Gaia is a cornerstone mission in the science programme of the European Space Agency (ESA). The spacecraft construction was approved in 2006, following a study in which the original interferometric concept was changed to a direct-imaging approach. Both the spacecraft and the payload were built by European industry. The involvement of the scientific community focusses on data processing for which the international Gaia Data Processing and Analysis Consortium (DPAC) was selected in 2007. Gaia was launched on 19 December 2013 and arrived at its operating point, the second Lagrange point of the Sun-Earth-Moon system, a few weeks later. The commissioning of the spacecraft and payload was completed on 19 July 2014. The nominal five-year mission started with four weeks of special, ecliptic-pole scanning and subsequently transferred into full-sky scanning mode. We recall the scientific goals of Gaia and give a description of the as-built spacecraft that is currently (mid-2016) being operated to achieve these goals. We pay special attention to the payload module, the performance of which is closely related to the scientific performance of the mission. We provide a summary of the commissioning activities and findings, followed by a description of the routine operational mode. We summarise scientific performance estimates on the basis of in-orbit operations. Several intermediate Gaia data releases are planned and the data can be retrieved from the Gaia Archive, which is available through the Gaia home page.

Abstract. The European Photon Imaging Camera (EPIC) consortium has provided the focal plane instruments for the three X-ray mirror systems on XMM-Newton. Two cameras with a reflecting grating spectrometer in the optical path are equipped with MOS type CCDs as focal plane detectors (Turner 2001), the telescope with the full photon flux operates the novel pn-CCD as an imaging X-ray spectrometer. The pn-CCD camera system was developed under the leadership of the Max-Planck-Institut für extraterrestrische Physik (MPE), Garching. The concept of the pn-CCD is described as well as the different operational modes of the camera system. The electrical, mechanical and thermal design of the focal plane and camera is briefly treated. The in-orbit performance is described in terms of energy resolution, quantum efficiency, time resolution, long term stability and charged particle background. Special emphasis is given to the radiation hardening of the devices and the measured and expected degradation due to radiation damage of ionizing particles in the first 9 months of in orbit operation.Key words. XMM-Newton -back illuminated pn-CCDs -radiation hardness -energy resolution -quantum efficiency -particle and flourescence background

In the past two decades, high amplitude electromagnetic outbursts have been detected from dormant galaxies and often attributed to the tidal disruption of a star by the central black hole 1,2 . X-ray emission from the Seyfert 2 galaxy GSN 069 (2MASX J01190869-3411305) at redshift z = 0.018 was first detected in 2010 July and implies an X-ray brightening of more than a factor of 240 over ROSAT observations performed 16 years earlier 3,4 . The emission has smoothly decayed over time since 2010, possibly indicating a long-lived tidal disruption event 5 . The X-ray spectrum is ultra-soft and can be described by accretion disc emission with luminosity proportional to the fourth power of the disc temperature during long-term evolution. Here we report observations of X-ray quasi-periodic eruptions from the nucleus of GSN 069 over the course of 54 days, 2018 December onwards. During these eruptions, the X-ray count rate increases by up to two orders of magnitude with event duration of just over 1 hour and recurrence time of about 9 hours. These eruptions are associated with fast spectral transitions between a cold and a warm phase in the accretion flow around a low-mass black hole (of approximately 4 × 10 5 solar masses) with peak Xray luminosity of ~ 5 × 10 42 ergs per second. The warm phase has a temperature of about 120 electronvolts, reminiscent of the typical soft X-ray excess, an almost universal thermal-like feature in the X-ray spectra of luminous active nuclei 6,7,8 . If the observed properties are not unique to GSN 069, and assuming standard scaling of timescales with black hole mass and accretion properties, typical active galactic nuclei with more massive black holes can be expected to exhibit high-amplitude optical to X-ray variability on timescales as short as months or years 9 .Since 2018 December 24, GSN 069 has exhibited peculiar high amplitude, short timescale X-ray variability, first detected during an XMM-Newton observation (XMM3, see Extended Data Table 1).The XMM-Newton light curve is characterized by two bright flares (bursts) with count rate increases by factors of ~ 22 and ~ 31 respectively in the 0.4-2 keV band ( Figure 1a). The bursts are separated by ~ 29.8 ks, their profile is close to symmetric with similar rise and decay times of ~ 1.8 ks, and the total event duration is ~ 4.5 ks. This unexpected X-ray variability prompted us to request a longer XMM-Newton Director Discretionary Time (DDT) observation performed on 2019 January 16/17 (XMM4).Five bursts are detected with varying amplitudes, corresponding to count rate increases by factors of ~ 19 to ~ 28, and longer recurrence time (~ 32.15 ks) than during XMM3 (Figure 1b). Finally, a Chandra DDT observation was performed on 2019 February 14/15 during which three further bursts are detected with count rate variations by factors of ~ 13 to ~ 22, and with recurrence time of ~ 32.7 ks (Figure 1c).We point out that no bursts were observed in a potentially long enough XMM-Newton exposure (83 ks) on 2014 December 5 (XMM2), i.e. 4 years before the X...

