Nous avons tout d’abord pris contact et échangé quelques mails explicatifs avec Shami Chatterjee, puis nous lui avons envoyé une liste de questions un peu plus formelle. Il nous a répondu point par point et on peut retrouver ci-dessous les questions et réponses de cet échange. Nous tenons à préciser que nous avons réussi à le contacter peu avant la fin de notre enquête. Contrairement à l’entretien avec MM. Zarka et Mottez, très explicatif, il s’agit ici de questions plus précises auxquelles nous avions du mal à trouver des réponses dans la presse, ou qui avaient été soulevées par nos professeurs.

About the detection of FRBs

• How do you actually detect a FRB using the radio telescope’s data? Is it close to a frequency analysis or more complicated? I think it is quite similar to the methods used to detect pulsars but don’t know to what extent. We also heard of a new method to detect them (at the UTMOST telescope in Canberra if I remember well), how does it differ from previous means of detection?
• How hard was the live detection of FRBs to achieve? Was there actually a team working on this for a long time or did they just implement an algorithm to treat the data directly after it was received instead doing some post-treatment?

There are two key things to understand.

Most pulsars and single pulse sources are detected by large single dish radio telescopes. This is “simple” – it’s a big light bucket, and it collects all the radio waves from a patch of sky. Then we look through the time-frequency plane, searching for pulses that are dispersed (swept in frequency) due to propagation through the interstellar medium (and in the case of FRBs, the intergalactic medium as well). Conceptually simple, but immensely data intensive, since we’re sifting massive amounts of data sampled very rapidly in time and with many frequency channels, and we have to deal with a lot of radio frequency interference as well.

The downside to this technique is that if we detect something (and most FRBs have been detected this way), we only conclude that they come from a given patch of sky. In the case of Arecibo, that patch of sky is 3 arc minutes in diameter and includes hundreds of sources, so we cat identify a counterpart or host. (For Parkes, it’s about 15 arcmin, so even tougher.)

Alternatively, one could use interferometers.  These combine signals from several smaller dishes and can achieve much higher resolution imaging of the radio sky. But typically, they dump data every few seconds, and we’re looking for a transient on millisecond timescales. So we need to run the correlators at their limits – for the Very Large Array, we were effectively making movies of the sky at 200 frames per second, which produces a Terabyte of data per hour on the sky. And then we also need to account for the pulses being swept (by an unknown amount) in frequency. So using an interferometer is a much, much more data intensive process, but it gives you much better localization – with the VLA, to ~arcseconds.

At the VLA, this relied on significant advancements in computing power, correlator capabilities, as well as a lot of grunt work actually looking at the data. And UTMOST is also an interferometer, although not yet operating at the capabilities of the VLA.

 

About the hypothesis

• Did the repetition of FRB 121102 really “kill“ all the cataclysmic hypothesis to explain FRBs? (neutron stars colliding, supernovas…) Or do scientist rather consider that there could be two kinds of FRBs, with different causes?

This is more of a philosophical issue at this point, until we get more detections of FRBs that repeat, or figure out that most FRBs do not actually repeat. There is no reason to rule out multiple classes of FRBs (why stop at two? Why not three?), but there is also no reason to jump to more complex explanations, as opposed to saying that all FRBs come from the same mechanism and we have just been unlucky with the others so far.

In our work, we are very careful to say that: at least in this ONE instance, we can conclude that the mechanism is not cataclysmic.

 

About you personal experience

•  Which part of your work time does take the study of FRBs?
• You published many articles with Victoria Kaspi, Jason Hessels and Laura Spitler, and you seem to collaborate less with Evan Keane, Emily Petroff and Matthew Bailes. Is there anything opposing those two groups of scientists?

We have existing collaborations based on projects at different telescopes. For Arecibo, we have been conducting a pulsar survey going back over a decade now (see Cordes et al. 2006, http://adsabs.harvard.edu/abs/2006ApJ…637..446C ); likewise, Bailes and others have been conducting pulsar surveys at Parkes in Australia, going back even further in time. There’s a lot of crossover between these groups   and the same cast of characters appears in surveys at the Green Bank telescope, etc.  It’s more a set of collaborations originally defined by geography than anything else. (Although again, there’s a lot of crossover between the groups.)

 

• Do you often share informations or ideas with astronomers across the globe (for example with the frenchs Philippe Zarka and Fabrice Mottez, or with Gao and Zhang in Beijing)?

Our regular collaboration teleconferences currently span from Hawaii to Germany, including a dozen people in Canada, USA, the Netherlands, and we have collaborators in India as well as various other places in Europe. Otherwise, we exchange ideas at meetings, such as the Aspen meeting you refer to, or a meeting at McGill in Montreal next week.

 

• What did you think of the Aspen conference? Would you say we can differentiate scientist focusing on experiments and observation from those working on pure theory?

Sure, there are a contingent of observers, and there are some pure theorists who are trying to explain the detected phenomena. But certainly the observers also dabble in theoretical models. And with FRB 121102, now that we have a host galaxy, there are experts in dwarf galaxies, supernovae, and magnetars all involved.

 

• Could you explain briefly what FRBs could imply concerning the theory of general relativity ?

I’m really not sure. Are you thinking of limits on the Equivalence Principle, as in papers like Tingay and Kaplan (2016; https://arxiv.org/abs/1602.07643 )? As far as I understand, GR continues to work beautifully at all the scales it’s been tested so far.