Interpreting the Recent Upper Limit on the Gravitational Wave Background from the Parkes Pulsar Timing Array

Abstract

Shannon et al. (25 Sept, 2015) report important constraints from the Parkes pulsar timing array (PPTA) on the gravitational-wave background (GWB) from supermassive black-hole binaries (SMBHBs) using data from four millisecond pulsars. We wish to clarify two points regarding their paper.The reported non-detection of the GWB, widely publicized as a case of "missing" GWs, is based on simple power-law models (1-3). These models assume the "last parsec problem" is optimally solved, meaning SMBHBs evolve, without stalling, to orbital frequencies where PTAs are sensitive, and that SMBHB environments (stars, gas) and eccentric orbits do not diminish the lowest frequency GWB signals. These effects have been an active area of research for several years (2-5) and current non-detections are unsurprising. With more millisecond pulsars, NANOGrav has placed new constraints on the shape of the GWB spectrum, and thereby on SMBHB environments (6).Shannon et al. conclude that GWB detection will require additional preciselytimed pulsars such as PSR J1909−3744, or higher-cadence observations targeted at GW frequencies above 0.2 yr −1 . Yet recent studies show that the best GWB detection strategy, regardless of GWB spectrum details, is to observe many pulsars over the longest possible timespans to maximize sensitivity to the lowest GW frequencies (7, 8). PTAs are most sensitive at frequencies near the inverse of the total observing timespan, with sensitivity decreasing at higher frequencies. No known mechanism can alter the GWB spectrum to allow a PTA detection at higher frequencies.The International Pulsar Timing Array (IPTA) aims to assemble the timing data necessary for GW detection by pooling contributions from the Parkes PTA, NANOGrav, and the European PTA. This approach, which leverages very high quality data from the Arecibo and Green Bank Telescopes, should enable a detection of the GWB within 10 years (8).

On the gravitational wave background from black hole binaries after the first LIGO detections

“…There can be various other contributions to the Ω GW (see e.g. [25,[35][36][37][38][39][40][41][42][43][44][45][46][47][48][49]). From this point on we are going to study only the contribution from BH binaries, since for those we already have observations and thus some first measurement of their local incidence rate.…”