CTA and Fermi Gamma Ray Space Telescope: probing the high energy Universe

An image to illustrate how the gamma ray telescope will probe the high energy universe
© iStock/coffeekai

Professor Julie McEnery, NASA’s Fermi Project Scientist, tells SciTech Europa about what the Cherenkov Telescope Array (CTA) will bring to astrophysics, and how Fermi Gamma ray Space Telescope and CTA can work together.

IN 2008, NASA launched the Fermi Gamma ray Space Telescope with the mission to study and observe the cosmos using the highest-energy form of light (ranging from 10 keV to 200 GeV). Now, the science enabled by Fermi will be complimented with that enabled by the Cherenkov Telescope Array (CTA), the next generation ground-based observatory for gamma-ray astronomy at very-high energies. CTA will be able to explore the entire sky by using 118 telescopes which are located in both the northern and southern hemispheres.
In May, SciTech Europa Quarterly attended the first CTA Science Symposium in Bologna and spoke with Julie McEnery, Fermi Project Scientist at the Astroparticle Physics Laboratory, Astrophysics Science Division of NASA’s Goddard Space Flight Center and Adjunct Professor of Physics at the University of Maryland’s College Park campus, about what CTA will bring to astrophysics, and how Fermi and CTA can work together.

As an astrophysicist, what are the most exciting things about CTA?

From a Fermi perspective, just about every TeV source has useful Fermi data. However, there are only perhaps a couple of hundred sources that can be seen at TeV energies by ground-based instruments. CTA, however, is going to be much more sensitive and that pushes its energy threshold down. As such, I hope that it will be possible to take almost all of the Fermi sources and use CTA to see what they are doing at the highest energies. A key question that you always want to ask is just how efficient and high energy the particles in these objects are, because you want to have observations of the highest possible energy, and I think CTA is going to add the final piece to the puzzle. There are several classes of sources that we can see with Fermi that haven’t yet had a counterpart at higher energies, and CTA is hopefully going to address that.

How would you like to see Fermi and CTA integrate more with each other?

I think each community should try and be proactively engaged with the other. This does happen to a certain extent already, but there is a sense that more could be done, such as by having CTA staff attending Fermi symposia, and, in turn, Fermi staff attending CTA events. Space-based observations are a very important compliment to what is being done on the ground. However, to ensure that space-based observations continue and that we retain adequate support, we need those who are active in ground-based observations to extol the virtues of space-based activities and to ensure that all stakeholders are aware that space-based activities are important to them. If this doesn’t happen then there is the possibility that space-based activities could eventually disappear.

This does indeed to be a problem, especially considering that Fermi’s lifespan is due to expire in 2020, meaning that CTA will potentially not have a space-based counterpart after this time. Have any ideas for a successor to Fermi yet been identified?

Propositions are being made, such as the ‘AMEGO’ (All Sky Medium Energy Gamma-ray Observatory) mission. We have done some preliminary engineering studies for this and are building a prototype but, despite this, it is not at all ready to move forwards. It should also be noted that even if we obtained approval to begin AMEGO next year, which is highly unlikely, it would still take until something like the end of the next decade before we would be ready to launch; even in the most optimistic scenario, we wouldn’t have anything new in time to replace Fermi in 2020. However, there is every chance that Fermi will continue to make observations after that time, as long as everything keeps working as it is at the moment.

One of the reasons why there isn’t yet a concrete replacement for Fermi is that people are not aware of the necessity of it. Furthermore, and this is true of both space- and ground-based observatories, we fall into the intersection between physics and astronomy, which can mean that if a review environment is dominated by astronomers, for instance, then they might not be overly familiar with what the kind of science that the physicists are doing. Indeed, there is a tendency to assume that if it is not being talked about in an individual’s department then it must not be worth discussing. The reality, however, is that it might not be being talked about in an astronomy department but that it is indeed being discussed in a physics department.

How will the science enabled by the CTA complement what is being done by Fermi? What will the main differences be?

