The REX survey: a search for Radio Emitting X-ray sources

We present the scientific goals, the strategy and the first results of the REX project, an effort aimed at creating a sizable and statistically complete sample of Radio Emitting X-ray sources (REX) using the available data from a VLA survey and the ROSAT PSPC archive. Through a positional cross-correlation of the two data sets we have derived a sample of about 1600 REX. Among the 393 REX identified so far a high fraction is represented by AGNs, typically radio loud QSOs and BL Lacs. The remaining sources are galaxies, typically radio galaxies isolated or in cluster. Thanks to the low flux limits in the radio and in the X-ray band and the large area of sky covered by the survey, we intend to derive a new complete and unbiased sample of BL Lacs which will contain both ``RBL'' and ``XBL'' type objects. In this way, the apparent dichotomy resulting from the current samples of BL Lacs will be directly analyzed in a unique sample. Moreover, the high number of BL Lacs expected in the REX sample (about 200) will allow an accurate estimate of their statistical properties. To date, we have discovered 15 new BL Lacs and 11 BL Lac candidates with optical properties intermediate between those of a typical elliptical galaxy and those of a typical BL Lac object. These objects could harbour weak sources of non-thermal continuum in their nuclei and, if confirmed, they could represent the faint tail of the BL Lac population. The existence of such ``weak'' BL Lacs is matter of discussion in recent literature and could lead to a re-assessment of the defining criteria of a BL Lac and, consequently, to a revision of their cosmological and statistical properties.Comment: 30 pages, 13 figures, accepted for publication to Ap

Emission Line AGNs from the REX survey: Results from optical spectroscopy

We present 71 Emission Line objects selected from the REX survey. Except for 3 of them, for which the presence of an active nucleus is dubious, all these sources are Active Galactic Nuclei (QSOs, Seyfert galaxies, emission line radiogalaxies). In addition, we present the spectra of other 19 AGNs included in a preliminary version of the REX catalog but not in the final one. The majority (80) of the 90 sources presented in this paper is newly discovered. Finally, we present the general properties in the radio and in the X-ray band of all the AGNs discovered so far in the REX survey.Comment: 27 pages. To be published in Astronomy and Astrophysics, Supplement Series. Better quality figures can be asked to the autho

Spectral observations of X Persei: Connection between H-alpha and X-ray emission

We present spectroscopic observations of the Be/X-ray binary X Per obtained during the period 1999 - 2018. Using new and published data, we found that during "disc-rise" the expansion velocity of the circumstellar disc is 0.4 - 0.7 km/s. Our results suggest that the disc radius in recent decades show evidence of resonant truncation of the disc by resonances 10:1, 3:1, and 2:1, while the maximum disc size is larger than the Roche lobe of the primary and smaller than the closest approach of the neutron star. We find correlation between equivalent width of H-alpha emission line ($W\alpha$) and the X-ray flux, which is visible when $15 \: \AA \: < W\alpha \le 40 \: \AA$. The correlation is probably due to wind Roche lobe overflow.Comment: Accepted for publication in Astronomy & Astrophysic

A Hard Medium Survey with ASCA. IV: the Radio-Loud Type 2 QSO AXJ0843+294 2

We discuss the X-ray, optical and radio properties of AX J0843+2942, a high luminosity Type 2 AGN found in the ASCA Hard Serendipitous Survey. The X-ray spectrum is best described by an absorbed power-law model with photon index of Gamma = 1.72 (+0.3 -0.6) and intrinsic absorbing column density of NH = 1.44 (+0.33 -0.52) x 10E23 cm-2. The intrinsic luminosity in the 0.5-10 keV energy band is ~ 3x10E45 erg s-1, well within the range of quasar luminosities. AX J0843+2942, positionally coincident with the core of a triple and strong (S_1.4 GHz ~ 1 Jy; P_1.4 GHz ~ 9 x 10E33 erg s-1 Hz-1) radio source, is spectroscopically identified with a Narrow Line object (intrinsic FWHM of all the permitted emission lines <= 1200 km s-1) at z=0.398, having line features and ratios typical of Seyfert-2 like objects. The high X-ray luminosity, coupled with the high intrinsic absorption, the optical spectral properties and the radio power, allow us to propose AX J0843+2942 as a Radio-Loud "Type 2 QSO". A discussion of the SED of this object is presented here together with a comparison with the SED of Ultra Luminous Infrared Galaxies, other "Type 2 QSO" candidates from the literature, and "normal" Radio-Quiet and Radio-Loud QSOs.Comment: 10 pages, 7 figures, Latex manuscript, Accepted for publication in Ast ronomy and Astrophysic

