EM counterparts to GW sources

Artist’s impression of two neutron stars colliding. Credit: NASA/Dana Berry.

Catching the flash from colliding neutron stars

When two neutron stars collide, they shake space-time itself, sending out gravitational waves — and often a brief flash of gamma rays. Only one such flash has ever been confirmed, in 2017. Daksha is expected to catch a few to a dozen every year, and to tell telescopes worldwide where to look within a minute.

For astronomers: technical details

Rates depend on the emission model: ~0.5 per year if every merger is as intrinsically faint as GRB 170817A, and ~12 per year for a Gaussian structured jet, for an A+ LIGO (Hanford, Livingston, India) + AdV + KAGRA network with 70% duty cycle and a BNS rate of 320 Gpc⁻³ yr⁻¹. Daksha triggers also enable sub-threshold GW searches, adding ~7 mergers per year. Details: Bhalerao et al. 2024, Science with the Daksha High Energy Transients Mission (Experimental Astronomy 57, 23), section “Science Driver I: Electromagnetic Counterparts to Gravitational Wave Sources”; joint GW–short GRB rates in Bhattacharjee et al. 2024 (MNRAS 528, 4255). All Daksha papers are listed on our Publications page.

What are neutron stars, and why do they collide?

A neutron star is what remains when a massive star explodes: more mass than the Sun, squeezed into a ball about 20 km across. Sometimes two neutron stars orbit each other. Over millions of years they spiral closer and closer, until they finally collide at a sizeable fraction of the speed of light.

The collision sends out gravitational waves — ripples in space-time, first detected in 2015 — which instruments such as LIGO, Virgo, KAGRA and the upcoming LIGO-India can pick up. It can also launch a narrow jet that produces a short burst of gamma rays, and fling out debris that glows for days as a “kilonova”, forging heavy elements such as gold and platinum.

The event that changed everything

On 17 August 2017, gravitational-wave detectors recorded a neutron-star collision, and 1.7 seconds later NASA’s Fermi and ESA’s INTEGRAL satellites detected a faint gamma-ray flash from the same direction. Telescopes around the world then found the kilonova. This one event proved that neutron-star collisions produce short gamma-ray bursts, showed that they forge heavy elements, and gave a new way to measure how fast the Universe is expanding.

But it was a lucky catch. The flash was so faint that it would have been missed had it been a little dimmer, and some sensitive telescopes missed it simply because the Earth was in the way. Gravitational-wave detectors have since found more neutron-star collisions — but none with a confirmed flash of light.

How Daksha will help

Sensitivity: Daksha could detect a flash like the one from 2017 about 50% further away than Fermi’s burst monitor can — covering about five times the volume of space.
Sky coverage: with two satellites on opposite sides of the Earth, very little of the sky is ever hidden from Daksha.
Speed: alerts within about a minute, so telescopes can start searching for the kilonova right away.
Help for gravitational-wave detectors: a Daksha flash tells scientists exactly when and where to look in the gravitational-wave data, helping them find collisions that would otherwise be too faint to spot.
Soft X-rays: Daksha can also measure the softer X-rays that the 2017 flash hinted at, which carry information about how the jet punches out through the debris.

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