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Middei R.,Third University of Rome | Vagnetti F.,University of Rome Tor Vergata | Bianchi S.,Third University of Rome | La Franca F.,Third University of Rome | And 5 more authors.
Astronomy and Astrophysics | Year: 2017

Context. Variability in the X-rays is a key ingredient in understanding and unveiling active galactic nuclei (AGN) properties. In this band, flux variations occur on short timescales (hours) as well as on larger timescales. While short timescale variability is often investigated in single source studies, only a few works are able to explore flux variation on very long timescales. Aims. This work aims to provide a statistical analysis of the AGN long term X-ray variability. We study variability on the largest time interval ever investigated for the 0.2-2 keV band, up to approximately 20 yr rest-frame for a sample of 220 sources. Moreover, we study variability for 2700 quasars up to approximatley eight years rest-frame in the same (soft) band. Methods. We built our source sample using the 3XMM serendipitous source catalogue data release 5, and data from ROSAT All Sky Survey Bright and Faint source catalogues. To ensure that we selected AGN only, we used the Sloan Digital Sky Survey quasar catalogues data releases 7 and 12. Combining ROSAT and XMM-Newton observations, we investigated variability using the structure function analysis which describes the amount of variability as a function of the lag between the observations. Results. Our work shows an increase of the structure function up to 20 yr. We find no evidence of a plateau in the structure function on these long timescales. Conclusions. The increase of the structure function at long time lags suggests that variability in the soft X-rays can be influenced by flux variations originated in the accretion disk or that they take place in a region large enough to justify variation on such long timescales. © 2017 ESO.

PubMed | University of Nottingham, International School for Advanced Studies, University of Helsinki, Stanford University and 58 more.
Type: Journal Article | Journal: Physical review letters | Year: 2015

We report the results of a joint analysis of data from BICEP2/Keck Array and Planck. BICEP2 and Keck Array have observed the same approximately 400deg^{2} patch of sky centered on RA 0 h, Dec. -57.5. The combined maps reach a depth of 57 nK deg in Stokes Q and U in a band centered at 150 GHz. Planck has observed the full sky in polarization at seven frequencies from 30 to 353 GHz, but much less deeply in any given region (1.2K deg in Q and U at 143 GHz). We detect 150353 cross-correlation in B modes at high significance. We fit the single- and cross-frequency power spectra at frequencies 150GHz to a lensed-CDM model that includes dust and a possible contribution from inflationary gravitational waves (as parametrized by the tensor-to-scalar ratio r), using a prior on the frequency spectral behavior of polarized dust emission from previous Planck analysis of other regions of the sky. We find strong evidence for dust and no statistically significant evidence for tensor modes. We probe various model variations and extensions, including adding a synchrotron component in combination with lower frequency data, and find that these make little difference to the r constraint. Finally, we present an alternative analysis which is similar to a map-based cleaning of the dust contribution, and show that this gives similar constraints. The final result is expressed as a likelihood curve for r, and yields an upper limit r_{0.05}<0.12 at 95% confidence. Marginalizing over dust and r, lensing B modes are detected at 7.0 significance.

Ajello M.,University of California at Berkeley | Romani R.W.,Kavli Institute for Particle Astrophysics and Cosmology | Gasparrini D.,Agenzia Spaziale Italiana Science Data Center | Gasparrini D.,Instituto Nazionale Of Astrofisica Osservatorio Astronomico Of Rome | And 14 more authors.
Astrophysical Journal | Year: 2014

