ligo.skymap is a Python library designed for working with sky localization maps of gravitational-wave (GW) events. Developed by the LIGO/Virgo Collaboration, it provides tools for reading, writing, analyzing, and visualizing sky maps in standard formats (particularly HEALPix), and is widely used in multi-messenger astrophysics for coordinating electromagnetic (EM) and neutrino follow-up observations.
Key Features
Support for HEALPix and Multi-Order Maps
ligo.skymap can read and write sky maps in both standard (fixed-resolution) and multi-resolution HEALPix formats (e.g., multi-order FITS). This flexibility is critical for handling large localization regions efficiently, especially for low-latency alerts.
Sky Localization Analysis
The library includes utilities to compute key properties of localization maps such as:
- Credible regions (e.g., 50%, 90% probability areas)
- Most probable sky positions
- Probability densities at specific coordinates
These features are essential for identifying high-probability regions on the sky for follow-up observations.
Map Arithmetic and Region Manipulation
ligo.skymap allows users to perform operations on sky maps, such as combining or intersecting maps (e.g., from GW and GRB detections), masking out regions (e.g., Earth-occulted areas), and applying selection filters based on observability constraints.
Galaxy Targeting Tools
Includes functionality to cross-match sky maps with galaxy catalogs, enabling targeted follow-up of potential host galaxies within a localization region. This is especially useful for searches of electromagnetic counterparts like kilonovae or short GRBs.
Integration with Astropy and SciPy
Built on standard Python scientific libraries, ligo.skymap integrates easily with tools like Astropy and SciPy, allowing for streamlined data analysis, transformation between coordinate systems, and advanced statistical operations.
Visualization
Provides high-quality, customizable plotting tools for visualizing sky maps using various projections (e.g., Mollweide, Cartesian). These visualizations are useful for observatory planning, public outreach, and scientific presentations.
Use Case
ligo.skymap is widely used in the GW and multi-messenger community to:
- Analyze GW sky localization regions in real time or offline.
- Plan follow-up observations by calculating optimal pointing directions.
- Combine and compare localizations from multiple messengers (e.g., GW, GRB, neutrino).
- Match sky maps to galaxy catalogs to identify likely host candidates.
- Produce publication-ready visualizations of GW event localizations.
By offering a robust and flexible toolkit for localization map analysis, ligo.skymap plays a central role in coordinating rapid, targeted, and efficient multi-messenger follow-up efforts. It is open-source and actively maintained to support both real-time alert processing and in-depth archival studies.