A new survey of X-ray clusters of galaxies with the Swift-X-ray Telescope

Clusters of galaxies are the largest virialized structures in the Universe, and despite the complex evolutionary process that shape them, they still keep track of the initial conditions of the primordial density fluctation field. For these reasons, they are at the crossroads of astrophysics and cosmology. A large effort is constantly put by the extragalactic community in studying clusters in greater details and to search for new ones, particularly at high redshifts.

The X-ray energy band (0.5-10 keV) is a privileged observational window to identify and study galaxy clusters. Most of their baryonic content is in the form of a hot, diffuse plasma, the intracluster medium (ICM), at temperatures reaching more than 10 keV (100 million K), and emitting in the X-rays with luminosities ranging from 10⁴³ to 10⁴⁵ erg/s. In the last decade, observations with the Chandra and XMM-Newton satellites, revealed a wealth of complex phenomena going on in the ICM. On the other hand, little progress has been made in the discovery of new clusters. A key aspect is that the major X-ray facilities existing today have a limited field of view (~0.1-0.2 deg²) and are mostly used in deep, targeted observations, which better fit their instrumental properties. Despite this, several cluster surveys based on compilation of serendipitous medium and deep-exposure observations with Chandra and XMM-Newton are currently ongoing, while only a few surveys are assembled with dedicated contiguous observations. As a result, the typical size of a complete and well defined cluster sample amounts at best to ~100 objects.

In this framework, an international collaboration between INAF-OAA, INAF-Brera, INAF-OATs, the University for Science and Technology of China-Hefei, the University of Ferrara, and the University of Massachusetts-Amherst, is undertaking a new cluster survey using an X-ray facility that was never exploited before for this goal: the X-ray Telescope (XRT) onboard the Swift satellite.  XRT is originally designed to follow-up Gammar Ray Burst, but despite its low collecting area (about one fifth of Chandra), XRT has characteristics which are optimal for cluster surveys: a low background, an almost constant angular resolution across the field of view, and a good spectral resolution. The latest release of the Swift-XRT cluster survey (SWXCS) has a sky coverage ranging from total 40 deg² to 1 deg² at a flux limit of about 10^-14 erg /s/cm².  The first catalog, including 72 sources, has been published in Tundo et al. (2012, A&A, 547, 57).

Recently, in the paper "The Swift X-ray Telescope Cluster Survey III: X-ray spectral analysis", Paolo Tozzi (Arcetri) and collaborators presented the X-ray spectral analysis of 46 out of the 72 sources in the SWXCS catalog of Tundo et al. (2012) to measure their temperature, total luminosity, and ICM metal abundance. They find that the sample is mostly constituted by clusters with temperatures between 3 and 10 keV, with a redshift distribution peaked around z~0.25 but extending up to z~1, with 60% of the sample at 0.1<z<0.4. They also measure the rest-frame, soft and bolometric luminosities and then compute the relation between temperature and luminosity, finding good agreement with previous studies.

Thanks to the unique properties of Swift/XRT, namely: the low background, the constant angular resolution across the field of view, and the good spectral response, the quality of the SWXCS sample is comparable to other samples available in the literature and obtained with much larger X-ray telescopes. The final goal is to assemble a sizeable sample with a good characterization, including mass proxies for the majority of the sample, and to use it for a robust determination of the cluster mass function. The results of this program have also interesting implications for the design of future X-ray survey telescopes, which must be characterised by good angular resolution over the entire field of view and low background.

Fig 1: Distribution on the sky (Aitoff projection) of the Gamma Ray Burst fields in the XRT archive as of April 2010. The 336 fields selected in Tundo et al. (2012) and used to build the SWXCS are those outside the Galactic plane (shaded area).	Fig 2: Relation between the ICM temperature and the X-ray luminosity  for the 46 sources with spectral analysis presented in Tozzi et al. (2014). The solid blue line is the best fit L ~ T2.8, while the dashed green line and the black, dot-dashed lines are for other cluster sample obtained from the Chandra archive. Error bars correspond to 1 sigma error both in temperature and luminosity.

The SWXCS team: S. Borgani (INAF Trieste, Universita' Trieste), S. Campana (INAF Brera), M. Giavalisco (UMASS), T. Liu (USTC), A. Moretti (INAF Brera), P. Rosati (Universita' di Ferrara), G. Tagliaferri (INAF Brera), P. Tozzi (INAF Arcetri), E. Tundo (INAF Arcetri), J.X. Wang (USTC).

Edited by P. Tozzi and A. Gallazzi