<p>It would be very interesting if it happened, but such an event would be very rare.</p>
<p>Black holes are relatively rare, and black hole binaries are rarer.  All the black hole mergers that we have detected have been in distant galaxies. The chance of a black hole merger happening in our galaxy is quite low.</p>
<p>Gravitational lensing occurs when a black hole (or other massive body like a neutron star or even just a brown dwarf or rogue planet) happens to pass directly in front of another star. The light from the background star is focused on the Earth, and for a short while the star appears much brighter than it had before. They are unpredictable events. To find a lensing you need to watch lots and lots of stars (with a computerised telescope) to see if any of them suddenly start to brighten.  With this in mind  microlensing events can be caused by black holes in our galaxy, acting as a lens, usually against field stars near the galactic bulge. Lensing is rare.  Although they are rare, the galaxy is big and there are probably lots of black holes, nevertheless only a handful of lensing events have been confidently ascribed to black holes. Most Other galaxies are too far away to have resolved stars that we can monitor for lensing events.</p>
<p>So given the rarity of these events, the chance that a merger will occur in our galaxy just after lensing is minute. Not impossible but very very unlikely.</p>

CC BY-SA 4.0
2021-07-31T22:31:03.463Z
Why you have narrowed your answer to this question just to the realm of our (Milky Way) galaxy?
Alex
CC BY-SA 4.0
2021-07-31T22:51:33.953Z
I'm not aware of any microlensing events observed by black holes in other galaxies.  The OGLE project looks for black holes in the milky way.  Looking for black holes in nearby galaxies by microlensing is probably possible. But all the black hole mergers have been in distant galaxies. Too far to resolve individual stars.  So it is very unlikely that a lensing event will occur at just the right time, though It would be interesting if it did.
James K
CC BY-SA 4.0
2021-07-31T23:04:11.617Z
So, if a lensing event is to be observed it must be in the milky way, or possibly a neighbouring galaxy like the LMC. We've never observed a merger from anything like as close.
James K
CC BY-SA 4.0
2021-07-31T23:18:15.550Z
Re "I'm not aware..."    Using ESA’s XMM-Newton and NASA’s NuSTAR space telescopes, an international team of scientists led by Dan Wilkins of Stanford University in the USA observed extremely bright flares of X-ray light coming from around a black hole. This supermassive black hole is 10 million times as massive as our Sun and located in the center of a nearby spiral galaxy called I Zwicky 1, 800 million light-years away from Earth. 
                                              
https://scitechdaily.com/light-echo-from-behind-a-black-hole-confirms-einsteins-theory-of-general-relativity/
Alex
CC BY-SA 4.0
2021-08-01T00:35:52.783Z
That's not gravitational lensing. That's about a X-ray flare in the accretion disc around a black hole.  Completely different phenomena.
James K
CC BY-SA 4.0
2021-08-01T00:50:59.053Z
I used GL in my question but really meant any observable image effect phenomenon difference caused the one (vs two) sufficiently dense massive object(s) including X-ray flare accretion around a black hole... 
Perhaps you could help me to edit/correct/generalize my question if possible?
Alex
CC BY-SA 4.0
2021-08-01T23:34:44.647Z
https://ccrg.rit.edu/content/grants/2018/manuela-campanelli-rit/advancing-computational-methods-understand-dynamics
Alex
CC BY-SA 4.0
2021-08-02T18:10:18.043Z
https://www.nasa.gov/mission_pages/chandra/news/black-hole-image-makes-history
Alex
CC BY-SA 4.0
2021-08-02T20:01:58.210Z
https://www.ozgrav.org/news/archives/04-2021
Alex
CC BY-SA 4.0
2023-07-01T19:25:50.233Z
Article by Souvik Jana, Parameswaran Ajith, Tejaswi Venumadhav Nerella et al, published in Physical Review Letters proposes to use gravitational lensing observed during the merge of two binary black holes for measuring cosmic expansion. The authors expect these advanced detectors to start their search for merging black holes in the next decade. They anticipate recording signals from a few million black hole pairs, a small fraction (about 10,000) of which will appear multiple times in the same detector due to gravitational lensing.
Alex
CC BY-SA 4.0
2023-07-18T19:43:38.833Z
https://www.space.com/gravitational-waves-lensing-universe-expansion
Alex
CC BY-SA 4.0
2023-07-18T20:50:08.493Z
Is there some point to all these links.  They are at most tangentially related to your question as asked.
James K
CC BY-SA 4.0
2023-07-19T14:09:13.347Z
I think there is a point.
Alex
CC BY-SA 4.0
2023-07-19T14:13:54.370Z
Are you aware of the following info: https://www.space.com/gravitational-waves-lensing-universe-expansion#xenforo-comments-62118 