Aims. We report on the production of a large area, shallow, sky survey, from XMM-Newton slews. The great collecting area of the mirrors coupled with the high quantum efficiency of the EPIC detectors have made XMM-Newton the most sensitive X-ray observatory flown to date. We use data taken with the EPIC-pn camera during slewing manoeuvres to perform an X-ray survey of the sky. Methods. Data from 218 slews have been subdivided into small images and source searched. This has been done in three distinct energy bands; a soft (0.2-2 keV) band, a hard (2-12 keV) band and a total XMM-Newton band (0.2-12 keV). Detected sources, have been quality controlled to remove artifacts and a catalogue has been drawn from the remaining sources. Results. A "full" catalogue, containing 4710 detections and a "clean" catalogue containing 2692 sources have been produced, from 14% of the sky. In the hard X-ray band (2-12 keV) 257 sources are detected in the clean catalogue to a flux limit of 4 × 10 −12 ergs s −1 cm −2 . The flux limit for the soft (0.2-2 keV) band is 6 × 10 −13 ergs s −1 cm −2 and for the total (0.2-12 keV) band is 1.2 × 10 −12 ergs s −1 cm −2 . The source positions are shown to have an uncertainty of 8 (1σ confidence).

Context. At about 1000 days after the launch of Gaia we present the first Gaia data release, Gaia DR1, consisting of astrometry and photometry for over 1 billion sources brighter than magnitude 20.7. Aims. A summary of Gaia DR1 is presented along with illustrations of the scientific quality of the data, followed by a discussion of the limitations due to the preliminary nature of this release. Methods. The raw data collected by Gaia during the first 14 months of the mission have been processed by the Gaia Data Processing and Analysis Consortium (DPAC) and turned into an astrometric and photometric catalogue. Results. Gaia DR1 consists of three components: a primary astrometric data set which contains the positions, parallaxes, and mean proper motions for about 2 million of the brightest stars in common with the Hipparcos and Tycho-2 catalogues -a realisation of the Tycho-Gaia Astrometric Solution (TGAS) -and a secondary astrometric data set containing the positions for an additional 1.1 billion sources. The second component is the photometric data set, consisting of mean G-band magnitudes for all sources. The G-band light curves and the characteristics of ∼3000 Cepheid and RR Lyrae stars, observed at high cadence around the south ecliptic pole, form the third component. For the primary astrometric data set the typical uncertainty is about 0.3 mas for the positions and parallaxes, and about 1 mas yr −1 for the proper motions. A systematic component of ∼0.3 mas should be added to the parallax uncertainties. For the subset of ∼94 000 Hipparcos stars in the primary data set, the proper motions are much more precise at about 0.06 mas yr −1 . For the secondary astrometric data set, the typical uncertainty of the positions is ∼10 mas. The median uncertainties on the mean G-band magnitudes range from the mmag level to ∼0.03 mag over the magnitude range 5 to 20.7. Conclusions. Gaia DR1 is an important milestone ahead of the next Gaia data release, which will feature five-parameter astrometry for all sources. Extensive validation shows that Gaia DR1 represents a major advance in the mapping of the heavens and the availability of basic stellar data that underpin observational astrophysics. Nevertheless, the very preliminary nature of this first Gaia data release does lead to a number of important limitations to the data quality which should be carefully considered before drawing conclusions from the data.