2025A&A...694A.160K

On the premise of a soft spectral distribution and absence of counterparts, the X-ray source eRASSU J065715.3+260428 was recently identified as a likely thermally emitting isolated neutron star (XINS) in a search in the SRG/eROSITA All-Sky Survey. We investigated the nature and evolutionary state of the neutron star through a dedicated multi-wavelength follow-up observational campaign with XMM-Newton, NICER, FAST, and ESO-VLT, complemented by the analysis of archival Fermi-LAT observations. The coherent timing analysis of the X-ray observations unveiled the rotation period of the XINS, P = 261.085400(4) ms, and its spin-down rate, $ \dot{P}=6^{+11}_{-4}\times10^{-15} $ s s^–1 (errors are 1σ confidence levels). The nearly sinusoidal pulse profile has a pulsed fraction of ∼15% (0.2 - 2 keV). No optical counterparts are detected down to 27.3 mag (5σ, R band) in the ESO-VLT FORS2 imaging, implying a large X-ray-to-optical flux ratio above 5200. The X-ray spectrum of the source is best described by a composite phenomenological model consisting of two thermal components, either a double blackbody continuum with temperatures 90 eV and 220 eV or a hydrogen neutron star atmosphere of temperature log(T/K)∼5.8 combined with a hot blackbody of 250 eV, in both cases modified by an absorption feature at low energies, ∼0.3 keV with an equivalent width of ∼100 eV. The presence of faint non-thermal hard X-ray tails is ruled out above (2.1 ± 1.8)% of the source unabsorbed flux. Radio searches at 1 - 1.5 GHz with FAST yielded negative results, with a deep upper limit on the pulsed flux of 1.4 μJy (10σ). Similarly, no significant spatial or pulsed signals were detected in sixteen years of Fermi-LAT observations. The most likely interpretation is that the source is a middle-aged spin-powered pulsar, which can also be identified as PSR J0657+2604. The absence of non-thermal X-ray, radio, or gamma-ray emission within current limits suggests either an unfavourable viewing geometry or unusual magnetospheric properties. Additional observations are needed to check for faint hard X-ray tails, investigate the presence of diffuse emission from a pulsar-wind nebula, and obtain a more accurately sampled timing solution.