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An illustration of THESEUS, a proposed medium-class ESA missions to detect and precisely locate gamma-ray bursts. (credit: ESA)

THESEUS: a high-energy proposal for a medium-sized mission

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THESEUS (Transient High-Energy Sky and Early and Universe Surveyor) is a space mission project aimed at detecting and characterizing gamma-ray bursts (GRBs) so as to investigate the early universe and advance multi-messenger and time-domain astrophysics. It is one of three finalists in the ESA’s latest call for medium-sized missions, along with EnVision and SPICA (see “EnVision and the Cosmic Vision decision”, The Space Review, March 2, 2020; and “SPICA: an infrared telescope to look back into the early universe”, The Space Review, May 4, 2020).

“THESEUS is the follow up to the Swift Observatory, a highly successful mission funded by the USA, Italy, and the UK. The UK team, with Paul O’Brien at Leicester University, is now working with us,” said Lorenzo Amati, principal investigator for THESEUS.

“THESEUS will be a huge improvement over SWIFT in terms of its ability to detect, localize, and identify very high redshift GRBs, and accurately localize short GRBs,” said Amati.

SWIFT was launched in 2004 to study GRBs. Its mission was only for two years, yet 15 years later it’s still making important contributions to the field. But the new era of multi-messenger astrophysics requires better machines. To this end, THESEUS’ instrument suite will include an X-ray imager with four “lobster-eye” cameras (SXI), an infrared telescope with both imaging and spectroscopy capabilities (IRT), and an X- and gamma-ray spectrometer/detector (XGIS).

“THESEUS will be a huge improvement over SWIFT in terms of its ability to detect, localize, and identify very high redshift GRBs, and accurately localize short GRBs,” said Amati. “In addition, it will be able to detect and characterize any sub-class of these events, as well as exploring with an unprecedented combination of field of view and sensitivity the soft X-ray sky. Like Swift, THESEUS will have a high degree of autonomy for the prompt follow-up of GRBs and transients, and will immediately communicate with ground facilities.”

The European astronomy community has taken on a principal role in high-energy astrophysics over the past decade, and the THESEUS proposal has grown out of that movement. A previous incarnation tried and failed to earn a place in the finalists of ESA’s earlier M4 mission call. But the science team has simply used the time since then to improve the mission’s specs. The current structural design of the satellite is the result of many conversations between the science team and the industry partners.

“The structural design is really related to the optical design,” said Diego Götz, science lead for the IRT. “So at the beginning we had a very simple Ritchey-Chrétien type telescope, but then we realized that we needed a different design, given our relatively large field of view. So, we moved to what we call a Korsch telescope. This also provided a simpler interface between us and the industry, which is responsible for delivering the telescope. It has allowed us to make an instrument which more compact, occupying less space within the satellite.”

Once launched, THESEUS will work like this: First, the SXI and the XGIS instruments will detect GRBs and transient sources over large fractions of the sky. SXI is made especially for the detection of GRBs at high redshift, optimized to provide a high sensitivity in the soft X-ray domain, while XGIS will provide spectral and timing characterization over an unprecedented photon energy range extending from soft X-rays to soft gamma-rays. In this way, GRBs can be distinguished from other classes of high-energy transients on the spot. Then, the IRT will utilize the satellite’s fast slewing capability (several degrees per minute) to automatically identify the source’s infrared counterparts. This will refine localization of events down to approximately arcsecond accuracy, determine redshift, and give a spectroscopic characterization of both the counterpart and its host galaxy. Then the data will be rapidly transmitted to ground and promptly processed by the mission ground segment before being distributed to the community. This is THESEUS’ most valuable offering: an automated system of detection, slewing, infrared verification, and prompt reporting that will enable fast and concerted follow-up campaigns with other ground and space observatories.

“In the last fifteen years only six gamma ray bursts have been detected at redshift higher than six, and only two at redshift higher than eight,” said Amati. “That is all, even considering a worldwide effort. With THESEUS we expect to detect and identify more or less 40–50 GRBs at redshift six in four years. So, if the mission lasts as expected, for five or ten years, it will measure more than one hundred of these gamma ray bursts. This will be a substantial step forward in the field.”

This is THESEUS’ most valuable offering: an automated system of detection, slewing, infrared verification, and prompt reporting that will enable fast and concerted follow-up campaigns with other ground and space observatories.

