The XXL Survey: I. Scientific motivations - XMM-Newton observing plan - Follow-up observations and simulation programme

Abstract: We present the XXL Survey, the largest XMM programme totaling some 6.9 Ms to date and involving an international consortium of roughly 100 members. The XXL Survey covers two extragalactic areas of 25 deg2 each at a point-source sensitivity of ~ 5E-15 erg/sec/cm2 in the [0.5-2] keV band (completeness limit). The survey's main goals are to provide constraints on the dark energy equation of state from the space-time distribution of clusters of galaxies and to serve as a pathfinder for future, wide-area X-ray missions. We review science objectives, including cluster studies, AGN evolution, and large-scale structure, that are being conducted with the support of approximately 30 follow-up programmes. We describe the 542 XMM observations along with the associated multi-lambda and numerical simulation programmes. We give a detailed account of the X-ray processing steps and describe innovative tools being developed for the cosmological analysis. The paper provides a thorough evaluation of the X-ray data, including quality controls, photon statistics, exposure and background maps, and sky coverage. Source catalogue construction and multi-lambda associations are briefly described. This material will be the basis for the calculation of the cluster and AGN selection functions, critical elements of the cosmological and science analyses. The XXL multi-lambda data set will have a unique lasting legacy value for cosmological and extragalactic studies and will serve as a calibration resource for future dark energy studies with clusters and other X-ray selected sources. With the present article, we release the XMM XXL photon and smoothed images along with the corresponding exposure maps. The XMM XXL observation list (Table B.1) is available in electronic form at the CDS. The present paper is the first in a series reporting results of the XXL-XMM survey.

Overview of the XXL Survey Paper

The XXL Survey paper elaborates on a significant astronomical endeavor, aimed at exploring the large-scale structure of the universe via X-ray observations. Fundamentally grounded on the contributions from the XMM-Newton observatory, this survey represents the culmination of a comprehensive research initiative involving an extensive international consortium.

Scientific Objectives and Observing Plan

The XXL Survey serves as the largest XMM programme to date, encompassing two extragalactic fields with a combined area of 50 square degrees. The survey's design is motivated by the need for detailed cosmological parameter constraints, primarily focusing on the equation of state of dark energy, a pivotal aspect of current cosmological models. By mapping out the space-time distribution of galaxy clusters and AGNs, the survey seeks to provide insights into dark energy’s effects on cosmic expansion.

Strategically, the observational plan deploys a point-source sensitivity target of approximately $5 \times 10^{-15}$ erg s$^{-1}$ cm$^{-2}$ in the [0.5-2] keV band, aiming to detect hundreds of galaxy clusters and tens of thousands of AGNs. This ambition is complemented by a robust follow-up and simulation program to enhance the depth of analysis and interpretation.

Data Processing and Numerical Simulations

The paper expounds on the instrumental methodologies, including detailed X-ray processing steps, quality control measures, and innovative tools for cosmological analysis. The survey includes 542 XMM observations and integrates significant multi-wavelength data and numerical simulations. The simulations are crucial for evaluating the selection functions of galaxy clusters and AGNs, facilitating their role in the cosmological analyses.

The utility of hydrodynamical and N-body simulations is underscored, aiding comprehensive evaluations of the observational strategy's efficacy and predicted yield. These simulations accommodate experimental approaches in managing extended sources, shedding light on the survey’s accuracy and operational integrity.

Empirical Contributions and Legacy Value

One of the survey's distinguishing merits is its empirical groundwork, essential for calibrating future wide-area X-ray missions. The survey’s database will furnish critical observational validation against theoretical models, serving as a terminus for subsequent DE investigations with future missions such as DES, Euclid, and eRosita.

Practical and Theoretical Implications

The implications of the XXL Survey extend far beyond its immediate findings. Practically, it provides a reference framework for understanding the intricacies involved in large-scale X-ray surveys. Theoretically, the survey underscores the nexus between observational cosmology and the broader astrophysical context, facilitating advancements in our comprehension of the universe’s structure.

Speculations on Future Developments

In the realm of AI and data analysis, the XXL dataset presents opportunities for algorithmic advancements. Given the complexity and volume of astronomical data, AI-driven solutions could optimize the analysis pipeline, enhance anomaly detection, and enable more agile pattern recognition within the cosmic structure data.

In conclusion, the XXL Survey is pivotal in the ongoing cosmological discourse, offering robust empirical data and methodological insights that advance the understanding of the universe's large-scale architecture.