When I look up at the sky, light from distant objects, like stars, reaches my eyes after travelling through vast amounts of space. When I stare into these objects, I look 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 electromagnetic spectrum. Imagine what lurks in the unseen!
In the height of the Cold War, Americans placed satellites in space that were capable of detecting very high energy photons, called gamma rays. They expected these high energy photons to be generated from nuclear weapon tested in space, carried out secretly by the USSR, which would violate a peace treaty between the USA and the USSR. Although the suspected violations were not detected, the USA discovered astronomical objects which are today called ‘Gamma Ray Bursts’. This discovery opened up the gamma-ray eye of humankind.
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, these GRBs remained a mystery until special satellites were sent around the Earth to detect and study them in greater detail. Scientists concluded that these bursts originate 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 vanish forever within fractions of seconds.
Most objects in the sky — stars, planets, galaxies — evolve over millions to billions of years. GRBs, however, die within seconds. When astronomers estimated the energy emitted by GRBs, they were astounded — a single GRB outshines the entire galaxy in which it exist in its minuscule lifetime! Astronomers pieced together the puzzles and figured out that these mysterious events are associated with deaths of stars at the end of their long, eventful lives. After decades of careful studies, they figured that long GRBs originate in the collapse of massive stars into black holes. On the other hand, short GRBs are formed when a pair of what are known as ‘neutron stars’ go around each other in a cosmic dance and finally collide to form a black hole. Neutron stars, astronomers knew, are made up entirely of neutrons, one of the three particles that makes up atoms. During the dance, the neutron stars give out waves of gravitational energy, called ‘gravitational waves’, as Albert Einstein had predicted in 1915.
Through these decades of working through the puzzles, scientists observed more and more GRBs with multiple detectors on-board Earth-orbiting satellites. Today, they observe GRBs routinely – whereas some detectors ‘see’ a GRB every day, some detect about a hundred every year. The detectors can measure the time-span of the GRBs, and the energies of the individual photons. How the number of photons vary with their energies is what scientists call the GRB’s ‘spectrum’. They analyse the GRB spectra to reveal important details of the GRB, including the environment of the newly formed black holes. As such detections pile up and more GRB-detectors are placed in orbit around the Earth, it becomes important to ask the following questions: How many GRBs can be formed in the Universe? How many can a certain detector see, given that each detector is different from the others?
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. Hence, astronomers use various techniques to measure distances to objects. As the distance becomes larger, it becomes more difficult to estimate them. Measuring the distance to GRBs depend on chance — only if astronomer are lucky to locate the galaxy from which it originates can they measure its distance. Thus, the distances to a large number of GRBs are not estimated. A method was proposed in 2004 by a group of Japanese scientists to estimate the distances to GRBs in the absence of direct measurements.
In the works during my PhD, I collected the data available publicly from all GRB detectors till date, and conducted a statistical study of all GRBs. The method proposed by the Japanese scientists was applicable only for the GRB-detector they had studied. I studied the validity of the method for all GRBs, and investigated necessary modifications that would let me apply the method to multiple detectors at the same time. I demonstrated that this method fails to account for the distances to individual GRBs. However, to my surprise, I found that 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. Thus, we can predict the number of GRBs that can be detected by new space-based GRB-missions, solving two problems at the same time.
On the 28th of September 2015, India placed an 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 the Indian Space Research Organisation (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 the date it was observed, pounded the CZTI 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 CZTI collects, it is impossible to manually search GRBs like we had done for the first one. Hence, my colleagues analysing the CZTI data resorted to searching for GRBs only when other international GRB missions, which do detect GRBs automatically, send robotic alerts over the web. That is, CZTI had no independent method of its own to search GRBs in its own data. My careful analysis of the data of GRB151006A led us to the conclusion that uncharacterised “noise” was present in the data — electronic aberrations that mimicked gamma-ray photons — which compounded the problem. The noise, we understood, was caused by misbehaving electronic components of CZTI, leading to a large number of false spikes in the data. It thus became clear to us that we should independently know the number of GRBs that CZTI can detect every year. My earlier 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 by carefully running numerical models of 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 their spectra. From the first two years of observation, about one-third of the long GRBs and and half of the short GRBs were missing in the CZTI data we had analysed. And thus I began a careful study of characterising noise and eliminating them from the CZTI data. Once I developed a method to do this, it was used to conduct automated searches for GRBs by my other colleagues of the CZTI team. Last heard, a significant number of the once-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, simultaneously releasing a part of their energy as gamma-ray photons. What had long been a carefully-arrived-at speculation was now an observational fact. This meant that I could also predict how many such pairs of neutron star “mergers” astronomers would observe with the help of 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.
Debdutta Paul 