The Swift X-ray Telescope Cluster Survey

II. X-ray spectral analysis ^⋆,⋆⋆

¹ INAF, Osservatorio Astrofisico di Firenze, Largo Enrico Fermi 5, 50125 Firenze, Italy
e-mail: ptozzi@arcetri.astro.it
² INAF, Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy
³ USTC, No. 96, JinZhai Road Baohe District, Hefei, 230026 Anhui, PR China
⁴ Università degli Studi di Ferrara, Dipartimento di Fisica e Scienze della Terra, via Saragat 1, 44121 Ferrara, Italy
⁵ INAF, Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 34143 Trieste, Italy
⁶ Dipartimento di Fisica dell’Università di Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italy
⁷ INFN-National Institute for Nuclear Physics, via Valerio 2, 34127 Trieste, Italy
⁸ INAF, Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate ( LC), Italy

Received: 9 October 2013
Accepted: 16 May 2014

Abstract

Aims. We present a spectral analysis of a new, flux-limited sample of 72 X-ray selected clusters of galaxies identified with the X-ray Telescope (XRT) on board the Swift satellite down to a flux limit of ~10^-14 erg s^-1 cm^-2 (SWXCS). We carry out a detailed X-ray spectral analysis with the twofold aim of measuring redshifts and characterizing the properties of the intracluster medium (ICM) for the majority of the SWXCS sources.

Methods. Optical counterparts and spectroscopic or photometric redshifts for some of the sources are obtained with a cross-correlation with the NASA/IPAC Extragalactic Database. Additional photometric redshifts are computed with a dedicated follow-up program with the Telescopio Nazionale Galileo and a cross-correlation with the SDSS. In addition, we also blindly search for the Hydrogen-like and He-like iron K_α emission line complex in the X-ray spectrum. We detect the iron emission lines in 35% of the sample, and hence obtain a robust measure of the X-ray redshift z_X with typical rms error 1–5%. We use z_X whenever the optical redshift is not available. Finally, for all the sources with measured redshift, background-subtracted spectra are fitted with a single-temperature mekal model to measure global temperature, X-ray luminosity and iron abundance of the ICM. We perform extensive spectral simulations to accounts for fitting bias, and to assess the robustness of our results. We derive a criterion to select reliable best-fit models and an empirical formula to account for fitting bias. The bias-corrected values are then used to investigate the scaling properties of the X-ray observables.

Results. Overall, we are able to characterize the ICM of 46 sources with redshifts (64% of the sample). The sample is mostly constituted by clusters with temperatures between 3 and 10 keV, plus 14 low-mass clusters and groups with temperatures below 3 keV. The redshift distribution peaks around z ~ 0.25 and extends up to z ~ 1, with 60% of the sample at 0.1 < z < 0.4. We derive the luminosity−temperature relation for these 46 sources, finding good agreement with previous studies.

Conclusions. Thanks to the good X-ray spectral quality and the low background of Swift/XRT, we are able to measure ICM temperatures and X-ray luminosities for the 46 sources with redshifts. Once redshifts are available for the remaining 26 sources, this sample will constitute a well-characterized, flux-limited catalog of clusters distributed over a broad redshift range (0.1 ≤ z ≤ 1.0) providing a statistically complete view of the cluster population with a selection function that allows a proper treatment of any measurement bias. The quality of the SWXCS sample is comparable to other samples available in the literature and obtained with much larger X-ray telescopes. Our results have interesting implications for the design of future X-ray survey telescopes, characterized by good-quality PSF over the entire field of view and low background.