23 research outputs found
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
Ensemble spectral variability study of Active Galactic Nuclei from the XMM-Newton serendipitous source catalogue
The variability of the X-ray spectra of active galactic nuclei (AGN) usually
includes a change of the spectral slope. This has been investigated for a small
sample of local AGNs by Sobolewska and Papadakis, who found that slope
variations are well correlated with flux variations, and that spectra are
typically steeper in the bright phase (softer when brighter behaviour). Not
much information is available for the spectral variability of high-luminosity
AGNs and quasars. In order to investigate this phenomenon, we use data from the
XMM-Newton Serendipitous Source Catalogue, Data Release 5, which contains X-ray
observations for a large number of active galactic nuclei in a wide luminosity
and redshift range, for several different epochs. This allows to perform an
ensemble analysis of the spectral variability for a large sample of quasars. We
quantify the spectral variability through the spectral variability parameter
, defined as the ratio between the change in spectral slope and the
corresponding logarithmic flux variation. We find that the spectral variability
of quasars has a softer when brighter behaviour, similarly to local AGNs.Comment: 5 pages, 2 figures. Prepared for the proceedings of the 6th Young
Researchers Meeting, L'Aquil
X-ray spectral variability of Active Galactic Nuclei from XMM-Newton data
The X-ray spectral variability of Active Galactic Nuclei typically follows a âsofter when brighterâ trend, which is believed to be originated mainly from the superposition of different spectral components, varying independently from each other, although some intrinsic variations of the continuum are also possible. We analyzed the MEXSAS sample, made up by more than 7800 observations from 2700 quasars from the fifth release of the XMM-Newton Serendipitous Source Catalogue (3XMM-SSC), cross-matched with two quasar catalogues, SDSS-DR7Q and SDSS-DR12Q. We developed a technique that is able to compute estimates of the photon index from approximate spectral fits, using the fluxes in the catalogue. Following Trevese & Vagnetti (2002), we quantify the spectral variability using ÎČ = ââÎ/â log F. We find an ensemble softer when brighter trend, extending therefore this result to quasars, and the same result is found for eight single sources extracted from the catalogue, plus one (PG 1114+445) discarded because of the presence of a prominent warm absorber, although with different extent. To investigate the reason for this range of different values of beta, we investigated a sample of X-ray
bright sources taken from the samples of Sobolewska & Papadakis (2009), plus M81, for which it is possible to obtain accurate photon indices. We compute both the accurate and approximate photon indices and we confront ÎČ obtained with both methods. Finally, we studied the spectra of 13 observations of PG 1114+445, finding multiple absorbers, one possibly being a highly ionized, ultra-fast outflow, with velocity of about 15% of the speed of light, which is observable in four
observations
A method for narrow-band searches of continuous gravitational wave signals
Targeted searches of continuous waves from spinning neutron stars normally
assume that the frequency of the gravitational wave signal is at a given known
ratio with respect to the rotational frequency of the source, e.g. twice for an
asymmetric neutron star rotating around a principal axis of inertia. In fact
this assumption may well be invalid if, for instance, the gravitational wave
signal is due to a solid core rotating at a slightly different rate with
respect to the star crust. In this paper we present a method for {\it
narrow-band} searches of continuous gravitational wave signals from known
pulsars in the data of interferometric detectors. This method assumes source
position is known to high accuracy, while a small frequency and spin-down range
around the electromagnetic-inferred values is explored. Barycentric and
spin-down corrections are done with an efficient time-domain procedure.
