747 research outputs found
An X-ray Survey in SA 57 with XMM-Newton
The maximum number density of Active Galactic Nuclei (AGNs), as deduced from
X-ray studies, occurs at z<~1, with lower luminosity objects peaking at smaller
redshifts. Optical studies lead to a different evolutionary behaviour, with a
number density peaking at z~2 independently of the intrinsic luminosity, but
this result is limited to active nuclei brighter than the host galaxy. A
selection based on optical variability can detect low luminosity AGNs (LLAGNs),
where the host galaxy light prevents the identification by non-stellar colours.
We want to collect X-ray data in a field where it exists an optically-selected
sample of "variable galaxies'', i.e. variable objects with diffuse appearance,
to investigate the X-ray and optical properties of the population of AGNs,
particularly of low luminosity ones, where the host galaxy is visible. We
observed a field of 0.2 deg^2 in the Selected Area 57, for 67ks with
XMM-Newton. We detected X-ray sources, and we correlated the list with a
photographic survey of SA 57, complete to B_J~23 and with available
spectroscopic data. We obtained a catalogue of 140 X-ray sources to limiting
fluxes 5x10^-16, 2x10^-15 erg/cm^2/s in the 0.5-2 keV and 2-10 keV
respectively, 98 of which are identified in the optical bands. The X-ray
detection of part of the variability-selected candidates confirms their AGN
nature. Diffuse variable objects populate the low luminosity side of the
sample. Only 25/44 optically-selected QSOs are detected in X-rays. 15% of all
QSOs in the field have X/O<0.1.Comment: 13 pages, 6 figures, 4 tables, A&A in pres
Evidence for T Tauri-like emission in the EXor V1118 Ori from near-IR and X-ray data
We present a near-IR study of the EXor variable V1118 Ori, performed by
following a slightly declining phase after a recent outburst. In particular,
the near-IR (0.8 - 2.3 micron) spectrum, obtained for the first time, shows a
large variety of emission features of the HI and HeI recombination and CO
overtone. By comparing the observed spectrum with a wind model, a mass loss
rate value is derived along with other parameters whose values are typical of
an accreting T Tauri star. In addition, we have used X-ray data from the XMM
archive, taken in two different epochs during the declining phase monitored in
IR. X-ray emission (in the range 0.5 - 10 keV) permits to derive several
parameters which confirm the T Tauri nature of the source. In the near-IR the
object maintains a low visual extinction during all the activity phases,
confirming that variable extinction does not contribute to brightness
variations. The lack of both a significant amount of circumstellar material and
any evidence of IR cooling from collimated jet/outflow driven by the source,
indicates that, at least this member of the EXor class, is in a late stage of
the Pre-Main Sequence evolution. In the X-ray regime, an evident fading is
present, detected in the post-outburst phase, that cannot be reconciled with
the presence of any absorbing material. This circumstance, combined with the
persistence (in the pre- and post-outburst phases) of a temperature component
at about 10 MK, suggests that accretion has some influence in regulating the
coronal activity
Blazar surveys with WMAP and Swift
We present the preliminary results from two new surveys of blazars that have
direct implications on the GLAST detection of extragalactic sources from two
different perspectives: microwave selection and a combined deep X-ray/radio
selection. The first one is a 41 GHz flux-limited sample extracted from the
WMAP 3-yr catalog of microwave point sources. This is a statistically well
defined sample of about 200 blazars and radio galaxies, most of which are
expected to be detected by GLAST. The second one is a new deep survey of
Blazars selected among the radio sources that are spatially coincident with
serendipitous sources detected in deep X-ray images (0.3-10 keV) centered on
the Gamma Ray Bursts (GRB) discovered by the Swift satellite. This sample is
particularly interesting from a statistical viewpoint since a) it is unbiased
as GRBs explode at random positions in the sky, b) it is very deep in the X-ray
band (\fx \simgt \ergs) with a position accuracy of a few
arc-seconds, c) it will cover a fairly large (20-30 square deg.) area of sky,
d) it includes all blazars with radio flux (1.4 GHz) larger than 10 mJy, making
it approximately two orders of magnitude deeper than the WMAP sample and about
one order of magnitude deeper than the deepest existing complete samples of
radio selected blazars, and e) it can be used to estimate the amount of
unresolved GLAST high latitude gamma-ray background and its anisotropy
spectrum.Comment: 3 pages, 3 figures, to appear in Proc. of the 1st GLAST Symposium,
Feb 5-8, 2007, Stanford, AIP, Eds. S. Ritz, P. F. Michelson, and C. Meega
VaR bounds for joint portfolios with dependence constraints
Based on a novel extension of classical Hoe ding-Fr\ue9chet bounds, we provide an upper VaR bound for joint risk portfolios with xed marginal distributions and positive dependence information. The positive dependence information can be assumed to hold in the tails, in some central part, or on a general subset of the domain of the distribution function of a risk portfolio. The newly provided VaR bound can be interpreted as a comonotonic VaR computed at a distorted con dence level and its quality is illustrated in a series of examples of practical interest
