Gaiaʼs first year of scientific observations
Last Friday, 21 August, ESAʼs billion-star surveyor, Gaia, completed its first year of science observations in its main survey mode. After launch on 19 December 2013 and a six-month long in-orbit commissioning period, the satellite started routine scientific operations on 25 July 2014. Located at the Lagrange point L2, 1.5 million km from Earth, Gaia surveys stars and many other astronomical objects as it spins, observing circular swathes of the sky. At the end of 28 days period for the initial calibration of the instruments, on 21 August, Gaia commenced its main survey operation, employing a scanning law designed to achieve the best possible coverage of the whole sky. Since then, the satellite recorded 272 billion positional or astrometric measurements, 54.4 billion brightness or photometric data points, and 5.4 billion spectra.
Last Friday, 21 August, ESAʼs billion-star surveyor, Gaia, completed its first year of science observations in its main survey mode. After launch on 19 December 2013 and a six-month long in-orbit commissioning period, the satellite started routine scientific operations on 25 July 2014. Located at the Lagrange point L2, 1.5 million km from Earth, Gaia surveys stars and many other astronomical objects as it spins, observing circular swathes of the sky. At the end of 28 days period for the initial calibration of the instruments, on 21 August, Gaia commenced its main survey operation, employing a scanning law designed to achieve the best possible coverage of the whole sky. Since then, the satellite recorded 272 billion positional or astrometric measurements, 54.4 billion brightness or photometric data points, and 5.4 billion spectra.
The Gaia team have spent a busy year processing and analysing these data, en route towards the development of Gaiaʼs main scientific products, consisting of enormous public catalogues of the positions, distances, motions and other properties of more than a billion stars. Because of the immense volume of data and their complex nature, this requires a huge effort from expert scientists and software developers distributed across Europe, combined in Gaiaʼs Data Processing and Analysis Consortium (DPAC). The group composed by about forty researchers and engineers from the University of Barcelona (UB), together with the Galician Group for Gaia and a group of experts from the Spanish National University for Distance Education (UNED).
Carme Jordi, researcher in the Institute of Cosmos Sciences of the UB (ICCUB-IEEC), affirms: “This first year has been crucial to test the scientific quality of data and the effectiveness of processing algorithms. Particularly, our team is responsible for initial data treatment and it has surpassed expectations in the management of the great volume of information”.
As one example of the ongoing validation, the Gaia team has been able to measure the parallax for an initial sample of two million stars. Parallax is the apparent motion of a star against a distant background observed over the period of a year and resulting from the Earth's real motion around the Sun; this is also observed by Gaia as it orbits the Sun alongside Earth. But parallax is not the only movement seen by Gaia: the stars are also really moving through space, which is called proper motion.
Only one year of observations is generally not enough to disentangle the parallax and proper motions. To overcome this, the scientists have combined Gaia data with positions extracted from the Tycho-2 catalogue, based on data taken between 1989 and 1993 by Gaia's predecessor, the Hipparcos satellite. This restricts the sample to just two million out of the more than one billion that Gaia has observed so far, but yields some useful early insights into the quality of its data.
Gaia has detected hundreds of transient sources so far, with a supernova being the very first on 30 August 2014. These detections are routinely shared with the community at large as soon as they are spotted in the form of ʻScience Alertsʼ, enabling rapid follow-up observations to be made using ground-based telescopes in order to determine their nature.
One transient source was seen undergoing a sudden and dramatic outburst that increased its brightness by a factor of five. It turned out that Gaia had discovered a so-called ʻcataclysmic variableʼ, a system of two stars in which one, a hot white dwarf, is devouring mass from a normal stellar companion, leading to outbursts of light as the material is swallowed.
Gaia has also discovered a multitude of stars whose brightness undergoes more regular changes over time. The satellite has delivered detailed light curves for dozens of RR Lyrae type variable stars in the Large Magellanic Cloud, and the fine details revealed in them testify to the very high quality of the data.
Another curious object covered during the same mission phase is the Catʼs Eye Nebula, a planetary nebula also known as NGC 6543, which lies close to the north ecliptic pole. Planetary nebulae are formed when the outer layers of an aging low-mass star are ejected and interact with the surrounding interstellar medium, leaving behind a compact white dwarf. Gaia made over 200 observations of the Catʼs Eye Nebula, and registered over 84 000 detections that accurately trace out the intricate gaseous filaments that such objects are famous for. As its observations continue, Gaia will be able to see the expansion of the nebular knots in this and other planetary nebulae.
Closer to home, Gaia has detected a wealth of asteroids, the small rocky bodies that populate our solar system, mainly between the orbits of Mars and Jupiter. Gaia scientists have developed special software to look for these ʻoutliersʼ, matching them with the orbits of known asteroids in order to remove them from the data being used to study stars. But in turn, this information will be used to characterise known asteroids and to discover thousands of new ones.
Finally, in addition to the astrometric and photometric measurements being made by Gaia, it has been collecting spectra for many stars. The basic use of these data is to determine the motions of the stars along the line-of-sight by measuring slight shifts in the positions of absorption lines in their spectra due to the Doppler shift. But in the spectra of some hot stars, the satellite has also seen absorption lines from gas in foreground interstellar material, which will allow the scientists to measure its distribution.
“These examples prove the revolution brought by the Gaia mission to astrophysics. We are looking forward for the publication of first scientific data in the summer of 2016”, emphasizes Edard Masana, researcher in the ICCUB-EEC.
UB participation in the mission
The Gaia UB team is composed by researchers from the Institute of Cosmos Sciences of the University of Barcelona (ICCUB), the Institute of Space Studies of Catalonia (IEEC) and the Department of Astronomy and Meteorology of the UB. The team has been involved in the Gaia mission since the very early phases. It has played a major role in the scientific and technological design of the instrumentation, database prototypes and data simulation. It has also developed a calibration algorithm of photometric data, and the system that will enable to daily process satelliteʼs data and store them in a database to later extract the first scientific results.
Furthermore, the group is developing tools for scientific exploitation, by means of data got from the Earth in order to complement those provided by Gaia. The Data Processing Center of Barcelona, which includes CESCA and the Barcelona Supercomputer Center, provides resources to carry out some operations throughout the mission and has been a necessary tool to carry out simulations in order to test the instrument.
For further information, please click on ESAʼs press release.