Session G4: Invited Session: Pulsar Timing Arrays and Gravitational Radiation

Pulsar Timing Arrays: No longer a Blunt Instrument for Gravitational Wave Detection

Pulsar timing now has a rich history in placing limits on the stochastic background of gravitational waves, and we plan soon to reach the sensitivity where we can detect, not just place limits on, the stochastic background. However, the capability of pulsar timing goes beyond the detection of a background. Herein I review efforts that include single source detection, localization, waveform recovery, a clever use of a ``time-machine" effect, alternate theories of gravity, and finally studies of the noise in our ``detector" that will allow us to tune and optimize the experiment. Pulsar timing arrays are no longer ``blunt" instruments for gravitational-wave detection limited to only detecting an amplitude of the background. Rather they are shrewd and tunable detectors, capable of a rich and dynamic variety of astrophysical measurements.   

Electromagnetic Emission from Supermassive Black Hole Binaries Resolved by Pulsar Timing Arrays

Pulsar timing arrays (PTAs) are expected to detect gravitational waves (GWs) from individual low--redshift ($z < 1.5$) compact supermassive ($M > 10^{9} {\rm M_\odot}$) black hole binaries with orbital periods of $\sim 0.1 - 10$ yrs. I will discuss the feasibility of identifying the electromagnetic counterparts of these sources. Because the host galaxies of resolved PTA sources are expected to be exceptionally massive and rare, it should be possible to find unique hosts of resolved sources out to redshift $z\approx 0.2$. At higher redshifts, the PTA error boxes are larger, and may contain as many as $\approx 100$ massive-galaxy interlopers, but the true counterpart may be identified from its peculiar spectrum and from its quasi-periodic variability. In particular, the binary's tidal torques expel the gas from the inner part of the circumbinary accretion disk, making the source appear unusually dim in soft X-rays, with unusually weak UV and broad optical emission lines. Additionally, if the orbital plane lies close to the line of sight, the UV lines would exhibit periodic Doppler shifts. These properties would make the PTA sources stand out among other optically luminous AGN.   

Testing alternative theories of gravity using Pulsar Timing Arrays

Pulsar timing experiments aimed at the detection of gravitational radiation have been performed for decades now. With the forthcoming construction of large arrays capable of tracking multiple millisecond pulsars, it is very likely we will be able to make the first detection of gravitational radiation in the nano-Hertz band, and test Einstein's theory of relativity by measuring the polarization components of the detected signals. Since a gravitational wave predicted by the most general relativistic metric theory of gravity accounts for six polarization modes (the usual two Einstein's tensor polarizations as well as two vector and two scalar wave components), we have estimated the single-antenna sensitivities to these six polarizations. We find pulsar timing experiments to be significantly more sensitive, over their entire observational frequency band (10$^{-9}$ - 10$^{-6}$ Hz), to scalar-longitudinal and vector waves than to scalar-transverse and tensor waves. At 10$^{-7}$ Hz and with pulsars at a distance of 1 kpc, for instance, we estimate an average sensitivity to scalar-longitudinal waves that is more than two orders of magnitude better than the sensitivity to tensor waves. Our results imply that a direct detection of gravitational radiation by pulsar timing will result into a test of the theory of general relativity that is more stringent than that based on monitoring the decay of the orbital period of a binary system.