What impact would a white dwarf-neutron star collision have on a civilization on a planet orbiting the white dwarf?

Question

An advanced civilization lives on an Earth-like planet orbiting a white dwarf. Their star is on a collision course with a neutron star. I imagine this collision will cause a supernova, which will obliterate the planet. But what other effects will this situation have on the civilization/planet, prior to the supernova event (e.g., increased radiation)? And at what point is the civilization likely to die out (probably much earlier than the supernova itself)? If it helps, I imagine the aliens to be an avian species, like large owls.

Answer 1

Gravitational waves

Presumably, there would be gravitational waves emitted as the two compact objects slowly came close to merging. Given their small cross-sections, a direct, head-on collision is highly unlikely. A more plausible scenario involves an interaction with a third massive body.$^{\dagger}$ The white dwarf and neutron star would be inserted into a tight orbit around each other, while that third object would be ejected from the system.

Any two bodies orbiting each other will emit gravitational waves. Usually, these are fairly insignificant up until the bodies merge. In particular, at a distance $r$ from the binary, the measured strain would be $$h\sim\frac{GM}{c^2}\frac{1}{r}\left(\frac{v}{c}\right)^2$$ If we take $M\approx3M_{\odot}$, $v/c\approx0.6$ (realistic shortly before the merger) and $r=1\text{ AU}$, we would find $h\sim10^{-8}$. This is roughly 13 orders of magnitude higher than the typical black hole-black hole merger detected by LIGO as measured on Earth - not negligible, but not enough to cause serious damage.

Thermal x-rays from an accretion disk

If we assume that the neutron star arrived with a companion, losing it in the three-body interaction, it's quite possible that it was accreting mass and thereby formed an accretion disk. This disk is presumably quite hot, with temperatures in the millions of Kelvin. This in turn would lead to thermal x-ray emission. Depending on the size and orientation of the disk, this could prove dangerous for the planet. (This statement is either quite conservative or the understatement of the year!) I'd assume there would also be non-thermal emission of some sort, but I don't know enough about the relevant processes to say anything intelligent on the matter. I'm certainly worried about the thermal x-rays, though.

Additional tidal activity

It's possible for tidal forces to mediate other interactions between the two objects before the merger. Mass transfer could actually occur, flowing from the white dwarf to the neutron star, and possibly even resulting in the complete tidal disruption of the white dwarf (Verbunt & Rappaport 1988). The resulting structure - either a disk or a torus - would provide another source of high-energy radiation.

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$^{\dagger}$ This third body could be either a then-ejected companion to the neutron star or else one of the planets in the system, which would then be tossed out. This also brings up another point: The odds of the system retaining the planets up to the merger seem quite low.