Abstract
Pulsar-timing arrays (PTAs) are seeking gravitational waves from supermassive-black-hole binaries, and there are prospects to complement these searches with stellar-astrometry measurements. Theorists still disagree, however, as to whether the local gravitational-wave background will be statistically isotropic, as arises if it is the summed contributions from many SMBH binaries, or whether it exhibits the type of statistical anisotropy that arises if the local background is dominated by a handful (or even one) bright source. Here we derive, using bipolar spherical harmonics, the optimal PTA estimators for statistical anisotropy in the GW background and simple estimates of the detectability of this anisotropy. We provide results on the smallest detectable amplitude of a dipole anisotropy (and several other low-order multipole moments) and also the smallest detectable amplitude of a "beam'' of gravitational waves. Results are presented as a function of the signal-to-noise with which the GW signal is detected and as a function of the number of pulsars (assuming uniform distribution on the sky and equal sensitivity per pulsar). We provide results first for measurements with a single time-domain window function and then show how the results are augmented with the inclusion of time-domain information. The approach here is intended to be conceptually straightforward and to complement the results of more detailed (but correspondingly less intuitive) modeling of the actual measurements.
Highlights
A longstanding effort [1,2,3,4,5,6] to detect a stochastic gravitational-wave background with pulsar-timing arrays consists of three major efforts—the Parkes Pulsar Timing Array (PPTA) [7, 8], North American Nanohertz Observatory for Gravitational Waves (NANOGrav) [9], and the European Pulsar Timing Array (EPTA) [10]– that collaborate through an International Pulsar Timing Array (IPTA) [11]
There are prospects to use complementary information from stellar astrometry [17,18,19,20,21] as the apparent position of distant stars will oscillate with a characteristic pattern on the sky due to GWs
It is still not understood, though, whether the local GW signal due to SMBH mergers will be the type of stochastic background that arises as the sum of a large number of cosmological sources, or whether it will be dominated by just a handful—or even just one—source [22,23,24,25,26]
Summary
A longstanding effort [1,2,3,4,5,6] to detect a stochastic gravitational-wave background with pulsar-timing arrays consists of three major efforts—the Parkes Pulsar Timing Array (PPTA) [7, 8], North American Nanohertz Observatory for Gravitational Waves (NANOGrav) [9], and the European Pulsar Timing Array (EPTA) [10]– that collaborate through an International Pulsar Timing Array (IPTA) [11]. A first obvious step, after the initial detection of a gravitational-wave signal, will be to seek the anisotropy in the background that may arise from a finite number of sources. Prior work [28,29,30] has developed tools to characterize and seek with PTAs anisotropy in the GW background that were implemented in a null search [31]. This anisotropy was characterized (as it is here ) in terms of an uncorrelated and unpolarized background of gravitational waves with a direction-dependent intensity parametrized in terms of spherical-harmonic expansion of the intensity.
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