Sensitivity and computational efficiency estimates are given and results of
tests done using simulated data are also discussed.Comment: 13 pages; 6 figures; accepted in PR
Unveiling Sub-pc Supermassive Black Hole Binary Candidates in Active Galactic Nuclei
The elusive supermassive black hole binaries (SMBHBs) are thought to be the penultimate stage of galaxy mergers, preceding a final coalescence phase. SMBHBs are sources of continuous gravitational waves, possibly detectable by pulsar timing arrays; the identification of candidates could help in performing targeted gravitational wave searches. Due to SMBHBsâ origin in the innermost parts of active galactic nuclei (AGN), X-rays are a promising tool for unveiling their presence, by means of either double Fe Kα emission lines or periodicity in their light curve. Here we report on a new method for selecting SMBHBs by means of the presence of a periodic signal in their Swift Burst Alert Telescope (BAT) 105 month light curves. Our technique is based on Fisherâs exact g-test and takes into account the possible presence of colored noise. Among the 553 AGN selected for our investigation, only the Seyfert 1.5 galaxy Mrk 915 emerges as a candidate SMBHB; from subsequent analysis of its light curve we find a period P0 = 35 ± 2 months, and the null hypothesis is rejected at the 3.7Ï confidence level. We also present a detailed analysis of the BAT light curve of the only previously X-ray-selected binary candidate source in the literature, the Seyfert 2 galaxy MCG+11-11-032. We find P0 = 26.3 ± 0.6 months, consistent with the one inferred from previously reported double Fe Kα emission lines
Gravitational waves: search results, data analysis and parameter estimation
The Amaldi 10 Parallel Session C2 on gravitational wave (GW) search results, data analysis and parameter estimation included three lively sessions of lectures by 13 presenters, and 34 posters. The talks and posters covered a huge range of material, including results and analysis techniques for ground-based GW detectors, targeting anticipated signals from different astrophysical sources: compact binary inspiral, merger and ringdown; GW bursts from intermediate mass binary black hole mergers, cosmic string cusps, core-collapse supernovae, and other unmodeled sources; continuous waves from spinning neutron stars; and a stochastic GW background. There was considerable emphasis on Bayesian techniques for estimating the parameters of coalescing compact binary systems from the gravitational waveforms extracted from the data from the advanced detector network. This included methods to distinguish deviations of the signals from what is expected in the context of General Relativity
Lunar Gravitational-Wave Antenna
Monitoring of vibrational eigenmodes of an elastic body excited by
gravitational waves was one of the first concepts proposed for the detection of
gravitational waves. At laboratory scale, these experiments became known as
resonant-bar detectors first developed by Joseph Weber in the 1960s. Due to the
dimensions of these bars, the targeted signal frequencies were in the kHz
range. Weber also pointed out that monitoring of vibrations of Earth or Moon
could reveal gravitational waves in the mHz band. His Lunar Surface Gravimeter
experiment deployed on the Moon by the Apollo 17 crew had a technical failure
rendering the data useless. In this article, we revisit the idea and propose a
Lunar Gravitational-Wave Antenna (LGWA). We find that LGWA could become an
important partner observatory for joint observations with the space-borne,
laser-interferometric detector LISA, and at the same time contribute an
independent science case due to LGWA's unique features. Technical challenges
need to be overcome for the deployment of the experiment, and development of
inertial vibration sensor technology lays out a future path for this exciting
detector concept.Comment: 29 pages, 17 figure
Science case study and scientific simulations for the enhanced X-ray Timing Polarimetry mission, eXTP
The X-ray astronomy mission eXTP (enhanced X-ray Timing Polarimetry) is designed to study matter under extreme conditions of density, gravity and magnetism. Primary
goals are the determination of the equation of state (EoS) of matter at supranuclear density,
the physics in extremely strong magnetic fields, the study of accretion in strong-field gravity
(SFG) regime. Primary targets include isolated and binary neutron stars, strong magneticfield systems like magnetars, and stellar-mass and supermassive black holes. In this paper
we report about key observations and simulations with eXTP on the primary objectives
involving accretion under SFG regimes and determination of NS-Eo
The large area detector onboard the eXTP mission
The Large Area Detector (LAD) is the high-throughput, spectral-timing instrument onboard the eXTP mission, a flagship
mission of the Chinese Academy of Sciences and the China National Space Administration, with a large European
participation coordinated by Italy and Spain. The eXTP mission is currently performing its phase B study, with a target
launch at the end-2027. The eXTP scientific payload includes four instruments (SFA, PFA, LAD and WFM) offering
unprecedented simultaneous wide-band X-ray timing and polarimetry sensitivity. The LAD instrument is based on the
design originally proposed for the LOFT mission. It envisages a deployed 3.2 m2 effective area in the 2-30 keV energy
range, achieved through the technology of the large-area Silicon Drift Detectors - offering a spectral resolution of up to
200 eV FWHM at 6 keV - and of capillary plate collimators - limiting the field of view to about 1 degree. In this paper
we will provide an overview of the LAD instrument design, its current status of development and anticipated
performance