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There's a recent paper in Nature about LB-1, a B-class star orbiting a massive black hole.

I don't understand how these two parts of the paper can be reconciled. On page 2, the authors argue that the broad $H\alpha$ line indicates there's a disk in the system, and then that the disk is around the black hole (as opposed to the star or the entire binary).

This supports the $H\alpha$ emission line not coming from a circumbinary disk, but from a disk around the black hole.

However in the last paragraph, they also say that this system doesn't emit X-rays:

Unlike every other known stellar black hole, LB-1 has not been detected in X-ray observations. We searched for X-ray emission from this system with a 10-ks observation with the Chandra X-ray Observatory, placing an upper limit for the X-ray luminosity of $\geq 2 × 10^{31} {\rm erg s}^{−1}$ (see Methods). This upper limit corresponds to about $10^{−9}$ of its Eddington luminosity, and suggests a mass accretion rate ̇$M \leq 10^{-11} M_{\odot} {\rm yr}^{-1}$ for a conversion efficiency of approximately $10^{−4}$ at such low luminosity.

Naively it seems to me that this non-detection is actually an argument that the $H\alpha$ line isn't coming from a disk around the black hole. If there's a disk around a black hole, it's presumably accreting. If it's accreting, the material presumably gets hot and produces X-rays. If it's producing X-rays, Chandra should have detected them.

What am I missing?

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    $\begingroup$ Short answer: because there is very little friction in the particular disk, leading to a low accretion rate. (Not an answer, because it is just shifting the problem.) $\endgroup$
    – TimRias
    Commented Nov 28, 2019 at 15:58
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    $\begingroup$ I'll note that there are at least three independent preprints out today arguing that the analysis was incorrect, with signs that the $\text{H}\alpha$ emission is not, in fact, coming from such a putative disk (e.g. arxiv.org/abs/1912.04185). Seems like the original group might have neglected to properly model the stellar companion's $\text{H}\alpha$ emission features, which might cast doubt on the disk's existence. $\endgroup$
    – HDE 226868
    Commented Dec 10, 2019 at 2:52

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As far as I know, accretion disks around black holes do emit X rays. If this X rays are not detected may depend on technology limitations.

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  • $\begingroup$ Black holes emit nothing! it's the matter, which is accelerated towards the black hole, who is emitting radiation. $\endgroup$
    – Dominique
    Commented Jan 22 at 15:02

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