Unveiling the X-ray faint end of the isolated neutron star population

Since the discovery of the first pulsar in 1967, the known population of Galactic isolated neutron stars (INSs) has grown to contain more than 3700 sources. Despite this large sample, thermally emitting INSs are still low in number. This is particularly problematic for the elusive class of X-ray dim isolated neutron stars (XDINSs) that are famous for their predominantly thermal spectra. With the potential to comprise a significant part of the overall Galactic INS population, the study of their intrinsic and population properties, their evolution, and their proposed evolutionary connections to other INS families is currently impeded by the simple fact that only seven confirmed members are known. However, due to its wide survey area and unprecedented sensitivity, the eROSITA All-Sky Survey (eRASS) is expected to unveil the XDINS population beyond the immediate solar vicinity. In this work, we aim to use the eRASS to clear the path towards an increased and flux-limited sample of these sources down to X-ray flux values above 10^-13 erg/s/cm^2. To this end, we selected candidates from the eRASS source catalogues that show soft, blackbody-like X-ray emission, large X-ray-to-optical flux ratios through the absence of any catalogued counterparts at optical/infrared wavelengths, and no hints of significant X-ray variability. These are all properties observed in the known XDINS population. We identified 33 candidates that abide by these constraints. The available extended Roentgen Survey with an Imaging Telescope Array and archival wide-area surveys alone do not allow us to unambiguously establish INS natures for the selected sources. Dedicated follow-up campaigns were launched, combining observations over the whole electromagnetic spectrum, to better characterise the emission properties of the candidates. While the majority of these campaigns are still ongoing, for two candidates sufficient observations have been obtained to characterise their nature. For the first source, eRASSU J065715.3+260428, the detection of a 261 ms spin period and multi-component thermal spectrum conforms with the nature of an middle-aged rotation-powered pulsar (RPPs). Even though this source will not add to the XDINS population, the absence of emission at radio and gamma-ray energies may imply emission properties that are rather unusual in comparison to most known RPPs. This highlights the power of extended Roentgen Survey with an Imaging Telescope Array to identify INSs that escape detection in conventional pulsar searches. The second source, eRASSU J131716.9-402647, presents a spin period of 12.8s. Along with its blackbody-like thermal spectrum and absorption features that imply high magnetic field strengths of B ~ 5x10^13 G, this source is stunningly similar to the known XDINS population. Thus, the identification of eRASSU J131716.9-402647 may mark the first XDINS discovery in more than two decades. We found that the search methodology may need improvements in the Galactic plane, where INSs are most abundant, but the effects of source confusion are also strongest. Nevertheless, the collected sample is a very important leap towards a larger population of XDINSs and thermally emitting INSs in general. This is further supported by preliminary results from the ongoing follow-up campaign that imply the discovery of at least four additional predominantly thermally emitting INSs. This demonstrates that searches at X-ray energies add otherwise underrepresented INS types to the population. While the full exploitation of the eRASS can be expected to take years, the resulting INS samples will shed new light on the properties and evolution of these enigmatic sources.