- The challenges of observing unpredictable cosmic events
- How the new SOXS instrument will react quickly to them
A star swallowed by a black hole, an exploding supernova… There are events in the Universe that have a very short expiry date. Such transient events must be observed quickly, sometimes within just hours or minutes, before they change or simply end. SOXS, a spectrograph soon to start observing on ESO’s New Technology Telescope at La Silla Observatory, is on a mission to capture the most unpredictable and short-lived events of the Universe, from its place on a legendary telescope.
It’s a son! How SOXS was born
Astrophysicists Sergio Campana and Paolo D’Avanzo of the Osservatorio Astronomico di Brera, INAF (Italian National Institute for Astrophysics), in Italy, had long been interested in transients. But they faced a major logistic challenge: it wasn’t always easy to get observing time in a suitable instrument as quickly as these fleeting events require.
The solution was clear. Together with Pietro Schipani, Director of INAF at the Osservatorio Astronomico di Capodimonte, in Italy, they gathered a team to design and build a state-of-the-art instrument that would be solely dedicated to the study of transients: the SOXS spectrograph (pronounced ‘socks’).
SOXS, meaning “Son of X-Shooter”, was built on the concepts and technology of its “father”, the X-shooter spectrograph mounted on ESO’s Very Large Telescope. Much like a prism, a spectrograph splits incoming light into its component colours or wavelengths. The resulting light spectrum may show bright or dark lines that can be used to unveil many properties of an object, such as its chemical composition or how far it is.
Unlike other spectrographs, X-shooter and SOXS can observe both visible and infrared light in one shot. This means astronomers don’t have to coordinate simultaneous observations with different telescopes, something hard to do when observing transient events.
While SOXS borrows some concepts from X-shooter, it’s not a mere copy, it was conceived as an entirely new instrument. “There is not a single screw that is identical between the two spectrographs,” Schipani points out.
Spectrographs are commonplace instruments at ESO’s telescopes, but there has never been one solely dedicated to transients. As astronomical surveys — giant maps of the sky — are becoming better and more common, more of these fleeting events are being discovered, making SOXS essential to follow them up as quickly as possible.
Blink and you miss it
Not all transient events are equal, though. Some events are recurring: we can predict when and where they will appear. If we miss them, though, we may have to wait years, decades or even hundreds of years to see them again. Think of a comet like Halley, paying us a visit every 75 years, or an exoplanet transiting in front of its host star every once in a while.
Other events are roughly predictable, but not with the same precision. Remember T Coronae Borealis? This is a binary star where one star is stealing material from its companion, bursting roughly every 80 years. Newspapers around the world announced an explosion would occur by the end of 2023 after astronomers saw that the double star system was dimming down, suggesting an explosion was imminent. One year later and the system was still dimming down. Sometimes the Universe is capricious.
There are also events we simply cannot predict. “We are speaking about highly variable objects, or unpredictable ones. Objects becoming suddenly super bright and nobody knows when and where these will happen,” says D’Avanzo. These include gamma-ray bursts — extreme explosions of high-energy radiation — or gravitational waves –– ripples in space-time caused by merging neutron stars or black holes. Some exotic phenomena, like fast blue optical transients, of which we only know a handful, have no clear explanation yet.
Always shooting at the sky: a novel approach
To capture transient events when most needed, scientists require a good instrument in a telescope with an extremely flexible schedule. But in telescopes like ESO’s Very Large Telescope, observation time is booked long in advance. A telescope may be scheduled to do a very precise observation of an object, but what happens when a supernova suddenly explodes elsewhere in the sky?
In those cases, telescopes have a procedure called “target of opportunity”: time that can be used on the fly to switch from the scheduled observations to a transient event, if one were to occur. These “targets of opportunity” are also booked in advance, meaning that even though astronomers won’t know from the get go when or where an event may happen, they still need to get their observations pre-approved. When the observations are finally triggered the telescope can leave all of its other duties on hold momentarily, sometimes with very short notice, but this cannot be done all the time.
SOXS works the opposite way. It’s always prepared for any transient event that may be happening in the sky, ready to point to a target with very little previous notice. “The fact that we can point the telescope to the transient that we discover in only half an hour, makes SOXS different,” Campana explains.
To fully understand transients it’s not enough to observe them immediately after they’ve been discovered. We must also follow them up over several days or weeks to study how they evolve, and again this isn’t always possible with current instruments. “Sometimes, when you have an exciting target, it is rather easy to override [ongoing] observations. But then, it can become difficult to keep following the same target,” explains D’Avanzo.
With a long waiting queue, other instruments may have to stop following a target rapidly. SOXS, on the contrary, will be following up these events. This mission is especially important because transient events can evolve in a matter of hours and days, and if not followed up, useful scientific information may be lost.
Working with SOXS
SOXS also owes much of its flexibility to its home: the New Technology Telescope (NTT) at ESO’s La Silla Observatory, in Chile. Built in 1989, the NTT has a place in the heart of many astronomers for breaking new ground with active optics, a technology that was a tipping point in the quality of astronomical images. “For me, it is an honour and it is also magical to go to La Silla and work on this telescope, because I always read articles about the NTT and active optics,” mentions Schipani.
Over the decades, the NTT has hosted various instruments with different capabilities, and SOXS will be a unique addition. “The idea of ESO was good because they decided to start a new life for the NTT [...] This is what happens when an organisation wants to keep the telescopes working at their best,” Schipani says.
Scientific proposals to observe objects will be added into a calendar that will automatically adapt every day in response to what is happening in the sky. And even then, the schedule can be easily disrupted if an extremely short-lived event, like a gamma-ray burst, occurs. “No time is lost, no target is lost. This is the main, novel approach. To my knowledge, it has never been applied to any ground telescope,” D’Avanzo concludes.
Preparing for the unexpected
After 11 years of hard work, SOXS is getting ready to start monitoring the sky thanks to the enthusiasm of everyone involved in its design and implementation. “It was not just good work, it was really fun to work with [the team]. I think we have a very good team made by real people working together,” says Campana. One July 1 we will host a SOXS Day at ESO's Headquarters in Garghing, Germany, with online participation available, to inform the astronomical community about the instrument and its capabilities. Stay tuned for more news soon!
Now, SOXS and the NTT are prepared for any potential discoveries. “We might have the possibility, if we are really quick, to observe the first light coming from a supernova, in what is called a shock breakout,” mentions Campana, referring to the very first flash that occurs when the shock of the explosion reaches the surface of the star. SOXS will also study stars destroyed by black holes, the sources of radio bursts that last less than a millisecond, and also closeby objects like asteroids or comets. Quite a list of phenomena to study, for just one telescope. Or as Schipani puts it: “The problem is that X-shooter will have only one son. If SOXS would have had many siblings, it would be much better for us.”
Biography Alejandro Izquierdo López
Alejandro is an evolutionary biologist from Spain who has been researching the ancestors of shrimps, centipedes and insects, trying to understand how evolution worked 500 million years ago. He has discovered several strange-looking extinct animals such as the “Pac-man crab” Pakucaris or the “Cambrian-beagle” Balhuticaris. He also loves communicating any type of science, including (of course), astronomy. At ESO, he aims to strengthen his communication skills while re-igniting his childhood passion for the cosmos.