Black Holes Destroy Dark Matter And Emit Gamma Rays
Black holes can cause dark matter to annihilate in their vicinity by concentrating the dark matter and enhancing the collision rate between dark matter particles.
The best observational candidates are supermassive black holes, such as the 4 million solar mass black hole found at the center of our Milky Way galaxy.
Some galaxies have much larger supermassive black holes, reaching as high as several billion or even tens of billions of solar masses. Most massive galaxies appear to have supermassive black holes in their centers.
We infer the existence of supermassive black holes through their effect on nearby stellar or molecular cloud orbits.
And we more directly detect supermassive black holes (SMBHs) by the radiation emitted from ordinary matter that is near the black hole (BH), but has not yet fallen into the BH’s event horizon.
Such matter will often form a hot accretion disk around the SMBH.
The disk or other infalling matter can be heated to millions of degrees by the strong gravitational potential of the BH as the kinetic energy of infall is converted to thermal energy by frictional processes.
Ordinary matter (OM) heated to such high temperatures will give off X-rays.
Now if OM is being pulled into a SMBH, so is dark matter, which pervades every galaxy. Dark matter (DM) responds to the same gravitational potential from the SMBH.
The difference is that OM is collisional since it easily interacts with other OM via the electromagnetic force, whereas DM is generally collisionless, since it does not interact via electromagnetism.
Nevertheless DM – DM collisions can occur, rarely, via a ‘direct hit’ (as if two bullets hit each other in mid-air) and this leads to annihilation.
Two DM particles meet directly and their entire energy content, from their rest mass as well as their kinetic energy of motion, is converted into other particles.
The cross-section strength is not known, but it must be small due to observational limits, yet is expected to be non-zero. The most likely candidates for decay products are expected to be photons, neutrinos, and electrons.
The leading candidate for DM is some sort of weakly interacting massive particle with a mass of perhaps 5 to 300 GeV; this is the range where DM searches from satellites and on Earth are focused.
So if two DM particles mutually annihilate, there is of order 10 GeV to 600 GeV of available rest mass energy to produce highly energetic gamma rays.
The likelihood of a direct hit is proportional to the square of the density of the DM.
A SMBH’s gravitational potential acts as a concentrator for DM, allowing the density to be high enough that there could be a significant number of annihilation events, resulting in a detectable flux of escaping photons reaching Earth.
Relativistic effects work to further increase the annihilation rate. And it is possible that the annihilation signal could scale as M³ (mass of the SMBH cubed), and thus the most massive SMBHs would be very strong gamma ray emitters.
These would be highly energetic gamma rays with well over 1 GeV of energy.
The search for gamma rays from annihilating DM around SMBHs is already underway. There is in fact a possible detection by the Fermi telescope at 130 GeV in our Milky Way galaxy, from the direction of the Sagittarius A* SMBH.
Future more sensitive gamma ray surveys may lead to many detections, helping us to better understand both dark matter and black holes.
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