Photometric Science Alerts From Gaia


Gaia is a European Space Agency (ESA) astrometry space mission, and a successor to the ESA Hipparcos mission. The main goal of the Gaia mission is to collect high-precision astrometric data (i.e. positions, parallaxes, and proper motions) for the brightest one billion objects in the sky. This data, complemented with G band, multi-epoch photometric and low resolution (lowers) spectroscopic data collected from the same observing platform, will allow astronomers to reconstruct the formation history, structure, and evolution of the Galaxy.

In addition, the Gaia satellite is an excellent transient discovery instrument, covering the whole sky (including the Galactic plane) for the next 5 years, at high spatial resolution (50 to 100 mas, similar to the Hubble space telescope (HST)) with precise photometry (1\(\%\) at G=19) and milliarcsecond astrometry (down to \(\sim\)20mag). Thus, Gaia provides a unique opportunity for the discovery of large numbers of transient and anomalous events, e.g. supernovae, black hole binaries and tidal disruption events. We discuss the validation of the alerts stream for the first six months of the Gaia observations, in particular noting how a significant ground based campaign involving photometric and spectroscopic followup of early Gaia alerts is now in place. We discuss the validation approach, and highlight in more detail the specific case of Type Ia supernova (SNe Ia) to be discovered by Gaia. The intense initial ground based validation campaign will ensure that the Gaia alerts stream for the remainder of the Gaia mission, are well classified.

What is a Photometric Science Alert?

A photometric science alert is the appearance of a new source, or a change in flux, which suggests we could learn something from prompt ground-based follow-up. This does not include: periodic variable stars (these sources may be better left to the end of the mission) and moving objects (however, astrometric microlensing would be an exception). The science alerts will be made public, within one to two days of Gaia detection, most of this time is due to downloading the data from the satellite.

Potential Triggers

Potential triggers for the the Gaia science alerts are objects of scientific interest which would benefit from fast ground based follow-up, as just discussed. Some examples of sources which maybe potential triggers include supernovae, super-luminous supernovae, tidal disruption events, cataclysmic variables, outbursts and eclipses from young stellar objects, X-ray binaries, microlensing events and other theoretical or unexpected phenomena. Figure \ref{fig:triggers} shows some of these potential triggers and the area of pars space they occupy for their brightness as a function of duration.

This shows the amplitude and duration of a range of potential triggers for the Gaia science alerts. \label{fig:triggers}

Gaia as a Transient Search Machine

Gaia is comparable to other transient search machines, such as the Catalina Sky Survey and the Palomar Transient Factory, as shown in Table \ref{tab:transient_machine}, which covers similar areas each day and similar limiting magnitude. The disadvantage of the Gaia survey is that the average cadence is only \(\sim\)30days whereas transient surveys usually have a cadence of approximately 3 to 5 days. However, there is also a shorter cadence of 106.5 mins from the two mirrors in the satellite, also sometimes a 253.5 mins cadence, and sometimes 3 or more observations are thus obtained (when close to the 45 degrees ecliptic latitude zones for example). This 106.5 mins cadence is a huge advantage and means that changes in brightness should be detected quickly. Also, Gaia will cover the whole sky (including the Galactic plane), which is a significant survey area increase over other transient searches. The Gaia transient alerts will also have high spatial resolution with precise photometry (1\(\%\) at G=19) and milliarcsecond astrometry (down to \(\sim\)20mag), lowres spectra for all objects brighter than \(\sim\)19mag and colours for fainter objects (see Jordi et al. (2010) for details of the photometry and lowres spectra).

Patient Gaia Catalina Sky Survey Palomar Transient Factory
deg2 day-1 \(\sim\)1230 1500 1000
Avg Cadence \(\sim\)30 days 14 days 5 days
Limiting mag 20 (21?) 19.5 21
fsky all sky 0.6 0.2