How can you tell for sure whether something is a black hole or a regular rock of the same mass, when you're too far away to resolve the size of the object in a telescope?
Do you have to send a probe close and throw something in it?
Do you have to send a probe close and throw something in it?
One possibility is gravitational lensing. The abstract also mentions detecting "annihilation signals from the dark matter microhalo around the PBH". Not that I understand that completely, and I don't believe (corrections welcome) that dark matter annihilation by black holes has been confirmed experimentally.
I believe high energy particles are released by black holes as they consume matter, so that might be another way to detect a small black hole instead of a similar mass rock.
From a distance there is no difference in lensing between a black hole and an regular object of the same mass.
From close there would be a difference, but if you could resolve something that small you would be able to directly image the object.
An earth mass blackhole is going to have 10, 1000s, or even millions of Gs depending on how close the light gets.
Seems like you'd get stars periodically blinking from places they shouldn't be, as light trajectory should have missed earth, but gets bent by the black hole. So while the black hole itself would be invisible at that distance, and occultations would be invisible, seeing stars in the wrong place would still be visible.
You are only bending a minute faction of the light from the star traveling toward you. It's not enough light to see, and it's too small to resolve (unless you could resolve the black hole in the first place).
Think about it: The redirected light only has the "size" of the gravitation field in question. For an earth sized mass thats 9mm. Even if you did 9km (which wouldn't bend much) you couldn't see it, never mind 9mm.
However if it's big enough to be messing with orbits around neptune it's likely pretty large. If it's large then the hawkings radiation isn't going to be visible.
So if it's moon mass it's going to be 0.1mm and at 2.7 kelvin or so. Earth mass is 9mm, but even colder. Generally for the orbital changes they are seeing in a wide variety of objects it seems like the mass is even larger still.
Even seeing pluto is hard, even a jupyter mass black hole is only 3 meters or so. So no I don't think we could detect it via hawking radiation.
We might however see high energy particles resulting from the somewhat messy feeding that black holes are known for. If we sent a probe it could plausibly get close enough to directly observe hawkings radiation. Pretty amazing thought, maybe we will get that lucky.
Surely a "large nuclear bomb" isn't very strong when you're talking about things on black hole scale?
Ah, looks like I was "a bit" low. Wiki claims 5×10^6 megatons of TNT when it finally evaporates. Apparently about the energy the shoemaker level struck Jupiter.
However a lensing event seems possible, it greatly increases the effective radius of detection and is much more distinctive. Not sure what the smallest lensing event witnessed is, all the ones I'm aware of were a solar mass or more.
I don't know the physics enough to know if a small (earth to neptune mass) blackhole could redirect enough star lights to detectable by an earth (or earth orbit) telescope.
With a huge database containing the time series of the sky you can start searching for unknown objects that stick out because they are a change from the previous exposures for that part of the sky.
With enough data mining of those previous exposures you could find likely candidates and get telescope time to check out where you expect it to be next.
This is similar to how things like Oumuamua and 2I/Borisov were found.
> This scenario could be confirmed through annihilation signals from the dark matter microhalo around the PBH
We'd basically see extremely energetic particles coming from a nearby source. Since no other known physical process can generate such high energy particles (not even fusion in main sequence stars), we'd have to conclude that there is probably some extremely dense object there.