Discriminate between a genuinely compact corona and an undetectably faint variable corona

Determine whether the absence of detected broadband variability and quasi-periodic oscillations in the soft-state NuSTAR observations of GRS 1739-278 is caused by a genuinely compact and weak corona or by coronal variability that is too faint to detect with the available count rates and exposures.

Background

The NuSTAR observations show no significant broadband noise, quasi-periodic oscillation, or soft-hard time lag, together with low fractional variability and a nearly flat rms spectrum. These results are consistent with a settled, disk-dominated soft state, but they do not uniquely determine the physical state of the corona.

The paper identifies two unresolved interpretations: the corona may genuinely be compact and weak, as observed during hard-to-soft transitions in other black-hole transients, or its variability may simply lie below the detection sensitivity of the available data. The authors explicitly state that the observations cannot distinguish between these alternatives.

References

On its own, this null result can not tell us which of two things is going on. Either the corona is genuinely compact and weak, similar to what Kara et al. found shrinking on timescales of days during the hard-to-soft transition of MAXI~J1820+070, later characterized further by Buisson et al. Or, alternatively, the corona's variability is simply too faint for us to pick up at the count rates and exposures available here. Our data can not distinguish between these two possibilities.

— NuSTAR View of the 2025 Mini-Outburst of the Black Hole X-ray Binary GRS 1739-278: Spectral and Timing Evolution in the Soft State  (2609.03624 - Hussain et al., 3 Sep 2026) in Section 5, Results and Discussion