Discovery of a > 13 Mpc long X-ray filament between two galaxy clusters beyond three times their virial radii

Context. A significant fraction of the missing baryons in the local Universe is expected to reside in large-scale filaments that may be observable in soft X-ray emission. Until now, however, very few candidate emission filaments have been found in individual systems, and none beyond three times the virial radius of the clusters at the nodes of these filaments. The new Spectrum Roentgen Gamma (SRG) eROSITA X-ray telescope has a superior response to extended soft X-rays, which makes it ideal for studying low X-ray surface brightness emission of cosmic filaments. Aims. We search for extended X-ray emission between the two nearby galaxy clusters Abell 3667 and Abell 3651, which are separated by a projected transverse distance of ~13 Mpc, using data from the SRG/eROSITA All-Sky Survey. Methods. We performed a detailed X-ray image analysis of the region between the two galaxy clusters and conducted a redshift analysis of the sources between them. We carried out a thorough surface brightness and spectral analysis between the clusters. The analysis was complemented with an X-ray pointed observation from XMM-Newton, infrared 2MASS data, and redshift information from NED. Results. We discover an emission filament beyond the known radio relic northwest of A3667 and even beyond three times its virial radius. It is smoothly connected to A3651. The X-ray emission in the direction of the filament shows an enhancement of (30 ± 3) % with a significance of 11 σ. The 2MASS map and redshift analysis show an alignment of galaxies along the filament and make a projection effect unlikely. Taking the redshift progression of galaxies within the filament into account, we estimate its three-dimensional length to be in the range of 25 Mpc–32 Mpc. The surface brightness analysis in combination with the temperature T = (0.91_‑0.11^+0.07) keV and metallicity Z = (0.10_‑0.08^+0.05) Z_⊙ from the spectral analysis leads to estimates of a total flux, gas mass, and central baryon overdensity of F_X = (7.4 ± 1.2)×10^‑12 erg s^‑1 cm^‑2, M_g = (2.7_‑0.8^+1.4) M_⊙ and δ₀ = 215_‑50⁺⁸⁶.