<p>Gravitational lensing happens when a massive object passes (from our point of view) in front of a light source. The light rays bend and the image of the source gets distorted. A light ray that passes at a distance <span class="math-container">$r$</span> from a spherical object of mass <span class="math-container">$M$</span> is deflected by an angle</p>
<p><span class="math-container">$$\theta = \frac{4GM}{rc^2}$$</span></p>
<p><strong>Strong lensing for a single black hole</strong></p>
<p>Consider a black hole that passes in front of a star. The maximal distortion of the image of the star is obtained when the star is directly behind the black hole. In that case instead of a point we see the star as a ring, called Einstein ring, with angular size given by</p>
<p><span class="math-container">$$\theta_E = \sqrt{\frac{GM}{c^2}\frac{d_s-d_b}{d_sd_b}}$$</span></p>
<p>where <span class="math-container">$d_s$</span> is the distance from us to the star and <span class="math-container">$d_b$</span> is the distance from us to the black hole. If the star is twice as distant than the black hole, <span class="math-container">$d_s = 2d_b$</span> and</p>
<p><span class="math-container">$$\theta_E = \sqrt{\frac{2GM}{c^2}\frac{1}{4d_b}} = \frac{1}{2}\sqrt{\frac{r_b}{d_b}} = \frac{1}{2} \sqrt{\frac{d_b}{r_b}} \theta_b$$</span></p>
<p>where <span class="math-container">$r_b$</span> is the Schwarzschild radius and <span class="math-container">$\theta_b = \frac{r_b}{d_b}$</span> is the angular radius of the black hole.</p>
<p>We may take <span class="math-container">$\theta_E$</span> as the characteristic angular size of a lensing event. If our telescope has an angular resolution <span class="math-container">$\theta_t &lt; \theta_E$</span> then we are able to detect that the image of the star is deformed.</p>
<p>We could try to search for black holes by looking towards the centre of the Milky Way, because the stars are denser and a star-black hole conjunction is more probable. The Sun is about <span class="math-container">$d=8$</span> kpc from the centre of the MW, if we assume a stellar black hole distant <span class="math-container">$d/2 = 4$</span> kpc passes in front of a star, we may write</p>
<p><span class="math-container">$$\theta_E \approx 0.0016{''} \times \left( \frac{d}{8 \text{kpc}}\right)^{-1/2} \left(\frac{M}{10M_{\odot}}\right)^{1/2}$$</span></p>
<p>Comparing this result with the angular resolution limit from Earth-based observations (seeing) <span class="math-container">$\approx 1 {''}$</span>, or even with Hubble's angular resolution <span class="math-container">$\approx 0.05 {''}$</span>, the observation of a distant black hole via strong lensing appears almost hopeless.</p>
<p><strong>Microlensing for a single black hole</strong></p>
<p>This is not the end of the story, tough, because spatially resolving the Einstein ring is not the only way to detect a lensing event. A gravitational lens not only deforms the shape of the background star, but also amplifies its brightness (just like a lens!). The if the angular separation between the star and the black hole is <span class="math-container">$\phi$</span>, then the amplification factor will be</p>
<p><span class="math-container">$$A(u) = \frac{u^2+2}{u\sqrt{u^2+4}};\ \ u = \frac{\phi}{\theta_E}$$</span></p>
<p>If a black hole happens to transit near a background star (<span class="math-container">$\phi \approx \theta_E$</span> or less), then we can detect a increase in the brightness of the star, even if we can't see any distortion in the shape of the star, which still looks like a fuzzy point to our telescope. The duration of the lensing effect depends on the time it takes for the black hole to transit through the distance <span class="math-container">$\theta_E$</span>, which is typically on the order of some months.</p>
<p>A microlensing event is much easier to detect and many have been found during the searches for MACHOs. Unfortunately it is not easy to infer the mass of the lensing object, because it depends on distance and the speed of the object, which are usually unknown. Therefore, it is often difficult to say if a given microlensing event was due to a black hole, or a neutron star, or a brown dwarf ecc...</p>
<p><strong>What about binary black holes?</strong></p>
<p>The question was specifically about binary black holes. Everything above also applies to binary systems, but here a new characteristic scale enters into the problem, which is the projected orbital distance <span class="math-container">$a$</span> between the two black holes, along with its corresponding angular size</p>
<p><span class="math-container">$$\theta_a = a/d_b \approx 0.0025 {''} \times \left(\frac{a}{10 \text{AU}}\right)  \left( \frac{d_b}{4 \text{kpc}}\right)^{-1}$$</span></p>
<p>If <span class="math-container">$\theta_a \approx \theta_E$</span> the light curve of a star passing behind the binary system would appear significantly different than the one due to a normal microlensing event.</p>
<p>If <span class="math-container">$\theta_a \gg \theta_E$</span> a transiting star could intercept only one of the two black holes, making it impossible to identify the system as a binary.</p>
<p>Otherwise, a particularly far system with a close orbit could have <span class="math-container">$\theta_a \ll \theta_E$</span>. In this case, unless the star passed really close to the black holes, we would detect a lensing event attributable to a single object with mass equal to the sum of the masses of the two black holes.</p>
<p>This is the closest thing to a &quot;minimum distance&quot; I could think of. Therefore, the minimum distance between the black holes to be able to tell it is a binary system is not an absolute value, but depends on the masses of the black holes, the distance of the system from Earth and the availability of background stars.</p>

