At the end of the mission, the eROSITA X-ray All-sky survey will deliver about 4 million point-like sources, to a flux limit of 1⋅10^-14 erg cm^-2s^-1 in the 0.2-5 keV and 2⋅10^-14 erg cm^-2s^-1 in the 2-10 keV energy bands, respectively. The brightest X-ray sources in the sky, however, were already observed by e.g., ROSAT and XMM but have been rarely used due to the large uncertainties in their positions, and due to the sparse multi-wavelength coverage, thus making the identification of their counterparts a demanding task with uncertain results. New all-sky multi-wavelength surveys of sufficient depth like AllWISE and Gaia allow us, for the first time, to provide reliable counterparts to the ROSAT and XMM Slew all-sky surveys, thanks also to the development of NWAY, a new algorithm based on Bayesian statistic. This algorithm extends the distance-based association work presented in Budavari et al. (2008) and Pineau et al. (2017), and by using one or more priors (e.g. colors, magnitudes, variability, morphology) it weights the probability that sources from two or more catalogs are simultaneously associated on the basis of their observable characteristics.

Using a WISE color-magnitude prior we have determined the counterparts of ROSAT/2RXS and XMMSlew2 outside the Galactic Plane. A reference sample of 4524 validated XMM and Chandra X-ray sources demonstrates a reliability of 94.7\% (2RXS2) and 97.4\% (XMMSL2). Combining our results with Chandra-COSMOS data, we propose a new separation between stars and AGN in the X-ray/WISE flux-magnitude plane, valid over six orders of magnitude ( Salvato et al. 2017).

Through this website we provide:

• The complete list of AllWISE counterparts to the ROSAT 2RXS catalog ( Boller et al. 2016), with |b|>15 degrees. Best and secondary candidate counterparts can be filtered following the instructions in Section 7.1 of Salvato et al. 2017.


• The complete list of AllWISE counterparts to the XMM Slew catalog (Release 2.0, 14th March 2017 ), with |b|>15 degrees. Best and secondary candidate counterparts can be filtered following the instructions in Section 7.1 of Salvato et al. 2017.Note that the file has been corrected and updated on June 9th. You should download it again if you did it before that day.


• The Github link to the NWAY distribution. The code is user friendly and comes with a user manual and vetted data for getting familiar with the algorithm. Its use is not limited to the search of counterparts to X-ray sources only. However, for every application it is the responsibility of the user to adopt the correct prior, depending on the data in hand. The analytical description of NWAY is presented in Salvato et al. 2017.


• The Web interface for accessing the X-ray data, X-ray properties, cone search and catalogs, as described in Boller et al. 2016.


• The SDSS counterparts to the ROSAT/2RXS and XMMSlew catalogs, as described in Dwelly et al. 2017.

Top: Positional uncertainties for the 2RXS (left) and XMMSL2 (right) samples as a function of X-ray flux. The flux of the XMMSL2 sources in the 0.2-12 keV band has been converted to the 0.5-2 keV band assuming Galactic NH =3e20 cm^-2 and a power-law of 1.7. For the 802 sources missing the flux in at 0.2-12 keV we converted either the flux in the 0.2-2 keV band (775 sources) or the flux in the (2-12 keV) band (27 sources). The detection likelihood reflects this procedure. Bottom: Density distribution of the AllWISE colors of counterparts to 2RXS (Left) and XMMSL2 (Right), plotted over the color-color diagram originally created by Chao-Wei Tsai, (used here with permission) but modified by adding the approximative locus of the counterparts to Fermi sources \citep[e.g.,][]{DAbrusco:2013kn}. See for more details Salvato et al. 2017