"Gravitational lensing is usually associated with light. But gravitational waves — ripples in space-time created by accelerating objects of enormous mass, such as two black holes spiraling toward each other — should be similarly affected. That means gravitational waves from these violent merger events should also display gravitational lensing, as light does."
Alex
CC BY-SA 4.0
2023-07-19T16:02:12.047Z
My question was covering three keywords: 1). GW; 2). Merge of massive and supermassive objects; 3). GL So any information which deals with the information showing connection between those 3 is relevant.
Alex
CC BY-SA 4.0
2023-07-19T19:08:18.110Z
in https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.130.261401 it also is asking for creating the catalog of all registered GW receptions events, which would contain detailed info/data about what was the cause of the GW. If you recall I was asking similar (in spirit) question about existence of the all known Black Holes catalog...
Alex
CC BY-SA 4.0
2021-07-31T22:09:17.103Z
2021-08-01T00:42:07.983Z
2021-08-01T00:42:07.983Z
James K
<p>There is a chance. If you observe gravitational lensing of two black holes close enough to us or far away and massive enough, like two supermassive black holes (look <a href="https://en.m.wikipedia.org/wiki/Binary_black_hole" rel="nofollow noreferrer">here</a>) then it depends on the situation of the binary system if you can observe gravitational waves. The detectable waves appear shortly before the holes form one hole (before they merge). If you observe the lensing then you must be very lucky to observe the waves also. You can aim the LIGO detector at the binary and wait for the waves. It depends on the resolution of your lensing observation if you can observe it is a system of two holes. This should in principle give a different lensing effect, as you can imagine.</p>
<p>So if you observe a double lensing effect you can aim your GW detecting device at them. But maybe it's better to make a calculation how long it takes before the merging actually happens. It's a way to find out where to look though instead of waiting for a wave to pass. Or, let's say it's a way to know from where the GW's came if they were detected as the GW-detector can't be aimed. It just waits for the waves to pass. You can anticipate an eventual burst if a binary hole is detected by the lensing effect. But then again, you can <em>always</em> anticipate such waves, of which LIGI is the very expression. Observing binaries doesnt say anything about when an eventual wave arrives. If a wave arrives you can use the lensing observations to confirm the source though. If there is nothing to see optically anymore after the wave has passed but still a lensing effect can be seen then you know that the source was a binary hole merger.</p>
<p>The chance is very small though. Two black holes can circle one another for a very long time. Like the Earth circles around the sun. A merger takes a few seconds. You can say: near zero chance.</p>
<p>But...As I found out by asking another question, black holes or binary systems of them, can't be detected by lensing. Only when you are very close or if the holes have a super size. In the last case they are accompanied by accretion discs and thus observable optically ( as the supermassive black hole recently photographed by the Event Horizon Telescope). It would be nice to see two supermassive holes merge! Spectacle!</p>

