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The soft X-ray instrument eROSITA on board the Spectrum-Roentgen-Gamma (SRG) observatory has successfully completed four of the eight planned all-sky surveys, detecting almost one million X-ray sources during the first survey (eRASS1). Despite the invaluable legacy of the eROSITA all-sky survey and the millions of cataloged X-ray sources, many non-detected sources are hidden in the noise of the observations. These non-detections can be faint X-ray objects or intrinsically variable X-ray sources. Based on X-ray aperture photometry, flux upper limits for eRASS1 are provided in the single detection band (0.2-2.3 keV) as well as in the three-band detection at soft (0.2-0.6 keV), medium (0.6-2.3 keV), hard (2.3-5.0 keV) and the combined 0.2-5.0 keV energy bands. These data are crucial for studying the X-ray properties of variable and transient objects, as well as non-detected sources in the eROSITA all-sky survey data.
The final data products consist of tables with the aperture photometry products (detected counts, background counts, and exposure time), a close-neighbour flag, and the flux upper limit based on an absorbed power-law spectral model (Γ = 2.0, NH = 3·1020 cm-2). The upper limits are calculated using the one-sided 3σ confidence interval (CL) of a normal distribution, representing CL = 99.87%. The aperture photometry products allow for an easy computation of upper limits at any other confidence interval and spectral model.
The eROSITA upper flux limits represent either the maximum flux of potential non-detections or the 3σ upper flux uncertainty of detected sources. We emphasise the importance of choosing the right spectral model that ought to match the spectral shape of the source of interest: the wrong spectral model can produce discrepancies of up to 30% in the final flux upper limit value.
We provide flux upper limits on the German half of the sky which covers the galactic longitudes between 180° < l < 360° (eROSITA-DE). The users can access the data via:
From these pages users can retrieve upper-limit products for any pair of coordinates on the eROSITA-DE footprint. Given an R.A. and Dec. and the energy band, the user can obtain photometric products and the flux upper limit for single or multiple positions in the sky.
If you want to retrieve upper limit raw data, go e.g. to the upper limit for single positions and then to the download area, choose the sky tile by clicking on the coordinates (first R.A. then DEC) and then on UPP_010. You will be taken to a directory where you can download the desired data.
The name of these upper limit tables is of the form PQii_jjjjjj_klm_nnnnnn_Rooo.fits and follows the Pipeline data product's file naming convention, except for klm which refers to the eROSITA-internal coding of the energy bands
and nnnnnn which is 'UpperLimitTable'.
Once the file is downloaded, you have to compute the correct HEALPix index for a given set of coordinates. One can make use of the Python implementation of the HEALPix algorithm astropy-healpix, which has the necessary tasks to perform the coordinate transformations. The HEALPix tesselation can be created as follows, hp = HEALPix(nside = 216, order='nested', frame='icrs'). This will create HEALPix cells with a resolution of ~3", similar to the eROSITA pixel size of 4". The coordinate conversion can be done with the functions skycoord_to_healpix(coords) and healpix_to_Skycoord(index), where coords is an array of astropy.coordinates and index is an array with HEALPix indexes. Finally, the corresponding upper limit can be retrieved by identifying the row of the computed HEALPix index (for the sky position of interest) in the downloaded table of the sky tile.
You can find the necessary commands for this calculation here.
Please see Tubín-Arenas et al. (2024) for further details.
The following document provides an overview of the eROSITA Upper Limit data products. It also gives suggestions to the users about how to use and interpret them. A full description can be found in Tubı́n-Arenas et al. (2024). We kindly request to cite Tubı́n-Arenas et al. (2024) and Merloni et al. (2024) if you use the upper limits products.
The upper limit products are stored in table1 format with the following column names:
a The photometric products are not corrected for the fraction of the PSF used (EEF = 0.75). These values (Counts, Bkg_counts, and Bkg_SourceMap) are provided so the user can compute upper limits at any confidence interval (CL) following the method expressed in Eq. 5 of the upper limit paper.
b Note that only the flux upper limits (UL_B and UL_S) are corrected by the fraction of the PSF (EEF = 0.75) used to extract the photometric data (See Eq. 7 and Sect. 3.4 in the upper limit paper for more details).
1 The tables are organized by sky tile and energy band. See Sect. 3.5 of the upper limit paper for a discussion about the different energy bands considered in the upper limit calculation.
We recommend the user to always first check the eROSITA source catalog(s) to investigate detected sources close to or at the positions of interest. If no sources are detected or if the user is interested in the upper flux limit of a hypothetical second source, the eROSITA upper limit server will provide the flux upper limit for the input coordinates, as well as the photometric products described above. In this section, we guide the users to a proper interpretation of the flux upper limits UL_B and UL_S.
Both upper limits are calculated based on the observed counts extracted from the circular aperture region, however, they consider different background counts. UL_B uses the source-free smoothed background counts while UL_S considers as a background the counts extracted from the source-map. The source-map is a combi- nation of the smoothed source-free background and a PSF model of the detected sources. See Sect. 3.1 of the upper limit paper for a more detailed explanation of the source-map.