What Can Joint Survey Processing (JSP) do for Time-Domain Studies? Ranga Ram Chary (Caltech/IPAC) for the JSP Team rchary@caltech.edu; Feb 2022 Abstract Joint Survey Processing (JSP) is the pixel level combination of ground- and space-based datasets such as Euclid (2023+), Rubin (2023+) and Roman (2027+). It is particularly important for mitigating source confusion which affects 50% of the sources in deep seeing-limited datasets such as those that will be taken by Rubin/LSST. By using location and morphology priors on static sources derived from space-datasets such as Euclid and Roman, and estimating accurate time- and location-dependent point spread functions for the ground- based data, one can model the scene in individual frames forward. The residuals after subtracting the modeled scene from the data, reveal transient events down to the shot noise limit of the individual images or in time- integrated coadds, even in crowded fields. This significantly improves the detectability of transient events, both in number and in faintness. JSP is therefore highly relevant for 1. detection of microlensing events, 2. moving objects like nearby stars and asteroids, 3. supernovae, and 4. tidal disruption events close to the high surface brightness centers of galaxies. In addition, by providing accurate multi-band photometry on galaxies one can derive accurate photometric redshifts and galaxy physical parameters which can be used to select host galaxies and transient electromagnetic counterparts for multi-messenger studies such as those where the positional error circles can be large (e.g. LIGO gravitational wave events, ultra high-energy cosmic rays or fast radio bursts). Figure 1: Map of the entire sky (from J-C. Cuillandre) in celestial coordinates showing the Euclid coverage (15000 sq-deg in yellow), and the Rubin coverage (20000 sq-deg) as the colored rectangles. There are also 3 deep fields totaling 40 sq-deg – two in the South and one in the North. Opportunities for time domain science come from the 7500 sq- deg overlap area which will have deep multi-wavelength optical and NIR data with sub- arcsecond resolution, and the 30 sq-deg high latitude deep fields in the South. In 2027+, the Roman microlensing survey towards the Galactic bulge, a potential Roman Galactic Plane survey and 15 sq-deg SN survey will also provide new opportunities to search for variable events. Figure 2: 5-year sensitivity of the Rubin/LSST ugrizy survey (blue) in comparison with the Euclid deep field sensitivities (green). Potentially the LSST deep drilling fields will be another 2.5 mags deeper than that shown while the Euclid wide field will be 2 mags shallower. The Euclid deep fields will have about 20 visits while the LSST area will have about ~100 visits. Figure 3: The blended fraction of sources in seeing-limited data (Rubin) as a function of apparent magnitude, in extragalactic fields. In the deep fields, more than half the sources are blended with other nearby sources. Combined with variable Rubin seeing, measuring variability in these faint sources is challenging. Forward modeling the scene using the high resolution space data (from Euclid or Roman), enables the detection of transients down to the shot noise limit of the surveys. Figure 4: An illustration of joint survey processing on Hubble/ACS and Subaru/HSC data from Faisst et al. (2022). By using priors on the position and morphologies of sources derived from the high resolution ACS data and fitting for these known sources in the HSC/Subaru data, faint transients which are on top of host galaxies (e.g. SNe or nuclear AGN) can be detected. This method also can help reveal moving objects like stars or asteroids if the time baseline is large enough. Summary The detection of variable sources at the sensitivity limit of upcoming, confusion-limited ground-based surveys requires forward modeling of the scene by using priors from the high resolution space-platform data. This requires joint pixel level pr