CC BY-SA 4.0
2021-07-31T22:40:40.010Z
I am looking for the quantitative estimation of such chance... I am also interested to find out via answers how the gravitational lensing caused by the two (sufficiently massive) black holes which are in the process of merging be observationally distinguished from gravitational lensing caused by the single "stable state" black hole...
Alex
CC BY-SA 4.0
2021-07-31T22:48:34.407Z
@Alex I don't think you can see a close merger resolved in two images by lensing. The merger event looks like one black hole. Only if they are at a sufficient distance from one another can they be resolved by observing lensing.
Deschele Schilder
CC BY-SA 4.0
2021-07-31T23:05:05.687Z
I am on contrary hope that there might be some peculiarities in observing light path behavior in the system of two black holes prior to their final merge event characterized by issuing gravitational wave.
Alex
CC BY-SA 4.0
2021-07-31T23:08:15.253Z
@Alex Hope there is always...only false hope is to reject
Deschele Schilder
CC BY-SA 4.0
2021-08-01T07:43:23.943Z
"You can aim the LIGO detector at the binary". No, you can't.
ProfRob
CC BY-SA 4.0
2021-08-01T07:47:13.353Z
Thats what I said. in the answer. It is fixed and waits. No directing. There is an orietation for which the detection is easiest.
Deschele Schilder
CC BY-SA 4.0
2021-08-01T08:37:23.210Z
Well take out the incorrect statement then.
ProfRob
CC BY-SA 4.0
2021-08-01T09:41:20.070Z
@ProfRob The wrong statement is used to accentuate the right one.
Deschele Schilder
CC BY-SA 4.0
2021-08-02T14:39:20.890Z
I find the aiming of LIGO confusing as well
Prallax
CC BY-SA 4.0
2023-07-01T19:26:21.757Z
Article by Souvik Jana, Parameswaran Ajith, Tejaswi Venumadhav Nerella et al, published in Physical Review Letters proposes to use gravitational lensing observed during the merge of two binary black holes for measuring cosmic expansion. The authors expect these advanced detectors to start their search for merging black holes in the next decade. They anticipate recording signals from a few million black hole pairs, a small fraction (about 10,000) of which will appear multiple times in the same detector due to gravitational lensing.
Alex
CC BY-SA 4.0
2021-07-31T22:17:35.760Z
2021-08-02T11:00:06.050Z
2021-08-02T11:00:06.050Z
Deschele Schilder
<p>What is the probability to observe gravitational lensing (or any other image related phenomena) from the pair of black holes (or pair of neutron stars or black hole - neutron star) which are about to merge and then catching the subsequent gravitational wave produced at their merge?
I am also interested to find out via answers how the observation of the gravitational lensing (or any other image related phenomena), caused by the two sufficiently massive objects (itemized above), which are in the process of merging, be distinguished from similar by its nature phenomenon caused by the single &quot;stable state&quot; object (again itemized above).</p>

CC BY-SA 4.0
2023-07-11T01:57:44.023Z
Extending my question to the gravitational lensing due to the merge of the objects much more massive than black holes and neutron stars - here is the link to the article describing lensing effect caused by the merge of Galaxy Clusters: https://scitechdaily.com/einsteins-monsters-hubble-space-telescope-observes-merging-galaxy-clusters/. I wonder if such merge causes low frequency gravitational waves, which are much more technically difficult to detect?
Alex
CC BY-SA 4.0
2023-07-11T17:12:06.973Z
The new evidence for ultra-low-frequency gravitational waves from supermassive black hole pairs.
https://iopscience.iop.org/article/10.3847/2041-8213/acdd02
Alex
CC BY-SA 4.0
2023-07-18T19:37:19.847Z
https://www.space.com/gravitational-waves-lensing-universe-expansion
Alex
<black-hole><gravitational-waves><neutron-star><gravitational-lensing><x-ray>
Could observation of GL or other image phenomena from system of 2 merging massive dense objects and of GW caused by their merge be connected?
Alex
# Could observation of GL or other image phenomena from system of 2 merging massive dense objects and of GW caused by their merge be connected?

(Asked by: username_0 on 2021-07-31)

What is the probability to observe gravitational lensing (or any other image related phenomena) from the pair of black holes (or pair of neutron stars or black hole - neutron star) which are about to merge and then catching the subsequent gravitational wave produced at their merge?
I am also interested to find out via answers how the observation of the gravitational lensing (or any other image related phenomena), caused by the two sufficiently massive objects (itemized above), which are in the process of merging, be distinguished from similar by its nature phenomenon caused by the single "stable state" object (again itemized above).

