Context. The X-ray telescope eROSITA on board the newly launched Spectrum-Roentgen-Gamma (SRG) mission serendipitously observed the galaxy cluster Abell 3408 (A3408) during the performance verification observation of the active galactic nucleus 1H 0707–495. The field of view of eROSITA is one degree, which allowed us to trace the intriguing elongated morphology of the nearby (z = 0.0420) A3408 cluster. Despite its brightness (F500 ≈ 7 × 10−12 ergs s−1 cm−2) and large extent (r200 ≈ 21'), it has not been observed by any modern X-ray observatory in over 20 yr. A neighboring cluster in the NW direction, A3407 (r200 ≈ 18', z = 0.0428), appears to be close at least in projection (~1.7 Mpc). This cluster pair might be in a pre- or post-merger state. Aims. We aim to determine the detailed thermodynamical properties of this special cluster system for the first time. Furthermore, we aim to determine which of the previously suggested merger scenarios (pre- or post-merger) is preferred. Methods. We performed a detailed X-ray spectro-imaging analysis of A3408. We constructed particle-background-subtracted and exposure-corrected images and surface brightness profiles in different sectors. The spectral analysis was performed out to 1.4r500 and included normalization, temperature, and metallicity profiles determined from elliptical annuli aligned with the elongation of A3408. Additionally, a temperature map is presented that depicts the distribution of the intracluster medium (ICM) temperature. Furthermore, we make use of data from the ROSAT all-sky survey to estimate some bulk properties of A3408 and A3407, using the growth-curve analysis method and scaling relations. Results. The imaging analysis shows the complex morphology of A3408 with a strong elongation in the SE-NW direction. This is quantified by comparing the surface brightness profiles of the NW, SW, SE, and NE directions, where the NW and SE directions show a significantly higher surface brightness than the other directions. We determine a gas temperature kBr500 = (2.23 ± 0.09) keV in the range 0.2r500 to 0.5r500 from the spectral analysis. The temperature profile reveals a hot core within two arcminutes of the emission peak, ${k_{\rm{B}}}T = 3.04_{- 0.25}^{+ 0.29}$ keV. Employing a mass–temperature relation, we obtain M500 = (9.27 ± 0.75) × 1013M⊙ iteratively. The r200 of A3407 and A3408 are found to overlap in projection, which makes ongoing interactions plausible. The two-dimensional temperature map reveals higher temperatures in the W than in the E direction. Conclusions. The elliptical morphology together with the temperature distribution suggests that A3408 is an unrelaxed system. The system A3407 and A3408 is likely in a pre-merger state, with some interactions already affecting the ICM thermodynamical properties. In particular, increased temperatures in the direction of A3407 indicate adiabatic compression or shocks due to the starting interaction.
Context. The most nearby clusters are the best places for studying physical and enrichment effects in the faint cluster outskirts. The Abell 3158 cluster (A3158), located at z = 0.059, is quite extended with a characteristic radius r 200 = 23.95 arcmin. The metal distribution in the outskirts of this cluster has previously been studied with XMM-Newton. In 2019, A3158 was observed as a calibration target in a pointed observation with the eROSITA telescope on board the Spektrum-Roentgen-Gamma mission. Bright large clusters, such as A3158, are ideal for studying the metal distribution in the cluster outskirts, along with the temperature profile and morphology. With the deeper observation time of the eROSITA telescope, these properties can now be studied in greater detail and at larger radii. Furthermore, bright nearby clusters are ideal X-ray instrumental cross-calibration targets as they cover a large fraction of the detector and do not vary in time. Aims. We first compare the temperature, metal abundance, and normalisation profiles of the cluster from eROSITA with previous XMM-Newton and Chandra data. Following this calibration work, we investigate the temperature and metallicity of the cluster out to almost r 200 , measure the galaxy velocity dispersion, and determine the cluster mass. Furthermore, we search for infalling clumps and background clusters in the field. Methods. We determined 1D temperature, abundance, and normalisation profiles from both eROSITA and XMM-Newton data as well as 2D maps of temperature and metal abundance distribution from eROSITA data. The velocity dispersion was determined and the cluster mass was calculated from the mass -velocity dispersion (M 200 -σ v ) relation. Galaxy density maps were created to enable a better understanding of the structure of the cluster and the outskirts. Results. The overall (i.e. in the range 0.2 − 0.5r 500 ) temperature was measured to be 5.158 ± 0.038 keV. The temperature, abundance, and normalisation profiles of eROSITA all agree to within a confidence level of about 10% with those we determined using XMM-Newton and Chandra data, and they are also consistent with the profiles published previously by the X-COP project. The cluster morphology and surface brightness profile of cluster Abell 3158 appear to be regular at a first glance. Clusters that have such profiles typically are relaxed and host cool cores. However, the temperature profile and map show that the cluster lacks a cool core, as was noted before. Instead, an off-centre cool clump lies to the west of the central cluster region, as reported previously. These are indications that the cluster may be undergoing some sloshing and merger activity. Furthermore, there is a bow-shaped edge near the location of the cool gas clump west of the cluster centre. Farther out west of the X-ray images of A3158, an extension of gas is detected. This largerscale extension is described here for the first time. The gas metallicity (∼0.2 solar) measured in the outskirts ( r 500 ) is consistent with an early-enrichmen...
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