The REX survey: a search for Radio Emitting X-ray sources

We present the scientific goals, the strategy and the first results of the REX project, an effort aimed at creating a sizable and statistically complete sample of Radio Emitting X-ray sources (REX) using the available data from a VLA survey and the ROSAT PSPC archive. Through a positional cross-correlation of the two data sets we have derived a sample of about 1600 REX. Among the 393 REX identified so far a high fraction is represented by AGNs, typically radio loud QSOs and BL Lacs. The remaining sources are galaxies, typically radio galaxies isolated or in cluster. Thanks to the low flux limits in the radio and in the X-ray band and the large area of sky covered by the survey, we intend to derive a new complete and unbiased sample of BL Lacs which will contain both ``RBL'' and ``XBL'' type objects. In this way, the apparent dichotomy resulting from the current samples of BL Lacs will be directly analyzed in a unique sample. Moreover, the high number of BL Lacs expected in the REX sample (about 200) will allow an accurate estimate of their statistical properties. To date, we have discovered 15 new BL Lacs and 11 BL Lac candidates with optical properties intermediate between those of a typical elliptical galaxy and those of a typical BL Lac object. These objects could harbour weak sources of non-thermal continuum in their nuclei and, if confirmed, they could represent the faint tail of the BL Lac population. The existence of such ``weak'' BL Lacs is matter of discussion in recent literature and could lead to a re-assessment of the defining criteria of a BL Lac and, consequently, to a revision of their cosmological and statistical properties.Comment: 30 pages, 13 figures, accepted for publication to Ap

Emission Line AGNs from the REX survey: Results from optical spectroscopy

We present 71 Emission Line objects selected from the REX survey. Except for 3 of them, for which the presence of an active nucleus is dubious, all these sources are Active Galactic Nuclei (QSOs, Seyfert galaxies, emission line radiogalaxies). In addition, we present the spectra of other 19 AGNs included in a preliminary version of the REX catalog but not in the final one. The majority (80) of the 90 sources presented in this paper is newly discovered. Finally, we present the general properties in the radio and in the X-ray band of all the AGNs discovered so far in the REX survey.Comment: 27 pages. To be published in Astronomy and Astrophysics, Supplement Series. Better quality figures can be asked to the autho

An X-ray review of MS1054-0321: hot or not?

XMM-Newton observations are presented for the z=0.83 cluster of galaxies MS1054-0321, the highest redshift cluster in the Einstein Extended Medium Sensitivity Survey (EMSS). The temperature inferred by the XMM-Newton data, T=7.2 (+0.7, -0.6) keV, is much lower than the temperature previously reported from ASCA data, T=12.3 (+3.1, -2.2) keV (Donahue et al. 1998), and a little lower than the Chandra temperature, T=10.4(+1.7, -1.5) keV, determined by Jeltema et al. 2001. The discrepancy between the newly derived temperature and the previously derived temperatures is discussed in detail. If one allows the column density to be a free parameter, then the best fit temperature becomes T=8.6 (+1.2, -1.1) keV, and the best fit column density becomes N_(H)=1.33 (+0.15 -0.14) x 10^20 atoms/cm^2. The iron line is well detected in the XMM-Newton spectrum with a value for the abundance of Z=0.33 (+0.19 -0.18) Zsol, in very good agreement with previous determinations. The derived XMM X-ray luminosity for the overall cluster in the 2-10 keV energy band is L_X=(3.81 +/- 0.19) x 10^44 h^-2 erg s^-1 while the bolometric luminosity is L_BOL=(8.05+/-0.40) x 10^44 h^-2 erg s^-1. The XMM-Newton data confirm the substructure in the cluster X-ray morphology already seen by ROSAT and in much more detail by Chandra. The central weak lensing clump is coincident with the main cluster component and has a temperature T=8.1 (+1.3, -1.2) keV. The western weak lensing clump coincides with the western X-ray component which is much cooler with a temperature T=5.6 (+0.8, -0.6)$ keV. Given the newly determined temperature, MS1054-0321 is no longer amongst the hottest clusters known.Comment: To appear in the A&A main Journal, 13 pages including 3 postscript figures and 4 tables. Figs. 1, 4, 5 and 7 are too large and are not given here. The whole paper as pdf file is posted at http://www.ira.cnr.it/~gioia/PUB/publications.htm