Simple Line Access Protocol

Purpose

The Simple Line Access Protocol (SLAP) is an IVOA Data Access protocol for retrieving spectral lines from various spectral line data collections through a uniform interface within the VO framework. These lines can be either observed or theoretical, and are typically used to identify emission or absorption features in astronomical spectra.

Current status

The SLAP 1.0 Recommendation was published in 2010. As of October 19th 2026, only a few SLAP 1.0 services are registered in the IVOA Registry.

SLAP 1.0 was designed as a simple protocol, but its data model proved too complex for simple use-cases (finding lines in a given wavelength range and labelling them textually) and too limited for complex ones (finding lines in a given wavelength range and identifying the corresponding quantum numbers). Meanwhile, the VAMDC infrastructure, developed shortly after SLAP 1.0 was released, addressed the complex use-cases more efficiently, but its extensive data model makes it impractical for simple ones. As a result, few services implemented SLAP 1.0.

SLAP 2.0 therefore aims to provide a simple protocol for simple use-cases, while relying on the VAMDC infrastructure for complex ones. It is based on a limited set of columns whose definitions match their VAMDC counterparts, so that users can move seamlessly from a SLAP 2.0 query to a full VAMDC query when they need more detail. Consequently, the Simple Spectral Line Data Model ( SSLDM) associated with SLAP 1.0 is deprecated in SLAP 2.0.

SLAP 2.0 is currently a Working Draft ( WD-SLAP-2.0-20261007).

A first reference implementation is available at: https://sesam.obspm.fr/26.09/vamdc/slap/capabilities

Both the /lines and /species resources can be validated with the PADC VOTAble validation service.