Chapter 3: Neutrons

GRB Pulses

I just described what could be called a generic GRB. But astronomical observations have shown that GRBs have a wide variety of energy signatures, typically coming in a series of pulses. There are several possible candidates for these pulses but, quite honestly, I don’t know which of them is the most important. When the first cloud of neutrons explodes, the blast will inevitably send yet another shock wave into the remaining neutron core. And that shock wave will also rebound off the center, bounce back to the surface, and rip off another layer of the core’s skin. This second cloud of neutrons could easily be even more powerful than the first, because, unlike the first cloud, it will not find any protons with which to form stable atoms. All the elements have already been blasted away by the first explosion. How many of these neutron blasts, shock waves, neutron expulsions, and subsequent explosions a given GRB can sustain, almost certainly depends on its mass (just like everything else about stellar phenomena). The size, strength, and speed of formation of the supermassive black hole at the center of the neutron core will also place an upper limit on the number of pulses. As the black hole swallows the core—a phenomenon known as a quasar—additional blasts and shock waves will be quickly attenuated.

It might seem at first that the pulses just mentioned would have to be at least ten minutes apart, since that is the time needed to account for two consecutive neutron decay events. However, once the skin of the neutron core is exposed, the neutrons near the surface begin decaying immediately. By the time the shock wave from the first blast tears a second cloud from that skin, the neutrons are already well on their way to decaying. They haven’t completely decayed yet because their “decay clock” runs somewhat slower in the extreme gravitational field of the star; neutron decay rate is related to the pressure they are under. Once blasted away from the star, this second batch of neutrons decays much more quickly than the first, because their decay is already well underway. Moreover, because all the neutrons in this entire phenomenon are subjected to very nearly the same conditions, the actual moment of decay will be much the same for all of them. The concept of a half-life, as we will see, is not a matter of quantum uncertainty, but of a lack of knowledge regarding the true state of the system in question. Therefore, it is reasonable to expect each successive pulse to be much closer together than the expected ten minutes.

Another possibility for some of the pulses (and these candidates are not mutually exclusive) is the successive decay of various unstable isotopes. The first big blast is caused primarily by free neutrons, but there are many atomic isotopes that decay within only seconds of their formation. If we consider the sheer quantity of such isotopes that are created when the neutron skin is thrown into the proton mantle, it is reasonable to assume that, as these isotopes decay in reverse order of stability, the ensuing explosions would appear as a series of energetic gamma ray pulses. The strength and duration of these pulses will depend very sensitively on the exact ratio of neutrons to protons during this rapid nucleosynthetic process. They will also depend on how far into the receding mantle matter the neutrons are flung. If the proton and neutron clouds are mixed together extensively then, all else being equal, a lower ratio of unstable isotopes is formed than if, say, the neutrons are tossed only about halfway into the proton cloud and are forced to make whatever they can of the few protons at their disposal. The strength of the ensuing explosions, in turn, dictates the power of any subsequent shock waves impinging on the remaining neutron core. And that dictates the strength of the next explosion. And on and on and on. Needless to say, the possibilities are practically endless, and so it is no surprise that GRBs come in so many varieties.

Here is yet another consideration. Some GRBs (as well as some supernovae) have a strong hydrogen spectral line while others do not. If the neutron skin is catapulted all the way through the proton cloud, virtually every proton will be captured to form complex atoms. If, on the other hand, the neutron cloud penetrates only half of the proton cloud (as in the diagram above), the leading edge of the GRB (or supernova) will be composed entirely of hydrogen. In all of the variations we are considering, the mass of the galactic star is decisive. In time, it should be possible to categorize these differences and use them to determine the relative masses of the parent stars.

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