Finding Black Holes

"Yesterday upon the stair "Yesterday upon the stair I met a man who wasn't there. I met a man who wasn't there. He wasn't there again today. He wasn't there again today. I wish that man would go away." I wish that man would go away." Hughes Mearns ( ) Hughes Mearns ( )

Three Interesting Mass Ranges 1. Stellar mass black holes – left behind after massive stars undergo supernovae. They could be: alone (the remnants of single stars), or alone (the remnants of single stars), or in a binary pair, with a normal star as a companion in a binary pair, with a normal star as a companion

Larger Still 2. Supermassive black holes, formed by the coalescence of myriads of stars. They might be found in the midst of a vast cluster of stars, or in the midst of a vast cluster of stars, or in the centre of a galaxy in the centre of a galaxy

On the Other Extreme 3. Mini black holes, ‘left over’ from the early days of the universe, when everything was more densely packed. [Black holes of this sort would have a mass like that of an asteroid or a mountain.] NOTE: such black holes could not be routinely created in the present-day universe.

Three Regimes; Thus, Varied Search Techniques

1. Stellar Mass Black Holes For historical reasons, consider those in binaries first. (This was the first success!)

Black Holes in Binary Stars Find a binary star in which a visible member is orbiting around a companion which is (a) not giving off any detectable light (b) but too massive to be a dim white dwarf or neutron star

They Will Be Rare! Only the most massive stars produce supernovae and black holes But such stars are rare! - for every O star, there are millions of faint red dwarf stars So very few binary pairs are likely to contain a black hole!

The Tell-tale Evidence The spectrum of the visible star will reveal that the star is moving to-and-fro. (We use the Doppler shift.) But we need to observe each target many times, to monitor for and discover changing velocities. That takes time and effort, even for just one target.

Which Stars are Promising Candidates? Maybe 1 or 2 of these, say… but which ones?

Narrowing the Field Remember Novae? (a binary with a white dwarf) In analogous fashion here: As a normal star evolves and expands, the gas spilling over can form an accretion disk around the black hole companion As a normal star evolves and expands, the gas spilling over can form an accretion disk around the black hole companion Additional infalling material hits the accretion disk and heats it to enormous temperatures, giving rise to X-ray emission Additional infalling material hits the accretion disk and heats it to enormous temperatures, giving rise to X-ray emission

…like so

Thus: the Search Strategy 1. Discover X-ray sources in the sky, and determine their precise positions. 2. Monitor the motion of any star found at that location. Is it orbiting around an unseen companion? 3. If so, use Newton’s laws to deduce the mass of the dark companion – perhaps a black hole?!

Cygnus (Long Exposure)

…as Seen by Eye

Overhead in the Summer

Success! Cyg X-1 is an X-ray source associated with a bright blue B star. In 1973, it was shown to be in a binary with a massive dark companion – a black hole! [This work was done at the David Dunlap Observatory, in Richmond Hill.] Many other examples are now known.

Sometimes We Even See Eclipses [the X-rays disappear when the black hole goes behind the star] Sometimes We Even See Eclipses [the X-rays disappear when the black hole goes behind the star]

So We’ve Found Black Holes in Binaries How About Isolated Black Holes? What would you suggest?

Is This A Good Black Hole Candidate?

How About This Very Dark Region?

Sorry, No! You probably thought of looking for a region in space from which no light emanates. This is the wrong thing to do. Remember that the black holes don’t ‘suck in’ light from a huge surrounding area and create a dark mark like an inkblot.

The Paradoxical Solution We look for and find invisible black holes by looking for the enhanced brightness of background stars. That is, identify stars that look brighter than they should. The rationale is that this could be caused by gravitational lensing if a dense lump in the foreground (perhaps a black hole?) lies between us and the star. The rationale is that this could be caused by gravitational lensing if a dense lump in the foreground (perhaps a black hole?) lies between us and the star.

Okay, But Which Star? Does one of these look ‘brighter than it should’? (Maybe the one just above the centre?) Maybe it is just closer than most of the others!!

Problem Solved! Remember that Objects Move! If a black hole drifts between us and a distant star, we will see a temporary brightening of the star and then a return to dimmer levels. This draws our attention to the phenomenon. (This is probably exactly the opposite of what you expected. It is the inverse of an eclipse!)

We are Looking for MACHOs In astro-speak, MACHOs are MAssive Compact Halo Objects [‘Halo’objects because we search for them in the outer parts – the ‘halo’ – of our galaxy]

1. Dim 2. Bright 3. Dim 1. Dim 2. Bright 3. Dim

This is Still Not Simple! 1. Black holes are rare, and only briefly lined up with any particular star. We need to study millions of stars for many years if we hope to catch even a few ‘in the act’. 2. Some stars vary in brightness anyway (eclipsing binaries, pulsating stars). How do we discriminate? 3. Any single event will never be repeated, so we can only work out statistical estimates of black hole masses and numbers.

For Efficiency Find a collection of many stars at some moderately large distance – like a nearby galaxy, say – so they can all be captured in a single big image. Then take picture after picture, year after year, and look for short-lived changes in brightness. Finally, automate the whole process!

One Very Helpful Thing The colour of a star does not change when it is seen through a gravitational lens. [This is because all light behaves the same way under gravity.] By contrast, pulsating stars undergo temperature (and thus colour) changes.

The MACHO Project [monitor the stars in the Large Magellanic Cloud]

2. Supermassive Black Holes in Dense Star Fields 2. Supermassive Black Holes in Dense Star Fields Here, we look for dynamical effects: (a) see if there is a strong concentration of stars which have been drawn in by the enormous gravity; or (better) (b) see if the stars and gas in these regions are moving in a way which proves the existence of a compact (but unseen) lump of material

At the Center of the Milky Way - a SMBH of several million solar masses

This is Still Rather Modest 10 6 solar masses is still only percent of the mass of the entire galaxy! But there is evidence for the existence of really supermassive black holes in external galaxies Quasars, for example, probably reside in the cores of big galaxies, and are black holes with a mass 10 9 x that of the sun. (Note: that’s one billion!)

3. Mini Black Holes We defer this until we discuss the fate of black holes (next).