This high number of high-redshift GRBs will allow THESEUS to achieve many of its primary scientific goals, including to undertake a complete census of the GRB population in the first billion years and to perform deep monitoring of X-ray transients. These observations will allow for better gravitational wave detections, the study of primordial star populations, investigations of the re-ionization epoch, a better understanding of the composition and distribution of material in the very early universe, and much more. But beyond the discoveries and insights THESEUS hopes to make on its own, the consortium has also argued a case for the project in terms of its collaborative potential.

“The unique science capability of THESEUS in this respect will be mostly on the location accuracy of short gamma-ray bursts,” Amati said. “Gravitational wave detectors produce very large error regions in the sky; the localization of wave signal is limited to an order of several tens or hundreds of square degrees in the sky. This is a huge uncertainty if you want to actually pinpoint the host galaxy or the source of gravitational waves. So, the way through is to capture and define localized electromagnetic counterparts, mostly short gamma ray bursts.”

The quest is on for more frequent and better characterized multi-messenger event observations, like the now legendary GW 170817 that Fermi, INTEGRAL, LIGO, and many others detected in 2017. By the 2030s, the sky will be routinely monitored by a network of second- and third-generation gravitational wave and neutrino detectors: aLIGO (USA), aVirgo (Italy), IndIGO (India), KAGRA (Japan), the Einstein Telescope, and Cosmic Explorer. They will provide a significant increase in sensitivity, but will still suffer from relatively narrow sky localization capabilities mainly based on triangulation methods. THESEUS sees itself as uniquely positioned to enable these massive projects, as well as others like the Extremely Large Telescope (ELT) and James Webb Space Telescope (JWST).

“All the major facilities have transients at the core of their science case,” said Paul O’Brien, lead of SXI instrument team. “But what they don't have is something to find the transients. And they need more information than we can gather with Swift. With Swift, we find about 90 GRBs a year. But we don't know the redshifts of any of them, we can't determine it on-board. And we're very limited in the amount of information other than giving a nice lightcurve. So, THESEUS is really about going beyond that; it’s not enough either just to see more. You can’t say to ELT or JWST, ‘Here’s a thousand transients.’ You need something rare and well-characterized and interesting. And finding more objects in general is necessary in order to find more of the rare ones.”

Astronomers have been avidly awaiting the age of multi-messenger astrophysics for decades. This work is more than just the sum of multiwavelength investigations. It is a new discipline which is already providing unique insights into the nature of the universe. These insights arise from the synthesis of information carried by photons, gravitational waves, neutrinos, and cosmic rays as they are ejected from astronomical events, like the merging of two neutron stars. But while this work may well be the future of astronomy, technical and functional inefficiencies currently limit the productivity of the field. Giant ground-based facilities are just part of the equation. Counterparts across the whole spectrum are necessary to build a complete picture of transient cosmic events, including X- and gamma-rays, which are impossible to capture inside of Earth’s atmosphere. This is why agencies all over the world are also making plans for the next generation of high-energy space observatories. And most of them, like ESA’s ATHENA, will be utilizing an exciting new technology to capture X-rays: micro-pore optics (MPO), also called lobster-eye detectors.

This innovation in X-ray imaging has been talked about since the 1980s, but only recently have advancements in materials science made it actually buildable. THESEUS will have four units: 8x8 spherical arrays of micro-channel plates. Each is made up of millions of tiny tubes measuring around 6–10 millimeters in diameter. X-rays are too small to reflect and detect with mirror telescopes, so the lobster eyes catch them by deflecting the particles through the tubes at an angle to the plate.

“Traditional X-ray telescopes have a very narrow field of view and they are very heavy,” said O’Brien. “They might weigh 50–100 kilograms at least. But we can build an MPO that weighs one kilogram because most of the plate is empty space. It’s new, but ESA has been funding the development for over a decade.”

BepiColumbo, an ESA/JAXA mission to Mercury that launched in 2018, had the first mission-ready microchannel optical plate in its instrument suite. SVOM, a Chinese-French collaboration set to launch in 2021, will also utilize a version of this technology. O’Brien’s group at Leicester University was involved with the development of BepiColumbo’s optics, and is also working with both SVOM and ATHENA. By the time THESEUS launches in the 2030s, the technology will have the benefit of another decade of R&D, allowing the team to learn and adapt from the trial and error of first adopters.

The community needs space telescopes with better localization specs, on-board redshift detection, automatic multi-wavelength verification, and fast response times.

