EM-GW

Gravitational wave (GW) astronomy has opened new windows into the cosmos, revealing some of the most energetic events in the universe. However, detecting the associated electromagnetic

Image of the kilonova observed by Hubble at NGC 4993 . Credit: NASA and ESA.

What Are Electromagnetic Counterparts?

Electromagnetic counterparts are light signals associated with gravitational wave events. They help in:

• Identifying the exact location of the source
• Understanding the astrophysical processes behind mergers
• Measuring cosmological distances using standard sirens
• Probing the equation of state of neutron stars

Sources of Electromagnetic Counterparts

Neutron Star Mergers and Kilonovae

The most well-known EM counterpart is a kilonova, an optical/infrared transient powered by radioactive decay of heavy elements from neutron star mergers.

• Gamma-ray bursts (GRBs): Short GRBs arise from relativistic jets in mergers.
• X-ray and radio afterglows: Reveal the merger's interaction with surrounding material.
• Optical and infrared kilonovae: Provide evidence of r-process nucleosynthesis.

Black Hole-Neutron Star Mergers

If the neutron star is tidally disrupted before falling into the black hole, it may produce a kilonova-like emission.

Black Hole Mergers

While stellar-mass black hole mergers were expected to be electromagnetically silent, some scenarios suggest weak EM counterparts, especially if they occur in accretion disks of active galactic nuclei.

Observing Electromagnetic Counterparts

Multi-Wavelength Follow-up

To detect EM counterparts, astronomers conduct rapid follow-up across multiple wavelengths:

• Gamma-ray telescopes: Swift, Fermi, INTEGRAL
• Optical telescopes: ZTF, GROWTH-India, DECam
• Infrared telescopes: JWST, UKIRT
• Radio telescopes: VLA, MeerKAT, ASKAP

Sky localization of the gravitational, gamma-ray and optical signals of GW170817/GRB 170817A. Left: LIGO (light green), LIGO-Virgo (dark green), Fermi and INTEGRAL (light blue), Fermi GBM (dark blue) localization. Right: localization of the host galaxy NGC 4993 by 1M2H Collaboration at 10.9 hours after the merger and the DLT40 pre-discovery image. Image Credit: LIGO, Virgo, Fermi, Swope, DLT40. Image text obtained from maravelias.info

Challenges in Detection

• Sky Localization: GW detectors provide broad sky maps (~100s of square degrees).
• Fading Signals: Kilonovae fade quickly, requiring rapid observations.
• Host Galaxy Identification: Helps refine merger properties.

Why Electromagnetic Counterparts Matter

Observing EM counterparts allows us to:

• Confirm the nature of compact object mergers
• Probe heavy element formation
• Use standard sirens to measure the Hubble constant
• Understand the physics of relativistic jets

Conclusion

Multi-messenger astronomy is a rapidly growing field, with each new detection bringing us closer to answering fundamental questions about compact objects, element formation, and cosmic expansion.