22,946 research outputs found
Dark-matter dynamical friction versus gravitational-wave emission in the evolution of compact-star binaries
The measured orbital period decay of compact-star binaries, with
characteristic orbital periods ~days, is explained with very high
precision by the gravitational wave (GW) emission of an inspiraling binary in
vacuum. However, the binary gravitational binding energy is also affected by an
usually neglected phenomenon, namely the dark matter dynamical friction (DMDF)
produced by the interaction of the binary components with their respective DM
gravitational wakes. The entity of this effect depends on the orbital period
and on the local value of the DM density, hence on the position of the binary
in the Galaxy. We evaluate the DMDF produced by three different DM profiles:
the Navarro-Frenk-White (NFW), the non-singular-isothermal-sphere (NSIS) and
the Ruffini-Arg\"uelles-Rueda (RAR) profile based on self-gravitating keV
fermions. We first show that indeed, due to their Galactic position, the GW
emission dominates over the DMDF in the NS-NS, NS-WD and WD-WD binaries for
which measurements of the orbital decay exist. Then, we evaluate the conditions
under which the effect of DMDF on the binary evolution becomes comparable to,
or overcomes, the one of the GW emission. We find that, for instance for
-- NS-WD, --~ NS-NS, and
--~ WD-WD, located at 0.1~kpc, this occurs at orbital
periods around 20--30 days in a NFW profile while, in a RAR profile, it occurs
at about 100 days. For closer distances to the Galactic center, the DMDF effect
increases and the above critical orbital periods become interestingly shorter.
Finally, we also analyze the system parameters for which DMDF leads to an
orbital widening instead of orbital decay. All the above imply that a
direct/indirect observational verification of this effect in compact-star
binaries might put strong constraints on the nature of DM and its Galactic
distribution.Comment: 15 pages, 12 figures, 2 tables, accepted for publication in Phys.
Rev. D, 201
Interpreting the time variable RM observed in the core region of the TeV blazar Mrk 421
In this work we interpret and discuss the time variable rotation measure (RM)
found, for the first time over a 1-yr period, in the core region of a blazar.
These results are based on a one-year, multi-frequency (15, 24, and 43 GHz)
Very Long Baseline Array (VLBA) monitoring of the TeV blazar Markarian 421 (Mrk
421). We investigate the Faraday screen properties and its location with
respect to the jet emitting region. Given that the 43 GHz radio core flux
density and the RM time evolution suggest a similar trend, we explore the
possible connection between the RM and the accretion rate. Among the various
scenarios that we explore, the jet sheath is the most promising candidate for
being the main source of Faraday rotation. During the one-year observing period
the RM trend shows two sign reversals, which may be qualitatively interpreted
within the context of the magnetic tower models. We invoke the presence of two
nested helical magnetic fields in the relativistic jet with opposite
helicities, whose relative contribution produce the observed RM values. The
inner helical field has the poloidal component () oriented in the
observer's direction and produces a positive RM, while the outer helical field,
with in the opposite direction, produces a negative RM. We assume
that the external helical field dominates the contribution to the observed RM,
while the internal helical field dominates when a jet perturbation arises
during the second observing epoch. Being the intrinsic polarization angle
parallel to the jet axis, a pitch angle of the helical magnetic field
is required. Additional scenarios are also considered to
explain the observed RM sign reversals.Comment: 6 pages, 2 figures. Published on MNRA
First clear evidence of quantum chaos in the bound states of an atomic nucleus
We study the spectral fluctuations of the Pb nucleus using the
complete experimental spectrum of 151 states up to excitation energies of
MeV recently identified at the Maier-Leibnitz-Laboratorium at Garching,
Germany. For natural parity states the results are very close to the
predictions of Random Matrix Theory (RMT) for the nearest-neighbor spacing
distribution. A quantitative estimate of the agreement is given by the Brody
parameter , which takes the value for regular systems and
for chaotic systems. We obtain which
is, to our knowledge, the closest value to chaos ever observed in experimental
bound states of nuclei. By contrast, the results for unnatural parity states
are far from RMT behavior. We interpret these results as a consequence of the
strength of the residual interaction in Pb, which, according to
experimental data, is much stronger for natural than for unnatural parity
states. In addition our results show that chaotic and non-chaotic nuclear
states coexist in the same energy region of the spectrum.Comment: 9 pages, 1 figur
Thermal van der Waals Interaction between Graphene Layers
The van de Waals interaction between two graphene sheets is studied at finite
temperatures. Graphene's thermal length controls
the force versus distance as a crossover from the zero temperature
results for , to a linear-in-temperature, universal regime for
. The large separation regime is shown to be a consequence of the
classical behavior of graphene's plasmons at finite temperature. Retardation
effects are largely irrelevant, both in the zero and finite temperature
regimes. Thermal effects should be noticeable in the van de Waals interaction
already for distances of tens of nanometers at room temperature.Comment: enlarged version, 9 pages, 4 figures, updated reference
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