• July, 2021

Now I am pretty sure you get a better idea about what is a gravitational wave, now let's talk about one of the most amazing examples of gravitational waves so far ...
It's the detection of GW170817 multimessenger signal - to understand that let me tell you the story in astrophysics style -

A day in Astronomy

On August 17, 2017 astronomers around the world were alerted to gravitational waves observed by the Advanced LIGO and Advanced Virgo detectors. This gravitational wave event appeared to be the result of the merger of two neutron stars. Less than two seconds after the signal, NASA's Fermi satellite observed a gamma-ray burst, and within minutes of these initial detections telescopes around the world began an extensive observing campaign. The Swope telescope in Chile was the first to report a bright optical source (SSS17a) in the galaxy NGC 4993 and several other teams independently detected the same transient over the next minutes and hours, so this one signal made the whole observational astronomy community alert for next few months...

After few weeks of observation it's is confirmed that its two neutron stars merged in NGC 4993 - producing gravitational waves, a short-duration gamma-ray burst, and a kilonova. This event was called GW170817 and it marks a new era of multi-messenger astronomy, where the same event is observed by both gravitational waves and electromagnetic waves.

Before we understand the event let's understand a bit more about neutron stars, GRB, kilonova.

Neutron Star
The idea of a neutron star was first presented over eighty years ago in 1934, but In 1967 X-ray emission from Scorpius X-1 was determined to be from a neutron star, Since then several binary neutron star systems have been discovered, which provided strong observational tests of General Relativity including the first firm evidence for the existence of gravitational waves (GWs).

In the mid-1960s gamma-ray bursts (GRBs) were discovered by the Vela satellites, finding the sources of GRBs has been one of the key challenges in high-energy astrophysics, The idea that GRBs might be related to binary neutron star mergers had been put forward early on and in 2005 and it was a major step forward when a short-duration gamma-ray burst (sGRB) was localized to a host galaxy, and multi-wavelength (X-ray, optical, radio) afterglows could be observed. These multi-wavelength observations provided evidence that sGRBs might be associated with binary neutron star mergers or the merger of a neutron star with a black hole.

On 17 August 2017

Now on 17 August, 2017 NASA's Fermi satellite and its Gamma-ray Burst Monitor (GBM) instrument sent an automatic alert about GRB170817A. It took about 6 minutes for automated LIGO data analysis to find that a candidate GW transient (later designated GW170817) had been detected at almost the same time at the LIGO-Hanford observatory. The GW was consistent with a binary neutron star merger occurring less than 2 seconds before GRB170817A and the LIGO-Virgo rapid-response team manually inspected the data and issued an alert, reporting that a highly significant GW candidate was associated with the time of the GRB. At the time of the alert for GW170817, the location of the source in the sky had set in Australia, but it was still well placed for observing by telescopes in South Africa and Chile. In the first few hours of Chilean darkness, the Swope telescope identified an optical transient (SSS17a) in the galaxy NGC 4993.

Next few weeks observation conformations..

Over the next two weeks, a network of ground-based telescopes and space-based observatories followed up the initial detection, spanning the ultraviolet (UV), optical (O), and near-infrared (IR) wavelengths. These observations carefully monitored the spectral energy distribution, revealing that this exceptional electromagnetic counterpart was a kilonova. This observation connects kilonovae with the binary neutron star merger, providing evidence supporting the idea that kilonovae result from the radioactive decay of the heavy elements formed by neutron capture during a BNS merger.

Finding neutrino - Neutrino observatories searched for coincident, high-energy neutrinos from the area of GW170817 as neutrinos are emitted in the relativistic outflow produced during a BNS merger. However, no neutrinos were identified that came from the direction of GW170817 and no supernova neutrino burst signal was detected coincident with the merger.


For the first time, both gravitational and electromagnetic waves from a single astrophysical source have been observed. This joint observation supports the hypothesis that the source is the merger of two neutron stars and it also allowed for the identification of the host galaxy. All of these observations provide the first global picture of the processes at play after compact star mergers that contain neutron stars, including a jet of high-energy particles and the interaction of this jet with the surrounding interstellar medium. This event also demonstrates the importance of collaborative, joint gravitational-wave, electromagnetic, and neutrino observations, and marks a new era in multi-messenger, time-domain astronomy.

Part 1 Journey to gravitational wave

Blog Topic suggested By Sakshi Srivastava