Explain the X-ray deficit in shortest-period disk-accreting ultracompacts

Determine the physical reasons why some ultracompact, disk-accreting AM Canum Venaticorum systems exhibit X-ray fluxes orders of magnitude below accretion-powered expectations, distinguishing between strong obscuration and intrinsically weak X-ray emission and quantifying the responsible mechanisms.

Background

Swift/XRT observations yielded deep non-detections of X-ray fluxes that are 3–4 orders of magnitude below simple accretion luminosity estimates for the reported systems, consistent with similar tensions found in ES Cet and many AM CVns below 30 minutes.

The authors propose two broad explanations—severe obscuration (e.g., by the inner disk or ISM for soft spectra) or genuinely low X-ray emission fractions—and note that the underlying reasons are not yet understood.

References

We are left to conclude that either 1) the X-ray flux in our systems is significantly obscured, perhaps by the inner disk or foreground ISM absorption, which is particularly severe for small blackbody temperatures ≲ 100 eV; or 2) only a very small fraction of the total accretion luminosity is even released in X-rays, for reasons not understood.

— Expanding the ultracompacts: gravitational wave-driven mass transfer in the shortest-period binaries with accretion disks (2411.12796 - Chakraborty et al., 19 Nov 2024) in Section 2.3 (X-ray upper limits with Swift/XRT)