Over the weekend Indian Space Research Organisation (ISRO) on social media posted that a team of scientists using the Indian space observatory AstroSat has made a fascinating discovery about a black hole system called Swift J1727.8-1613.

During an unusual outburst of energy from this system, they observed that the high-energy X-ray photons emitted were not following a regular pattern, but rather a more chaotic, ‘aperiodic’ modulation, meaning that the intensity of the X-rays was fluctuating in a way that could not be predicted by a simple, repeating cycle. This discovery could help scientists better understand the complex processes that happen around black holes during these energetic events.

To understand what exactly the Indian scientists have discovered, and why it is significant, there are multiple concepts one needs to know about.

Introducing the AstroSat

India’s first space telescope, AstroSat, was launched in 2015. It is a unique observatory designed to study celestial objects by looking at their light in different colours, including X-rays, visible light and ultraviolet light. This allows scientists to get a more complete picture of what is happening in space. AstroSat is managed from the ground by the Mission Operations Center at ISTRAC in Bengaluru.

A team of Indian scientists, including researchers from the URSC/ISRO, IIT-Guwahati, University of Mumbai and TIFR, have made a groundbreaking discovery about a black hole system. They used the powerful AstroSat telescope to observe this system and their findings are published in a prestigious scientific journal, called Monthly Notices of the Royal Astronomical Society (MNRAS).

Black Hole X-ray Binary

“Out in the vast universe, there is a cosmic dance happening called a black hole X-ray binary. It is a duet between two stars: a black hole, a super-dense object with gravity so strong that even light cannot escape, and a normal star, much like our own Sun. These two stars are locked in a gravitational embrace, orbiting each other. 

The black hole, being the heavier partner, exerts a powerful pull on the normal star, literally sucking matter away from it. This stolen matter does not fall straight into the black hole, but, instead, forms a swirling disk around it, like water swirling down a drain. As this disk spins faster and faster, it gets incredibly hot, reaching millions of degrees. This intense heat causes the matter to glow brightly, emitting powerful X-rays that we can detect here on Earth with our telescopes,” explains Girish Linganna, Bengaluru based defence and aerospace analyst.

Adding, “These X-ray binaries are a reminder that, even though black holes are often thought of as dark and mysterious, they can create spectacular displays of light and energy, revealing their presence across vast distances.”

The Powerful X-Ray Beacon

Imagine a black hole as a silent, invisible monster lurking in space. “We cannot see it directly, but it is giving off clues about its presence. These clues are like little messages sent out in the form of powerful X-rays. These X-rays are like a beacon, letting us know that a black hole is hiding nearby, even though we cannot see it directly. It is like finding a hidden treasure by following a trail of breadcrumbs. The X-rays are our breadcrumbs, leading us to the hidden black hole,” Linganna states.

Why Are These Significant?

Think of black hole X-ray binaries (BH-XRBs) as cosmic laboratories. They are like giant, natural experiments happening in space, giving us a glimpse into some of the most extreme, and powerful, forces in the universe. These systems help us understand how matter behaves when it is pulled towards a black hole, and how this process releases incredible amounts of energy. It is like studying a powerful engine. But, in this case, the engine is a black hole and the fuel is the matter it pulls in.

According to Linganna, “By studying these systems, we can learn about fundamental physics, the laws that govern the universe, in ways that we cannot replicate on Earth. It is like having a front-row seat to some of the most dramatic and exciting events in the cosmos.”

Swift J1727.8-1613 Blackhole

On August 24, 2023, scientists discovered a new black hole, called Swift J1727.8-1613, using the Swift/BAT space telescope. This black hole was incredibly bright, emitting a huge amount of X-rays. In fact, it was one of the brightest black holes ever seen, releasing energy equivalent to about 7 ‘Crab’ units. The ‘Crab’ is a standard measurement for how bright an X-ray source is, like a cosmic measuring stick.

This discovery is exciting because it gives us a chance to study a powerful black hole in detail. It is like finding a new, super-bright star in the sky, but, instead of light, it is emitting powerful X-rays that tell us a lot about the black hole itself.

AstroSat Takes a Closer Look

Based on the information in the public domain, after the initial discovery of the black hole, Swift J1727.8-1613, scientists wanted to learn more about it. So, they used another space telescope, called AstroSat, to take a closer look. AstroSat started observing the black hole on September 2 and continued for several days—from September 8 to September 14, 2023. In total, they spent about 207,000 seconds (about 2.4 days!) observing this powerful object.

Think of it like taking a series of pictures of the black hole to see how it changes over time. By studying these observations, scientists can learn more about the black hole’s behaviour and how it releases energy.

Black Hole’s Wobbly X-ray

Black holes are known to release powerful X-rays, but these bursts are not always perfectly regular. They have a kind of ‘wobble’ to them, which scientists call ‘aperiodic modulation’.

Linganna explains, “This study found that, even though the X-ray bursts are irregular, they create a pattern when you look at how often they happen (frequency) and how strong they are (power). This pattern is called a Quasi-Periodic Oscillation (QPO).”

“What’s even more remarkable is that this QPO pattern changes over time. In just 7 days, the frequency of the X-ray bursts from black hole Swift J1727.8-1613 shifted from 1.4 times per second (1.4 hz) to 2.6 times per second (2.6hz),” he adds. “This is the first time anyone has seen this kind of change in a black hole that releases X-rays.”

This discovery is exciting because it could help scientists understand how black holes spin and release energy, which is a fundamental mystery in astrophysics.

Compton Scattering Drives QPOs

A black hole is surrounded by a spinning disk of hot gas called an accretion disk. This disk is like a giant, swirling whirlpool of superheated material. This disk releases two types of X-rays:

Soft X-rays: These are like the ‘cooler’ X-rays coming from the outer parts of the disk

Hard X-rays: These are the ‘hotter’ X-rays and come from the inner parts of the disk where things are much more intense.

“This study found that the hard X-rays are actually created when the soft X-rays from the outer disk get ‘bumped’ into by super-hot electrons in the inner disk. This bumping process is called Compton scattering, and makes the X-rays more energetic,” Linganna elaborates.

During the observations, black hole Swift J1727.8-1613 was mostly releasing hard X-rays (about 90%) and only a small amount of soft X-rays (about 10%). This means that Compton scattering was the main way the black hole was producing X-rays.

“And here’s the important part,” adds Linganna. “The study found these hard X-rays from Compton scattering are the ones that have the ‘wobbly’ pattern, or ‘aperiodic modulation’, which creates the QPOs. This tells us that the QPOs are related to the way the black hole is releasing energy through Compton scattering, and that is a big clue to understanding how black holes work.”

2 Telescopes Unveil Black Hole Secrets

To study these ‘wobbles’ in X-rays from black holes, scientists used special space telescopes. They used AstroSat, which has a special instrument called LAXPC that can detect very small changes in X-rays over time (like a super-fast camera). They also used data from another telescope, called NICER, which is on the International Space Station.

“By combining these telescopes,” continues Linganna, “they could get a clearer picture of the X-rays coming from the black hole, including how much energy they had. This allowed them to study the ‘wobbles’ (QPOs) in more detail, especially at higher energy levels. It is like having two different types of microscopes to study the same object, giving them a more complete view.”