156 research outputs found
Efficient Gravitational Wave Searches with Pulsar Timing Arrays using Hamiltonian Monte Carlo
Pulsar timing arrays (PTAs) detect low-frequency gravitational waves (GWs) bylooking for correlated deviations in pulse arrival times. Current Bayesiansearches use Markov Chain Monte Carlo (MCMC) methods, which struggle to samplethe large number of parameters needed to model the PTA and GW signals. As thedata span and number of pulsars increase, this problem will only worsen. Analternative Monte Carlo sampling method, Hamiltonian Monte Carlo (HMC),utilizes Hamiltonian dynamics to produce sample proposals informed byfirst-order gradients of the model likelihood. This in turn allows it toconverge faster to high dimensional distributions. We implement HMC as analternative sampling method in our search for an isotropic stochastic GWbackground, and show that this method produces equivalent statistical resultsto similar analyses run with standard MCMC techniques, while requiring 100-200times fewer samples. We show that the speed of HMC sample generation scales as where is the number ofpulsars, compared to for MCMC methods. Thesefactors offset the increased time required to generate a sample using HMC,demonstrating the value of adopting HMC techniques for PTAs.<br
Microarcsecond VLBI pulsar astrometry with PSRPI I. Two binary millisecond pulsars with white dwarf companions
Model-independent distance constraints to binary millisecond pulsars (MSPs)
are of great value to both the timing observations of the radio pulsars, and
multiwavelength observations of their companion stars. Very Long Baseline
Interferometry (VLBI) astrometry can be employed to provide these
model-independent distances with very high precision via the detection of
annual geometric parallax. Using the Very Long Baseline Array, we have observed
two binary millisecond pulsars, PSR J1022+1001 and J2145-0750, over a two-year
period and measured their distances to be 700 +14 -10 pc and 613 +16 -14 pc
respectively. We use the well-calibrated distance in conjunction with revised
analysis of optical photometry to tightly constrain the nature of their massive
(M ~ 0.85 Msun) white dwarf companions. Finally, we show that several
measurements of their parallax and proper motion of PSR J1022+1001 and PSR
J2145-0750 obtained by pulsar timing array projects are incorrect, differing
from the more precise VLBI values by up to 5 sigma. We investigate possible
causes for the discrepancy, and find that imperfect modeling of the solar wind
is a likely candidate for the timing model errors given the low ecliptic
latitude of these two pulsars.Comment: 14 pages, 9 figures, 6 tables; minor revisions in response to referee
comments to match version accepted by Ap
The NANOGrav 11-Year Data Set: Limits on Gravitational Waves from Individual Supermassive Black Hole Binaries
Observations indicate that nearly all galaxies contain supermassive black
holes (SMBHs) at their centers. When galaxies merge, their component black
holes form SMBH binaries (SMBHBs), which emit low-frequency gravitational waves
(GWs) that can be detected by pulsar timing arrays (PTAs). We have searched the
recently-released North American Nanohertz Observatory for Gravitational Waves
(NANOGrav) 11-year data set for GWs from individual SMBHBs in circular orbits.
As we did not find strong evidence for GWs in our data, we placed 95\% upper
limits on the strength of GWs from such sources as a function of GW frequency
and sky location. We placed a sky-averaged upper limit on the GW strain of at nHz. We also developed a
technique to determine the significance of a particular signal in each pulsar
using ``dropout' parameters as a way of identifying spurious signals in
measurements from individual pulsars. We used our upper limits on the GW strain
to place lower limits on the distances to individual SMBHBs. At the
most-sensitive sky location, we ruled out SMBHBs emitting GWs with
nHz within 120 Mpc for , and
within 5.5 Gpc for . We also determined that
there are no SMBHBs with emitting
GWs in the Virgo Cluster. Finally, we estimated the number of potentially
detectable sources given our current strain upper limits based on galaxies in
Two Micron All-Sky Survey (2MASS) and merger rates from the Illustris
cosmological simulation project. Only 34 out of 75,000 realizations of the
local Universe contained a detectable source, from which we concluded it was
unsurprising that we did not detect any individual sources given our current
sensitivity to GWs.Comment: 10 pages, 11 figures. Accepted by Astrophysical Journal. Please send
any comments/questions to S. J. Vigeland ([email protected]
The scientific potential of space-based gravitational wave detectors
The millihertz gravitational wave band can only be accessed with a
space-based interferometer, but it is one of the richest in potential sources.
