Could we see a gamma ray source on the Moon?

If a gamma ray source were placed on the moon and oriented properly, could it be detected on earth?

Gamma rays are one of the highest energy forms of light in the Universe - its wavelength is around a picometer (that’s 10^-12 meters), which is roughly the same order of magnitude as the size of a hydrogen atom. These wavelengths are a thousand times shorter than that of visible light.

The high energies of gamma rays means that this form of light is extremely damaging to life. Ultraviolet (UV) light doesn’t penetrate far into your skin - it can damage the surface layers of your skin, giving you a sunburn, but it won’t damage anything past the first few layers of your skin. (UV light will, however, increase your chances of skin cancer, if you get severe sunburns.) Gamma rays can penetrate much further into your body, and can destroy or alter the DNA within your cells, causing drastic changes to the replication instructions of your cells. This can cause radiation sickness and/or cancer to appear, but with no surface burning of your skin.

Fortunately, our atmosphere is opaque to gamma rays, so the average level of gamma radiation coming towards our planet is blocked out by our atmosphere. Our atmosphere does a wonderful job of protecting us from the beating our cells would otherwise be taking.

On the other hand, the fact that our atmosphere is such an effective wall to this wavelength of light means that we can’t observe any of the gamma rays produced elsewhere in the universe (which is a very interesting field with a lot of science to be done) from the surface of the Earth. In order to look at this light, we have to put our telescopes into space, outside of our atmosphere. At the moment, we have the Fermi Gamma Ray Space Telescope observing the whole sky for gamma ray sources; that telescope produced the map of the sky at the top of the page. The bright line through the middle is all gamma radiation from within our own galaxy. Anything far away from that central region is probably coming from another galaxy. Individual bright points in our galaxy are likely to be coming from the aftermath of a supernova.

The moon, unlike the Earth, doesn’t have an atmosphere to protect it from any kind of battering, which means that there’s a constant flow of high energy light and small particles pounding into the surface of the moon. One of these objects smacking into the surface of the moon is called a cosmic ray (which is a highly accelerated tiny piece of grit - most of them are protons). Cosmic rays are constantly streaming throughout the universe. When they smash into the surface of the moon, they can create gamma radiation as they come to a stop.

This means that the moon itself glows faintly in gamma rays, and this is detectable by our space based telescopes, which can observe gamma rays. (Again, we couldn’t detect this from the surface of the Earth, because our atmosphere gets in the way.) The image below was produced by the Compton Gamma Ray Observatory, which was in orbit around the Earth from 1991 - 2000, when it was instructed to crash itself back into our ocean. The moon, as imaged below, is actually brighter than our sun in gamma rays - our sun, very fortunately, does not produce much in the way of gamma radiation. (If it did, the gamma radiation from such a close source might have destroyed our atmosphere, removing our shelter from this wavelength.)



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How are gamma ray jets being generated from black holes?

There are two ways to create gamma rays from a black hole, and the two are distinguished by the size of black hole that creates them. The first is called a Gamma Ray Burst, and is sometimes formed during the creation of a “stellar mass” black hole, as a star collapses at the end of its life. Gamma Ray Bursts are the most intense bursts of gamma radiation in the universe, but are pretty rare and therefore hard to spot. These bursts happens over the period of less than a minute - incredibly short, especially given the lengthy timescales that astronomy usually operates on. They are created during what’s called a hypernova - the amped up cousin of a supernova, where the original star is at least 15 times the mass of our sun. In addition to the high mass of the star, in order to get the gamma rays out, you also need to have the star rapidly rotating before it explodes & collapses.

The second way that black holes can produce gamma radiation is via objects we call quasars - an abbreviated form of a “quasi-stellar object”. This is a remnant from the days in which we kept finding these super bright, point-like objects (like stars) but had really bizarre spectra (definitely not like stars). They were dubbed “quasi-stellar” in nature due to them looking like a star, but not behaving like a star. We now believe that quasars are the signature of supermassive black holes at the centers of galaxies that are currently attempting to grow by inefficiently pulling in material from a rapidly spinning disk of material that surrounds the black hole.

The formation of gamma rays in both Gamma Ray Bursts and in quasars occurs in a very similar manner. For a Gamma Ray Burst, the rapid rotation of the dying star twists up the magnetic field of the dying star, and means that the easiest means of escape for any of the particles being produced in the supernova is along a very narrow ‘beam’ at the poles of rotation, since the magnetic field can’t get as tangled in that direction. However, many of the particles being given off are electrons, not photons; getting from a beam of electrons to a beam of gamma ray photons requires a few transfers of energy. There are a couple of ways of doing this. One method of energy transfer happens when electrons travel along the untangled magnetic field lines extending outwards along the beam. The magnetic field causes the electrons to travel in a helix, as though they were tracing the path of an extremely long spiral staircase. These spiraling electrons give off high energy particles of light; if the electrons are moving fast enough, they can spit out gamma rays. The other way you can create gamma ray photons is more straightforward: via collision. If you take a extremely speedy electron and crash it into a photon, the photon can gain enough energy to become a gamma wave. The end result of both of these processes is a pencil beam of gamma radiation speeding outward in two jets at the north and south poles of the spinning star. These jets are only stable for a few tens of seconds before the black hole forms, and they vanish.

Getting gamma radiation out of a quasar happens in a very similar way to the Gamma Ray Burst. We believe there must be a magnetic field surrounding the disk of material surrounding the black hole (called the accretion disk), and this disk must be rapidly rotating. The black hole itself may also be rapidly spinning, but that depends on which theoretical physicist you speak to. The rapid spin of the accretion disk, in combination with the magnetic field, produces jets, which can accelerate particles (again, largely electrons) all the way up to gamma ray energies. These quasar jets, in contrast to those produced by Gamma Ray Bursts, are very stable, and can reach lengths of hundreds of thousands of light years. If we are able to spot them, it means that that quasar just so happened to have been aimed in our direction as it shot a bright beam of light across the cosmos.

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