SX /L IR ? = −4.3 and an average hard X-ray to infrared ratio of log?L HX /L IR ?=−3.8.WereportthattheX-ray/IRcorrelationisapproximatelylinearthrough the entire range of L IR and z probed and, although broadly consistent with the local (z < 0.1) one, it does display some discrepancies. We suggest that these discrepancies are unlikely to be physical, i.e. due to an intrinsic change in the X-ray properties of star-forming galaxies with cosmic time, as there is no significant evidence for evolution of the L X /L IR ratio with redshift. Instead, they are possibly due to selection effects and remaining AGN contamination.
We also examine whether dust obscuration in the galaxy plays a role in attenuating X-rays from star formation, by investigating changes in the L X /L IR ratio as a function of the average dust temperature. We conclude that X-rays do not suffer any measurable attenuation in the host galaxy
Deepest view of AGN X-ray variability with the 7 Ms Chandra Deep Field-South survey
We systematically analyze X-ray variability of active galactic nuclei (AGNs)
in the 7~Ms \textit{Chandra} Deep Field-South survey. On the longest timescale
( years), we find only weak (if any) dependence of X-ray
variability amplitudes on energy bands or obscuration. We use four different
power spectral density (PSD) models to fit the anti-correlation between
normalized excess variance () and luminosity, and obtain a
best-fit power law index for the low-frequency
part of AGN PSD. We also divide the whole light curves into 4 epochs in order
to inspect the dependence of on these timescales, finding
an overall increasing trend. The analysis of these shorter light curves also
infers a of that is consistent with the above-derived
, which is larger than the frequently-assumed value of . We
then investigate the evolution of . No definitive
conclusion is reached due to limited source statistics but, if present, the
observed trend goes in the direction of decreasing AGN variability at fixed
luminosity toward large redshifts. We also search for transient events and find
6 notable candidate events with our considered criteria. Two of them may be a
new type of fast transient events, one of which is reported here for the first
time. We therefore estimate a rate of fast outbursts and a tidal disruption
event~(TDE) rate assuming the other four long outbursts to be
TDEs.Comment: 20 pages, 16 figures. Accepted for publication in Ap
Variability-selected low-luminosity active galactic nuclei candidates in the 7 Ms Chandra Deep Field-South
In deep X-ray surveys, active galactic nuclei (AGNs) with a broad range of
luminosities have been identified. However, cosmologically distant
low-luminosity AGN (LLAGN, erg s)
identification still poses a challenge due to significant contamination from
host galaxies. Based on the 7 Ms Chandra Deep Field-South (CDF-S) survey, the
longest timescale ( years) deep X-ray survey to date, we utilize an
X-ray variability selection technique to search for LLAGNs that remain
unidentified among the CDF-S X-ray sources. We find 13 variable sources from
110 unclassified CDF-S X-ray sources. Except for one source which could be an
ultraluminous X-ray source, the variability of the remaining 12 sources is most
likely due to accreting supermassive black holes. These 12 AGN candidates have
low intrinsic X-ray luminosities, with a median value of erg
s. They are generally not heavily obscured, with an average effective
power-law photon index of 1.8. The fraction of variable AGNs in the CDF-S is
independent of X-ray luminosity and is only restricted by the total number of
observed net counts, confirming previous findings that X-ray variability is a
near-ubiquitous property of AGNs over a wide range of luminosities. There is an
anti-correlation between X-ray luminosity and variability amplitude for
high-luminosity AGNs, but as the luminosity drops to erg
s, the variability amplitude no longer appears dependent on the
luminosity. The entire observed luminosity-variability trend can be roughly
reproduced by an empirical AGN variability model based on a broken power-law
power spectral density function.Comment: 18 pages, 11 figures, accepted for publication in Ap
Investigating A Dose Response Relationship between High Fat Diet Consumption and the Contractile Performance of Isolated Mouse Soleus, EDL and Diaphragm Muscles
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Metabolic Risk Factors as Differential Predictors of Profiles of Neurocognitive Impairment Among Older HIV+ and HIV- Adults: An Observational Study.
