First Search for Nontensorial Gravitational Waves from Known Pulsars

B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration)

Phys. Rev. Lett. 120, 031104 – Published 19 January 2018

Abstract

We present results from the first directed search for nontensorial gravitational waves. While general relativity allows for tensorial (plus and cross) modes only, a generic metric theory may, in principle, predict waves with up to six different polarizations. This analysis is sensitive to continuous signals of scalar, vector, or tensor polarizations, and does not rely on any specific theory of gravity. After searching data from the first observation run of the advanced LIGO detectors for signals at twice the rotational frequency of 200 known pulsars, we find no evidence of gravitational waves of any polarization. We report the first upper limits for scalar and vector strains, finding values comparable in magnitude to previously published limits for tensor strain. Our results may be translated into constraints on specific alternative theories of gravity.

Received 12 October 2017
Revised 16 November 2017

DOI:https://doi.org/10.1103/PhysRevLett.120.031104

Physics Subject Headings (PhySH)

Alternative gravity theories Astrophysical studies of gravity Experimental studies of gravity Gravitational wave detection Gravitational waves

Neutron stars & pulsars

Gravitation, Cosmology & Astrophysics

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Vol. 120, Iss. 3 — 19 January 2018

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Figure 1
Log(odds) vs emission frequency. Log(odds) comparing the any-signal hypothesis to the instrumental (top) and Gaussian noise (bottom) hypotheses, as a function of assumed gravitational-wave frequency, $f_{GW} = 2 f_{rot}$ , for each pulsar. Looking at the top plot for $\log_{10} O_{I}^{S}$ , notice that the instrumental noise hypothesis is clearly favored for all pulsars except one, for which the analysis is inconclusive. (This is $J 1932 + 17$ , the same nonsignificant outlier identified in Ref. [25].) These results were obtained without incorporating any information on the source orientation, and are tabulated in Table II in the Supplemental Material [45]. Expressions for both odds are given in Eq. (10) and Eq. (12).
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Figure 2
Log(odds) distributions. Distributions of log(odds) comparing the any-signal hypothesis to the instrumental (ordinate axis, right) and Gaussian noise (abscissa axis, top) hypotheses for all pulsars. This plot contains the same information as Fig. 1 and displays the same nonsignificant outlier. These results were obtained without incorporating any information on the source orientation, and are tabulated in Table II in the Supplemental Material [45]. Expressions for both odds in this plot are given in Eq. (10) and Eq. (12). We underscore that, although this plot looks similar to Fig. 2 in Ref. [25], the signal hypothesis here incorporates scalar, vector, and tensor modes, in all possible combinations.
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Figure 3
Sub-hypothesis odds. Box plots for the distribution of the signal vs noise log(odds) for each of the sub-hypotheses considered, for all of the pulsars analyzed. The sub-hypotheses are ( $s t$ ), vector-tensor ( $t v$ ), scalar-vector-tensor tensor-only ( $t$ ), scalar-only ( $s$ ), vector-only ( $v$ ), scalar-vector ( $s v$ ), scalar-tensor ( $s t$ ), vector-tensor ( $v t$ ), and scalar-vector-tensor ( $s t v$ ); these are all combined into the signal hypothesis ( $S$ ). The quantity represented is $\log_{10} B_{N}^{m}$ , which is the same as $\log_{10} O_{N}^{m}$ if neither $H_{m}$ nor $H_{N}$ are favored a priori (hence, the label on the ordinate axis). The horizontal red line marks the median of the distribution, while each gray box extends from the lower to upper quartile, and the whiskers mark the full range of the distribution of $\log_{10} O_{N}^{m}$ for the 200 pulsars analyzed. These results were produced without incorporating any information on the source orientation, and are tabulated in Table II in the Supplemental Material [45].
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Figure 4
Non-GR upper limits vs emission frequency. Circles mark the 95%-credible upper limit on the scalar, $h_{s}^{95 %}$ (top), and the effective vector, $h_{v}^{95 %}$ (middle), and tensor $h_{t}^{95 %}$ (bottom) strain amplitudes as a function of assumed gravitational-wave frequency for each of the 200 pulsars in our set. The upper limits are obtained assuming a signal model including all five independent polarizations ( $H_{s t v}$ ), and incorporating no information on the orientation of the source (Table II in the Supplemental Material [45]). The effective amplitude spectral density (ASD) of the detector noise is also displayed for reference; this is the harmonic mean of the H1 and L1 spectra; the scaling is obtained from linear regression to the upper limits.
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