2022A&A...661A..26I

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 x 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 k_Br500=(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_BT=3.04_–0.25^+0.29keV. Employing a mass-temperature relation, we obtain M₅₀₀ = (9.27 ± 0.75) x 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.