“The lobster-eye technology gives us the unique capability of focusing X-rays on a huge field of view,” Amati said. “This is what is missing in the field. There is no other technology for monitoring the sky over a large field of view at this level of sensitivity and location accuracy. Usually when you think of focusing X-ray telescopes, you have a sight of about one degree, but one degree is already too much, becoming thousands of square degrees in the sky. Theseus will cover half of a steradian. This is a giant technological leap and of huge interest to the whole scientific community.”

The search for transients suffers not from a lack of interest, will, or ideas, but technical inefficiencies. The community needs space telescopes with better localization specs, on-board redshift detection, automatic multi-wavelength verification, and fast response times. THESEUS is a proposal designed specifically for this purpose, which is why they already have the support of all the major gravitational wave facilities, current and upcoming.

THESEUS’ annual conference, which was scheduled for May 11 in Malaga, Spain, was instead held online on June 3–4 due to the coronavirus pandemic. The purported reasons for large in-person group meetings (like those held by EnVision in Paris this February, and SPICA’s meeting in March which was ultimately held via Zoom) are to invite interested outsiders for talks, build civic engagement, and provide opportunities for the consortium to work closely. According to Amati, THESEUS already has considerable interest and support from the high-energy community, and the group members are accustomed by now to meeting online.

“The scientific working groups have regular discussions,” Amati said. “The aim of the conference in Malaga was for meeting, and involving outside scientists, and the community, and advertising the mission as much as possible. It was more a social event than a strictly scientific event, so not easily moved online. This is why we rescheduled it for March of next year. Till then the work continues.”

The unfinished fallout of the pandemic has impaired work across the globe and astrophysicists are no exception. While theorists have felt the effects mainly in terms of closed classrooms and offices, most collaborative projects, especially those in the lab, are still caught in the knock-on effects of COVID-19’s massive event horizon. Engineering work on the instrument prototypes for all three finalists has been temporarily slowed or suspended. But the THESEUS team is not worried that this will cause a delay in the ESA’s decision.

“ESA trusts the viability of our technology,” O’Brien said. “The three instruments are based on either flight heritage or known elements. At this stage you shouldn’t be proposing technology if a loss of six months development time is a problem. It (the pandemic) has certainly slowed down things, obviously. But ESA hasn't given any indication that they're going to delay the selection, quite the opposite. In fact, they keep telling us it's the same deadline, get on with it.”

The three finalists, EnVision, SPICA, and THESEUS were chosen in May 2018 for further study because they were judged to be ahead of the pack, in terms of programmatic compliance and technical preparedness. It is likely that the events of this year may push back launches down the line, the M5 mission included. But since the winner isn’t scheduled to fly for at least another decade, ESA decision makers seem to be of the opinion that this “pause” is not significant in terms of the committee’s ability to make a fair selection among the candidates next summer.

“All the three missions have very interesting science cases,” Amati said. “But THESEUS is the only fully European proposal.”

“For those of us who've been involved in many missions it's clear that ESA is adopting a more rigorous approach than in the past,” O’Brien said. “They're treating this pre-selection phase like a post-selection phase. It’s difficult to keep missions on time and on budget. So they're trying to develop a process now where they do as much of the retiring of risk early on as possible. That's not a bad thing. That means when you get to mission selection, the panel can be more certain that what they're looking at really will happen.”

EnVision, SPICA, and THESEUS are all very different missions. They are each the culmination of decades of scientific aspiration and preparation, and in an ideal world all three would launch. Only one will, though. ESA will have both a very difficult and a very easy job. On the one hand, no matter what they decide it will be good for astronomy. On the other, two worthy projects must walk away empty handed.

“All the three missions have very interesting science cases,” Amati said. “But THESEUS is the only fully European proposal and, as such, provides a unique opportunity for exploiting the scientific and technological heritage of the European community, and for the supporting leadership of our scientists, universities and industry.”

THESEUS is a mission that is on the frontier of one the hottest topics in astrophysics and cosmology, multi-messenger astrophysics. It aims to provide a unique synergy to the large facilities of the future. European astronomy is currently putting a lot of effort and money into this field, especially with ATHENA, for which a machine like THESEUS is needed in order to efficiently find and characterize some of the mission's most wanted, rare, and distant objects.

“Many missions are now planned on ground and in space to explore the very early phases of the universe,” Amati said. “They will do very precise and deep investigations to find the first stars and the first galaxies, but will be limited to a small fraction of the sky. With its wide field of view, THESEUS will be the machine which pinpoints and guides the large observatories of the future.”

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