A new window in the gravitational wave (GW) spectrum was opened recently at nano-Hertz frequencies with the recent announcement of evidence for such waves from stochastic gravitational wave background (SGWB) by the European Pulsar Timing Array (EPTA) and the Indo-Japanese Pulsar Timing Array (InPTA) collaboration coordinated with a similar announcement by the North American NanOhertz GRAVitational waves observatory (NANOGrav), the Parkes Pulsar Timing Array (PPTA), and the Chinese Pulsar Timing Array (CPTA) experiments.

GWs are transverse waves, generated by the motion of matter, such as the motion of a binary star system like double neutron stars and are quadrupolar in nature. Unlike the transient nature of GWs detected between 10 and 500 Hz by the Advanced Laser Interferometery Gravitational Wave Observatory, ultra-low frequency GWs in the nano-Hertz band come from an isotropic stochastic gravitational wave background (SGWB) formed by the superposition of continuous GWs from an ensemble of supermassive black hole binary systems (SMBHB). This signal with a power-law spectrum with an expected index of 2/3 was being searched by the pulsar timing array (PTA) experiments, such as EPTA and InPTA, for the last two decades using their most sensitive six telescopes. PTA experiments employ a collection of massive and compact stars called radio pulsars to form a Galactic-sized GW detector. A PTA measures the variations in the apparent frequency of clock-like radio pulses due to a passing GW, which are correlated across pulsar pairs in a characteristics spatial correlation called the Hellings-Down overlap function for an isotropic SGWB [5, 6].

The InPTA data were obtained using the upgraded Giant Meterwave Radio Telescope (uGMRT) using its unique sub-array capability to form two co-located telescopes to concurrently observe 14 pulsars at 300 − 500 and 1260 − 1460 MHz [7]. These unique capabilities provide high precision estimates of Dispersion measures and constrain the slow variations in the ionized interstellar medium, which are dominant below 500 MHz. The InPTA data were observed over 3.5 years [8]. The EPTA experiment observed 25 pulsars over 24.7 years using the five largest telescopes located in Europe with observations carried out mostly between 1 and 8 GHz. While the InPTA data complement the EPTA data by extending the frequency coverage, the longer time baseline of the EPTA pulsars better estimates of rotational, astrometric, and binary parameters of the pulsar sample. The data combination used the InPTA data for 10 pulsars, which overlap with the EPTA 25 pulsars. The SGWB search was carried out using the joint dataset with the search proceeding in the latest 10.3-year dataset (DR2new +) and the full joint dataset (DR2full +) separately [9, 10].

The main difference in this analysis from the past efforts was the use of more advanced noise models. Three different types of time-correlated or “red” noise processes were considered—(a) achromatic spin noise, (b) time-correlated chromatic DM noise with an observing frequency dependence of ν − 2, and (c) a chromatic scattering noise with ν − 4. These advanced noise models, particularly the chromatic models, benefitted from the addition of low-frequency InPTA data and have helped in improving the significance of evidence of SGWB in these data [10].

The main result of this analysis is the first evidence for both the expected spectrum and the spatial correlation suggesting the presence of ultra-long wavelength GWs [9]. Evidence for a SGWB with a Bayes factor of 60 and a false alarm probability of about 0.1% was obtained with a 10.3-year subset [9]. While the correlated spectrum was detected with high significance in the 10.3-year data, it is in mild tension with the expected spectral index of 13/3. Similar results were simultaneously published by NANOGrav, PPTA, and CPTA collaborations [11,12,13]. While the significance of evidence for SGWB varied between different experiments, broadly the results of different experiments are consistent.

These results have opened the ultra-low frequency window of the GW astronomy. The mild tension seen in the spectral index reported by three experiments appears to suggest an origin of SGWB other than that due to a superposition of GWs from an ensemble of SMBHBs. The non-stationary SGWB hinted by one experiment is intriguing and needs to be investigated further. A data combination is currently ongoing for an IPTA Data release 3 planned next year, where some of these questions are likely to be answered. Thus, an exciting era of GW astronomy is opening up indeed.