Davide Gerosa

A PTA Primer

The next few years are expected to be a golden age for pulsar timing array (PTA) science. The recent tentative claim of a detection of an astrophysical signal in the NANOGrav 12.5-year data set is expected to be confirmed, thereby opening a new observational window on supermassive black holes. In order to better follow these developments, we will run a spring journal club in which we aim to review some key papers in the field. 

We will meet at 2.00 pm on Fridays starting on May 7th, 2021 and running for approximately 7 weeks.


A rough plan/agenda, plus minutes and useful references will be kept up to date on this overleaf

The zoom link is available on our wiki (Birmingham ASR login required). If you can’t see it, please ask either Chris or Davide.

  • [May 7, 2pm] Episode 1. The birth of an idea.
  • [May 14, 2pm] Episode 2. The basic physics of PTAs.
  • Episode 3. Correlations are key, the Hellings and Downs curve.
  • Episode 4. Pulsars in general.
  • Episode 5. The astro side of things, predicting the nHz GW spectrum.
  • Episode 6: A first detection of GWs?
  • Episode 7: Into the wild, fundamental physics and exotica with PTAs.

Episode 1. The birth of an idea

Led by Alberto. May 7, 2021.

Minutes by Chris

  • Alberto highlighted the importance of 3 very early papers which are very commonly cited as the origin of the idea for PTAs: it seems Sazhin (1978) was the fi rst to suggest something along these lines, but it was Detweiler (1979) who got all the details arranged in a way that we would immediately recognize today, and [quite a bit later] it was Foster & Backer (1990) who made the first attempt to actually construct a PTA.
  • The basic effect of GWs delaying/advancing light from some distant object was studied even earlier [Estabrook & Wahlquist 1975], but in the context of Doppler tracking distant spacecraft. In fact, it seems to us that the idea for PTAs owes quite a bit to the older idea of Doppler tracking spacecraft.
  • An amazing paper: Vecchio, Bertotti, and Iess. “Coalescing Binaries and Doppler Experiments” (1998).
  • Moving on to the main paper for this week, we discussed the details of the timing model (eq. 3) and the importance of the time standard (i.e. clocks = monopole) and locating the SSB (i.e ephemeris = dipole).
  • We also discussed dispersion quite a bit. Including how observations at several frequencies allow you to extrapolate to an ideal “infinite frequency” time of arrival.
    We also discussed (but didn’t reach a conclusion) on why the Earth’s atmosphere doesn’t appear to in uence the time of arrival in the timing model.

Episode 2. The basic physics of PTAs