75 research outputs found
Realistic calculations of correlations and final state interaction effects in the A(e,e'p)X process off complex nuclei
A linked cluster expansion for the calculation of ground state observables of
complex nuclei with realistic interactions has been used to calculate the
ground state energy, density and momentum distribution of 16O and 40Ca. Using
the same cluster expansion and the wave function and correlation parameters
obtained from the energy calculation, we have evaluated the semi inclusive
reaction A(e,e'p)X taking final state interaction (FSI) into account by a
Glauber type approach; the comparison between the distorted and undistorted
momentum distributions provides an estimate of the transparency of the nuclear
medium to the propagation of the hit proton. The effect of color transparency
is also included by considering the Finite Formation Time (FFT) that the hit
hadron needs to reach its asymptotic physical state
Discovery, identification, and study of variability in gamma-ray point sources
We present preliminary results of a statistical analysis obtained with a
sample of blazars observed at the Perugia Astronomical Observatory since 1992.
We briefly show how these statistical results can be useful to discriminate
faint variable sources against the background noise. This technique, together
with the more traditional ones, may be used to discover and identify
high-energy point sources
Mid-infrared photometry of carbon stars and perspectives for surveys in the Magellanic Clouds from Dome-C
A preliminary analysis of the data from the MSX space infrared satellite,
seems to confirm that the [8.8]-[12.5] micron color index is well correlated
with the mass-loss rates in carbon stars of our Galaxy. The extension of this
mid-infrared observation criterion to the Magellanic Clouds, with a small-size
telescope, able to perform a continuous survey from Dome-C on the Antarctic
Plateau, could be crucial to trace the local AGB population and evolution
Synchrotron-Self Compton Spectral Evolution of PKS 2155-304
The high frequency peaked blazar PKS 2155-304 is one of the brightest and
most intensively studied prototype of BL Lac objects. Gamma-rays from PKS
2155-304 have been detected from the MeV to TeV ranges. We computed a
synchrotron self-Compton (SSC) model, based on the temporal behavior of the
particles distribution, responsible for the high-energy emission. Using the
available simultaneous multiwavelength data, we simulated the overall spectral
energy distribution (SED) and the spectral variability of this source
Blazars: the next gamma-ray view of GLAST
Blazars, the extreme family of AGN, can be strong gamma-ray emitters and
constitute the largest fraction of identified point sources of EGRET. The next
Gamma-ray Large Area Space Telescope (GLAST) is a high energy (30MeV-300GeV)
gamma-ray astronomy mission, planned for launch at the end of 2006. GLAST
performances will allow to detect few thousands of gamma-ray blazars, with a
broad band coverage and temporal resolution, also in quiescent emission phases,
providing probably many answers about these sources
Holographic D3-probe-D5 model of a double layer Dirac semimetal
The possibility of inter-layer exciton condensation in a holographic D3-probe-D5 brane model of a strongly coupled double monolayer Dirac semi-metal in a magnetic field is studied in detail. It is found that, when the charge densities on the layers are exactly balanced so that, at weak coupling, the Fermi surfaces of electrons in one monolayer and holes in the other monolayer would be perfectly nested, inter-layer condensates can form for any separation of the layers. The case where both monolayers are charge neutral is special. There, the inter-layer condensate occurs only for small separations and is replaced by an intra-layer exciton condensate at larger separations. The phase diagram for charge balanced monolayers for a range layer separations and chemical potentials is found. We also show that, in semi-metals with multiple species of massless fermions, the balance of charges required for Fermi surface nesting can occur spontaneously by breaking some of the internal symmetry of the monolayers. This could have important consequences for experimental attempts to find inter-layer condensates
Four Years of Fermi LAT Observations of Narrow-Line Seyfert 1 Galaxies
Before the launch of the Fermi satellite only two classes of AGN were known
to generate relativistic jets and thus emit up to the gamma-ray energy range:
blazars and radio galaxies, both hosted in giant elliptical galaxies. The first
two years of observations by the Large Area Telescope (LAT) on board Fermi
confirmed that these two are the most numerous classes of identified sources in
the extragalactic gamma-ray sky, but the discovery of variable gamma-ray
emission from 5 radio-loud Narrow-Line Seyfert 1 galaxies (NLSy1s) revealed the
presence of a possible emerging third class of AGN with relativistic jets.
