July 5 () –
Echoes of light from jets coming out of black holes offer a new way to specify the distance to these objects and study an unnoticed population at the center of the galaxy.
It could also even help determine the expansion rate of the Universe.
The technique, developed by a team at Newcastle University and tested on the archetypal black hole Cygnus X-1, was presented by postgraduate researcher and team member Patrick O’Neill at the British National Astronomy Meeting in Cardiff.
Most black holes are the compact remnants of stars that ended their lives in supernova explosions. They have such a strong gravitational field that not even light can escape their reach, hence the description of them as black. Despite that, the influence on its environment can be very obvious, since the material orbiting a black hole is concentrated in a disk and can get very hot. This means they are strong sources of X-rays, and many also have associated jets that spew gas and dust great distances.
The calculated distance to most black holes is based on their X-ray brightness and associated measurements of their mass, which can be inferred from how fast material is swirling around them. O’Neill and the other team members take a different approach.
Light from the black hole’s jet is emitted in all directions, so it reaches the disk. Like a mirror, the disk reflects a part of the incoming light. Starting from the innermost part of the disk, the reflected light will ripple outward as the light emitted in the jet it takes longer to reach the outer parts of the disk. This “reverberation” of light is similar to a sound echo.
This means that we see the light originating from the jet in two ways: the light that travels directly towards us and the light that reflects off the disk. By simultaneously monitoring the brightness of the light traveling directly towards us and the light being reflected, it is possible to deduce how far above the disk the jet is. It also tells astronomers how close the inner edge of the disk is to the black hole itself. Closer in the black hole’s gravitational field disrupts the shape of the disk.
Tracking the light emitted by the black hole jet and the surrounding disk together allows the team to calculate the size of the disk and the fraction of light it reflects. That gives an absolute measure of the disk’s brightness, and therefore the distance to the black hole-disk system.
Dense clouds of gas and dust normally block infrared, visible, and ultraviolet light emitted from the centers of galaxies (including our own), restricting our vision. By contrast, X-rays can cross these regions unimpeded, so it should be possible to measure the distance to supermassive black holes. If that can be done, it will be a new way to determine how fast the universe is expanding, something that has yet to be resolved 94 years after the discovery of the expansion itself.
It is also a powerful tool for probing the population of black holes at the center of the galaxy. Until now, astronomers have tended to look at black holes that are relatively light and far from the plane of the Galaxy where most stars lie (our Galaxy has spiral arms on a flat disk emerging from a central bar).
Sometimes a black hole and a massive star orbit each other in a binary system. If the massive star explodes as a supernova, the black hole may be ejected from the plane of the galaxy. The heavier the black hole, the lower the acceleration, so the most massive black holes will be closer to the galactic plane and in the galactic center.
O’Neill said it’s a statement: “We often limit ourselves to observations of distant galaxies to make inferences about the Milky Way. This cutting-edge technique offers a method for probing the previously hidden galactic center, offering new insights into the evolution of our own galaxy and how black holes accumulate material[MOU1] . It’s also exciting to think that we could help establish the rate at which the Universe is expanding and gain a better understanding of its future.”
The team now wants to build a picture of the black hole population at the center of the galaxy. This could help find objects like intermediate-mass black holes, objects thought to result from the merger of black holes from individual stars and a step on the path to the formation of monster-sized supermassive black holes found in the center of most galaxies.