Science & Technology

ISRO satellite detected 500 black hole formations in 6 years. Mission scientist tells us how they did it

Dipankar Bhattacharya talks about how AstroSat uses gamma ray bursts to detect formations of black holes

By Rohini Krishnamurthy
Published: Thursday 02 June 2022

In May 2022, Pune-based Inter-University Centre for Astronomy and Astrophysics (IUCAA) announced that AstroSat, India’s first dedicated multiwavelength space telescope, had detected 500 black hole formations in over six years of its operation. The count increased to 506 as of June 2, 2022.

The instrument behind the discovery is the Cadmium Zinc Telluride Imager (CZTI), one of the five gadgets on board of the telescope operated by Indian Space Research Organisation (ISRO).

Down To Earth spoke with Dipankar Bhattacharya, the principal investigator of CZTI and faculty member at Ashoka University as well as IUCAA, to understand the science behind the detection and the future of the mission.

Rohini Krishnamurthy: How do you identify a black hole formation?

Dipankar Bhattacharya: To do that, we study gamma-ray bursts (GRB), bright explosions that release gamma-ray light. They are the most energetic form of light, a million times brighter than the sun. But they typically last for only seconds. And so much energy is put out in such a little time.

This brings us to the question: What produced it? We have a partial answer to that because we know that at least in some gamma bursts, there is also a connection with the supernova — a bright explosion happening when a star reaches its end and forms a black hole.

GRB also happens when two neutron stars merge. It can occur when a neutron star joins with the black hole also, in principle. But we don't know of a specific case.

So, now, at the end of this process, you either have a black hole or in some cases, you may be left with a high-speed, spinning, strongly magnetised neutron star, which goes by the name millisecond magnetar. Millisecond refers to the spin period, and magnetar means it is very strongly magnetised.

But what powers this burst of light? What mechanisms convert this energy into gamma rays — these are some of the questions that we are interested in.

RK: How do you identify a GRB’s source: Whether it is from a black hole or a millisecond magnetar neutron star?

DB: It is not easy to identify the source. We look at how the brightness or intensity of light changes with time.

So, if the GRB has extracted energy from a millisecond magnetar, one may expect the rate of change of the brightness to be a little slow in the beginning and then it gathers speed.

In the case of energy extracted from a black hole, the brightness will continuously drop at a specific rate. 

Satellites, meant explicitly for gamma-ray studies, immediately catch this. Because GRBs don’t last long, the information needs to be communicated very quickly. National Aeronautics and Space Administration’s (NASA) Fermi or Swift satellites have been specially designed to do this.

The data it communicates contains fundamental parameters such as where the GRB is occurring, how much energy the satellite has received, and the spectral distribution of energy or the distribution of various wavelengths of light.

But this will not help us understand the generation mechanisms of GRB. We will have to download the data and analyse it. We then perform simulations, which involve making various guesses on how the GRB was generated and running simulations to test each of them.

RK: How is ISRO’s AstroSat different from NASA’s Fermi and Swift satellites?

DB: AstroSat was not meant to study GRBs. Its main business is to observe compact objects like neutron stars and black holes and supermassive black holes at the galaxy’s centre using visible, ultraviolet and X-ray light.

AstroSat satellite carries five instruments onboard the satellite: Ultraviolet Imaging Telescopes (UVIT), Large Area Xenon Proportional Counters, CZTI, Soft X-ray Telescope and a Scanning Sky Monitor.

With UVIT, we can study newly forming stars or galaxies with some additional capability. X-ray is more critical while studying compact stars feeding on materials from their surroundings.

With CZTI, we can look at GRBs. They also measure the polarisation of the rays, something that NASA’s satellites cannot do. Measuring polarisation is a vital quantity as it will tell us about the mechanism driving this gamma-ray generation.

Other than that, there is sensitivity. Fermi is more sensitive to higher energy, while Swift is more sensitive to lower energy. CZTI’s peak sensitivity lies somewhere in the middle. It bridges the gap between swift and fermi. 

When the information gathered by CZTI is combined with that from Swift and Fermi, we get a broader range of data.

RK: How does CZTI detect GRBs?

DB: CZTI is made of Cadmium Zinc Telluride (CZT), a semiconductor that can detect photons of light. Every photon that strikes the semiconductor will produce a charge that is proportional to its energy. By looking at how much charge has been made, you can estimate the energy that has struck the semiconductor.

We chose CZT as a semiconductor to limit interference from optical light. That’s the central element of the instrument.

Additionally, we have built housing which sits on top of the semiconductor. Its role is to restrict the field of view to about 5 degrees to help focus on a point and filter out radiations coming from other directions. 

RK: AstroSat was given a lifetime of five years. It has entered its sixth year of operation. Will the satellite continue to run?

DB: We earlier gave a lifespan of five years because we expected that the gas-based detectors would degrade. Two out of five LAXPC detectors are not working as well as before. Only one of SSM’s detectors is working at complete sensitivity; others are not.

On the other hand, CZTI and the other three instruments are solid-state detectors. They have longer lifetimes. The idea is that the mission will go on as long as we can do useful science.

RK: ISRO is working on the second version of AstroSat called AstroSat 2. What is it going to study?

DB: It is in a conceptual stage currently. Multiple proposals have been given seed funding to build prototype detectors. Based on that, they’ll decide which ones will get the nod and in what order.

I think it is unlikely that AstroSat 2 will study many wavelengths like AstroSat. I expect they’ll do the same job as its predecessor — but better. 

Also, some new kinds of science have been proposed, such as studying gravitation wave sources and ultraviolet spectra in greater detail. One has proposed measuring fast intensity variation and polarisation in X rays. 

AstroSat 2 is expected to be more sensitive than the earlier version. There’s also a proposal to look for planets outside our solar system.

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