CC BY-SA 4.0
2021-08-03T15:36:26.837Z
Are there direct (not by capture of gravitational waves during their merge/collision) observational methods allowing to detect presence of binary massive dense objects (such as 2 black holes or 2 neutron stars or black hole + neutron star) and what would be their maximum distance from Earth to be detected?
Alex
CC BY-SA 4.0
2021-08-03T16:00:43.113Z
yes, gravitational lensing, pulsar timing (if one of the objects is a pulsar), x-ray emission (if there is an accretion disk)
Prallax
CC BY-SA 4.0
2021-08-02T13:35:36.237Z
2021-08-02T13:35:36.237Z
Prallax
<p>Suppose two black holes are observable by means of their lensing effect.</p>
<p>If these holes are orbiting around each other, then is there a minimum distance from each other to be still observable as separate black holes? Has this distance to do with the background of stars, the image of which they deform by lensing? Is there a minimum distance at all?</p>

CC BY-SA 4.0
2021-08-01T09:20:58.723Z
What do you mean "observable each by means of their lensing effect."  The lensing effect is a transitory effect as the black hole passes in front of a star.  You can't observe a black hole by this effect. You observe a star suddenly brighten, and then dim and deduce that a black hole must have drifted in front of it. Such events are rare.
James K
CC BY-SA 4.0
2021-08-01T09:27:58.880Z
@JamesK Then I have a wrong idea of lensing. Cant you observe a BH on its own? Doesnt the hole distort the stars behind? Of course it has to move wrt to the stars in order for that.
Deschele Schilder
CC BY-SA 4.0
2021-08-01T09:32:19.433Z
Black holes are too far away and too small to see this distortion.  Black holes are *tiny* and the distortion only occurs in a ring close to the event horizon. The exception is the EHT image of the black hole in M81, in which you could see the accretion disc lensed behind the black hole, (but you couldn't see any stars)
James K
CC BY-SA 4.0
2021-08-01T09:38:46.210Z
@JamesK But near the center light can even turn almost around it. Doesnt this give a distorted picture of the stars behind it? Say a massive hole moves along the sky. Wouldnt this make the image of the stars behind change in time?
Deschele Schilder
CC BY-SA 4.0
2021-08-01T09:57:27.333Z
Yes but that distortion is a few km across, So yeah, if you travel off until you are close to the black hole, then you'll see this distortion, but it is too small to be seen from Earth.
James K
CC BY-SA 4.0
2021-08-01T10:01:04.613Z
@JamesK I just asked a question...Im not sure I understand. What if light from galaxies bends around it? Wouldnt a small change give rise to a big change?
Deschele Schilder
CC BY-SA 4.0
2021-08-01T17:19:59.747Z
The gravitational lenses we know are whole galaxies. See https://en.wikipedia.org/wiki/Gravitational_lens In the Gallery section of that article it's mentioned that gravitational lensing can be used to observe features as "small" as 100 light-years, or less. Also see https://en.wikipedia.org/wiki/Twin_Quasar FWIW, a lot of gravitational lensing work is done using radioastronomy.
PM 2Ring
<observational-astronomy><black-hole><gravitational-lensing><microlensing>
Is there a minimum distance between two black holes in a binary system beneath which it can't be observed as a binary?
Deschele Schilder
# Is there a minimum distance between two black holes in a binary system beneath which it can't be observed as a binary?

