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Supermassive Black Holes and Binary Systems
Context:
Astrophysicists have long suspected that some galaxies harbor not just one, but two supermassive black holes at their centers.
More on News:
- These binary black hole systems are formed when galaxies collide and merge, bringing their central black holes close enough to orbit each other. However, detecting these elusive pairs is no easy feat.
- The detection of the 20-year pattern revealed that the two supermassive black holes in PG 1553+153 have a mass ratio of 2.5:1, with orbits that are nearly circular.
Supermassive black holes (SMBHs) are colossal entities with masses millions to billions of times that of our Sun, typically found at the centers of galaxies. When two galaxies merge, their central black holes can form a binary system, orbiting each other in a gravitational dance. These binary systems are not only fascinating but also crucial for understanding galaxy evolution and the nature of gravity.
The Challenge of Detection:
- Unlike cracking an egg to see if it has one or two yolks, identifying binary black holes requires sophisticated technology and indirect methods.
- Supermassive black holes are regions of space where gravity is so strong that not even light can escape from their vicinity.
- They form when the core of a massive star collapses on itself, and they act as cosmic vacuum cleaners.
Gravitational Waves: A Clue to Binary Black Holes:
- One of the primary methods for detecting binary black holes is through the observation of gravitational waves.
- According to Einstein’s theory of general relativity, these ripples in space-time travel at the speed of light, causing space itself to stretch and squeeze around them.
- Specialised observatories, such as LIGO and Virgo, have been instrumental in detecting these waves, which are emitted when black holes merge.
Pulsar Timing Arrays:
- Another method involves using pulsar timing arrays, which rely on the precise timing of radio waves emitted by pulsars—dense, bright cores of collapsed stars.
- By looking for gaps and anomalies in the pattern of these radio waves, researchers can detect the collective gravitational wave signal from binary black hole systems.
Active Galactic Nuclei:
- Astronomers also search for periodic signals from the centres of active galaxies, which emit significantly more energy than expected from their stars, gas, and dust.
- These active galactic nuclei can provide indirect evidence of binary black holes through the process of accretion, where black holes pull nearby gas inward, causing it to heat up and glow brightly in optical, ultraviolet, and X-ray light.
PG 1553+153: A Case Study
- PG 1553+153, an active galaxy, has periodic light variations every 2.2 years, which could indicate a binary black hole system.
- Other causes for this variation, like wobbly jets or changes in gas flow, were ruled out.
- Simulations suggested that clumps of gas orbiting the black holes should create longer periodic variations (every 10-20 years).
Future Confirmation:
While this data strengthens the hypothesis of a binary system, final confirmation may require the sensitivity of pulsar timing arrays to detect gravitational waves directly from PG 1553+153.
Historical Data from DASCH
- Astronomers used over 120 years of data from the DASCH (Digital Access to a Sky Century at Harvard) project, which digitised photographic plates taken by observatories from 1900 onwards.
- They found a 20-year periodic signal in PG 1553+153, supporting the binary black hole theory.
