An SRG/eROSITA Study of Be X-ray binaries in Quiescence

Be X-ray binaries are mostly transient X-ray binaries with usually a neutron star as the compact object and a Be star as their optical companion. The new analysis still awaits peer review, but it already lays out the central claim clearly.

This matters because astrophysics becomes persuasive only when an observed signal can be tied to a physically defensible explanation. Compact objects such as neutron stars and black holes are natural laboratories for extreme physics, but the distance and complexity of these systems make interpretation difficult without multi-wavelength coverage and careful modeling. A detection without a mechanism is only half a result. the other half comes from showing that the signal fits quantitatively inside a coherent physical picture rather than merely being consistent with a broad family of models. Be X-ray binaries (BeXRBs) are mostly transient X-ray binaries with usually a neutron star as the compact object and a Be star as their optical companion. We performed a comprehensive analysis of BeXRBs in quiescence with the extended ROentgen Survey with an Imaging Telescope Array (eROSITA), the soft X-ray instrument on board the.

The transient BeXRBs spend majority of time in their quiescent state, but are often studied during their outbursts. A quiescence study is useful to probe the emission mechanism of BeXRBs at low mass accretion rates.

The spectra of BeXRBs detected in the eRASS1 main catalogue could be well fitted with a powerlaw as well as a blackbody model. In the absence of detection of a BeXRB, we report the luminosity upper limit for its position in the sky.

The X-ray luminosity for the quiescent BeXRBs detected with eROSITA have $L_{X}\sim10^{33-36}$ erg s$^{-1}$ for both absorbed powerlaw and blackbody models in most cases. For the absorbed blackbody model, the emission seems to come from a fraction of the neutron star's surface.

The broader interest lies in turning an observational clue into something that can be weighed against competing models of the underlying physics. Astrophysics does not have the luxury of controlled experiments; everything is inferred from radiation that traveled across cosmic distances under conditions that cannot be reproduced in a terrestrial laboratory. This makes the interpretation chain longer and more uncertain than in bench science, but it also means that a well-constrained measurement of an extreme object carries theoretical information that no earthbound experiment can provide.

The quiescence luminosity of BeXRBs are used to distinguish between different accretion models at low mass accretion rates, like accretion in the propeller regime or from a cold. The detection space of BeXRBs in quiescence for the eRASS1 data is compared against previously reported luminosities across multiple X-ray observatories.

Because this is still a preprint, the result should be read with genuine interest and proportionate caution. Peer review is not a guarantee of correctness, but it is a process that forces authors to respond to technical criticism from specialists who have no stake in a particular outcome. Preprints that survive that process, often with substantive revisions, emerge with a stronger evidential base than the version that first appeared. Until that stage is complete, the responsible reading keeps uncertainty explicitly visible rather than treating the claims as established findings.

The next step is to see whether independent datasets and physical modeling converge on the same interpretation. Multi-wavelength follow-up, combining X-ray, radio and optical data where possible, is typically what separates a compelling detection from a robust physical characterization. In high-energy astrophysics, results that initially looked definitive have been revised when data from a second messenger arrived; the current result should be read with that history in mind. Until peer review and independent follow-up address those open questions, skepticism is not a failure of appreciation for the work; it is part of how science decides what to keep.

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