(Asked by: username_0 on 2021-08-01)

Suppose two black holes are observable by means of their lensing effect.

If these holes are orbiting around each other, then is there a minimum distance from each other to be still observable as separate black holes? Has this distance to do with the background of stars, the image of which they deform by lensing? Is there a minimum distance at all?

**Tags:** observational-astronomy, black-hole, gravitational-lensing, microlensing

> **Comments:**
> **username_1**: What do you mean "observable each by means of their lensing effect."  The lensing effect is a transitory effect as the black hole passes in front of a star.  You can't observe a black hole by this effect. You observe a star suddenly brighten, and then dim and deduce that a black hole must have drifted in front of it. Such events are rare.  
> **username_0**: @username_1 Then I have a wrong idea of lensing. Cant you observe a BH on its own? Doesnt the hole distort the stars behind? Of course it has to move wrt to the stars in order for that.  
> **username_1**: Black holes are too far away and too small to see this distortion.  Black holes are *tiny* and the distortion only occurs in a ring close to the event horizon. The exception is the EHT image of the black hole in M81, in which you could see the accretion disc lensed behind the black hole, (but you couldn't see any stars)  
> **username_0**: @username_1 But near the center light can even turn almost around it. Doesnt this give a distorted picture of the stars behind it? Say a massive hole moves along the sky. Wouldnt this make the image of the stars behind change in time?  
> **username_1**: Yes but that distortion is a few km across, So yeah, if you travel off until you are close to the black hole, then you'll see this distortion, but it is too small to be seen from Earth.  
> **username_0**: @username_1 I just asked a question...Im not sure I understand. What if light from galaxies bends around it? Wouldnt a small change give rise to a big change?  
> **username_2**: The gravitational lenses we know are whole galaxies. See https://en.wikipedia.org/wiki/Gravitational_lens In the Gallery section of that article it's mentioned that gravitational lensing can be used to observe features as "small" as 100 light-years, or less. Also see https://en.wikipedia.org/wiki/Twin_Quasar FWIW, a lot of gravitational lensing work is done using radioastronomy.  

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## 1 answers

### Answer by username_3 on 2021-08-02. Score: 1

Gravitational lensing happens when a massive object passes (from our point of view) in front of a light source. The light rays bend and the image of the source gets distorted. A light ray that passes at a distance $r$ from a spherical object of mass $M$ is deflected by an angle

$$\theta = \frac{4GM}{rc^2}$$

**Strong lensing for a single black hole**

Consider a black hole that passes in front of a star. The maximal distortion of the image of the star is obtained when the star is directly behind the black hole. In that case instead of a point we see the star as a ring, called Einstein ring, with angular size given by

$$\theta\_E = \sqrt{\frac{GM}{c^2}\frac{d\_s-d\_b}{d\_sd\_b}}$$

where $d\_s$ is the distance from us to the star and $d\_b$ is the distance from us to the black hole. If the star is twice as distant than the black hole, $d\_s = 2d\_b$ and

$$\theta\_E = \sqrt{\frac{2GM}{c^2}\frac{1}{4d\_b}} = \frac{1}{2}\sqrt{\frac{r\_b}{d\_b}} = \frac{1}{2} \sqrt{\frac{d\_b}{r\_b}} \theta\_b$$

where $r\_b$ is the Schwarzschild radius and $\theta\_b = \frac{r\_b}{d\_b}$ is the angular radius of the black hole.

