Beating the Spin-Down Limit on Gravitational Wave Emission From the Vela Pulsar

Abstract

We present direct upper limits on continuous gravitational wave emission from the Vela pulsar using data from the Virgo detector's second science run. These upper limits have been obtained using three independent methods that assume the gravitational wave emission follows the radio timing. Two of the methods produce frequentist upper limits for an assumed known orientation of the star's spin axis and value of the wave polarization angle of, respectively, 1.9×10 −24 and 2.2×10 −24 , with 95% confidence. The third method, under the same hypothesis, produces a Bayesian upper limit of 2.1 × 10 −24 , with 95% degree of belief. These limits are below the indirect spin-down limit of 3.3 × 10 −24 for the Vela pulsar, defined by the energy loss rate inferred from observed decrease in Vela's spin frequency, and correspond to a limit on the star ellipticity of ∼10 −3. Slightly less stringent results, but still well below the spin-down limit, are obtained assuming the star's spin axis inclination and the wave polarization angles are unknown.

Narrow-band search of continuous gravitational-wave signals from Crab and Vela pulsars in Virgo VSR4 data

“…The uncertainty on the amplitude will contribute to the uncertainty on the upper limit on signal amplitude, together with that coming from the finite size of the Monte Carlo simulation used to compute it, see Sec. V. The calibration error on the phase can be shown to have a negligible impact on the analysis [3].…”

“…The detection statistic we use is based on the so-called 5-vectors, the same used for pulsar targeted searches [3,4], and is here briefly described. Once barycentric and spindown corrections have been applied, a CW signal with frequency f 0 present in the data would be monochromatic, apart from an amplitude and phase sidereal modulation due to the time-varying detector beam pattern functions, and given by Eq.…”

“…are built, see [3,12] for more details. They correspond to computing matched filters between the data and the signal templates, and it can be shown, assuming the noise is Gaussian with mean value zero, that they are estimators of the signal plus and cross complex amplitudes H 0 e ıΦ0 H + , H 0 e ıΦ0 H × .…”

“…Several targeted searches for CW have been conducted on data from first generation interferometric detectors. No evidence for a signal has been found, but interesting upper limits have been placed in a few cases [2][3][4]. Given the uncertainties on gravitational wave emission mechanisms and also the lack of a full detailed picture of the electro-magnetic emission geometry, it is not obvious at all that the gravitational-wave emission takes place at exactly twice the star measured pulse rate, or that such relation holds for observation times of months to years.…”

Gravity Talks: Observing the Universe with Gravitational Waves

“…This consideration sets an upper limit on the expected amplitude (the spindown limit). For two pulsars, the Crab and Vela, current gravitational wave observations have constrained the amplitudes more tightly than the spindown limit [18,19]. Advanced detectors may detect such radiation or constrain many more systems.…”