The International Pulsar Timing Array second data release: Search for an isotropic gravitational wave background

Abstract

We searched for an isotropic stochastic gravitational wave background in the second data release of the International Pulsar Timing Array, a global collaboration synthesizing decadal-length pulsar-timing campaigns in North America, Europe, and Australia. In our reference search for a power law strain spectrum of the form hc = A(f/1 yr−1)α, we found strong evidence for a spectrally-similar low-frequency stochastic process of amplitude $A = 3.8^{+6.3}_{-2.5}\times 10^{-15}$ and spectral index α = −0.5 ± 0.5, where the uncertainties represqent 95% credible regions, using information from the auto- and cross-correlation terms between the pulsars in the array. For a spectral index of α = −2/3, as expected from a population of inspiralling supermassive black hole binaries, the recovered amplitude is $A = 2.8^{+1.2}_{-0.8}\times 10^{-15}$. Nonetheless, no significant evidence of the Hellings-Downs correlations that would indicate a gravitational-wave origin was found. We also analyzed the constituent data from the individual pulsar timing arrays in a consistent way, and clearly demonstrate that the combined international data set is more sensitive. Furthermore, we demonstrate that this combined data set produces comparable constraints to recent single-array data sets which have more data than the constituent parts of the combination. Future international data releases will deliver increased sensitivity to gravitational wave radiation, and significantly increase the detection probability.

A Parallelized Bayesian Approach To Accelerated Gravitational-Wave Background Characterization

“…All of these results are statistically consistent with one another, and broadly in alignment with theoretical expectations of the GWB spectrum from SMBHBs [33]. Furthermore, the IPTA has also recently announced the discovery of a similar stochastic process with amplitude 2.8 × 10 −15 [26], where this result derives from the synthesis of older NANOGrav, PPTA, and EPTA datasets. However, none of these results exhibit significant inter-pulsar correlations, and as such these processes could still be noise or other systematic processes of non-GW origin.…”

“…Hence, the FL technique enables fast, parallelizable recovery of the amplitude and cross-correlation significance of a GWB in PTA data, thereby circumventing many of the sampling and computational limitations that PTAs will encounter as data volume grows. This technique has already seen broad uptake within the PTA community for analyzing NANOGrav [30], PPTA [31], EPTA [32], and IPTA [26] flagship datasets and other studies [56] (having been developed by the lead author), but has lacked a formal methodology until now.…”

“…There are several collaborations that have now been contributing to this goal for over a decade, including the North American Nanohertz Observatory for Gravitational waves (NANOGrav) [19], the Parkes Pulsar Timing Array (PPTA) [20,21], and the European Pulsar Timing Array (EPTA) [22]. Together with the Indian Pulsar Timing Array (InPTA) [23], these collaborations also constitute the International Pulsar Timing Array (IPTA) [24][25][26]. The Chinese Pulsar Timing Array [27], and telescope-centered efforts such as CHIME/Pulsar [28] and MeerTime [29], will be important future contributors.…”

New physics from the polarised light of the cosmic microwave background

“…New mission concepts were submitted for ESA's Voyage 2050 planning of Large Class science missions in the timeframe 2035-2050, which illustrated various laser and atomic interferometer designs covering f 10 −6 to 10 Hz [177][178][179][180] . We can also measure GW-induced motion of astrophysical bodies such as pulsars 181,182 (rotating neutron stars emitting pulses of radio emission) and stars 183 within our Galaxy to detect GW of λ tens of light-years (10 −9 Hz). However, none of these techniques can be used to detect GW of λ tens of billions of light-years.…”

Stochastic Gravitational-Wave Backgrounds: Current Detection Efforts and Future Prospects

“…Publications that adopted alphabetical author list are denoted by asterisks. The fourth column contains 95% upper limits on A for the GWB and measurements of A of the commonspectrum process (CP) with credible levels of 1σ in [26] and 5-95% in [54,276,277]. The values in the last two columns are the characteristics of the data set: the total observation time T obs and the number of pulsars in the array N psr .…”