389,135 research outputs found
Measuring the parameters of massive black hole binary systems with Pulsar Timing Array observations of gravitational waves
The observation of massive black hole binaries (MBHBs) with Pulsar Timing
Arrays (PTAs) is one of the goals of gravitational wave astronomy in the coming
years. Massive (>10^8 solar masses) and low-redshift (< 1.5) sources are
expected to be individually resolved by up-coming PTAs, and our ability to use
them as astrophysical probes will depend on the accuracy with which their
parameters can be measured. In this paper we estimate the precision of such
measurements using the Fisher-information-matrix formalism. We restrict to
"monochromatic" sources. In this approximation, the system is described by
seven parameters and we determine their expected statistical errors as a
function of the number of pulsars in the array, the array sky coverage, and the
signal-to-noise ratio (SNR) of the signal. At fixed SNR, the gravitational wave
astronomy capability of a PTA is achieved with ~20 pulsars; adding more pulsars
(up to 1000) to the array reduces the source error-box in the sky \Delta\Omega
by a factor ~5 and has negligible consequences on the statistical errors on the
other parameters. \Delta\Omega improves as 1/SNR^2 and the other parameters as
1/SNR. For a fiducial PTA of 100 pulsars uniformly distributed in the sky and a
coherent SNR = 10, we find \Delta\Omega~40 deg^2, a fractional error on the
signal amplitude of ~30% (which constraints only very poorly the chirp mass -
luminosity distance combination M_c^{5/3}/D_L), and the source inclination and
polarization angles are recovered at the ~0.3 rad level. The ongoing Parkes PTA
is particularly sensitive to systems located in the southern hemisphere, where
at SNR = 10 the source position can be determined with \Delta\Omega ~10 deg^2,
but has poorer performance for sources in the northern hemisphere. (Abridged)Comment: 20 pages, 12 figures, 2 color figures, submitted to Phys. Rev.
Linking gravitational waves and X-ray phenomena with joint LISA and Athena observations
The evolution of cosmic structures, the formation and growth of the first
black holes and the connection to their baryonic environment are key unsolved
problems in astrophysics. The X-ray Athena mission and the gravitational-wave
Laser Interferometer Space Antenna (LISA) offer independent and complementary
angles on these problems. We show that up to 10 black hole binaries in the mass
range 10^5 - 10^8 Msun discovered by LISA at redshift <~ 3.5 could be detected
by Athena in an exposure time up to 100 ks, if prompt X-ray emission of ~ 1% -
10% of the Eddington luminosity is present. Likewise, if any LISA-detected
extreme mass ratio inspirals occur in accretion disks, Athena can detect
associated electromagnetic emission out to redshift ~ 1. Finally, warned by
LISA, Athena can point in advance and stare at stellar-mass binary black hole
mergers at redshift <~ 0.1. These science opportunities emphasise the vast
discovery space of simultaneous observations from the two observatories, which
would be missed if they were operated in different epochs.Comment: Published in Nature Astronom
The astrophysical science case for a decihertz gravitational-wave detector
We discuss the astrophysical science case for a decihertz gravitational-wave
mission. We focus on unique opportunities for scientific discovery in this
frequency range, including probes of type IA supernova progenitors, mergers in
the presence of third bodies, intermediate mass black holes, seeds of massive
black holes, improved sky localization, and tracking the population of merging
compact binaries
The stochastic gravitational-wave background from massive black hole binary systems: implications for observations with Pulsar Timing Arrays
Massive black hole binary systems, with masses in the range ~10^4-10^10
\msun, are among the primary sources of gravitational waves in the frequency
window ~10^-9 Hz - 0.1 Hz. Pulsar Timing Arrays (PTAs) and the Laser
Interferometer Space Antenna (LISA) are the observational means by which we
will be able to observe gravitational radiation from these systems. We carry
out a systematic study of the generation of the stochastic gravitational-wave
background from the cosmic population of massive black hole binaries. We
consider a wide variety of assembly scenarios and we estimate the range of
signal strength in the frequency band accessible to PTAs. We show that, taking
into account the uncertainties surrounding the actual key model parameters, the
amplitude lies in the interval h_c(f = 10^-8 Hz)~5x10^-16 - 8x10^-15. The most
optimistic predictions place the signal level at a factor of ~3 below the
current sensitivity of Pulsar Timing Arrays, but within the detection range of
the complete Parkes PTA for a wide variety of models, and of the future
Square-Kilometer-Array PTA for all the models considered here. We also show
that at frequencies >10^-8 Hz the frequency dependency of the generated
background follows a power-law significantly steeper than f^-2/3, that has been
considered so far. Finally we show that LISA observations of individual
resolvable massive black hole binaries are complementary and orthogonal to PTA
observations of a stochastic background from the whole population in the
Universe. In fact, the detection of gravitational radiation in both frequency
windows will enable us to fully characterise the cosmic history of massive
black holes.Comment: 21 pages, 14 figures, minor revisions, accepted for publication in
MNRA
Conformity-Driven Agents Support Ordered Phases in the Spatial Public Goods Game
We investigate the spatial Public Goods Game in the presence of
fitness-driven and conformity-driven agents. This framework usually considers
only the former type of agents, i.e., agents that tend to imitate the strategy
of their fittest neighbors. However, whenever we study social systems, the
evolution of a population might be affected also by social behaviors as
conformism, stubbornness, altruism, and selfishness. Although the term
evolution can assume different meanings depending on the considered domain,
here it corresponds to the set of processes that lead a system towards an
equilibrium or a steady-state. We map fitness to the agents' payoff so that
richer agents are those most imitated by fitness-driven agents, while
conformity-driven agents tend to imitate the strategy assumed by the majority
of their neighbors. Numerical simulations aim to identify the nature of the
transition, on varying the amount of the relative density of conformity-driven
agents in the population, and to study the nature of related equilibria.
Remarkably, we find that conformism generally fosters ordered cooperative
phases and may also lead to bistable behaviors.Comment: 13 pages, 5 figure
Capacity-achieving CPM schemes
The pragmatic approach to coded continuous-phase modulation (CPM) is proposed
as a capacity-achieving low-complexity alternative to the serially-concatenated
CPM (SC-CPM) coding scheme. In this paper, we first perform a selection of the
best spectrally-efficient CPM modulations to be embedded into SC-CPM schemes.
Then, we consider the pragmatic capacity (a.k.a. BICM capacity) of CPM
modulations and optimize it through a careful design of the mapping between
input bits and CPM waveforms. The so obtained schemes are cascaded with an
outer serially-concatenated convolutional code to form a pragmatic
coded-modulation system. The resulting schemes exhibit performance very close
to the CPM capacity without requiring iterations between the outer decoder and
the CPM demodulator. As a result, the receiver exhibits reduced complexity and
increased flexibility due to the separation of the demodulation and decoding
functions.Comment: Submitted to IEEE Transactions on Information Theor
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