A clustering approach and a rule of thumb for risk aggregation
The problem of establishing reliable estimates or bounds for the (T)VaR of a joint risk portfolio is a relevant subject in connection with the computation of total economic capital in the Basel regulatory framework for the finance sector as well as with the Solvency regulations for the insurance sector. In the computation of total economic capital, a financial institution faces a considerable amount of model uncertainty related to the estimation of the interdependence amongst the marginal risks. In this paper, we propose to apply a clustering procedure in order to partition a risk portfolio into independent subgroups of positively dependent risks. Based on available data, the portfolio partition so obtained can be statistically validated and allows for a reduction of capital and the corresponding model uncertainty. We illustrate the proposed methodology in a simulation study and two case studies considering an Operational and a Market Risk portfolio. A rule of thumb stems from the various examples proposed: in a mathematical model where the risk portfolio is split into independent subsets with comonotonic dependence within, the smallest VaR-based capital estimate (at the high regulatory probability levels typically used) is produced by assuming that the infinite-mean risks are comonotonic and the finite-mean risks are independent. The largest VaR estimate is instead generated by obtaining the maximum number of independent infinite-mean sums
Broadband Observations of the Compton-thick Nucleus of NGC 3393
We present new NuSTAR and Chandra observations of NGC 3393, a galaxy reported
to host the smallest separation dual AGN resolved in the X-rays. While past
results suggested a 150 pc separation dual AGN, three times deeper Chandra
imaging, combined with adaptive optics and radio imaging suggest a single,
heavily obscured, radio-bright AGN. Using VLA and VLBA data, we find an AGN
with a two-sided jet rather than a dual AGN and that the hard X-ray, UV,
optical, NIR, and radio emission are all from a single point source with a
radius <0.2". We find that the previously reported dual AGN is most likely a
spurious detection resulting from the low number of X-ray counts (<160) at 6-7
keV and Gaussian smoothing of the data on scales much smaller than the PSF
(0.25" vs. 0.80" FWHM). We show that statistical noise in a single Chandra PSF
generates spurious dual peaks of the same separation (0.550.07" vs. 0.6")
and flux ratio (399% vs. 32% of counts) as the purported dual AGN. With
NuSTAR, we measure a Compton-thick source (NH=
cm) with a large torus half-opening angle, {\theta}=79 which we
postulate results from feedback from strong radio jets. This AGN shows a 2-10
keV intrinsic to observed flux ratio of 150. Using simulations, we find that
even the deepest Chandra observations would severely underestimate the
intrinsic luminosity of NGC 3393 above z>0.2, but would detect an unobscured
AGN of this luminosity out to high redshift (z=5).Comment: Accepted for publication in ApJ. 15 Figures and 4 table
The seven year Swift-XRT point source catalog (1SWXRT)
Swift is a multi-wavelength observatory specifically designed for gamma-ray
burst (GRB) astronomy that is operational since 2004. Swift is also a very
flexible multi-purpose facility that supports a wide range of scientific fields
such as active galactic nuclei, supernovae, cataclysmic variables, Galactic
transients, active stars and comets. The Swift X-ray Telescope (XRT) has
collected more than 150 Ms of observations in its first seven years of
operations. We present the list of all the X-ray point sources detected in XRT
imaging data taken in photon counting mode during the first seven years of
Swift operations. All these point-like sources, excluding the Gamma-Ray Bursts
(GRB), will be stored in a catalog publicly available (1SWXRT). We consider all
XRT observations with exposure time > 500 s taken in the period 2005-2011. Data
were reduced and analyzed with standard techniques and a list of detected
sources for each observation was produced. A careful visual inspection was
performed to remove extended, spurious and piled-up sources. Positions, count
rates, fluxes and the corresponding uncertainties were computed. We have
analyzed more than 35,000 XRT fields, with exposures ranging between 500 s and
100 ks, for a total exposure time of 140 Ms. The catalog includes ~ 89,000
entries, of which almost 85,000 are not affected by pile-up and are not GRBs.
Since many XRT fields were observed several times, we have a total of ~36,000
distinct celestial sources. We computed count rates in three energy bands:
0.3-10 keV (Full, or F), 0.3-3 keV (Soft, or S) and 2-10 keV (Hard, or H). Each
entry has a detection in at least one of these bands. In particular, we detect
~ 80,000, ~ 70,000 and ~ 25,500$ in the F, S and H band, respectively. Count
rates were converted into fluxes in the 0.5-10, 0.5-2 and 2-10 keV bands. Some
possible scientific uses of the catalog are also highlighted.Comment: 11 pages, 15 ps figures, accepted for publication in A&A. Abstract
shortened with respect to the original versio
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