Abstract
Aims. The sample of serendipitous sources detected in all Swift-XRT images pointing at gamma ray bursts (GRBs) constitutes the largest existing medium-deep survey of the X-ray sky. To build such dataset we analysed all Swift X-ray images centred on GRBs and observed over a period of 15 years using automatic tools that do not require any expertise in X-ray astronomy. Besides presenting a new large X-ray survey and a complete sample of blazars, this work aims to be a step in the direction of achieving the ultimate goal of the Open Universe Initiative, which is to enable non-expert people to benefit fully from space science data, possibly extending the potential for scientific discovery, which is currently confined within a small number of highly specialised teams, to a much larger population. Methods. We used the Swift_deepsky Docker container encapsulated pipeline to build the largest existing flux-limited and unbiased sample of serendipitous X-ray sources. Swift_deepsky runs on any laptop or desktop computer with a modern operating system. The tool automatically downloads the data and the calibration files from the archives, runs the official Swift analysis software, and produces a number of results including images, the list of detected sources, X-ray fluxes, spectral energy distribution data, and spectral slope estimations. Results. We used our source list to build the LogN-LogS of extra-galactic sources, which perfectly matches that estimated by other satellites. Combining our survey with multi-frequency data, we selected a complete radio-flux-density-limited sample of high energy peaked blazars (HBL). The LogN-LogS built with this data set confirms that previous samples are incomplete below ∼20 mJy.
Highlights
X-ray sky surveys have played a major role in astrophysics ever since the early days of X-ray astronomy (e.g. Giacconi et al 1979)
– The use of the Swift_deepsky pipeline, which does not require any expertise in X-ray astronomy, associated with effective cleaning algorithms, allowed us to build a sample of serendipitous X-ray sources that is sufficiently clean to be used for statistical purposes without any visual or manual intervention
– We have built a deep radio-flux limited sample of 23 high energy peaked blazars (HBL) blazars that, combined with larger samples selected using the RASS survey, is suitable for detailed statistical analyses to be presented in a future paper (Chang et al, in prep.)
Summary
X-ray sky surveys have played a major role in astrophysics ever since the early days of X-ray astronomy (e.g. Giacconi et al 1979). Studies (Rector et al 2000; Wolter & Celotti 2001) based on small X-ray selected samples have shown that BL Lacs, and in particular those of the HBL class, display no or even negative cosmological evolution This peculiar behaviour has been confirmed in radio-flux-limited samples of the most extreme HBLs (Giommi et al 1999). More recently Ajello et al (2014) showed that even in the case of a γ-ray selected sample, low-luminosity HBL BL Lacs show strong negative evolution Despite their extreme rareness, HBL blazars play a crucial role in current and future high- and very-high-energy γ-ray surveys, (The Fermi-LAT collaboration 2020; CTA Consortium 2019). This work demonstrates that complex data analysis projects can in principle be carried out by non-experts, one of the main goals of the United Nations Open Universe initiative
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