Hunting nano-Hertz Gravitational Waves with the Indian Pulsar Timing Array Experiment

Gravitational waves (GWs) are the latest messenger in astrophysics, which bring the music of unseen masses in the universe to us.  The bass tones of this music, the ultra-low frequency GWs, are covered by  Pulsar Timing Array (PTA) experiments, unlike the higher frequencies discovered by terrestrial GW detectors, such as Advanced Laser Interferometer Gravitational Observatory (aLIGO). The ultra-low frequency GWs are produced by a superposition of continuous waves radiated by a randomly distributed collection of  super massive black hole binaries (SMBHBs) or by inspiralling SMBHBs.  PTA experiments, which use radio monitoring observations of an ensemble of radio millisecond pulsars, are tantalizingly close  to discovering these sub-micro Hertz GWs. In this talk, I will describe an ongoing experiment using TIFR’s two premiere radio observatories, the upgraded GMRT and the Ootucmund Radio Telescope. This experiment is  conducted by an Indo-Japanese collaboration of over 40 astronomers called the Indian Pulsar Timing Array (InPTA) and provides low radio frequency observations of about 15 millisecond pulsars since the last 7 years. The InPTA has routinely provided over the last 3.5 years a measure of the variations in the line-of-sight electron density with the highest precision obtained so far. Such observations are  critical for enhancing the sensitivity of PTA experiments by precision modeling of interstellar medium variations, which are  noise sources covariant with GW signal given that  both the GWs and the ISM vary over a similar time-scale of a few years. The InPTA pools its data with international pulsar timing array consortia (IPTA), as do three other major experiments in Australia, Europe and US. After a brief review of the techniques used in these experiments, I will present the latest results from InPTA and the contribution of the InPTA in developing the GW community in India and Japan.  While nano-Hertz GWs are yet to be detected, the limits obtained by the PTAs already yield interesting results about galaxy merger rates and solar system dynamics. Work in the current year on combining data sets over 15 to 18 years by different PTAs is already providing hints of an emerging GW signal from the noise making a detection imminent in the coming days. I will describe some of these recent efforts  and  put the InPTA effort in this context. I will conclude by outlining the ongoing work and possible future directions  in this emerging new messenger.