Improving timing sensitivity in the microhertz frequency regime: limits from PSR J1713$+$0747 on gravitational waves produced by super-massive black-hole binaries

TL;DR

This paper enhances the sensitivity of pulsar timing to detect gravitational waves from super-massive black-hole binaries in the microhertz range, setting new upper limits on wave amplitudes using high-cadence observations of PSR J1713+0747.

Contribution

It introduces high-cadence pulsar timing observations and compares Bayesian algorithms to improve limits on gravitational wave strains in the microhertz regime.

Findings

Placed 95% upper limit on strain amplitude at 1 μHz: ~3.5×10^{-13}

Placed 95% upper limit on strain amplitude at 20 nHz: ~1.4×10^{-14}

Demonstrated high-cadence observations improve sensitivity in the microhertz frequency range.

Abstract

We search for continuous gravitational waves (CGWs) produced by individual super-massive black-hole binaries (SMBHBs) in circular orbits using high-cadence timing observations of PSR J1713$+$0747. We observe this millisecond pulsar using the telescopes in the European Pulsar Timing Array (EPTA) with an average cadence of approximately 1.6 days over the period between April 2011 and July 2015, including an approximately daily average between February 2013 and April 2014. The high-cadence observations are used to improve the pulsar timing sensitivity across the GW frequency range of $0.008-5$ $\mu$Hz. We use two algorithms in the analysis, including a spectral fitting method and a Bayesian approach. For an independent comparison, we also use a previously published Bayesian algorithm. We find that the Bayesian approaches provide optimal results and the timing observations of the pulsar place a 95 per cent upper limit on the sky-averaged strain amplitude of CGWs to be $\lesssim3.5\times10^{-13}$ at a reference frequency of 1 $\mu$Hz. We also find a 95 per cent upper limit on the sky-averaged strain amplitude of low-frequency CGWs to be $\lesssim1.4\times10^{-14}$ at a reference frequency of 20~nHz.