Explain the anomalous class-φ X-ray spectrum

Investigate the physical origin and spectral properties of the class-φ X-ray variability in GRS 1915+105, including why its low-flux, soft spectrum is not reproduced by the existing spectral models.

Background

Class φ is identified as especially difficult to interpret within the variable TADAF framework because it combines unusually low X-ray flux with a soft spectrum. The paper suggests that this state may correspond to a large TADAF radius but a small turbulence parameter, yet acknowledges that existing models fail to describe its spectrum. A dedicated investigation is required to determine whether this proposed TADAF configuration can account for the observations.

References

In fact, \citet{Naik2002} found that the spectrum in class $\phi$ does not fit any of the models and requires further investigation.

— Re-interpreting the GRS 1915+105 observations within the variable TADAF scenario  (2609.37088 - Xu, 29 Sep 2026) in Section 3.1, paragraph discussing the X-ray variability classes

However, how the radiation spectrum evolves with $\eta$ has not been fully studied. In particular, when a light curve class such as class $\phi$ in GRS 1915+105 is observed with supposedly small $\eta$ values, its spectrum cannot yet be well modeled.

— Re-interpreting the GRS 1915+105 observations within the variable TADAF scenario  (2609.37088 - Xu, 29 Sep 2026) in Section 4, paragraph beginning “The radiation spectrum from the TADAF”