At what point would we discover a black hole headed directly toward us?

CCWilson

Say a three solar mass black hole. Would someone notice it before it reached the Kuiper Belt, or only after there were deflections of known bodies in the Kuiper Belt?

Drakkith

I'd hope we would see some sort of stellar anomalies or something well before then. A 3 stellar mass black hole should have made its gravitation noticed through the interactions with nearby stars well in advance. Or so I would think.

CCWilson

The black hole approaching us would be much closer than any star, in this scenario, back when we began photographing the night sky for later comparisons. What might be noticeable is that as the black hole got closer to us, the amount of deflection of the light from a star in that direction would change, and the apparent position of that star would shift. But I have no idea as to how noticeable that would be.

Drakkith

Are you inventing a scenario where a black hole is already on top of us?

Chronos

The gravitational lensing would become pretty obvious by the time it got within about 100 light years.

CCWilson

Wouldn't a black hole heading directly toward us maintain almost the same position relative to the stars near its path, from our vantage point? If so, wouldn't the apparent position changes caused by gravitational lensing be subtle? Or would the rotation of the Milky Way cause the black hole to appear (if we could see it) to move in relation to the stars?

snorkack

Black hole participates or does not participate in Milky Way rotation like all other stars with their peculiar motions do.

So assume we have a Population II stellar mass black hole. The young Milky Way must have formed lots of them.

It would be orbiting the Milky Way on a high inclination orbit - like many red dwarfs (Barnard´s Runner for one) and a few red giants (Arcturus for one). We are speaking of peculiar velocity around 100 km/s.

Say it is passing through Local Bubble. No stellar satellite. The sole source of energy would be infalling interstellar matter.

There is relatively not much of it in Local Bubble. A high speed star would not be awfully efficient in capturing what it does encounter - it would pass the gas before it has time to fall in. Only a small amount of gas near the path would be captured.

Would even this gas form an accretion disc?

An accretion disc needs to be rotating. If a black hole is moving through uniform gas, what would be the direction of accretion disc rotation?

CCWilson

I doubt that such a black hole would encounter enough interstellar matter to cause detectable radiation until it got to the Oort Cloud. Before that, I suspect, gravitational lensing would be the only way to detect it, and I don't know how likely that is. But this is a rank amateur speaking.

aerrowknows

easiest, safest and fastest method would be to watch for anomalies in background galactic radiation. As you know , the galaxy emits a left over gamma or other wavelength radiation from deep space. If a black were to approach us to any reasonable distance , it will block that part out and create a "black spot" on radiation graphs.

Easy as 1 2 3.

mfb

I calculated that a while ago, and 100 light years is very close to my result (multiplied by 3 to account for the higher mass here). It is not "pretty obvious", however, unless you have a really good lensing event and not just a change in the position of a star.
In any way, it would be obvious before it comes closer than Alpha Centauri and we would have thousands of years to prepare.

The same as the rotation of the black hole itself.

The actual black disk has a negligible diameter - impossible to resolve with current technology unless it is in the solar system itself.

CCWilson

I don't know how long ago that photographs were first made of the heavens of sufficient precision as to allow us now to compare with current imaging and noticing gravitational lensing. But no more than 150 years or so, and maybe much less.

The scenario I'm asking about is where a black hole is going to be approaching us within the next few years. So it's not going to be passing by or through any galaxies other than our own. Is it inevitable that a black hole headed directly toward us would be close enough in line with a star or stars in our galaxy for gravitational lensing to be picked up by observers?

mfb

Some years? Sure. It would give a significant deviation for many stars observed by the Hipparcos mission (and probably other survey missions as well) within the observation time.
You don't need observations from 150 years ago - their precision is bad compared to current observations, and this is more significant than the advantage of the longer timescale.

Drakkith

Remember parallax. It's going to move a very large amount in contrast to the background stars. If it's several years out I'd estimate it to be several dozen arcseconds of parallax at least, which is easily noticeable in even amateur telescopes.

snorkack

Your Gaia is a fictional device. It has never existed.

Hipparcos has. But there are fewer than 120 000 HIP objects in the whole sky.

The sky is over 42 000 arc degrees. But the disc of Sun or Moon is less than 1/5 arc degrees - so the sky has black spots between HIP objests several times the size of solar disc.

Sun at 1 AU deflects light of stars 15 arc minutes away by mere 1,75 seconds.

100 km/s means about 20 AU per year. A black hole 1 century away, at 2000 AU... Sure, 100 arcsecond parallax - of the black hole itself. But if we assume that the black hole is unseen - the only effect of nearby stars is increasing their parallax. But if the hole is centuries away then increasing the parallax merely means the stars seem to be nearer than they are. No one would know they are actually not as close unless they follow the parallax over time.

mfb

It will exist in the near future with a high probability.
Actually, I would expect that it already exists here on earth, as it is supposed to launch in 2013.

You don't have to follow the parallax itself, it is sufficient to follow the stellar positions.

1 AU radius corresponds to the disk of the moon in a distance of ~200 AU. If the black hole has more mass (like ~5 solar masses), you get the same deflection with ~1000 AU distance or ~50 years. As the black hole moves so quickly, deflection would change during the observation time. Not 0<->100%, but at least so much that it cannot be closer than ~10 years away.

snorkack

You get 1,75´´ deflection at distance of 5 solar radii, or 3,5 million km.
Deflection itself changing with time is indistinguishable from proper motion.

mfb

For a single star, in linear approximation: Sure.

If multiple stars show a common pattern, or non-linear effects of the deflection are visible, it is possible to notice something unusual. If that would be detected and interpreted as black hole is a different question.


1000 AU refer to the distance of the object where 5 AU correspond to the apparent size of moon and sun.