On non-local propositional and weak monodic quantified CTL

Accepted versio

Mutual independence of critical temperature and superfluid density under pressure in optimally electron-doped superconducting LaFeAsO1−x_{1-x}Fx_{x}

The superconducting properties of LaFeAsO$_{1-x}$F$_{x}$ in conditions of optimal electron-doping are investigated upon the application of external pressure up to $\sim 23$ kbar. Measurements of muon-spin spectroscopy and dc magnetometry evidence a clear mutual independence between the critical temperature $T_{c}$ and the low-temperature saturation value for the ratio $n_{s}/m^{*}$ (superfluid density over effective band mass of Cooper pairs). Remarkably, a dramatic increase of $\sim 30$ % is reported for $n_{s}/m^{*}$ at the maximum pressure value while $T_{c}$ is substantially unaffected in the whole accessed experimental window. We argue and demonstrate that the explanation for the observed results must take the effect of non-magnetic impurities on multi-band superconductivity into account. In particular, the unique possibility to modify the ratio between intra-band and inter-bands scattering rates by acting on structural parameters while keeping the amount of chemical disorder constant is a striking result of our proposed model.Comment: 8 pages (Main text: 5 pages. Paper merged with supplemental information), 5 figure

The connection between radio and high energy emission in black hole powered systems in the SKA era

Strong evidence exists for a highly significant correlation between the radio flux density and gamma-ray energy flux in blazars revealed by Fermi. However, there are central issues that need to be clarified in this field: what are the counterparts of the about 30% of gamma-ray sources that are as yet unidentified? Are they just blazars in disguise or they are something more exotic, possibly associated with dark matter? How would they fit in the radio-gamma ray connection studied so far? With their superb sensitivity, SKA1-MID and SKA1-SUR will help to resolve all of these questions. Even more, while the radio-MeV/GeV connection has been firmly established, a radio-VHE connection has been entirely elusive so far. The advent of CTA in the next few years and the expected CTA-SKA1 synergy will offer the chance to explore this connection, even more intriguing as it involves the opposite ends of the electromagnetic spectrum and the acceleration of particles up to the highest energies. We are already preparing to address these questions by exploiting data from the various SKA pathfinders and precursors. We have obtained 18 cm European VLBI Network observations of E>10 GeV sources, with a detection rate of 83%. Moreover, we are cross correlating the Fermi catalogs with the MWA commissioning survey: when faint gamma-ray sources are considered, pure positional coincidence is not significant enough for selecting counterparts and we need an additional physical criterion to pinpoint the right object. It can be radio spectral index, variability, polarization, or compactness, needing high angular resolution in SKA1-MID; timing studies can also reveal pulsars, which are often found from dedicated searches of unidentified gamma-ray sources. SKA will be the ideal instrument for investigating these characteristics in conjunction with CTA. (abridged)Comment: 12 pages, to be published in the proceedings of "Advancing Astrophysics with the Square Kilometre Array", PoS(AASKA14)15

The superconducting gaps in LiFeAs: Joint study of specific heat and ARPES

We present specific heat, c_P, and ARPES data on single crystals of the stoichiometric superconductor LiFeAs. A pronounced anomaly is found in c_P at the superconducting transition. The electronic contribution can be described by two s-type energy gaps with magnitudes of approximately Delta1 = 1.2 meV and Delta2 = 2.6 meV and a normal-state gamma coefficient of 10 mJ/mol K^2. All these values are in remarkable agreement with ARPES results.Comment: 4 pages, 3 figure