Considering also that NLSy1s are typically hosted in spiral galaxy, this
finding poses intriguing questions about the nature of these objects, the onset
of production of relativistic jets, and the cosmological evolution of
radio-loud AGN. Here, we report on a preliminary investigation of the
properties of this sample of radio-loud NLSy1 at MeV-GeV photon energies,
utilizing the four-year accumulation of Fermi LAT data. In addition we briefly
discuss some radio-to-gamma-rays properties of the gamma-ray emitting NLSy1 in
the context of the blazar scenario
Optical Luminosities and Mass--to--Light Ratios of Nearby Galaxy Clusters
We analyze a sample of 105 clusters having virial mass homogeneously
estimated and for which galaxy magnitudes are available with a well defined
high degree of completeness. In particular, we consider a subsample of 89
clusters with B_j band galaxy magnitudes taken from the COSMOS/UKST Southern
Sky Object Catalogue. We compute cluster luminosities L_{B_j} within several
clustercentric distances and within the virialization radius R_{vir}. In
particular, we use the luminosity function and background counts estimated by
Lumsden et al. (1997) on the Edinburgh/Durham Southern Galaxy Catalogue. We
analyze the effect of several uncertainties connected to photometric data,
fore/background removal, and extrapolation below the completeness limit of the
photometry, in order to assess the robustness of our cluster luminosity
estimates. We draw our results on the relations between luminosity and
dynamical quantities from the COSMOS sample by considering mass and
luminosities determined within the virialization radius. We find a very good
correlation between cluster luminosity, L_{B_j}, and galaxy velocity
dispersion, sigma_v, with L_{B_j} proportional to sigma_v^{2.1--2.3}. Our
estimate of typical value for the mass-to-light ratio is M/L_{B_j} about 250
(in solar units). We do not find any correlation of M/L_{B_j} with cluster
morphologies, i.e. Rood--Sastry and Bautz--Morgan types, and only a weak
significant correlation with cluster richness. We find that mass has a slight,
but significant, tendency to increase faster than the luminosity does, M
proportional to L_{B_j}^{1.2--1.3}. We verify the robustness of this relation
against a number of possible systematics
The Observational Mass Function of Nearby Galaxy Clusters
We present a new determination of the mass function of galaxy clusters, based
on optical virial mass estimates for a large sample of 152 nearby (z\le0.15)
Abell-ACO clusters, as provided by Girardi et al. (1998). This sample includes
both data from the literature and the new ENACS data. The resulting mass
function is reliably estimated for masses larger than M_{lim}\simeq 4\times
10^{14} \msun, while it is affected by sample incompleteness at smaller masses.
We find N(>M_{lim})=(6.3\pm 1.2) 10^{-6} (h^{-1} Mpc)^{-3} for cluster masses
estimated within a 1.5 h^{-1} radius. Our mass function is intermediate between
the two previous estimates by Bahcall & Cen (1993) and by Biviano et al.
(1993). Based on the Press-Schechter approach, we use this mass function to
constrain the amplitude of the fluctuation power spectrum at the cluster scale.
After suitably convolving the PS predictions with observational errors on
cluster masses and COBE-normalizing the fluctuation power spectrum, we find
sigma_8=(0.60\pm 0.04) Omega_0^{-0.46+0.09 Omega_0} for flat low-density
models and sigma_8=(0.60\pm 0.04) Omega_0^{-0.48+0.17 Omega_0} for open models
(at the 90% c.l.)
Long-term monitoring of PKS 0537-441 with Fermi-LAT and multiwavelength observations
We report on multiwavelength observations of the blazar PKS 0537-441 (z =
0.896) obtained from microwaves through gamma rays by SMA, REM, ATOM, Swift and
Fermi during 2008 August-2010 April. Strong variability has been observed in
gamma rays, with two major flaring episodes (2009 July and 2010 March) and a
harder-when-brighter behaviour, quite common for FSRQs and
low-synchrotron-peaked BL Lacs, in 2010 March. In the same way the SED of the
source cannot be modelled by a simple synchrotron self-Compton model, as
opposed to many BL Lacs, but the addition of an external Compton component of
seed photons from a dust torus is needed. The 230 GHz light curve showed an
increase simultaneous with the gamma-ray one, indicating co-spatiality of the
mm and gamma-ray emission region likely at large distance from the central
engine. The low, average, and high activity SED of the source could be fit
changing only the electron distribution parameters, but two breaks in the
electron distribution are necessary. The ensuing extra spectral break, located
at NIR-optical frequencies, together with that in gamma rays seem to indicate a
common origin, most likely due to an intrinsic feature in the underlying
electron distribution. An overall correlation between the gamma-ray band with
the R-band and K-band has been observed with no significant time lag. On the
other hand, when inspecting the light curves on short time scales some
differences are evident. In particular, flaring activity has been detected in
NIR and optical bands with no evident gamma-ray counterparts in 2009 September
and November. Moderate variability has been observed in X-rays with no
correlation between flux and photon index. An increase of the detected X-ray
flux with no counter part at the other wavelengths has been observed in 2008
October, suggesting once more a complex correlation between the emission at
different energy bands
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