We may take $\theta\_E$ as the characteristic angular size of a lensing event. If our telescope has an angular resolution $\theta\_t < \theta\_E$ then we are able to detect that the image of the star is deformed.

We could try to search for black holes by looking towards the centre of the Milky Way, because the stars are denser and a star-black hole conjunction is more probable. The Sun is about $d=8$ kpc from the centre of the MW, if we assume a stellar black hole distant $d/2 = 4$ kpc passes in front of a star, we may write

$$\theta\_E \approx 0.0016{''} \times \left( \frac{d}{8 \text{kpc}}\right)^{-1/2} \left(\frac{M}{10M\_{\odot}}\right)^{1/2}$$

Comparing this result with the angular resolution limit from Earth-based observations (seeing) $\approx 1 {''}$, or even with Hubble's angular resolution $\approx 0.05 {''}$, the observation of a distant black hole via strong lensing appears almost hopeless.

**Microlensing for a single black hole**

This is not the end of the story, tough, because spatially resolving the Einstein ring is not the only way to detect a lensing event. A gravitational lens not only deforms the shape of the background star, but also amplifies its brightness (just like a lens!). The if the angular separation between the star and the black hole is $\phi$, then the amplification factor will be

$$A(u) = \frac{u^2+2}{u\sqrt{u^2+4}};\ \ u = \frac{\phi}{\theta\_E}$$

If a black hole happens to transit near a background star ($\phi \approx \theta\_E$ or less), then we can detect a increase in the brightness of the star, even if we can't see any distortion in the shape of the star, which still looks like a fuzzy point to our telescope. The duration of the lensing effect depends on the time it takes for the black hole to transit through the distance $\theta\_E$, which is typically on the order of some months.

A microlensing event is much easier to detect and many have been found during the searches for MACHOs. Unfortunately it is not easy to infer the mass of the lensing object, because it depends on distance and the speed of the object, which are usually unknown. Therefore, it is often difficult to say if a given microlensing event was due to a black hole, or a neutron star, or a brown dwarf ecc...

**What about binary black holes?**

The question was specifically about binary black holes. Everything above also applies to binary systems, but here a new characteristic scale enters into the problem, which is the projected orbital distance $a$ between the two black holes, along with its corresponding angular size

$$\theta\_a = a/d\_b \approx 0.0025 {''} \times \left(\frac{a}{10 \text{AU}}\right) \left( \frac{d\_b}{4 \text{kpc}}\right)^{-1}$$

If $\theta\_a \approx \theta\_E$ the light curve of a star passing behind the binary system would appear significantly different than the one due to a normal microlensing event.

If $\theta\_a \gg \theta\_E$ a transiting star could intercept only one of the two black holes, making it impossible to identify the system as a binary.

Otherwise, a particularly far system with a close orbit could have $\theta\_a \ll \theta\_E$. In this case, unless the star passed really close to the black holes, we would detect a lensing event attributable to a single object with mass equal to the sum of the masses of the two black holes.

This is the closest thing to a "minimum distance" I could think of. Therefore, the minimum distance between the black holes to be able to tell it is a binary system is not an absolute value, but depends on the masses of the black holes, the distance of the system from Earth and the availability of background stars.

> **Comments:**
> **username_4**: Are there direct (not by capture of gravitational waves during their merge/collision) observational methods allowing to detect presence of binary massive dense objects (such as 2 black holes or 2 neutron stars or black hole + neutron star) and what would be their maximum distance from Earth to be detected?  
> **username_3**: yes, gravitational lensing, pulsar timing (if one of the objects is a pulsar), x-ray emission (if there is an accretion disk)  

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