Discovery of two promising isolated neutron star candidates in the SRG/eROSITA All-Sky Survey

¹ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
e-mail: jkurpas@aip.de
² Potsdam University, Institute for Physics and Astronomy, Karl-Liebknecht-Straße 24/25, 14476 Potsdam, Germany
³ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
⁴ South African Astronomical Observatory, PO Box 9 Observatory, 7935 Cape Town, South Africa
⁵ Department of Physics, University of the Free State, PO Box 339 Bloemfontein, 9300 South Africa
⁶ Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch, 7701 South Africa

Received: 10 March 2023
Accepted: 15 April 2023

Abstract

We report the discovery of the isolated neutron star (INS) candidates eRASSU J065715.3+260428 and eRASSU J131716.9−402647 from the Spectrum Roentgen Gamma (SRG) eROSITA All-Sky Survey. Selected for their soft X-ray emission and absence of catalogued counterparts, both objects were recently targeted with the Large Binocular Telescope and the Southern African Large Telescope. The absence of counterparts down to deep optical limits (25 mag, 5σ) and, as a result, large X-ray-to-optical flux ratios in both cases strongly suggest an INS nature. The X-ray spectra of both sources are well described by a simple absorbed blackbody, whereas other thermal and non-thermal models (e.g., a hot-plasma emission spectrum or power law) are disfavoured by the spectral analysis. Within the current observational limits, and as expected for cooling INSs, no significant variation (> 2σ) has been identified over the first two-year time span of the survey. Upcoming dedicated follow-up observations will help us to confirm the candidates’ nature.