1,770 research outputs found
Testing the binary hypothesis: pulsar timing constraints on supermassive black hole binary candidates
The advent of time domain astronomy is revolutionizing our understanding of
the Universe. Programs such as the Catalina Real-time Transient Survey (CRTS)
or the Palomar Transient Factory (PTF) surveyed millions of objects for several
years, allowing variability studies on large statistical samples. The
inspection of 250k quasars in CRTS resulted in a catalogue of 111
potentially periodic sources, put forward as supermassive black hole binary
(SMBHB) candidates. A similar investigation on PTF data yielded 33 candidates
from a sample of 35k quasars. Working under the SMBHB hypothesis, we
compute the implied SMBHB merger rate and we use it to construct the expected
gravitational wave background (GWB) at nano-Hz frequencies, probed by pulsar
timing arrays (PTAs). After correcting for incompleteness and assuming virial
mass estimates, we find that the GWB implied by the CRTS sample exceeds the
current most stringent PTA upper limits by almost an order of magnitude. After
further correcting for the implicit bias in virial mass measurements, the
implied GWB drops significantly but is still in tension with the most stringent
PTA upper limits. Similar results hold for the PTF sample. Bayesian model
selection shows that the null hypothesis (whereby the candidates are false
positives) is preferred over the binary hypothesis at about and
for the CRTS and PTF samples respectively. Although not decisive,
our analysis highlights the potential of PTAs as astrophysical probes of
individual SMBHB candidates and indicates that the CRTS and PTF samples are
likely contaminated by several false positives.Comment: 14 pages, 11 figures, 3 tables. Resubmitted to the Astrophysical
Journal after some major revision of the results including a proper estimate
of the intrinsic mass of the binary candidate
Patterns of urban development in Budapest after 1989
This paper will focus on various interrelated, intermingled but often divergent processes and phenomena condensed around the fall of communism in Budapest regarding the spatial structure and development of society and economy. It will demonstrate and analyze major tendencies, such as urban decay, gentrification, suburbanization, emergence of polycentric spatial patterns, effects of globalization, and economic proceedings that have fundamentally altered the position and picture of Budapest and its metropolitan area. It will point out local characteristics as well as the effects of general trends of socio-spatial development on Budapest and its surroundings
DISRUPTED GLOBULAR CLUSTERS CAN EXPLAIN the GALACTIC CENTER GAMMA-RAY EXCESS
The Fermi satellite has recently detected gamma-ray emission from the central regions of our Galaxy. This may be evidence for dark matter particles, a major component of the standard cosmological model, annihilating to produce high-energy photons. We show that the observed signal may instead be generated by millisecond pulsars that formed in dense star clusters in the Galactic halo. Most of these clusters were ultimately disrupted by evaporation and gravitational tides, contributing to a spherical bulge of stars and stellar remnants. The gamma-ray amplitude, angular distribution, and spectral signatures of this source may be predicted without free parameters, and are in remarkable agreement with the observations. These gamma-rays are from fossil remains of dispersed clusters, telling the history of the Galactic bulge. © 2015. The American Astronomical Society. All rights reserved.
Measurement Accuracy of Inspiraling Eccentric Neutron Star and Black Hole Binaries Using Gravitational Waves
In a recent paper, we determined the measurement accuracy of physical
parameters for eccentric, precessing, non-spinning, inspiraling, stellar-mass
black hole - black hole (BH-BH) binaries for the upcoming second-generation
LIGO/VIRGO/KAGRA detector network at design sensitivity using the Fisher matrix
method. Here we extend that study to a wide range of binary masses including
neutron star - neutron star (NS-NS), NS-BH, and BH-BH binaries with BH masses
up to . The measurement error of eccentricity at a gravitational-wave (GW) frequency of is in the range
for NS-NS, NS-BH, and BH-BH
binaries at a luminosity distance of if .
