Abstract
Context. Since July 2014, the Gaia mission has been engaged in a high-spatial-resolution, time-resolved, precise, accurate astrometric, and photometric survey of the entire sky. Aims. We present the Gaia Science Alerts project, which has been in operation since 1 June 2016. We describe the system which has been developed to enable the discovery and publication of transient photometric events as seen by Gaia. Methods. We outline the data handling, timings, and performances, and we describe the transient detection algorithms and filtering procedures needed to manage the high false alarm rate. We identify two classes of events: (1) sources which are new to Gaia and (2) Gaia sources which have undergone a significant brightening or fading. Validation of the Gaia transit astrometry and photometry was performed, followed by testing of the source environment to minimise contamination from Solar System objects, bright stars, and fainter near-neighbours. Results. We show that the Gaia Science Alerts project suffers from very low contamination, that is there are very few false-positives. We find that the external completeness for supernovae, CE = 0.46, is dominated by the Gaia scanning law and the requirement of detections from both fields-of-view. Where we have two or more scans the internal completeness is CI = 0.79 at 3 arcsec or larger from the centres of galaxies, but it drops closer in, especially within 1 arcsec. Conclusions. The per-transit photometry for Gaia transients is precise to 1% at G = 13, and 3% at G = 19. The per-transit astrometry is accurate to 55 mas when compared to Gaia DR2. The Gaia Science Alerts project is one of the most homogeneous and productive transient surveys in operation, and it is the only survey which covers the whole sky at high spatial resolution (subarcsecond), including the Galactic plane and bulge.
Highlights
On 19 December 2013, the European Space Agency (ESA) launched its Gaia satellite, which was the start of an ambitious project to measure the parallaxes of a billion stars in Classification tables are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc. u-strasbg.fr/viz-bin/cat/J/A+A/652/A76.the Milky Way
– The list of young stellar objects (YSOs) described in Sect. 2.7.7; – To aid the rejection of spurious transients arising from contamination by Solar System objects, we display data on nearby planets, their satellites, and minor planets
In the early phases of Gaia science alerts (GSA) we used SkyBot (Berthier et al 2006), but we exploit ephemerides shared within DPAC; – Results of positional cross-match against our own tables of transient events, assembled from the hourly parsing of a significant collection of other publicly available transient surveys
Summary
On 19 December 2013, the European Space Agency (ESA) launched its Gaia satellite, which was the start of an ambitious project to measure the parallaxes of a billion stars in Classification tables are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc. The Milky Way. Gaia started scientific operations in July 2014 and completed the 5-yr nominal mission on 16 July 2019, but the spacecraft is in good health and the data collection and processing is still ongoing as an extended mission phase. The final data release of the nominal mission is still to come (DR4, the extended mission will be released as DR5), the survey has already had a transformational impact on a broad range of fields, including white dwarfs (Gentile Fusillo et al 2019), hypervelocity stars (Boubert et al 2018), cosmological.
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