**Tags:** black-hole, gravitational-waves, neutron-star, gravitational-lensing, x-ray
> **Comments:**
> **username_0**: Extending my question to the gravitational lensing due to the merge of the objects much more massive than black holes and neutron stars - here is the link to the article describing lensing effect caused by the merge of Galaxy Clusters: https://scitechdaily.com/einsteins-monsters-hubble-space-telescope-observes-merging-galaxy-clusters/. I wonder if such merge causes low frequency gravitational waves, which are much more technically difficult to detect?  
> **username_0**: The new evidence for ultra-low-frequency gravitational waves from supermassive black hole pairs. https://iopscience.iop.org/article/10.3847/2041-8213/acdd02  
> **username_0**: https://www.space.com/gravitational-waves-lensing-universe-expansion  

---

## 2 answers

### Answer by username_1 on 2021-07-31. Score: 3 | Accepted answer

It would be very interesting if it happened, but such an event would be very rare.

Black holes are relatively rare, and black hole binaries are rarer. All the black hole mergers that we have detected have been in distant galaxies. The chance of a black hole merger happening in our galaxy is quite low.

Gravitational lensing occurs when a black hole (or other massive body like a neutron star or even just a brown dwarf or rogue planet) happens to pass directly in front of another star. The light from the background star is focused on the Earth, and for a short while the star appears much brighter than it had before. They are unpredictable events. To find a lensing you need to watch lots and lots of stars (with a computerised telescope) to see if any of them suddenly start to brighten. With this in mind microlensing events can be caused by black holes in our galaxy, acting as a lens, usually against field stars near the galactic bulge. Lensing is rare. Although they are rare, the galaxy is big and there are probably lots of black holes, nevertheless only a handful of lensing events have been confidently ascribed to black holes. Most Other galaxies are too far away to have resolved stars that we can monitor for lensing events.

So given the rarity of these events, the chance that a merger will occur in our galaxy just after lensing is minute. Not impossible but very very unlikely.

> **Comments:**
> **username_0**: Why you have narrowed your answer to this question just to the realm of our (Milky Way) galaxy?  
> **username_1**: I'm not aware of any microlensing events observed by black holes in other galaxies.  The OGLE project looks for black holes in the milky way.  Looking for black holes in nearby galaxies by microlensing is probably possible. But all the black hole mergers have been in distant galaxies. Too far to resolve individual stars.  So it is very unlikely that a lensing event will occur at just the right time, though It would be interesting if it did.  
> **username_1**: So, if a lensing event is to be observed it must be in the milky way, or possibly a neighbouring galaxy like the LMC. We've never observed a merger from anything like as close.  
> **username_0**: Re "I'm not aware..."    Using ESA’s XMM-Newton and NASA’s NuSTAR space telescopes, an international team of scientists led by Dan Wilkins of Stanford University in the USA observed extremely bright flares of X-ray light coming from around a black hole. This supermassive black hole is 10 million times as massive as our Sun and located in the center of a nearby spiral galaxy called I Zwicky 1, 800 million light-years away from Earth.                                                 https://scitechdaily.com/light-echo-from-behind-a-black-hole-confirms-einsteins-theory-of-general-relativity/  
> **username_1**: That's not gravitational lensing. That's about a X-ray flare in the accretion disc around a black hole.  Completely different phenomena.  
> **username_0**: I used GL in my question but really meant any observable image effect phenomenon difference caused the one (vs two) sufficiently dense massive object(s) including X-ray flare accretion around a black hole...  Perhaps you could help me to edit/correct/generalize my question if possible?  
> **username_0**: https://ccrg.rit.edu/content/grants/2018/manuela-campanelli-rit/advancing-computational-methods-understand-dynamics  
> **username_0**: https://www.nasa.gov/mission_pages/chandra/news/black-hole-image-makes-history  
> **username_0**: https://www.ozgrav.org/news/archives/04-2021  
> **username_0**: Article by Souvik Jana, Parameswaran Ajith, Tejaswi Venumadhav Nerella et al, published in Physical Review Letters proposes to use gravitational lensing observed during the merge of two binary black holes for measuring cosmic expansion. The authors expect these advanced detectors to start their search for merging black holes in the next decade. They anticipate recording signals from a few million black hole pairs, a small fraction (about 10,000) of which will appear multiple times in the same detector due to gravitational lensing.  
> **username_0**: https://www.space.com/gravitational-waves-lensing-universe-expansion  
> **username_1**: Is there some point to all these links.  They are at most tangentially related to your question as asked.  
> **username_0**: I think there is a point.  
> **username_0**: Are you aware of the following info: https://www.space.com/gravitational-waves-lensing-universe-expansion#xenforo-comments-62118   "Gravitational lensing is usually associated with light. But gravitational waves — ripples in space-time created by accelerating objects of enormous mass, such as two black holes spiraling toward each other — should be similarly affected. That means gravitational waves from these violent merger events should also display gravitational lensing, as light does."  
> **username_0**: My question was covering three keywords: 1). GW; 2). Merge of massive and supermassive objects; 3). GL So any information which deals with the information showing connection between those 3 is relevant.  
> **username_0**: in https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.130.261401 it also is asking for creating the catalog of all registered GW receptions events, which would contain detailed info/data about what was the cause of the GW. If you recall I was asking similar (in spirit) question about existence of the all known Black Holes catalog...  

