16,749 research outputs found
The evolution of the disc variability along the hard state of the black hole transient GX 339-4
We report on the analysis of hard-state power spectral density function (PSD)
of GX 339-4 down to the soft X-ray band, where the disc significantly
contributes to the total emission. At any luminosity probed, the disc in the
hard state is intrinsically more variable than in the soft state. However, the
fast decrease of disc variability as a function of luminosity, combined with
the increase of disc intensity, causes a net drop of fractional variability at
high luminosities and low energies, which reminds the well-known behaviour of
disc-dominated energy bands in the soft state. The peak-frequency of the
high-frequency Lorentzian (likely corresponding to the high-frequency break
seen in active galactic nuclei, AGN) scales with luminosity, but we do not find
evidence for a linear scaling. In addition, we observe that this characteristic
frequency is energy-dependent. We find that the normalization of the PSD at the
peak of the high-frequency Lorentzian decreases with luminosity at all
energies, though in the soft band this trend is steeper. Together with the
frequency shift, this yields quasi-constant high frequency (5-20 Hz) fractional
rms at high energies, with less than 10 percent scatter. This reinforces
previous claims suggesting that the high frequency PSD solely scales with BH
mass. On the other hand, this constancy breaks down in the soft band (where the
scatter increases to ~30 percent). This is a consequence of the additional
contribution from the disc component, and resembles the behaviour of optical
variability in AGN.Comment: 12 pages, 8 figures, accepted for publication in MNRA
Tracing the reverberation lag in the hard state of black hole X-ray binaries
We report results obtained from a systematic analysis of X-ray lags in a
sample of black hole X-ray binaries, with the aim of assessing the presence of
reverberation lags and studying their evolution during outburst. We used
XMM-Newton and simultaneous RXTE observations to obtain broad-band energy
coverage of both the disc and the hard X-ray Comptonization components. In most
cases the detection of reverberation lags is hampered by low levels of
variability signal-to-noise ratio (e.g. typically when the source is in a soft
state) and/or short exposure times. The most detailed study was possible for GX
339-4 in the hard state, which allowed us to characterize the evolution of
X-ray lags as a function of luminosity in a single source. Over all the sampled
frequencies (~0.05-9 Hz) we observe the hard lags intrinsic to the power law
component, already well-known from previous RXTE studies. The XMM-Newton soft
X-ray response allows us to detail the disc variability. At low-frequencies
(long time scales) the disc component always leads the power law component. On
the other hand, a soft reverberation lag (ascribable to thermal reprocessing)
is always detected at high-frequencies (short time scales). The intrinsic
amplitude of the reverberation lag decreases as the source luminosity and the
disc-fraction increase. This suggests that the distance between the X-ray
source and the region of the optically-thick disc where reprocessing occurs,
gradually decreases as GX 339-4 rises in luminosity through the hard state,
possibly as a consequence of reduced disc truncation.Comment: 15 pages, 9 figures, 2 tables, accepted for publication in Ap
Dressed Polyakov loop and phase diagram of hot quark matter under magnetic field
We evaluate the dressed Polyakov loop for hot quark matter in strong magnetic
field. To compute the finite temperature effective potential, we use the
Polyakov extended Nambu-Jona Lasinio model with eight-quark interactions taken
into account. The bare quark mass is adjusted in order to reproduce the
physical value of the vacuum pion mass. Our results show that the dressed
Polyakov loop is very sensitive to the strenght of the magnetic field, and it
is capable to capture both the deconfinement crossover and the chiral
crossover. Besides, we compute self-consistently the phase diagram of the
model. We find a tiny split of the two aforementioned crossovers as the
strength of the magnetic field is increased. Concretely, for the largest value
of magnetic field investigated here, , the split is of the order
of . A qualitative comparison with other effective models and recent
Lattice results is also performed.Comment: 10 pages, 3 figures, RevTeX4-1 styl
Contrasting Supersymmetry and Universal Extra Dimensions at Colliders
We contrast the experimental signatures of low energy supersymmetry and the
model of Universal Extra Dimensions and discuss various methods for their
discrimination at hadron and lepton colliders. We study the discovery reach of
hadron colliders for level 2 Kaluza-Klein modes, which would indicate the
presence of extra dimensions. We also investigate the possibility to
differentiate the spins of the superpartners and KK modes by means of the
asymmetry method of Barr. We then review the methods for discriminating between
the two scenarios at a high energy linear collider such as CLIC. We consider
the processes of Kaluza-Klein muon pair production in universal extra
dimensions in parallel to smuon pair production in supersymmetry. We find that
the angular distributions of the final state muons, the energy spectrum of the
radiative return photon and the total cross-section measurement are powerful
discriminators between the two models.Comment: 6 pages, 8 figures, to appear in the proceedings of the 2005
International Linear Collider Workshop, Stanford, US
Estimates of multipolar coefficients to search for cosmic ray anisotropies with non-uniform or partial sky coverage
We study the possibility to extract the multipolar moments of an underlying
distribution from a set of cosmic rays observed with non-uniform or even
partial sky coverage. We show that if the degree is assumed to be upper bounded
by , each multipolar moment can be recovered whatever the coverage, but with
a variance increasing exponentially with the bound if the coverage is zero
somewhere. Despite this limitation, we show the possibility to test predictions
of a model without any assumption on by building an estimate of the
covariance matrix seen through the exposure function.Comment: 20 pages, 8 figure
Chiral magnetic effect in the PNJL model
We study the two-flavor Nambu--Jona-Lasinio model with the Polyakov loop
(PNJL model) in the presence of a strong magnetic field and a chiral chemical
potential which mimics the effect of imbalanced chirality due to QCD
instanton and/or sphaleron transitions. Firstly we focus on the properties of
chiral symmetry breaking and deconfinement crossover under the strong magnetic
field. Then we discuss the role of on the phase structure. Finally the
chirality charge, electric current, and their susceptibility, which are
relevant to the Chiral Magnetic Effect, are computed in the model.Comment: Some reference added. Minor revisions. One figure added. To appear on
Phys. Rev.
Magnetic Susceptibility of the Quark Condensate and Polarization from Chiral Models
We compute the magnetic susceptibility of the quark condensate and the
polarization of quarks at zero temperature and in a uniform magnetic
background. Our theoretical framework consists of two chiral models that allow
to treat self-consistently the spontaneous breaking of chiral symmetry: the
linear model coupled to quarks, dubbed quark-meson model, and the
Nambu-Jona-Lasinio model. We also perform analytic estimates of the same
quantities within the renormalized quark-meson model, both in the regimes of
weak and strong fields. Our numerical results are in agreement with the recent
literature; moreover, we confirm previous Lattice findings, related to the
saturation of the polarization at large fields.Comment: 13 pages, 4 figure
Chiral magnetic effect in the PNJL model
We study the two-flavor Nambu--Jona-Lasinio model with the Polyakov loop
(PNJL model) in the presence of a strong magnetic field and a chiral chemical
potential which mimics the effect of imbalanced chirality due to QCD
instanton and/or sphaleron transitions. Firstly we focus on the properties of
chiral symmetry breaking and deconfinement crossover under the strong magnetic
field. Then we discuss the role of on the phase structure. Finally the
chirality charge, electric current, and their susceptibility, which are
relevant to the Chiral Magnetic Effect, are computed in the model.Comment: Some reference added. Minor revisions. One figure added. To appear on
Phys. Rev.
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