2025/01/13 by Kurpas, J., Pires, A. M., Schwope, A. D. +6
#FOS: Physical sciences #High Energy Astrophysical Phenomena (astro-ph.HE)
paper · doi:10.48550/arxiv.2501.07347
The X-ray source eRASSU J065715.3+260428 was identified as a likely thermally emitting isolated neutron star in a search in the SRG/eROSITA All-Sky Survey. We investigated the nature and evolutionary state of the source through a dedicated multi-wavelength follow-up campaign with XMM-Newton, NICER, FAST, and ESO-VLT, complemented by the analysis of archival Fermi-LAT observations. The X-ray observations unveiled the rotation period, P=261.085400(4) ms, and spin-down rate, P=6+11-4×10-15 s s-1, of the source. No optical counterparts are detected down to 27.3 mag (5σ, R band), 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). 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 source is most likely a middle-aged spin-powered pulsar and 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.