black-hole. Image from NASA

Astronomers think they have seen an invisible black hole for the first time.

In 2019, astronomers captured the first direct photograph of a black hole, thanks to material burning in its proximity. Many black holes, on the other hand, are nearly hard to detect. Now, another team utilizing the Hubble Space Telescope claims to have discovered something no one has ever seen before: a fully undetectable black hole. The study has yet to be peer-reviewed despite being published online and submitted to the Astrophysical Journal. The article related to this was published in theconversation.

When huge stars die and their cores collapse, what’s left are black holes. They are extraordinarily dense, having so powerful gravity that nothing, including light, can escape them. Black holes are fascinating to astronomers because they can reveal a lot about how stars die. We can learn about what happened in the dying moments of stars when their cores collapsed and their outer layers were evacuated by analyzing the masses of black holes.

What is the definition of a black hole? A black hole is an astronomical object with such a tremendous gravitational pull that nothing can escape it, not even light. The event horizon of a black hole is the boundary where the velocity required to escape exceeds the speed of light, which is the universe’s speed limit. Matter and radiation are drawn in, yet they are unable to leave.

There are two types of black holes that have been studied extensively. Supermassive monsters weighing 100,000 to billions of solar masses are found in the centers of most big galaxies, including ours. Stellar-mass black holes with three to dozens of times the Sun’s mass are found throughout our Milky Way galaxy, while supermassive monsters weighing 100,000 to billions of solar masses are found in the centers of most big galaxies, including ours.

Simulation of hot gas surrounding and falling into a black hole. (Credit: NASA’s Goddard Space Flight Center/J. Schnittman, J. Krolik (JHU) and S. Noble (RIT))

It may appear that black holes are by definition invisible – after all, their capacity to capture light is how they got their name. However, due to their strong gravity, we can still identify them by the way they interact with other objects. The interaction of hundreds of tiny black holes with other stars has been discovered.

There are two methods for detecting this type of threat. The gravitational field of a black hole can draw material from its companion in “X-ray binary stars,” in which a star and a black hole orbit a common center while generating X-rays. As the material rounds the black hole, it heats up due to friction. Before being pulled into the black hole and gone, the heated material flashes brightly in X-ray light, revealing the black hole. Pairs of black holes can also be detected as they merge, spiraling downwards and generating a brief flash of gravitational waves, which are spacetime ripples.

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Many rogue black holes, on the other hand, are wandering across space without colliding with anything, making them difficult to detect. This is a concern since we can’t learn about how black holes develop or the deaths of the stars from whence they came if we can’t identify isolated black holes.

The team of scientists had to combine two different sorts of data over several years to discover such an invisible black hole. This remarkable result heralds a new approach to discovering the hitherto elusive class of isolated black holes.

According to Einstein’s General Theory of Relativity, heavy things bend light as it passes through them. That means that any light passing very close to an invisible black hole – but not close enough to enter it – will be bent in the same way that light passing through a lens would be distorted. When a foreground object aligns with a background object, bending its light, this is known as gravitational lensing. The technique has already been used to investigate everything from galaxy clusters to planets orbiting other stars.

The Topsy-Turvy Galaxy NGC 1313*
The central parts of the starburst galaxy NGC 1313. The very active state of this galaxy is evident from the image, showing many star formation regions. A great number of supershell nebulae, that is, cocoons of gas inflated and etched by successive bursts of star formation, are visible. The green nebulosities are regions emitting in the ionized oxygen lines and may harbor clusters with very hot stars. This color-composite is based on images obtained with the FORS1 instrument on one of the 8.2-m Unit Telescope of ESO’s Very Large Telescope, located at Cerro Paranal. The data were obtained on the night of 16 December 2003, through different broad- (R, B, and z) and narrow-band filters (H-alpha, OI, and OIII).

In their quest for black holes, the authors of this new study used two types of gravitational lensing observations. It began with them noticing light from a faraway star abruptly amplify, making it appear brighter for a brief moment before returning to normal. However, they were unable to discern any foreground objects that were producing the amplification due to gravitational lensing. That suggested the object could be a solitary black hole, something never seen before. The issue was that it could have simply been a dim star.

It took a lot of work to figure out if it was a black hole or a dim star, which is where the second type of gravitational lensing measurements came in. For six years, the authors used Hubble to take photos, measuring how far the star appeared to travel as its light was deflected.

They were able to compute the mass and distance of the object that generated the lensing effect as a result of this. They discovered that it had a mass of nearly seven times that of our Sun and was positioned around 5,000 light-years away, which seems far but is actually quite close. We should be able to see a star of such magnitude and proximity. They determined that it must be an isolated black hole because we can’t see it.

It’s not easy to make that many observations with an observatory like Hubble. The telescope is quite popular, and there is fierce rivalry for its viewing time. Given how difficult it is to confirm an object like this, you might suppose the chances of discovering more are slim. Fortunately, astronomy is on the verge of a revolution. This is thanks to a new generation of facilities, like the Gaia survey, the planned Vera Rubin Observatory, and the Nancy Grace Roman Space Telescope, which will all take repeated measurements of broad portions of the sky in unprecedented detail.

A black hole has two basic parts. The singularity is at the center and is where the mass resides. The event horizon is the boundary that marks where the escape velocity from the mass is the speed of light. (Credit: NASA’s Imagine the Universe)

That will have a major impact on astronomy in general. With regular, high-precision measurements of so much of the sky, we will be able to explore things that change on extremely short timescales en masse. We’ll investigate asteroids, supernovae (stars that explode), and planets orbiting other stars in novel ways.

When it comes to the quest for unseen black holes, this means that rather than celebrating the discovery of only one, we may soon discover a slew of them. This will allow us to fill in the gaps in our knowledge of star deaths and black hole formation.

In the end, the galaxy’s invisible black holes will find it much more difficult to hide.

Source: theconversation / NASA

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