The second ROSAT all-sky survey (2RXS) catalogue

We present the second ROSAT all-sky survey source catalogue, hereafter referred to as the 2RXS catalogue. The 2RXS catalogue is the second publicly released ROSAT catalogue of point-like sources obtained from the ROSAT all-sky survey observations performed between June 1990 and August 1991, and is an extended and revised version of the 1RXS catalogue. We have developed shell scripts containing a collection of individual MIDAS/EXSAS commands to create the new data products in addition to the source detection parameters. X-ray control images were produced, which show the source and background extraction regions, selected by the timing and spectral analysis procedures, to allow the user to validate the derived parameters. Thirty-one large extended regions with diffuse emission and embedded point sources were identified and excluded from the present analysis. The 2RXS catalogue contains about 135,000 X-ray detections down to a detection likelihood of 6.5 in the 0.1-2.4 keV energy band. Our simulations show that the expected spurious content of the catalogue is a strong function of detection likelihood, and the full catalogue is expected to contain about 30 % spurious detections. A more conservative likelihood threshold of 9, on the other hand, yields about 71,000 detections with a 5 % spurious fraction. We recommend thresholds appropriate to the scientific application. X-ray images with overlaid X-ray intensity contours provide an additional product to allow the user to evaluate the detection likelihood. Intra-day variability in the X-ray light curves was quantified based on the normalised excess variance and a maximum amplitude variability analysis. X-ray spectral fits were performed using three basic models, a power law, a thermal plasma emission model, and black-body emission. The 2RXS catalogue provides the deepest and cleanest X-ray all-sky survey catalogue before eROSITA data will become available.

Aitoff projection in Galactic coordinates of the sky distribution of 2RXS sources. The size of the symbols scales with the source count rate and the colours represent different spectral characteristics (increasing hardness ratio 1 from red (soft) to blue (hard).

The ROSAT X-ray all-sky survey

The ROSAT all-sky survey (RASS) was the first to scan the whole sky with a powerful imaging X-ray telescope operating in the 0.1 - 2.4 keV band (Trümper 1982). The Wolter type I mirror system (Aschenbach 1988) was exceptionally well suited for sky survey operation, due to the very low micro-roughness of the mirrors (< 0.3 nm), which resulted in an excellent contrast of the X-ray images. In addition, the fast optics provided a low non-X-ray (particle) background. The focal plane detector used for the sky survey, the Position Sensitive Proportional Counter (PSPC), had a five-sided anti-coincidence system which reduced the particle background with an efficiency of 99.85% (Pfeffermann 1986, Pfeffermann 2003.  Another reason for the exceptionally low particle background of ROSAT was the low orbit of about 550 km.

The full paper appeared in Astronomy & Astrophysics in April 2016, Volume 588,  A103 .

The motivations for creating the 2RXS catalogue

  1. Reduction of systematic time delays between star tracker and photon arrival time.
  2. Use of an improved detection algorithm.
  3. A significantly improved reduction of spurious sources by a careful visual screening of each catalogue entry and the exclusion of large extended emission regions, in particular from the background map creation process.
  4. The provision of X-ray images of 1378 sky fields (6.4 degrees x 6.4 degrees) covering the whole sky.
  5. The provision of local maps (40 arcmin x 40 arcmin) for each detected X-ray source.
  6. The creation of source spectra and light curves and deduction of characteristic parameters.
  7. The creation of new photon event tables with astrometric errors corrected that are present in the publicly available event files (originating from the RASS-3 processing).
  8. The delivery of a documented and reproducible point source catalogue.
  9. Performing extensive simulations to estimate the amount of spurious detections in the 2RXS catalogue as a function of the detection likelihood and other source parameters.

The 2RXS data products

For the spatial analysis the full sky was divided into 1378 sky fields with a size of 6.4 degrees x 6.4 degrees.
X-ray images in units of count rates for the 1378 sky fields are available as pdf documents. The figure shows the image for field 930101.
For all 2RXS sources we have produced 40 arcmin by 40 arcmin X-ray images around the location of the source. The source (centred on the image) and  other nearby sources are marked with green circles. The regions considered for background extraction to produce light curves and spectra are marked by yellow and cyan circles. In this example the region marked with yellow is not used for the background as it is contaminated by another 2RXS source.

In addition, for each source X-ray images were created in six energy bands. For each of the 1378 sky fields an overview for all sources deteced in the field is provided as pdf document with a one-page summary for each source. Here an example for a sky field source summary can be found.

Additional data products are X-ray light curves, and plots with spectral fits using simple models like power law, thermal plasma emission (mekal) and black-body. Examples are shown below.

X-ray light curve for source 10 in field 930101.
Power-law fit to the ROSAT PSPC spectrum of source 10 in field 930101.
As in the left figure, but using a thermal plasma emission model (mekal).
... and for a black-body fit.

The online 2RXS catalogue

The full 2RXS catalogue (FITS format) and associated data products are provided online. Information from the catalogue together with associated products can be accessed via a cone search interface.

At CDS the catalogue is available via ftp (ASCII)  together with a detailed description of all columns in the catalogue table. The catalogue can be searched using VizieR.

The 6.4x6.4 degrees count rate images are available here (285 MB).

Software

To analyse the ROSAT all-sky survey photon event files and create the products, the publicly available software packages ESO-MIDAS (version P03SEPpl1.2) and EXSAS (version 05APR_EXP) were used on a Linux platform. For more detailed information see the EXSAS users guide, together with additional technical reports available for EXSAS commands which we used for our analysis:
DETECT/LOCAL
CREATE/BG_IMAGE
DETECT/MAP
DETECT/MAXLIK

Related projects

The High Energy group at MPE has committed to provide homogenoeus access to the X-ray surveys that we are working on. For each of the surveys (e.g. (E)CDFS, GOODS-S, EGS, XMM-LSS, etc.) we  provide reduced X-ray data, X-ray catalogues together with catalogues of the optical counterparts, photometric redshift and redshift probability distribution.
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