Pulsar timing in the GC
Pulsars are highly stable rotators in the universe, rivaling the best atomic clocks on the earth. Thus, they are excellent tools to probe any perturbations in space-time. Pulsar timing is the technique to monitor the rotation of pulsars by precisely measuring the arrival time of their signals at the earth. With this technique, astrophysical effects that influence the arrival time of the pulsar signal, including relativistic orbital motion of pulsars, can be described within the framework of a timing model, and the model parameters can be measured with great precision (see Fig. 1 for an illustration).
Precision pulsar timing in the Galactic Centre (GC) is in particular challenging. At low frequencies (<5 GHz) where pulsar timing are typically performed at, the high-density interstellar medium in the GC can greatly smear out the pulsation signals from the pulsars, which is known as scattering. At higher frequencies, the intrinsic strength of the pulsar signals largely decreases as pulsar radio emission usually has a steep spectrum. Therefore, an optimal observing frequency for timing experiment in the GC would lie in between these two constraints (see Fig. 2 for more details).
Given the large distance from the earth to the GC (approximately 8 kpc), observing and timing GC pulsars require the most sensitive radio telescopes on the ground. At frequency around 10 GHz, the Square-Kilometre-Array, once available, will provide the highest sensitivity and deliver the capability to achieve timing precision as good as 100 microsecond with GC pulsars. In the mm-wavelength regime, the Atacama-Large-Millimeter-Array in phase-up mode, will be the most powerful instrument for pulsar timing experiment.