4,759 research outputs found
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 () and the X-ray
flux, which is visible when . 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
Mutual independence of critical temperature and superfluid density under pressure in optimally electron-doped superconducting LaFeAsOF
The superconducting properties of LaFeAsOF in conditions of
optimal electron-doping are investigated upon the application of external
pressure up to kbar. Measurements of muon-spin spectroscopy and dc
magnetometry evidence a clear mutual independence between the critical
temperature and the low-temperature saturation value for the ratio
(superfluid density over effective band mass of Cooper pairs).
Remarkably, a dramatic increase of % is reported for at
the maximum pressure value while 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
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