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As I understand it, objects falling into an old/inactive/dead/non-pulsar neutron star would be ripped apart, form an accretion disc and eventually emit a burst of radiation as they fall onto the surface of the star.

If this happened to:

  1. a comet
  2. a dwarf planet
  3. an ice giant

...roughly how much energy/radiation would each produce? Also, if possible, over what kind of time frame? I'm only looking for ballpark figures in all cases.

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    $\begingroup$ Roughly 1 atomic bomb per kg of mass. Don't worry, they are too far away to be even visible. Note, collision is very rare, it requires a direct hit. Otherways it will go on a hyperbolic orbit and never collides the neutron star. Hopefully you will get a more detailed answer, too. $\endgroup$ – peterh says reinstate Monica Mar 18 at 7:57
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Back of the envelope? The energy released is about $GM/R$ per unit mass. For a 1.5 solar mass neutron star of radius 10 km, this amounts to $2\times 10^{16}$ J/kg, or approximately 2/9 of the rest mass energy of the accreted material.

Of order half of this would be radiated and half would go into heating the neutron star.

Timescales are more difficult. Starting when? Once the material gets to the innermost stable circular orbit at around 15 km, then the final 5km to the surface takes a tiny fraction of a second. But from where an object is shredded (much further out), much would depend on its angular momentum.

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