7 research outputs found
The X-ray/UV ratio in Active Galactic Nuclei: dispersion and variability
The relation between the index and the optical/UV luminosity
(), a by product of the X-ray - optical/UV luminosity relation, is
affected by a relatively large dispersion, due to variability in the
within single sources (intra-source dispersion) and variations of
fundamental physical parameters from source to source (inter-source
dispersion). We use archival data from the XMMSSC and from the XMMOM-SUSS3. We
select a sub-sample in order to decrease the dispersion of the relation due to
the presence of Radio-Loud and Broad Absorption Line objects, and to
absorptions in both X-ray and optical/UV bands. We analyse the dependence of
the residuals of the relation on various physical parameters in order to
characterise the inter-source dispersion. We find a total dispersion of ~0.12
and, using the Structure Function, we find that intrinsic variability
contributes for 56% of the variance of the relation. We find weak but
significant dependences of the residuals of the relation on black-hole (BH)
mass and on Eddington ratio, confirmed by a multivariate regression analysis of
as a function of optical/UV luminosity and the above quantities.
We find a weak positive correlation of both the and the residuals
of the relation with inclination indicators (FWHM(H) and EW[O])
suggesting a weak increase of X-ray/UV ratio with the viewing angle. Our
results suggest the possibility of selecting a sample of objects, based on
their viewing angle and/or BH mass and Eddington ratio, for which the
relation is as tight as possible, in light of the use of
the X-ray - optical/UV luminosity relation to build a distance modulus (DM) -
plane and estimate cosmological parameters.Comment: accepted for publication in A&
The MEXSAS2 Sample and the Ensemble X-ray Variability of Quasars
We present the second Multi-Epoch X-ray Serendipitous AGN Sample (MEXSAS2),
extracted from the 6th release of the XMM Serendipitous Source Catalogue
(XMMSSC-DR6), cross-matched with Sloan Digital Sky Survey quasar catalogues
DR7Q and DR12Q. Our sample also includes the available measurements for masses,
bolometric luminosities, and Eddington ratios. Analyses of the ensemble
structure function and spectral variability are presented, together with their
dependences on such parameters. We confirm a decrease of the structure function
with the X-ray luminosity, and find a weak dependence on the black hole mass.
We introduce a new spectral variability estimator, taking errors on both fluxes
and spectral indices into account. We confirm an ensemble softer when brighter
trend, with no dependence of such estimator on black hole mass, Eddington
ratio, redshift, X-ray and bolometric luminosity.Comment: 6 pages, 3 figures, to appear in Frontiers in Astronomy and Space
Science
Hard X-ray selected giant radio galaxies - II. Morphological evidence of restarted radio activity
About 6 per cent of radio galaxies (RGs) can reach linear sizes larger than 0.7 Mpc, and are then classified as giant radio galaxies (GRGs). The conditions that make possible the formation of such big structures are still not clear - either core accretion properties or environmental factors. Recent studies have shown that GRGs can be up to four times more abundant in hard X-ray selected (I.e. from INTEGRAL/IBIS and Swift/BAT at >20 keV) RG samples. Moreover, a high fraction of young radio sources found in their cores suggest a recently restarted activity, as suggested from the discrepancy between the measured jet and lobes power, with respect to the one expected from core X-ray luminosity. Here, we present a radio morphological study of a sample of 15 hard X-ray selected GRGs, discussing low-frequency images from our GMRT campaign complemented with others from the literature: among them, 7/15 show evidence of restarted radio activity either in the form of double-double/X-shaped morphology, or as a cocoon emission embedding more recent jets. This, together with the objects from this sample already found hosting a young radio source in their core, suggests that at least 13 over 15 of these hard X-ray selected GRGs show features that are consistent with the possibility of restarted radio activity
Hard-X-ray-selected active galactic nuclei - II. Spectral energy distributions in the 5-45 GHz domain
A wide-frequency radio study of active galactic nuclei (AGN) is crucial to evaluate the intervening radiative mechanisms responsible for the observed emission and relate them with the underlying accretion physics. We present wide-frequency (5â45 GHz), high-sensitivity (few ÎŒJybeamâ1â ), (sub)-kpc Jansky Very Large Array (JVLA) observations of a sample of 30 nearby (â 0.003â€zâ€0.3â ) AGN detected by the International Gamma-Ray Astrophysics Laboratory (INTEGRAL)/Imager on Board the INTEGRAL Satellite (IBIS) at hard X-ray. We find a high detection fraction of radio emission at all frequencies, i.e. â„95 per cent at 5, 10, and 15 GHz and â„80 per cent at 22 and 45 GHz. Two sources out of 30 remain undetected at our high sensitivities. The nuclear radio morphology is predominantly compact, sometimes accompanied by extended jet-like structures, or more complex features. The radio spectral energy distributions (SEDs) of the radio cores appear either as single or as a broken power law, a minority of them exhibit a peaked component. The spectral slopes are either flat/inverted or steep, up to a break/peak or over the whole range. The sample mean SED shows a flat slope up to 15 GHz that steepens between 15 and 22 GHz and becomes again flat above 22 GHz. Significant radioâX-ray correlations are observed at all frequencies. About half of the sample features extended emission, clearly resolved by the JVLA, indicating low-power jets or large-scale outflows. The unresolved cores, which often dominate the radio power, may be of jet, outflow, and/or coronal origin, depending on the observed frequency
Performance of earthquake early warning systems during the 2016-2017 Mw5-6.5 central Italy sequence
Earthquake early warning systems (EEWSs) are nowadays contributing
to seismic risk mitigation actions, both in terms of
losses and societal resilience, by issuing an alert promptly after
the earthquake origin and before the ground-shaking impacts
the target to be protected. In this work, we analyze the performance
of network-based and stand-alone (on-site) early warning
systems during the 2016â2017 central Italy sequence,
characterized by events with magnitude as large as 6.5. For the
largest magnitude event, both systems predict well the ground
shaking nearby the event source, with a rate of success in the
85%â90% range, within the potential earthquake damage zone.
