<p>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.</p>

CC BY-SA 4.0
2024-01-22T15:02:49.917Z
Black holes emit nothing! it's the matter, which is accelerated towards the black hole, who is emitting radiation.
Dominique
CC BY-SA 4.0
2024-01-19T09:53:09.283Z
2024-01-23T11:43:59.267Z
2024-01-23T11:43:59.267Z
Juan Casado
<p>There's <a href="https://www.nature.com/articles/s41586-019-1766-2" rel="noreferrer">a recent paper</a> in <em>Nature</em> about LB-1, a B-class star orbiting a massive black hole.</p>
<p>I don't understand how these two parts of the paper can be reconciled. On page 2, the authors argue that the broad <span class="math-container">$H\alpha$</span> 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).</p>
<blockquote>
<p>This supports the <span class="math-container">$H\alpha$</span> emission line not coming from a circumbinary disk, but from a disk around the black hole.</p>
</blockquote>
<p>However in the last paragraph, they also say that this system doesn't emit X-rays:</p>
<blockquote>
<p>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 <span class="math-container">$\geq 2 × 10^{31} {\rm erg s}^{−1}$</span> (see Methods). This upper limit corresponds to about <span class="math-container">$10^{−9}$</span> of its Eddington luminosity, and suggests a mass accretion rate ̇<span class="math-container">$M \leq 10^{-11} M_{\odot} {\rm yr}^{-1}$</span> for a conversion efficiency of approximately <span class="math-container">$10^{−4}$</span> at such low luminosity.</p>
</blockquote>
<p>Naively it seems to me that this non-detection is actually an argument that the <span class="math-container">$H\alpha$</span> 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, <a href="https://en.wikipedia.org/wiki/Chandra_X-ray_Observatory" rel="noreferrer">Chandra</a> should have detected them.</p>
<p>What am I missing?</p>

CC BY-SA 4.0
2019-11-28T15:58:55.020Z
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.)
TimRias
CC BY-SA 4.0
2019-12-10T02:52:17.977Z
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. https://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.
HDE 226868
<black-hole><accretion-discs><x-ray><chandra>
Why would a black hole have a disk, but not emit x-rays?
Allure
# Why would a black hole have a disk, but not emit x-rays?

(Asked by: username_0 on 2019-11-28)

There's [a recent paper](https://www.nature.com/articles/s41586-019-1766-2) 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](https://en.wikipedia.org/wiki/Chandra_X-ray_Observatory) should have detected them.

What am I missing?

**Tags:** black-hole, accretion-discs, x-ray, chandra
> **Comments:**
> **username_1**: 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.)  
> **username_2**: 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. https://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.  

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## 1 answers

### Answer by username_3 on 2024-01-19. Score: 1

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.

> **Comments:**
> **username_4**: Black holes emit nothing! it's the matter, which is accelerated towards the black hole, who is emitting radiation.  

---

