\section{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 \citet{Jordi:2010} for details of the photometry and lowres spectra).


\begin{center}
\begin{tabular}{c|c|c|c}  
Patient &  Gaia & Catalina Sky Survey  &  Palomar Transient Factory\\ 
\hline
 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\\
\end{tabular}
\caption{\label{tab:transient_machine} Predicted numbers for the Gaia transient search compared to some ongoing surveys.}
\end{center}



