647 research outputs found
X-ray Photons in the CO 2-1 'Lacuna' of NGC 2110
A recent ALMA study of the Seyfert 2 Active Galactic Nucleus (AGN) NGC 2110
by Rosario et al. (2019) has reported a remarkable lack of CO 2-1 emission from
the circumnuclear region, where optical lines and H2 emission are observed,
leading to the suggestion of excitation of the molecular clouds by the AGN.
Since interaction with X-ray photons could be the cause of this excitation, we
have searched the archival Chandra data for corroborating evidence. We report
an extra-nuclear ~1'' (~170 pc) feature found in the soft (<1.0 keV) Chandra
data of the Seyfert 2 Active Galactic Nucleus (AGN) NGC 2110. This feature is
elongated to the north of the nucleus and its shape matches well that of the
optical lines and H2 emission observed in this region, which is devoid of CO
2-1 emission. The Chandra image completes the emerging picture of a multi-phase
circumnuclear medium excited by the X-rays from the AGN, with dense warm
molecular clouds emitting in H2 but depleted of CO 2-1 emission.Comment: ApJ Letters - in pres
Unidentifed gamma-ray sources: hunting gamma-ray blazars
One of the main scientific objectives of the ongoing Fermi mission is
unveiling the nature of the unidentified gamma-ray sources (UGSs). Despite the
large improvements of Fermi in the localization of gamma-ray sources with
respect to the past gamma-ray missions, about one third of the Fermi-detected
objects are still not associated to low energy counterparts. Recently, using
the Wide-Field Infrared Survey Explorer (WISE) survey, we discovered that
blazars, the rarest class of Active Galactic Nuclei and the largest population
of gamma-ray sources, can be recognized and separated from other extragalactic
sources on the basis of their infrared (IR) colors. Based on this result, we
designed an association method for the gamma-ray sources to reognize if there
is a blazar candidate within the positional uncertainty region of a generic
gamma-ray source. With this new IR diagnostic tool, we searched for gamma-ray
blazar candidates associated to the UGS sample of the second Fermi gamma-ray
catalog (2FGL). We found that our method associates at least one gamma-ray
blazar candidate as a counterpart each of 156 out of 313 UGSs analyzed. These
new low-energy candidates have the same IR properties as the blazars associated
to gamma-ray sources in the 2FGL catalog.Comment: 24 pages, 4 figures, Accepted for publication on the Astrophysical
Journa
Identification of roughness with optimal contact response with respect to real contact area and normal stiffness
Additive manufacturing technologies are a key point of the current era of Industry 4.0, promoting the production of mechanical components via the addition of subsequent layers of material. Then, they may be also used to produce surfaces tailored to achieve a desired mechanical contact response. In this work, we develop a method to prototype profiles optimizing a suitable trade-off between two different target mechanical responses. The mechanical design problem is solved relying on both physical assumptions and optimization methods. An algorithm is proposed, exploiting an analogy between genetics and the multiscale characterization of roughness, where various length-scales are described in terms of rough profiles, named chromosomes. Finally, the proposed algorithm is tested on a representative example, and the topological and spectral features of roughness of the optimized profiles are discussed
Hidden AGNs in Early-Type Galaxies
We present a stacking analysis of the complete sample of Early Type Galaxies
(ETGs) in the \textit{Chandra} COSMOS (C-COSMOS) survey, to explore the nature
of the X-ray luminosity in the redshift and stellar luminosity ranges
and {10}^{9}. Using established
scaling relations, we subtract the contribution of X-ray binary populations, to
estimate the combined emission of hot ISM and AGN. To discriminate between the
relative importance of these two components, we (1) compare our results with
the relation observed in the local universe for
hot gaseous halos emission in ETGs, and (2) evaluate the spectral signature of
each stacked bin. We find two regimes where the non-stellar X-ray emission is
hard, consisten t with AGN emission. First, there is evidence of hard, absorbed
X-ray emission in stacked bins including relatively high z () ETGs
with average high X-ray luminosity (L_{X-LMXB}\gtrsim 6\times{10}^{42}\mbox{
erg}/\mbox{s}). These luminosities are consistent with the presence ofhighly
absorbed "hidden" AGNs in these ETGs, which are not visible in their optical-IR
spectra and spectral energy distributions. Second, confirming the early
indication from our C-COSMOS study of X-ray detected ETGs, we find
significantly enhanced X-ray luminoaity in lower stellar mass ETGs
(L_K\lesssim{10}^{11}L_{\astrosun}), relative to the local
relation. The stacked spectra of these ETGs also
suggest X-ray emission harder than expected from gaseous hot halos. This
emission is consistent with inefficient accretion
onto M_{BH}\sim
{10}^{6}-{10}^{8}\,M_{\astrosun}.Comment: 22 pages, 7 figures, 2 tables. Accepted for publications on Ap
Unveiling the nature of the unidentified gamma-ray sources III: gamma-ray blazar-like counterparts at low radio frequencies
About one third of the gamma-ray sources listed in the second Fermi LAT
catalog (2FGL) have no firmly established counterpart at lower energies so
being classified as unidentified gamma-ray sources (UGSs). Here we propose a
new approach to find candidate counterparts for the UGSs based on the 325 MHz
radio survey performed with Westerbork Synthesis Radio Telescope (WSRT) in the
northern hemisphere. First we investigate the low-frequency radio properties of
blazars, the largest known population of gamma-ray sources; then we search for
sources with similar radio properties combining the information derived from
the Westerbork Northern Sky Survey (WENSS) with those of the NRAO VLA Sky
survey (NVSS). We present a list of candidate counterparts for 32 UGSs with at
least one counterpart in the WENSS. We also performed an extensive research in
literature to look for infrared and optical counterparts of the gamma-ray
blazar candidates selected with the low-frequency radio observations to confirm
their nature. On the basis of our multifrequency research we identify 23 new
gamma-ray blazar candidates out of 32 UGSs investigated. Comparison with
previous results on the UGSs are also presented. Finally, we speculate on the
advantages on the use of the low-frequency radio observations to associate UGSs
and to search for gamma-ray pulsar candidates.Comment: 15 pages, 13 figures, 3 tables, ApJS accepted for publication
(version pre-proof corrections
Fatigue degradation and electric recovery in Silicon solar cells embedded in photovoltaic modules
Cracking in Silicon solar cells is an important factor for the electrical power-loss of photovoltaic modules. Simple geometrical criteria identifying the amount of inactive cell areas depending on the position of cracks with respect to the main electric conductors have been proposed in the literature to predict worst case scenarios. Here we present an experimental study based on the electroluminescence (EL) technique showing that crack propagation in monocrystalline Silicon cells embedded in photovoltaic (PV) modules is a much more complex phenomenon. In spite of the very brittle nature of Silicon, due to the action of the encapsulating polymer and residual thermo-elastic stresses, cracked regions can recover the electric conductivity during mechanical unloading due to crack closure. During cyclic bending, fatigue degradation is reported. This pinpoints the importance of reducing cyclic stresses caused by vibrations due to transportation and use, in order to limit the effect of cracking in Silicon cells
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