ObjectiveNeurocognitive performance among older persons, including those living with HIV (people living with HIV [PLWH]), exhibits significant heterogeneity, suggesting subpopulations with differing profiles of neurocognitive impairment (NCI). Metabolic factors are associated with NCI, but their relationships to cluster-derived NCI profiles are unknown.MethodParticipants (144 PLWH and 102 HIV uninfected) aged 50+ years completed a neuropsychological battery assessing seven cognitive domains. Latent class analysis (LCA) identified NCI profiles separately by HIV serostatus and in a combined sample. Obtained classes were examined against the Montreal Cognitive Assessment (MoCA) and diagnoses of HIV-associated neurocognitive disorders (HAND). Multinomial regression identified metabolic predictors of classification.ResultsLCA identified three latent classes in each participant sample: Class1Multidomain NCI (high probability of impairment across multiple domains), Class 2Learning & Recall NCI (high probability of impairment in learning and recall), and Class 3NC Unimpaired (low probability of NCI across all domains). Severity of NCI implied by classes corresponded with MoCA scores and HAND diagnoses. In analyses on the combined sample, compared to HIV-uninfected individuals, PLWH were more likely to be in Class1Multidomain NCI. Among PLWH, those with dyslipidemia and hypertension had greater odds of classification in Class 1Multidomain NCI while those with central obesity had higher odds of classification in Class 2Learning & Recall NCI; metabolic syndrome approached significance as a differential predictor. Regardless of HIV status, individuals with diabetes were more likely to be in Class 1Multidomain NCI.ConclusionsMetabolic risk factors confer heightened risk of NCI in HIV infection. Interventions to reduce metabolic risk may improve neurocognitive outcomes among PLWH
GMRT HI observations of the Eridanus group of galaxies
The GMRT HI 21cm-line observations of galaxies in the Eridanus group are
presented. The Eridanus group, at a distance of ~23 Mpc, is a loose group of
\~200 galaxies. The group extends more than 10 Mpc in projection. The velocity
dispersion of the galaxies in the group is ~240 km/s. The galaxies are
clustered into different sub-groups. The overall population mix of the group is
30% (E+S0) and 70% (Sp+Irr). The observations of 57 Eridanus galaxies were
carried out with the GMRT for ~200 hour. HI emission was detected from 31
galaxies. The channel rms of ~1.0 mJy beam^{-1} was achieved for most of the
image-cubes made with 4 hour of data. The corresponding HI column density
sensitivity (3-sigma) is ~1x10^{20} cm^{-2} for a velocity-width of ~13.4 km/s.
The 3-sigma detection limit of HI mass is ~1.2x10^{7} M_sun for a line-width of
50 km/s. Total HI images, HI velocity fields, global HI line profiles, HI mass
surface densities, HI disk parameters and HI rotation curves are presented. The
velocity fields are analysed separately for the approaching and the receding
sides of the galaxies. This data will be used to study the HI and the radio
continuum properties, the Tully-Fisher relations, the dark matter halos, and
the kinematical and HI lopsidedness in galaxies.Comment: 75 pages including HI atlas; Accepted for publication in Journal of
Astroph. & Astron. March, 200
The XMM deep survey in the CDFS XI. X-ray properties of 185 bright sources
We present X-ray spectra of 185 bright sources detected in the XMM-Newton
deep survey of the Chandra Deep Field South, combining the three EPIC cameras.
The 2-10 keV flux limit of the sample is 2e-15 erg/s/cm2. The sources are
distributed over a redshift range of z=0.1-3.8. Eleven new X-ray redshift
measurements are included. A spectral analysis was performed using a simple
model to obtain absorbing column densities, rest-frame 2-10 keV luminosities
and Fe K line properties of 180 sources at z>0.4. Obscured AGN are found to be
more abundant toward higher redshifts. Using the XMM-Newton data alone, seven
Compton-thick AGN candidates are identified, which makes the Compton-thick AGN
fraction to be ~4%. An exploratory spectral inspection method with two
rest-frame X-ray colours and an Fe line strength indicator is introduced and
tested against the results from spectral fitting. This method works reasonably
well to characterise a spectral shape and can be useful for a pre-selection of
Compton-thick AGN candidates. We found six objects exhibiting broad Fe K lines
out of 21 unobscured AGN of best data quality, implying a detection rate of
~30%. Five redshift spikes, each of which has more than six sources, are
identified in the redshift distribution of the X-ray sources. Contrary to the
overall trend, the sources at the two higher-redshift spikes at z=1.61 and
z=2.57 have puzzlingly low obscuration.Comment: Accepted for publication in Astronomy & Astrophysics. Table 1 and
Fig. 2 are available in electronic form the CDS. Typos corrected and some
language editing and references adde
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