Hawking radiation does not require an accretion disk.
Edit: with an accretion disk, I'd assume we'd have noticed it by now. Outside of that, as a black hole gets smaller, the hawking radiation wavelength should go down. It is rather weak overall, but it would be rather close. I haven't actually done the math, and, this is not my area of expertise. Would a relatively close by small black hole's hawking radiation be brighter than further X-ray emitters? Dunno.
The hawking radiation of a 9x earth mass is somewhere in the neighborhood of 0.01 K where the cosmic background is 2.7 K. It would appear to be a very tiny very cold ball.
The break even point for hawking radiation vs the cosmic background is about the mass of the moon.
I'm not going to do much more searching or calculation but I would be willing to bet a black hole small enough to emit xrays would have a remaining lifespan measured in nanoseconds at most.
A black hole bright enough to see its hawking radiation has to be tiny and would be quite short lived with an origin of unknown mechanism.
> A black hole bright enough to see its hawking radiation has to be tiny and would be quite short lived with an origin of unknown mechanism.
Depends on what distance you want to see it from.
A black hole that glows as hot as an incandescent filament would have as much mass as a 250km cube of rock and it would last for 10^35 years. It would have a radius of 60nm and emit less than a microwatt.
> a black hole small enough to emit xrays
To reach the low end of xrays, 100 electron volts, we'd need a black hole 100 times smaller. It would still have 10^29 years of lifetime, and would be emitting 2 milliwatts of xrays.
To reach the high end of xrays, 100k electron volts, we'd need a black hole 100k times smaller. It would still have 10^20 years of lifetime, and would be emitting 2 kilowatts of xrays.
To go the other way around, if I calculate a black hole that has "only" 10 billion years of lifetime left, weighing a dainty 190 million tons, it would be emitting 10 gigawatts of gamma rays. At 10 gigawatts of output, it would shrink by 1 ton every 285 years. The speed of light squared is an enormous number.
If I’m remembering right, compressing all of Earth’s mass would give a Schwarzschild radius of roughly 9 mm.
I think Uranus (15x heavier than earth) would be like basketball.
