This powerful event, now named Gaia14aaa, took place in a distant galaxy some 500 million light-years away, and was revealed via a sudden rise in the galaxy’s brightness between two Gaia observations separated by 1 month.
Gaia, which began its scientific work on 25 July, repeatedly scans the entire sky, so that each of the roughly billion stars in the final catalogue will be examined an average of 70 times over the next 5 years.
This kind of repeated survey comes in handy for studying the changeable nature of the sky,” comments Simon Hodgkin from the Institute of Astronomy in Cambridge, UK.
Many astronomical sources are variable: some exhibit a regular pattern, with a periodically rising and declining brightness, while others may undergo sudden and dramatic changes.
As Gaia goes back to each patch of the sky over and over, we have a chance to spot thousands of ‘guest stars’ on the celestial tapestry,
notes Dr Hodgkin.
These transient sources can be signposts to some of the most powerful phenomena in the Universe, like this supernova.
Dr Hodgkin is part of Gaia’s Science Alert Team, which includes astronomers from the Universities of Cambridge and Warsaw, who are combing through the scans in search of unexpected changes.
It did not take long until they found the first ‘anomaly’ in the form of a sudden spike in the light coming from a distant galaxy, detected on 30 August. The same galaxy appeared much dimmer when Gaia first looked at it just a month before.
We immediately thought it might be a supernova, but needed more clues to back up our claim,
explains Łukasz Wyrzykowski from the Warsaw University Astronomical Observatory, Poland.
Other powerful cosmic events may resemble a supernova in a distant galaxy, such as outbursts caused by the mass-devouring supermassive black hole at the galaxy centre.
However, in Gaia14aaa, the position of the bright spot of light was slightly offset from the galaxy’s core, suggesting that it was unlikely to be related to a central black hole.
So, the astronomers looked for more information in the light of this new source. Besides recording the position and brightness of stars and galaxies, Gaia also splits their light to create a spectrum. In fact, Gaia uses two prisms spanning red and blue wavelength regions to produce a low-resolution spectrum that allows astronomers to seek signatures of the various chemical elements present in the source of that light.
In the spectrum of this source, we could already see the presence of iron and other elements that are known to be found in supernovas,
says Nadejda Blagorodnova, a PhD student at the Institute of Astronomy in Cambridge.
In addition, the blue part of the spectrum appears significantly brighter than the red part, as expected in a supernova. And not just any supernova: the astronomers already suspected it might be a ‘Type Ia’ supernova – the explosion of a white dwarf locked in a binary system with a companion star.
While other types of supernovas are the explosive demises of massive stars, several times more massive than the Sun, Type Ia supernovas are the end product of their less massive counterparts.
Low-mass stars, with masses similar to the Sun’s, end their lives gently, puffing up their outer layers and leaving behind a compact white dwarf. Their high density means that white dwarfs can exert an intense gravitational pull on a nearby companion star, accreting mass from it until the white dwarf reaches a critical mass that then sparks a violent explosion.
To confirm the nature of this supernova, the astronomers complemented the Gaia data with more observations from the ground, using the Isaac Newton Telescope (INT) and the robotic Liverpool Telescope on La Palma, in the Canary Islands, Spain.
A high-resolution spectrum, obtained on 3 September with the INT, confirmed not only that the explosion corresponds to a Type Ia supernova, but also provided an estimate of its distance. This proved that the supernova happened in the galaxy where it was observed.
This is the first supernova in what we expect to be a long series of discoveries with Gaia,
says Timo Prusti, ESA’s Gaia Project Scientist.
Supernovas are rare events: only a couple of these explosions happen every century in a typical galaxy. But they are not so rare over the whole sky, if we take into account the hundreds of billions of galaxies that populate the Universe.
Astronomers in the Science Alert Team are currently getting acquainted with the data, testing and optimising their detection software. In a few months, they expect Gaia to discover about three new supernovas every day.
In addition to supernovas, Gaia will discover thousands of transient sources of other kinds – stellar explosions on smaller scale than supernovas, flares from young stars coming to life, outbursts caused by black holes that disrupt and devour a nearby star, and possibly some entirely new phenomena never seen before.
The UK and Gaia
The UK has two major roles in the Gaia mission: building the spacecraft, and delivering the science.
UK industry and science institutes won some €80 million of industrial contracts to build Gaia, with leadership roles in building the heart of Gaia, the array of 106 CCDs, the control avionics and the critical micro-propulsion system, as well as playing a critical role in the development of the Gaia spectrometer.
Airbus Defence and Space at Stevenage was responsible for the spacecraft’s super precision guidance and control system as well as the powerful on-board computers needed to process the torrent of data it will produce.
The ‘eye’ of Gaia’s camera has the most sensitive set of light detectors ever assembled for a space mission. It is also the largest focal plane array ever to be flown in space and contains a mosaic of 106 large area, high performance Charged Coupled Device (CCD) CCD91-72 image sensors, which are custom designed, manufactured and tested by UK company e2v. Without these image sensors the Gaia mission would not be possible. These detectors were calibrated with the Gaia electronics at Mullard Space Science Laboratory.
SciSys UK Ltd is responsible for the spacecraft’s operational simulator.
Gaia data will be processed and analysed ready for release to the scientific community and public at six data centres, including one in the UK, operating software developed and tested by a consortium of 400 people across Europe, including some 50 people at 6 Institutes in the UK (Cambridge, UCL-MSSL, Leicester, Edinburgh, The Open University, STFC RAL Space and Bristol).
University College London’s Mullard Space Science Laboratory (MSSL) has a major role in spectroscopic science, and in conjunction with The Open University, is involved in software development including architecture, integration and validation, pre-processing development, spectra extraction and calibration.
The UK hosts the Cambridge data processing centre, where the Gaia imaging data are processed. These data provide brightness, colour, and position information, which enable the core Gaia science, determining distances motions and the intrinsic properties of each star. Gaia also has a spectrograph, to measure the radial velocities of 300 million stars, delivering their complete 3-D positions and 3-D velocities. The spectra will be processed at the French data centre, using software in part developed in the UK.