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Tell me about the physics of material "falling onto a neutron star and emitting hard x-rays" (lemmy.world)

submitted 2 years ago by j4k3 to c/askscience

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https://piped.video/watch?v=simuXjzxlGI&t=380

This video by Scott Manley is about object impact physics. The relevant clip is 6:20~6:50. Scott collides a neutron star with a simulated Earth. He mentions the ejecta should be falling onto the neutron star and emitting hard x-rays.

How would forces balance in real life?

I'm trying to mentally picture how the objects interact as gravity wells and how this might interact with the impact and velocity. Is the gravity well even relevant at high velocity?

Any neutron star is as close as possible to a black hole. Its gravity well should be capable of accelerating material at a significant fraction of the speed of light right? So does the Roche limit really obliterate the planet long before the impact as simulated? What prevents all of the planetary matter from collapsing onto the neutron star? Why were the "hard x-rays" specifically mentioned?

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[–] [email protected] 2 points 2 years ago

As a quick calculation using the Boltzman formula:

E = 3/2 k_B T

Say we imagine that the entire kinetic energy of bulk material from Earth (let's say iron) impacting the star at 10000km/s is converted into thermal kinetic energy of individual iron atoms (atomic weight 56).

1/2 m v**2 = 3/2 k_B T
T = 1/3 m v**2 / k_B
k_B = 1.38e-23 J/K
m = 0.056 kg / 6.02e23
v = 1e7 m/s
T = 1/3 * .056/6.02e23 * 1e7**2 / 1.38e-23

T = 225 GK

Looking at the black body temperature chart that 225 gigakelvin corresponds precisely to gamma rays from neutron star collisions.