However, the lead time, that is, the time available for security
actions, is significantly larger for the network-based system. For
the regional system, it increases to more than 10 s at 40 km
from the event epicenter. The stand-alone system performs better
in the near-source region, still showing a positive albeit
small lead time (60 km),
the performances slightly degrade, mostly owing to the large
uncertainty in the attenuation relationships. This study opens
up the possibility for making an operational EEWS in Italy,
based on the available acceleration networks, provided that the
delay due to data telemetry has to be reduced.Published1-125T. Sismologia, geofisica e geologia per l'ingegneria sismicaJCR Journa
The Near Fault Observatory community in Europe: a new resource for faulting and hazard studies
The Near Fault Observatories (NFOs) community is one of the European Plate Observing System (EPOS, http://www.epos-eu.org) Thematic Communities, today consisting of six research infrastructures that operate in regions characterised by high seismic hazard originating from different tectonic regimes. Earthquakes respond to complex natural systems whose mechanical properties evolve over time. Thus, in order to understand the multi-scale, physical/chemical processes responsible for the faulting that earthquakes occur on, it is required to consider phenomena that intersect different research fields, i.e., to put in place multidisciplinary monitoring. Hence, NFOs are grounded on modern and multidisciplinary infrastructures, collecting near fault high resolution raw data that allows generation of innovative scientific products. The NFOs usually complement regional backbone networks with a higher density distribution of seismic, geodetic, geochemical and other geophysical sensors, at surface and sometimes below grade. These dense and modern networks of multi-parametric sensors are sited at and around active faults, where moderate to large earthquakes have occurred in the past and are expected in the future. They continuously monitor the underlying Earth instability processes over a broad time interval. Data collected at each NFO results in an exceptionally high degree of knowledge of the geometry and parameters characterizing the local geological faults and their deformation pattern. The novel data produced by the NFO community is aggregated in EPOS and is made available to a diverse set of stake-holders through the NFO Federated Specific Data Gateway (FRIDGE). In the broader domain of the Solid Earth sciences, NFOs meet the growing expectations of the learning and communication sectors by hosting a large variety of scientific information about earthquakes as a natural phenomenon and a societal issue. It represents the EPOS concept and objective of aggregating and harmonising the European research infrastructures capabilities to facilitate broader scientific opportunity. The NFOs are at the cutting edge of network monitoring. They conduct multidisciplinary experiments for testing multi-sensor stations, as well as realise robust and ultra-low latency, transmis-sion systems that can routinely accommodate temporary monitoring densification. The effort to continuously upgrade the technological efficiency of monitoring systems positions the NFO at the centre of marketing opportunities for the European enterprises devoted to new sensor technology. The NFOs constitute ideal test beds for generating expertise on data integration, creating tools for the next generation of multidisciplinary research, routine data analysis and data visualization. In particular focus is often on near-real time tools and triggering alarms at different levels are tested and implemented, strengthening the cooperation with the Agencies for risk management. NFOs have developed innovative operational actions such as the Testing Centre for Earthquake Early Warning and Source Characterisation (CREW) and detailed fast ground shaking and damage characterization. Complementing the recent growth of modern laboratory and computational models, the NFOs can provide interdisciplinary observations of comparable high resolution to describe the behaviour of fault slip over a vast range of spatial and temporal scales and aiding to provide more accurate earthquake hazard characterizations.ISSN:1593-521