Digging deep into cosmic explosions

When I look up at the sky, light from distant objects reaches my eyes after travelling through vast amounts of space. Staring into these objects, I am looking at them just as they were thousands to millions to billions of years ago! This realisation gives me goosebumps every time I stare at the sky. Each snapshot of the sky is a history of the cosmos. What is more, our visions are limited to what physicists call the ‘visible’ part of the light energies. Imagine what lurks in the unseen!

In the height of the Cold War, the Americans placed satellites in space that were capable of detecting very high energy photons, called gamma rays, which would be generated if the Soviet Russians were secretly testing nuclear weapons in space thus violating the peace treaty. Although such violations were not detected, they discovered astronomical objects which are today called ‘Gamma Ray Bursts’ – opening up the gamma-ray eye of mankind.

As the name suggests, Gamma Ray Bursts, or ‘GRBs’ as they came to be fondly called, are sudden bursts of gamma rays, coming from space and pounding the detectors for a few seconds before vanishing forever. For decades they remained a mystery until special satellites were sent around the Earth to detect and study them in greater detail. Scientists concluded that these bursts are events originating in distant galaxies, and that they come in at least two kinds – the ‘long’ GRBs and the ‘short’ GRBs. Whereas the former lasts for tens to hundreds of seconds, the latter vanishes forever within fractions of seconds.

Most objects in the sky: stars, planets, galaxies – evolve very slowly, over millions to billions of years. GRBs die within seconds. When astronomers calculated the energy emitted by GRBs, they were astounded – they outshine the entire galaxy in which they reside in their minuscule lifetime! They pieced together the puzzles and understood that these mysterious events are associated with deaths of stars at the end of their long, eventful lives. After decades of careful studies, long GRBs were understood to originate during the collapse of massive stars into black holes. On the other hand, short GRBs were understood to be formed when a pair of what are known as ‘neutron stars’ – stars made up entirely of neutrons, a particle that makes up the nucleus of atoms – go around each other in a cosmic dance and finally collide to form a black hole. During this dance, they give out waves of gravitational energy as well, called ‘gravitational waves’, as predicted by Albert Einstein in 1915.

These startling revelations came after a lot of hard work by scientists over decades, but more importantly, with observations of more and more GRBs with multiple satellites. Today, observations of GRBs are routine – whereas some detectors ‘see’ a GRB per day, some detect about a hundred per year. They measure the time-span of the GRBs, the distribution of energies of the photons, etc. These let astronomers study the environment of the newly formed black holes. As such detections pile up and more detectors of GRBs are placed in orbit around the Earth, it becomes important to ask the questions: How many GRBs are there in the Universe? How many can a certain detector see, given that each detector is different from the others?

In my works during my PhD, I collected the data made publicly available by all GRB detectors till date, and conducted a thorough statistical study of all GRBs. In astronomy, it is difficult to measure the distances of objects. For example, there is no direct way of measuring the distance to even the Sun or the Moon, and astronomers use various techniques to measure distances to objects. As the distance becomes larger, it becomes more difficult to measure. The measurement of the distance to GRBs depend on chance – only if we are lucky to see the galaxy from which it originates, can we measure its distance. Thus, the distances to a large fraction of GRBs cannot be measured. I studied a method proposed first in 2004 by a group of Japanese scientists, to estimate the distances to GRBs in the absence of direct measurements. The method was proposed only for one detector, and I investigated necessary modifications that would let me apply to multiple detectors at the same time. I demonstrated that although this method fails to account for the distances to individual GRBs, it does a great job for the whole population, allowing me to calculate the intrinsic distribution of GRBs in the cosmos. This is a remarkable result – it allows us to know how many GRBs are forming in different parts of the universe, and thus predict the number that can be detected by the space-based GRB missions, solving two problems at the same time.

On the 28th of September 2015, India placed its completely indigenous multi-wavelength astronomy satellite, AstroSat, in orbit around the earth. Built, tested and integrated for over more than a decade, the day arrived and ISRO made history. On the 6th of October, one of the six instruments on-board, the Cadmium Zinc Telluride Imager or the CZTI, headed by my PhD advisor in Tata Institute of Fundamental Research, Professor A. R. Rao, was the first to be opened for acquiring data – and boom! GRB151006A, named after its date, pounded the detector with photons at precisely the same time and exactly the same way as other gamma-ray detectors on board NASA satellites. In the coming few months, CZTI would go on to detect GRBs routinely. Exactly how many? In the first year, the number was close to a hundred. But, there was a problem.

Given the large volumes of data, it is impossible to manually look for GRBs like the first one. CZTI thus resorts to looking for GRBs only when other international GRB missions send automatic alerts over the web, in the absence of such a method of its own. This is also because my careful analysis of the data of GRB151006A led to the conclusion that uncharacterised “noise” was present in the data – electronic aberrations that mimicked gamma-ray photons. This leads to the possibility of a large number of false spikes created due to misbehaving electronic components. Thus, it is immediately practical to be able to independently know the number of GRBs that CZTI can detect in a year. My work enables one to do exactly that.

With independent studies of long and short GRBs, I predicted that AstroSat is supposed to detect about 140 long and 30 short GRBs per year. I was able to do this via carefully running numerical models over the population of GRBs, both long and short, and finding the best models that could explain the multiple properties of this population: their distances, total energy output, and the distribution of energies at which they emit. From the first two years of observation, about one-third of the former and half of the latter, were missing in the CZTI database. And thus I began a careful study of characterising noise and eliminating them from the CZTI data, which was then subjected to automated searches for GRBs by other members of the CZTI team. Last heard, a significant number of the missing GRBs have been recovered!

On the 17th of August 2017, history was made in astronomy: for the first-time ever, astronomers had direct observational evidence of a pair of “neutron stars” going around each other while emitting “gravitational waves” as predicted by Albert Einstein in 1915, simultaneously releasing a part of their energy as gamma-ray photons. What had long been a speculation was now an observational fact. This meant that I could also predict how many such pairs of neutron star “mergers” would be “heard” by the next-generation detectors of gravitational waves – the “advanced LIGO/VIRGO collaboration” – of which India will be a part from 2025. As for now, the next observations of gravitational waves have just begun after instrumental upgrades. My predictions are that there will be at least two such mergers detected per year. The veracity of this prediction, only time will tell.

This is a short summary of my Thesis work carried out at the Tata Institute of Fundamental Research, Mumbai, India — without the scientific jargon and hence without many details.

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