Fermi has provided the largest sample of γ-ray-selected blazars to date. In this work we use a uniformly selected set of 211 BL Lacertae (BL Lac) objects detected by Fermi during its first year of operation. We obtained redshift constraints for 206 out of the 211 BL Lac objects in our sample, making it the largest and most complete sample of BL Lac objects available in the literature. We use this sample to determine the luminosity function of BL Lac objects and its evolution with cosmic time. We find that for most BL Lac classes the evolution is positive, with a space density peaking at modest redshift (z ≈ 1.2). Low-luminosity, high-synchrotron-peaked (HSP) BL Lac objects are an exception, showing strong negative evolution, with number density increasing for z ≲ 0.5. Since this rise corresponds to a drop-off in the density of flat-spectrum radio quasars (FSRQs), a possible interpretation is that these HSPs represent an accretion-starved end state of an earlier merger-driven gas-rich phase. We additionally find that the known BL Lac correlation between luminosity and photon spectral index persists after correction for the substantial observational selection effects with implications for the so-called "blazar sequence." Finally, by estimating the beaming corrections to the luminosity function, we find that BL Lac objects have an average Lorentz factor of , and that most are seen within 10° of the jet axis. © 2014. The American Astronomical Society. All rights reserved.

Potgieter M.S.,North West University South Africa | Vos E.E.,North West University South Africa | Munini R.,National Institute of Nuclear Physics, Italy | Boezio M.,National Institute of Nuclear Physics, Italy | And 2 more authors.
Astrophysical Journal | Year: 2015

The last solar minimum activity period, and the consequent minimum modulation conditions for cosmic rays, was unusual. The highest levels of Galactic protons were recorded at Earth in late 2009 in contrast to expectations. A comprehensive model was used to study the proton modulation for the period from 2006 to 2009 in order to determine what basic processes were responsible for solar modulation during this period and why it differs from proton modulation during previous solar minimum modulation periods. This established model is now applied to studying the solar modulation of electron spectra as observed for 80 MeV-30 GeV by the PAMELA space detector from mid-2006 to the end of 2009. Over this period the heliospheric magnetic field had decreased significantly until the end of 2009 while the waviness of the heliospheric current sheet decreased moderately and the observed electron spectra increased by a factor of ∼1.5 at 1.0 GeV to ∼3.5 at 100 MeV. In order to reproduce the modulation evident from seven consecutive semesters, the diffusion coefficients had to increase moderately while maintaining the basic rigidity dependence. It is confirmed that the main diffusion coefficients are independent of rigidity below ∼0.5 GV, while the drift coefficient had to be reduced below this value. The 2006-2009 solar minimum epoch indeed was different than previously observed minima, at least since the beginning of the space exploration era. This period could be called "diffusion-dominated" as was also found for the modulation of protons. © 2015. The American Astronomical Society. All rights reserved.

Spinoglio L.,National institute for astrophysics | Pereira-Santaella M.,National institute for astrophysics | Dasyra K.M.,French National Center for Scientific Research | Calzoletti L.,Agenzia Spaziale Italiana Science Data Center | And 5 more authors.
Astrophysical Journal | Year: 2015

We observed the far-IR fine-structure lines of 26 Seyfert galaxies with the Herschel-PACS spectrometer. These observations are complemented with Spitzer Infrared Spectrograph and Herschel SPIRE spectroscopy. We used the ionic lines to determine electron densities in the ionized gas and the [C I] lines, observed with SPIRE, to measure the neutral gas densities, while the [O I] lines measure the gas temperature, at densities below ∼104 cm-3. Using the [O I]145 μm/63 μm and [S III]33/18 μm line ratios, we find an anti-correlation of the temperature with the gas density. Various fine-structure line ratios show density stratifications in these active galaxies. On average, electron densities increase with the ionization potential of the ions. The infrared lines arise partly in the narrow line region, photoionized by the active galactic nucleus (AGN), partly in H II regions photoionized by hot stars, and partly in photo-dissociated regions. We attempt to separate the contributions to the line emission produced in these different regions by comparing our observed emission line ratios to theoretical values. In particular, we tried to separate the contribution of AGNs and star formation by using a combination of Spitzer and Herschel lines, and we found that besides the well-known mid-IR line ratios, the line ratio of [O III]88 μm/[O IV]26 μm can reliably discriminate the two emission regions, while the far-IR line ratio of [C II]157 μm/[O I]63 μm is only able to mildly separate the two regimes. By comparing the observed [C II]157 μm/[N II]205 μm ratio with photoionization models, we also found that most of the [C II] emission in the galaxies we examined is due to photodissociation regions. © 2015. The American Astronomical Society. All rights reserved.