Observations in this band have amazing scientific potential. The mergers
between massive black holes with mass in the range 10 thousand to 10 million
solar masses, which are expected to occur following the mergers of their host
galaxies, produce strong millihertz gravitational radiation. Observations of
these systems will trace the hierarchical assembly of structure in the Universe
in a mass range that is very difficult to probe electromagnetically. Stellar
mass compact objects falling into such black holes in the centres of galaxies
generate detectable gravitational radiation for several years prior to the
final plunge and merger with the central black hole. Measurements of these
systems offer an unprecedented opportunity to probe the predictions of general
relativity in the strong-field and dynamical regime. Millihertz gravitational
waves are also generated by millions of ultra-compact binaries in the Milky
Way, providing a new way to probe galactic stellar populations. ESA has
recognised this great scientific potential by selecting The Gravitational
Universe as its theme for the L3 large satellite mission, scheduled for launch
in ~2034. In this article we will review the likely sources for millihertz
gravitational wave detectors and describe the wide applications that
observations of these sources could have for astrophysics, cosmology and
fundamental physics.Comment: 18 pages, 2 figures, contribution to Gravitational Wave Astrophysics,
the proceedings of the 2014 Sant Cugat Forum on Astrophysics; v2 includes one
additional referenc
Evidence for alignment of the rotation and velocity vectors in pulsars
We present strong observational evidence for a relationship between the
direction of a pulsar's motion and its rotation axis. We show carefully
calibrated polarization data for 25 pulsars, 20 of which display linearly
polarized emission from the pulse longitude at closest approach to the magnetic
pole. Such data allow determination of the position angle of the linear
polarisation which in turn reflects the position angle of the rotation axis. Of
these 20 pulsars, 10 show an offset between the velocity vector and the
polarisation position angle which is either less than 10\degr or more than
80\degr, a fraction which is very unlikely by random chance. We believe that
the bimodal nature of the distribution arises from the presence of orthogonal
polarisation modes in the pulsar radio emission. In some cases this orthogonal
ambiguity is resolved by observations at other wavelengths so that we conclude
that the velocity vector and the rotation axis are aligned at birth.
Strengthening the case is the fact that 4 of the 5 pulsars with ages less than
3 Myr show this relationship, including the Vela pulsar. We discuss the
implications of these findings in the context of the Spruit & Phinney
(1998)\nocite{sp98} model of pulsar birth-kicks. We point out that, contrary to
claims in the literature, observations of double neutron star systems do not
rule out aligned kick models and describe a possible observational test
involving the double pulsar system.Comment: MNRAS, In Pres
Multi-Messenger Gravitational Wave Searches with Pulsar Timing Arrays: Application to 3C66B Using the NANOGrav 11-year Data Set
When galaxies merge, the supermassive black holes in their centers may form
binaries and, during the process of merger, emit low-frequency gravitational
radiation in the process. In this paper we consider the galaxy 3C66B, which was
used as the target of the first multi-messenger search for gravitational waves.
Due to the observed periodicities present in the photometric and astrometric
data of the source of the source, it has been theorized to contain a
supermassive black hole binary. Its apparent 1.05-year orbital period would
place the gravitational wave emission directly in the pulsar timing band. Since
the first pulsar timing array study of 3C66B, revised models of the source have
been published, and timing array sensitivities and techniques have improved
dramatically. With these advances, we further constrain the chirp mass of the
potential supermassive black hole binary in 3C66B to less than using data from the NANOGrav 11-year data set. This
upper limit provides a factor of 1.6 improvement over previous limits, and a
factor of 4.3 over the first search done. Nevertheless, the most recent orbital
model for the source is still consistent with our limit from pulsar timing
array data. In addition, we are able to quantify the improvement made by the
inclusion of source properties gleaned from electromagnetic data to `blind'
pulsar timing array searches. With these methods, it is apparent that it is not
necessary to obtain exact a priori knowledge of the period of a binary to gain
meaningful astrophysical inferences.Comment: 14 pages, 6 figures. Accepted by Ap
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