# Does gravitational lensing cause a black-hole to be the main 'source' of light in a given area?

Light gets bent around high-gravity sources, such as a black-hole. A source of light, say A, is directly observed in its 'true' position and some of its light is bent around the black-hole, X, which gives a second observation of A, called A'.

Does gravitational lensing cause the amount of emissions (detected as coming from the (point/position/direction) of X) to far exceed anything else in that vicinity?

Does this then make the 'black-hole' by far the brightest, noisiest thing to be observed? (kind of like waving & shouting in a quiet room)

[edit - based on astromax's answer]
I follow what is meant regarding super massive black-holes.
Consider the following scenario: A suitable massive sun on dying contracts into a (small) black-hole. This black-hole will now cause a lensing-effect, but since it is small the amount of emissions so 'lensed' around it is very small and maybe too small to detect? or, it is detected but erroneously ascribed to be a type of sun or other type of emission source?

• Something tells me you will be very eager to read this question about how black holes are detected, gravitational lensing (the process your picture shows) is one of the methods: astronomy.stackexchange.com/q/24/96
– user96
Oct 29 '13 at 9:07
• where has your diagram come from? It seems to be suggesting that light would be bent 120 degrees around a massive object x. Feb 18 '14 at 4:20
• @Jeremy: home-drawn :) and it does exaggerate the paths ... About the bending: why not? Imagine a series of black holes each bending the light a fraction, then with enough of them you can get the light returning to source ... Feb 18 '14 at 18:03
• But there aren't a series of perfectly arranged black holes to do that. A massive object will bend the light around it like your "B" example. What you don't show, is that the result is that you will see a B1 and B2 image above and below X. You show this for A,C, and D, but those are all wildly exaggerated and unrealistic. Feb 19 '14 at 1:20
• @Jeremy: "But there aren't a series of perfectly arranged black holes to do that" - how do you know this? Also: Imagine => en.wikipedia.org/wiki/Thought_experiment Feb 19 '14 at 8:09

So the question is, Can black holes be really be extraordinarily bright due to lensing of background objects?

Let's first also specify that these black holes must certainly be super massive. If not, we're talking about stellar mass black holes (or intermediate mass black holes), and the lensing signal would be much weaker. More mass = more lensing generally speaking (there are some exceptions, e.g. - microlensing, where the lensing is done by stars in our own Milky Way galaxy and the signal is enhanced because the source and lens are aligned nearly perfectly).

This is an important statement to make because virtually every galaxy has been identified with a super massive black hole at its center. The reverse is also true - every super massive black hole is associated with a galaxy. It's also important to note that, realistically speaking, galaxies have total masses of about $\sim10^{11} - 10^{12} M_{\odot}$, whereas super massive black holes typically have masses of millions to hundreds of millions (maybe even billions) of solar masses. This is a smallish fraction of the total mass. This means that most of the lensing will be done by the galactic halo and not the super massive black hole.

If we modeled the center of the lens galaxy as a point-lens, and the galactic halo as a singular isothermal sphere, the relevant question I would ask is what are their Einstein radii (which is a measure of how effective or efficient they are at lensing) individually, and what are they in combination? Essentially, how much does the existence of a central super massive black hole matter to the system as a whole. Strong lensing features (arcs, rings, or multiple images) generally occur at around the Einstein radius of the object.

My best guess:

Quite honestly I don't see the central super massive black hole mattering all that much when it comes to lensing. Many of these mass profile models for the lens galaxy halo are singular, or rise very rapidly to a central core. Furthermore, I've never really heard of a situation where a lone super massive black hole (not associated with a galaxy - call it a 'rogue' smbh if you will) has been found floating around in space to do this sort of lensing. They generally hide at the centers of galaxies, or show themselves only if they're actively accreting material. Correct me if one has been found (maybe it would come from a merging of two smbh's where one is kicked out of the system).

• +1 Nice answer. I know it's not directly relevant to the question (not due to gravitational lensing), but perhaps it wouldn't go amiss also mentioning AGNs since the brightness of supermassive black holes is discussed. ;) Oct 29 '13 at 15:41
• Absolutely. In fact, the best probes of inner lens systems are background quasars, typically in quad image configurations called cusps, folds, and crosses. Oct 29 '13 at 21:45
• Thanks for answering; I extended my question based on your answer. Oct 30 '13 at 12:10