Puzia T.H.,University of Santiago de Chile | Puzia T.H.,National Research Council Canada | Paolillo M.,University of Naples Federico II | Paolillo M.,National Institute of Nuclear Physics, Italy | And 5 more authors.
Astrophysical Journal | Year: 2014

We present a comprehensive high spatial resolution imaging study of globular clusters (GCs) in NGC 1399, the central giant elliptical cD galaxy in the Fornax galaxy cluster, conducted with the Advanced Camera for Surveys (ACS) aboard the Hubble Space Telescope (HST). Using a novel technique to construct drizzled point-spread function libraries for HST/ACS data, we accurately determine the fidelity of GC structural parameter measurements from detailed artificial star cluster experiments and show the superior robustness of the GC half-light radius, rh, compared with other GC structural parameters, such as King core and tidal radius. The measurement of rh for the major fraction of the NGC 1399 GC system reveals a trend of increasing r h versus galactocentric distance, R gal, out to about 10 kpc and a flat relation beyond. This trend is very similar for blue and red GCs, which are found to have a mean size ratio of r h, red/r h, blue = 0.82 ± 0.11 at all galactocentric radii from the core regions of the galaxy out to ∼40 kpc. This suggests that the size difference between blue and red GCs is due to internal mechanisms related to the evolution of their constituent stellar populations. Modeling the mass density profile of NGC 1399 shows that additional external dynamical mechanisms are required to limit the GC size in the galaxy halo regions to rh ≈ 2 pc. We suggest that this may be realized by an exotic GC orbit distribution function, an extended dark matter halo, and/or tidal stress induced by the increased stochasticity in the dwarf halo substructure at larger galactocentric distances. We compare our results with the GC rh distribution functions in various galaxies and find that the fraction of extended GCs with rh ≥ 5 pc is systematically larger in late-type galaxies compared with GC systems in early-type galaxies. This is likely due to the dynamically more violent evolution of early-type galaxies. We match our GC rh measurements with radial velocity data from the literature and split the resulting sample at the median rh value into compact and extended GCs. We find that compact GCs show a significantly smaller line-of-sight velocity dispersion, 〈σcmp〉 = 225 ± 25 km s-1, than their extended counterparts, 〈σ ext〉 = 317 ± 21 km s-1. Considering the weaker statistical correlation in the GC rh color and the GC r h-R gal relations, the more significant GC size-dynamics relation appears to be astrophysically more relevant and hints at the dominant influence of the GC orbit distribution function on the evolution of GC structural parameters. © 2014. The American Astronomical Society. All rights reserved.

Menci N.,National institute for astrophysics | Grazian A.,National institute for astrophysics | Grazian A.,Agenzia Spaziale Italiana Science Data Center | Castellano M.,National institute for astrophysics | Sanchez N.G.,Paris-Sorbonne University
Astrophysical Journal Letters | Year: 2016

We show that the recently measured UV luminosity functions of ultra-faint lensed galaxies at z ≈ 6 in the Hubble Frontier Fields provide an unprecedented probe for the mass m X of the warm dark matter (WDM) candidates independent of baryonic physics. Comparing the measured abundance of the faintest galaxies with the maximum number density of dark matter halos in WDM cosmologies sets a robust limit of m X ≥ 2.9 keV for the mass of thermal relic WDM particles at a 1σ confidence level, m X ≥ 2.4 keV at 2σ, and m X ≥ 2.1 keV at 3σ. These constraints are independent of the baryonic physics involved in galaxy formation and constitute the tightest constraints on WDM particle mass derived to date. We discuss the impact of our results on the production mechanism of sterile neutrinos. In particular, if sterile neutrinos are responsible for the 3.5 keV line reported in observations of X-ray clusters, our results firmly rule out the Dodelson-Widrow production mechanism and yield m sterile ≳ 6.1 keV for sterile neutrinos produced via the Shi-Fuller mechanism. © 2016. The American Astronomical Society. All rights reserved.

