On the eve of Dhanteras in the festival of light Diwali, when many Indian think fondly of their stack of gold jewellery, the origin of gold in the cosmos has been nailed through a fascinating story of coordinated effort of gravitational waves (GW) observations and the conventional Electromagnetic (EM) waves observatories across the entire spectrum ranging from very energetic Gamma & X-rays, visible light to the much softer radio waves.
In this major breakthrough, gravitational waves & bright Gamma ray light flash were simultaneously observed from merging Neutron stars on Aug 17, 2017.
The event dubbed, GW170817, puts to rest any speck of skepticism that may have harboured about the reality of GW discoveries that till now that featured black hole mergers. Black hole mergers are not expected to emit any light and, indeed, none was detected in the previous discoveries. That all changes today with this cosmic sound and light show observation that now formally launches gravitational-wave multi-messenger astronomy.
Neutron stars are the smallest, densest stars known to exist and are formed when massive stars explode in supernovas. Typical neutron stars are heavier than the sun, but packed into a astonishing small diameter of just about 20 kilometers.
(The radius of the sun is about 1.4 million km). The two Laser Interferometer GW Observatories (LIGO) in USA and Virgo observatory in Europe could track these neutron stars, weighing about 1.1 to1.6 times the mass of the sun, for about 100 seconds as they spiraled towards each other in a final deadly dance and collided. These observations contain important clues to the nature of the dense matter that constitute these stars.
The collision of the two neutron starts created a prompt flash of gamma rays that was detected by earth-orbiting satellites just two seconds after the gravitational waves. This is the first conclusive evidence that short gamma ray bursts, often seen by orbiting satellites, are indeed created by colliding neutron stars — something that had only been theoretically speculated for decades. The near simultaneous arrival of gravitational waves and gamma rays from a source that is 130 million light years away confirms that gravitational waves indeed travel with the speed of light, as predicted by Einstein’s theory to one-part in a million-billion. These joint observations also provided scientists an independent way of measuring the expansion rate of the universe that has been published in the Nature journal by LIGO Science Collaboration (LSC).
In the days that followed, astronomers pinpointed the source on the sky and studied it extensively in various forms of electromagnetic radiation, including X-ray, ultraviolet, optical, infrared, and radio waves. These joint observations clearly show that at least some short gamma-ray bursts, the energetic flashes of gamma rays, are generated by the merging of neutron stars — something that was only theorised before. These studies showed signatures of newly synthesized elements, confirming that such mergers are indeed the birthplaces of half of the elements heavier than iron – including most of the gold and platinum in the universe. These results are published in two letters to the Astrophysical Journal.
There are 40 scientists from 13 Indian institutions are part of the LIGO-Virgo discovery paper in Physical Review Letters. Indian scientists contributed to the fundamental algorithms crucial to search for merging binaries in noisy data from multiple detectors, in computing waveforms for these signals by solving Einstein’s equations, in separating astrophysical signals from numerous instrumental and environmental artefacts, in interpretation of joint gravitational-wave and gamma-ray observations, tests of Einstein’s theory and many other aspects of the data analysis. In addition, what is new is that several Indian telescopes such as AstroSat, Giant Metrewave Radio Telescope (GMRT) and the Himalayan Chandra Telescope (HCT) participated in the search for electromagnetic flashes and, hence, there are some more authors from the Indian Astronomy and Astrophysics community in a multi-messenger astronomy letter published in the Science Journal. The sensitive CZTI instrument on AstroSat helped arrow down the location of the gamma-ray flashes. HCT obtained optical images at locations of neutrinos detected by other telescopes at the same time as the burst, and showed that they were unrelated to the gravitational-wave trigger. GMRT played a key role in understanding jet physics and refining models of radio emission from the remnant formed by the merging neutron stars.
The Indian team in LIGO includes scientists from CMI Chennai, ICTS-TIFR Bangalore, IISER Kolkata, IISER Trivandrum, IIT Bombay, IIT Gandhinagar, IIT Hyderabad, IIT Madras, IPR Gandhinagar, IUCAA Pune, RRCAT Indore, TIFR Mumbai and UAIR Gandhinagar. Astronomers from IISER Pune, IIT Bombay, IUCAA Pune, TIFR Mumbai, PRL Ahmedabad, IIT Hyderabad, IIA Bangalore, NCRA-TIFR Pune, ARIES Nainital and IIST Trivandrum participated in the electromagnetic follow-up of this event using a variety of telescopes.
The planned LIGO-India detector, being funded by Department of Atomic Energy (DAE) and the Department of Science & Technology (DST), will increase the sensitivity of the international gravitational-wave network and produce many fold improvement to the localisation of the sources. Astronomers will then be able to identify the exact location of the cosmic explosion a lot quicker, and promptly follow from the first moments in every frequency band of the electromagnetic spectrum.
- LIGO-Virgo detection (Physical Review Letters)
- Multi-Messenger Astronomy (Astrophysical Journal Letters)
- GW-GRB connection (Astrophysical Journal Letters)
- EM follow-up including Astrosat (Science)
- Hubble constant (Nature)