.. _counterpart:

===========
Counterpart
===========

When two compact objects such as neutron stars (:term:`NS`) or black holes (:term:`BH`) merge,
they can produce both gravitational-waves and electromagnetic signals.
However, the nature and visibility of these signals depend strongly on the type of merging objects and the details of the event.
This section explains what kinds of multi-messenger signals we can expect from different types of compact binary mergers,
and how these observations reveal the physical processes at work in the most extreme environments in the universe.


.. dropdown:: What can we expect from different types of compact binary mergers?

  Not all compact binary mergers emit both gravitational waves and electromagnetic signals.
  The type of signal we can expect depends on the components of the binary system:

  .. admonition:: Signals from Different CBC Type
    :class: info

    .. tab-set::

      .. tab-item:: BBH

        - **Gravitational Waves:** Strong, clear signals

        - **EM Counterpart:**  None (no matter involved)

      .. tab-item:: BNS

        - **Gravitational-waves:** Strong signals with :term:`NS` structure effects

        - **Electromagnetic Counterpart:** :term:`GRB`, :term:`KNe`, Afterglows (X-ray to radio)

      .. tab-item:: NSBH

        - **Gravitational Waves:** Strong signals

        - **Electromagnetic Counterpart:**  Possible emission if :term:`NS` is disrupted by :term:`BH` (depends on :term:`BH` mass and spin)


  .. figure:: ../_static/cbc-merger-scenario.png
    :align: center
    :scale: 40%

    Multi-messenger outcome depends on the type of merger. **Left**: a :term:`BNS`
    merger ejects matter from tidal tails and disk winds, producing both gravitational-waves
    and isotropic kilonova light. **Right**: in a :term:`NSBH` merger, matter may be swallowed directly
    by the :term:`BH` or ejected, depending on the system's geometry. Only disrupted systems yield observable light.

    Credit: This figure is adapted from work by Matthew R. Mumpower



.. dropdown:: **What about GW170817?**

  This :term:`BNS` merger, detected in 2017, was the first confirmed multi-messenger event:

   - gravitational-waves detection by LIGO and Virgo
   - A short :term:`GRB` (GRB170817A) detected simultaneously
   - A bright :term:`KN` (AT2017gfo) observed from ultraviolet to infrared
   - Localized in galaxy NGC 4993 (~40 Mpc)

  It revealed that neutron star mergers are key sites for the synthesis of heavy elements (gold, platinum)
  via the rapid neutron capture process :math:`r`-process.

.. note::

    The visibility of electromagnetic counterparts (especially :term:`KNe`) depends on:

    - Ejecta mass and composition (lanthanide-rich vs. lanthanide-poor)
    - :term:`NS` disruption (more likely in :term:`NSBH` if the :term:`BH` is light and fast-spinning)
    - Viewing angle and detector sensitivity


.. dropdown:: Gamma-ray Bursts

  :term:`GRB` are the most energetic explosions in the universe:

  - **Short GRBs** (< 2 seconds, harder gamma-ray spectra) result from :term:`CBC` involving neutron stars (e.g., GW170817).
  - **Long GRBs** (> 2 seconds, softer gamma-ray spectra) result from the collapse of massive stars.

  GRBs are extremely bright, detectable across electromagnetic wavelengths, from gamma-rays to optical and :term:`NIR` afterglows.


.. dropdown:: Kilonovae

  :term:`KNe` are transient astrophysical phenomena emitting from :term:`UV` to :term:`IR` wavelengths, following :term:`NS` mergers (:term:`BNS` or :term:`NSBH`). These events:

  - **Produce heavy elements** via :math:`r`-process nucleosynthesis (e.g., gold, platinum).
  - Emit relatively isotropic EM radiation, unlike the highly directional GRBs.
  - Offer clues about neutron star interiors, dense matter physics, and cosmic chemical evolution.

  The brightness and color (blue vs. red :term:`KNe`) depend primarily on the ejected matter’s **electron fraction (Ye)**:

  - **Blue kilonovae** (Ye > 0.25): Lanthanide-poor, less opaque ejecta emitting at shorter wavelengths (visible and :term:`UV`).
  - **Red kilonovae** (Ye < 0.25): Lanthanide-rich, highly opaque ejecta emitting primarily at longer, IR wavelengths.


.. dropdown:: Multi-wavelength Observations

  Observations across multiple wavelengths are essential to fully characterize merger events:

  - **Ground-based observatories:** :term:`ZTF`, :term:`LSST` for optical follow-up.
  - **Space-based observatories:** Swift (:term:`GRB`), JWST (infrared), upcoming ULTRASAT (:term:`UV`).
  - **GW detectors:** LIGO-Virgo-KAGRA (ground-based), LISA (future space-based detector).



.. important::

  Combining gravitational-waves with electromagnetic signals from compact binary mergers allows astronomers to:

  - Probe extreme physics and dense matter properties in neutron stars.
  - Understand cosmic nucleosynthesis and the origin of heavy elements.
  - Refine cosmological parameters, including the expansion rate of the universe.

  Multi-messenger astronomy promises transformative insights into the most energetic and mysterious phenomena in our universe.