For events with masses and distances similar to the detected 10 GW transients,
we show that nonzero orbital eccentricities may be detected if . Consequently, the LIGO/VIRGO/KAGRA detector network at design
sensitivity will have the capability to distinguish between eccentric waveforms
and circular waveforms. In comparison to circular inspirals, we find that the
chirp mass measurement precision can improve by up to a factor of and
for NS-NS and NS-BH binaries with BH companion masses , respectively. The identification of eccentric sources may
give information on their astrophysical origin; it would indicate merging
binaries in triple or higher multiplicity systems or dynamically formed
binaries in dense stellar systems such as globular clusters or galactic nuclei.Comment: 22 pages, 1 table, 12 figures, version accepted for publication in
Ap
Diffusion and Mixing in Globular Clusters
Collisional relaxation describes the stochastic process with which a self-gravitating system near equilibrium evolves in phase-space due to the fluctuating gravitational field of the system. The characteristic timescale of this process is called the relaxation time. In this paper, we highlight the difference between two measures of the relaxation time in globular clusters: (1) the diffusion time with which the isolating integrals of motion (i.e., energy E and angular momentum magnitude L) of individual stars change stochastically and (2) the asymptotic timescale required for a family of orbits to mix in the cluster. More specifically, the former corresponds to the instantaneous rate of change of a star's E or L, while the latter corresponds to the timescale for the stars to statistically forget their initial conditions. We show that the diffusion timescales of E and L vary systematically around the commonly used half-mass relaxation time in different regions of the cluster by a factor of ∼10 and ∼100, respectively, for more than 20% of the stars. We define the mixedness of an orbital family at any given time as the correlation coefficient between its E or L probability distribution functions and those of the whole cluster. Using Monte Carlo simulations, we find that mixedness converges asymptotically exponentially with a decay timescale that is ∼10 times the half-mass relaxation time. © 2018. The American Astronomical Society. All rights reserved.
Toward reliable morphology assessment of thermosets via physical etching: Vinyl ester resin as an example
The morphology of peroxide-cured, styrene crosslinked, bisphenol A-based vinyl ester (VE) resin was investigated by atomic force microscopy (AFM) after ‘physical’ etching with different methods. Etching was achieved by laser ablation, atmospheric plasma treatment and argon ion bombardment. Parameters of the etching were varied to get AFM scans of high topography resolution. VE exhibited a nanoscaled nodular structure the formation of which was ascribed to complex intra- and intermolecular reactions during crosslinking. The microstructure resolved after all the above physical etching techniques was similar provided that optimized etching and suitable AFM scanning conditions were selected. Nevertheless, with respect to the ‘morphology visualization’ these methods follow the power ranking: argon bombardment > plasma treatment > laser ablation
Compact Object Mergers Driven by Gas Fallback
Recently, several gravitational wave detections have shown evidence for compact object mergers. However, the astrophysical origin of merging binaries is not well understood. Stellar binaries are typically at much larger separations than what is needed for the binaries to merge due to gravitational wave emission, which leads to the so-called final AU problem. In this Letter we propose a new channel for mergers of compact object binaries which solves the final AU problem. We examine the binary evolution following gas expansion due to a weak failed supernova explosion, neutrino mass loss, core disturbance, or envelope instability. In such situations the binary is possibly hardened by ambient gas. We investigate the evolution of the binary system after a shock has propagated by performing smoothed particle hydrodynamics simulations. We find that significant binary hardening occurs when the gas mass bound to the binary exceeds that of the compact objects. This mechanism represents a new possibility for the pathway to mergers for gravitational wave events
Detecting Triple Systems with Gravitational Wave Observations
The Laser Interferometer Gravitational Wave Observatory (LIGO) has recently discovered gravitational waves (GWs) emitted by merging black hole binaries. We examine whether future GW detections may identify triple companions of merging binaries. Such a triple companion causes variations in the GW signal due to: (1) the varying path length along the line of sight during the orbit around the center of mass; (2) relativistic beaming, Doppler, and gravitational redshift; (3) the variation of the light-travel time in the gravitational field of the triple companion; and (4) secular variations of the orbital elements. We find that the prospects for detecting a triple companion are the highest for low-mass compact object binaries which spend the longest time in the LIGO frequency band. In particular, for merging neutron star binaries, LIGO may detect a white dwarf or M-dwarf perturber at a signal-to-noise ratio of 8, if it is within 0.4 R⊙ distance from the binary and the system is within a distance of 100 Mpc. Stellar mass (supermassive) black hole perturbers may be detected at a factor 5 × (103×) larger separations. Such pertubers in orbit around a merging binary emit GWs at frequencies above 1 mHz detectable by the Laser Interferometer Space Antenna in coincidence
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