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., 2024) in Section 2.3 (X-ray upper limits with Swift/XRT)

Several open questions remain. The identification of the near-infrared line as blueshifted Brγ is not secure. The absence of additional hydrogen and helium lines is also notable, particularly given the rich emission and absorption line spectrum of SS 433 (D. R. Gies et al. 2002; Y. Fuchs et al. 2002; S. Fabrika 2004; E. L. Robinson et al. 2017). The absence of, e.g., moving lines associated with the baryonic jets in SS 433 is expected in VT J1906+0849 if the jets are leptonic. Multi-epoch, high-resolution (R ≳ 10, 000) near-infrared spectroscopy would help determine whether the broad line traces a stable wind geometry, orbital motion, or a more complex outflow. A deep hard X-ray observation would test whether an absorbed or reflected central source emerges above the soft X-ray band, and is the natural next step in the study of VT J1906+0849.

— VLASS Discovery of a Luminous Galactic Radio Transient Evolving on Decade Timescales  (2608.21320 - Miller et al., 21 Aug 2026) in Section 4.1.2, final paragraph of the jet–wind interaction discussion