Pizzolotto C.,National Institute of Nuclear Physics, Italy | Pizzolotto C.,Agenzia Spaziale Italiana Science Data Center
Nuclear Physics B - Proceedings Supplements | Year: 2013

The Alpha Magnetic Spectrometer (AMS-02) is a large acceptance particle physics detector installed on board the International Space Station (ISS) since May 19th 2011 to perform a high statistic and long duration measurement of the spectra of primary charged cosmic rays. This paper describes the characteristics of the AMS-02 detector and reviews its in orbit performances. © 2013 Elsevier B.V.

PubMed | McGill University, University of Texas at Arlington, Lawrence Livermore National Laboratory, Agenzia Spaziale Italiana Science Data Center and 13 more.
Type: Journal Article | Journal: Nature | Year: 2014

Asymmetry is required by most numerical simulations of stellar core-collapse explosions, but the form it takes differs significantly among models. The spatial distribution of radioactive (44)Ti, synthesized in an exploding star near the boundary between material falling back onto the collapsing core and that ejected into the surrounding medium, directly probes the explosion asymmetries. CassiopeiaA is a young, nearby, core-collapse remnant from which (44)Ti emission has previously been detected but not imaged. Asymmetries in the explosion have been indirectly inferred from a high ratio of observed (44)Ti emission to estimated (56)Ni emission, from optical light echoes, and from jet-like features seen in the X-ray and optical ejecta. Here we report spatial maps and spectral properties of the (44)Ti in Cassiopeia A. This may explain the unexpected lack of correlation between the (44)Ti and iron X-ray emission, the latter being visible only in shock-heated material. The observed spatial distribution rules out symmetric explosions even with a high level of convective mixing, as well as highly asymmetric bipolar explosions resulting from a fast-rotating progenitor. Instead, these observations provide strong evidence for the development of low-mode convective instabilities in core-collapse supernovae.

PubMed | German Electron Synchrotron, Montpellier University, Kavli Institute for Particle Astrophysics and Cosmology, University of Rome Tor Vergata and 30 more.
Type: Journal Article | Journal: Physical review letters | Year: 2016

The Fermi Large Area Telescope (LAT) Collaboration has recently released a catalog of 360 sources detected above 50GeV (2FHL). This catalog was obtained using 80 months of data re-processed with Pass 8, the newest event-level analysis, which significantly improves the acceptance and angular resolution of the instrument. Most of the 2FHL sources at high Galactic latitude are blazars. Using detailed MonteCarlo simulations, we measure, for the first time, the source count distribution, dN/dS, of extragalactic -ray sources at E>50GeV and find that it is compatible with a Euclidean distribution down to the lowest measured source flux in the 2FHL (810^{-12}phcm^{-2}s^{-1}). We employ a one-point photon fluctuation analysis to constrain the behavior of dN/dS below the source detection threshold. Overall, the source count distribution is constrained over three decades in flux and found compatible with a broken power law with a break flux, S_{b}, in the range [810^{-12},1.510^{-11}]phcm^{-2}s^{-1} and power-law indices below and above the break of _{2}[1.60,1.75] and _{1}=2.490.12, respectively. Integration of dN/dS shows that point sources account for at least 86_{-14}^{+16}% of the total extragalactic -ray background. The simple form of the derived source count distribution is consistent with a single population (i.e., blazars) dominating the source counts to the minimum flux explored by this analysis. We estimate the density of sources detectable in blind surveys that will be performed in the coming years by the Cherenkov Telescope Array.

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