Establishing the X-ray source detection strategy for eROSITA with simulations

Abstract

Context. The eROSITA X-ray telescope on board the Spectrum-Roentgen-Gamma satellite has started to detect new X-ray sources over the full sky at an unprecedented rate. Understanding the performance and selection function of the source detection is important for the subsequent scientific analysis of the eROSITA catalogs. Aims. Through simulations, we test and optimize the eROSITA source detection procedures, and we characterize the detected catalog quantitatively. Methods. Taking the eROSITA Final Equatorial-Depth Survey (eFEDS) as an example, we ran extensive photon-event simulations based on our best knowledge of the instrument characteristics, the background spectrum, and the population of astronomical X-ray sources. We introduce a method of analyzing source detection completeness, purity, and efficiency based on the origin of each photon. Results. According to the source detection efficiency measured in the simulation, we chose a two-pronged strategy to build eROSITA X-ray catalogs, creating a main catalog using only the most sensitive band (0.2–2.3 keV) and an independent hard-band-selected catalog using multiband detection in a range up to 5 keV. Because our mock data are highly representative of the real eFEDS data, we used the mock catalogs to measure the completeness and purity of the eFEDS catalogs as a function of multiple parameters, such as detection likelihood, flux, and luminosity. These measurements provide a basis for choosing the eFEDS catalog selection thresholds. The mock catalogs (available with this paper) can be used to construct the selection function of active galactic nuclei and galaxy clusters. A direct comparison of the output and input mock catalogs also gives rise to a correction curve that converts the raw point-source flux distribution into the intrinsic number counts distribution.

Cosmological Constraints from Galaxy Clusters and Groups in the eROSITA Final Equatorial Depth Survey

“…The inclusion of an exposure-time dependence will likely be helpful in the modeling of the eROSITA All-Sky Survey sample at some future date. Note that Bulbul et al (2022) recently derived the completeness of the eFEDS clusters in terms of the count rate by analyzing the population of mis-identified clusters in the point-source catalog (Salvato et al 2022) and the simulations (Liu et al 2022b), showing a similar functional form as in equation ( 35). However, we stress that the completeness presented in Bulbul et al (2022) cannot be directly utilized in a cosmological analysis.…”

“…In what follows, we introduce one novel aspect of this work regarding the 𝜂-𝑀-𝑧 relation. It is known that there exists a deficit of the cluster population with low extent likelihoods (L ext 10) compared to that predicted by the eFEDS simulations (Liu et al 2022b). Specifically, the distribution of L ext in the eFEDS catalog shows a peak at L ext ≈ 10 and decays at L ext 10, which is distinctly different from the simulations that show a power-law distribution of L ext without a turnover.…”

“…In this work, we impose the threshold 𝑓 cont < 0.3, reducing the cluster catalog from 542 to 477 clusters. The contamination due to point sources in the initial cluster catalog is estimated to be ≈ 20% (Liu et al 2022a) using the simulations (Liu et al 2022b); therefore, the residual contamination after the optical confirmation is expected to be at a level of ≈ 0.3 × 20% ≈ 6% (Klein et al 2022). A more stringent cut of 𝑓 cont < 0.2, as previously used in the weak-lensing mass calibration (Chiu et al 2022), would further discard ≈ 20 clusters (corresponding to a 5% difference in the sample size) and reduce the contamination rate to ≈ 4%.…”

“…This would result in a smaller observed flux per halo mass for groups than massive clusters. In the eFEDS simulations (Liu et al 2022b), the low-mass clusters were injected into the X-ray imaging as though they are scaled-down versions of massive clusters (see Section 3.1.1 in Comparat et al 2020), hence the population of synthetic galaxy groups might be neither complete nor accurately capturing the changing impact of ICM physics at the lowest mass scales. Motivated by this, we introduce a broken power-law feature at the group scale as ln…”

“…For instance, an X-ray selection function directly derived from simulations and the resulting cosmological constraints would be biased if the simulated clusters do not accurately and precisely reflect the observed cluster population. In fact, this is the case for the eFEDS simulation (Liu et al 2022b), where the number of synthetic clusters with low fluxes was over-predicted (we return to discuss this in terms of the X-ray scaling relation and the selection function in Section 4.1.1 and Section 4.2, respectively). Second, the modeling is primarily determined by what the data require and therefore fully captures the behavior of the data.…”