The two fundamental key differences – and perhaps synergies – are that the space-based instruments can extend the energy range down lower (which means that the combination of space- and ground-based observations basically cover a huge swathe of the electromagnetic spectrum) and the pattern with which we view the sky. That is, the space-based instruments see pretty much all of the sky all of the time; we are very good at telling the ground where they need to look. The ground-based instruments, however, have a much greater sensitivity to seeing very rapid variability. If we get a hint that something is interesting and flaring on the sky, and if we can get CTA to look at it, then it is going to make some great observations.

With some science cases, it is important to be able to quickly look at the right place in the sky. However, ground-based observatories are limited because they have to wait until there are gamma ray bursts, otherwise it’s not worth doing. But for a lot of objects, Fermi has the potential to see that something is happening and then, when night falls, CTA can already have a plan in place for the observations they intend to do. This could be what we would do with a stellar nova, for example.

A nova is a white dwarf star that is accreting material from a companion, and when it accretes enough material, it triggers a thermonuclear explosion in the binary system. It has always been assumed that the amount of energy in these explosions is not sufficient to accelerate particles at the highest energies. But one of the things that we have discovered with Fermi (completely serendipitously), is that not only can it produce high energy gamma rays in order to do this, but that they routinely do so. This has entirely changed our understanding of what is happening in stellar novae because we went on to realise that the shocks that were sent up between the system are responsible for most of the radiation that we are seeing. If we could get CTA observations here, then this would tell us what is responsible for producing those gamma rays.

What are your thoughts on how the CTA will benefit advances in multi-messenger astronomy? How will the CTA benefit the work that NASA undertakes?

Extreme explosions produce a huge amount of energy in a small region of space. That produces gravitational waves and extreme accelerators. We are getting a clear picture now of the most energetic particles in the Universe which are being produced in these events, and we also know that those sites will produce neutrinos.

CTA is an absolutely natural partner for these extreme accelerators because the same process that produce neutrinos also produces gamma rays. Indeed, every single gamma ray source in the sky is a potential site for the production of neutrinos. In the next year, we will potentially see an upgrade to the IceCube experiment, which is finding a significant number of neutrinos, and this coupled with CTA as a much more capable gamma ray instrument means that it should be possible to really take the study of neutrinos and the discovery of their sources to the next level, using both the gamma ray and neutrino information to really learn something about these extreme sources.

The situation with gravitational waves is a little more complicated. The most natural counterpart for a gravitational wave source is a gamma ray burst. CTA operates at an energy that is much higher than that at which gamma ray bursts are typically produced. The connection seems therefore to be a little less direct, and so perhaps CTA will be able to find counterparts to gravitational wave events, just as it is likely to find higher energy counterparts to gamma ray bursts themselves.

When two neutron stars spiral around each other until they lose enough energy and momentum that they merge, the very first photons that are emitted out of that system are gamma rays. This unique measurement allows us to understand whether gravitational waves and light feel the gravitational potential of our galaxy in the same way, and that is interesting because modified theories of gravity have been developed in order to explain dark matter without having to invent new particles.

Many of those models predict that gravitational waves do not see the gravitational potential of our galaxy in the same way that light does, and if those models are correct, then the gravitational waves that propagate from such an event should therefore arrive at the Earth at a different time to the photons. While we now know what happens with regard to lower energies, it is difficult to say what we are going to see with higher energy gamma rays. If it turns out that there is a signal that CTA is able to find, then it will be able to do all that science. And in some senses maybe even a little bit more, because if CTA sees photons from a gamma ray burst, it really will be sampling the highest energies.

Do you plan on utilising CTA in your own research projects?

I imagine so. It is a little far off into the future and I haven’t done any more than just monitor the progress being made. I am nevertheless fascinated by the possibilities that CTA offers, and I personally think that with new projects and experiments it makes a lot of sense not to cling vigorously to one particular science topic and try and do that science topic with every instrument. I therefore want to take a step back and look at what this experiment does really well, and then go and modify my science interests in order to take advantage of what is there.

What do you think about CTA being proposal driven?

I’m fascinated by this, too. My background is in ground-based gamma rays, and then I moved to NASA to work on space-based observatories. The space-based community has already made the transition from being led by a consortium to a more open system, but it was a controversial transition and not everybody was comfortable with it at the time. It will be interesting to watch another community go through that process.

Professor Julie McEnery
Fermi Project Scientist
NASA
Tweet @NASAFermi
https://fermi.gsfc.nasa.gov

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