Ascertain the nature of the hard-emitting Be-star candidate Cl Pismis 17 3

Determine the long-term X-ray behavior and physical nature of Cl Pismis 17 3 through future observations, despite its unexpectedly low NuSTAR flux and contamination by stray light.

Background

A subset of apparently isolated Be stars displays unusually hard or luminous X-ray emission comparable to that of the least luminous BeXRBs. Such emission may indicate an undetected compact-object companion, although alternative explanations include Cas-star behavior or other forms of stellar activity.

Cl Pismis 17 3 was observed with NuSTAR as part of an ongoing campaign aimed at probing the hard X-ray emission of these candidate systems. The observation yielded a lower-than-expected flux and was affected by stray light, preventing a reliable interpretation. Its upper limit remains compatible with Cas-star behavior, so the source’s nature is unresolved.

References

Another, Cl Pismis 17 3, has also been observed by NuSTAR. The observation revealed a lower flux than expected, and is further plagued by stray light, such that its behaviour cannot currently be ascertained without future work.

— The low luminosity end of Galactic HMXBs with eROSITA: Establishing a luminosity floor for accreting BeXRBs  (2608.13259 - Zainab et al., 13 Aug 2026) in Section 4, “Comparison to Be stars”