---

### Answer by username_2 on 2021-07-31. Score: -2

There is a chance. If you observe gravitational lensing of two black holes close enough to us or far away and massive enough, like two supermassive black holes (look [here](https://en.m.wikipedia.org/wiki/Binary_black_hole)) then it depends on the situation of the binary system if you can observe gravitational waves. The detectable waves appear shortly before the holes form one hole (before they merge). If you observe the lensing then you must be very lucky to observe the waves also. You can aim the LIGO detector at the binary and wait for the waves. It depends on the resolution of your lensing observation if you can observe it is a system of two holes. This should in principle give a different lensing effect, as you can imagine.

So if you observe a double lensing effect you can aim your GW detecting device at them. But maybe it's better to make a calculation how long it takes before the merging actually happens. It's a way to find out where to look though instead of waiting for a wave to pass. Or, let's say it's a way to know from where the GW's came if they were detected as the GW-detector can't be aimed. It just waits for the waves to pass. You can anticipate an eventual burst if a binary hole is detected by the lensing effect. But then again, you can *always* anticipate such waves, of which LIGI is the very expression. Observing binaries doesnt say anything about when an eventual wave arrives. If a wave arrives you can use the lensing observations to confirm the source though. If there is nothing to see optically anymore after the wave has passed but still a lensing effect can be seen then you know that the source was a binary hole merger.

The chance is very small though. Two black holes can circle one another for a very long time. Like the Earth circles around the sun. A merger takes a few seconds. You can say: near zero chance.

But...As I found out by asking another question, black holes or binary systems of them, can't be detected by lensing. Only when you are very close or if the holes have a super size. In the last case they are accompanied by accretion discs and thus observable optically ( as the supermassive black hole recently photographed by the Event Horizon Telescope). It would be nice to see two supermassive holes merge! Spectacle!

> **Comments:**
> **username_0**: I am looking for the quantitative estimation of such chance... I am also interested to find out via answers how the gravitational lensing caused by the two (sufficiently massive) black holes which are in the process of merging be observationally distinguished from gravitational lensing caused by the single "stable state" black hole...  
> **username_2**: @Alex I don't think you can see a close merger resolved in two images by lensing. The merger event looks like one black hole. Only if they are at a sufficient distance from one another can they be resolved by observing lensing.  
> **username_0**: I am on contrary hope that there might be some peculiarities in observing light path behavior in the system of two black holes prior to their final merge event characterized by issuing gravitational wave.  
> **username_2**: @Alex Hope there is always...only false hope is to reject  
> **username_3**: "You can aim the LIGO detector at the binary". No, you can't.  
> **username_2**: Thats what I said. in the answer. It is fixed and waits. No directing. There is an orietation for which the detection is easiest.  
> **username_3**: Well take out the incorrect statement then.  
> **username_2**: @ProfRob The wrong statement is used to accentuate the right one.  
> **username_4**: I find the aiming of LIGO confusing as well  
> **username_0**: Article by Souvik Jana, Parameswaran Ajith, Tejaswi Venumadhav Nerella et al, published in Physical Review Letters proposes to use gravitational lensing observed during the merge of two binary black holes for measuring cosmic expansion. The authors expect these advanced detectors to start their search for merging black holes in the next decade. They anticipate recording signals from a few million black hole pairs, a small fraction (about 10,000) of which will appear multiple times in the same detector due to gravitational lensing.  

---

