When a polarized signal propagates through the magnetized medium, the rotation of polarization angle is frequency dependent. This is the Faraday rotation discovered by M. Faraday in 1844. The rotation measure (RM, in the unit of rad/m²), is an integrated or averaged quantity of thermal electron density n_e (in the unit of cm^-3) and magnetic fields B (in the unit of μG) over the path from the source to us. Only the projected component of the magnetic field on the path to the line of sight works for the Faraday rotation.

Observations for RMs have good scientifical merits. RMs of radio sources in small sky regions have been observed for probing the magnetic fields in galaxy clusters, in stellar bubbles or HII regions and even SNRs in our Milky Way galaxy. RMs of radio sources behind the Galactic disk have been observed for revealing magnetic structure in the disk. The RM distribution over the whole sky have been used for delineat the magnetic fields in the Galactic halo.

Many radio sources have their RMs well-determined via multi-frequency polarization observations. In early days, polarization observations were carried out for strong radio sources by single dish radio telescopes, and the measurements of polarization angles at several frequencies were used to estimate RMs. Later, the synthesis radio telescopes have been used for polarization observations with excellent resolutions, so that RMs of different emission components of radio sources can be measured separatedly. Recent developments is the radio observations in a spectropolarimeter mode within a wideband, so that the RMs can be determined from measured ψ values in many frequency channels or directly from the Q and U values of the channel maps which is called as RM synthesis.

The most often used early compilations of RM data of galaxies or quasars were conducted for 555 objects by Simard-Normandin, Kronberg, & Button (1981) and 674 objects by Broten, MacLeod, & Vallee (1988). Recent years RMs of several thousands of radio sources have been observed. In addition, Taylor, Stil, & Sunstrum (2009, TSS09) published 37,543 RMs by re-calculating the NVSS data.

At present, we have compiled a catalog for well-determined RMs of 4553 point sources from literature, and also RMs for 97 extended sources or point sources with only 2-band measurements. We also supplement the published RMs of 669 pulsars and 37543 RMs from the NVSS RM catalog by TSS09.

In this web-site, we provide users the catalog data files and basic tools:

• The RM data in a small region can be extracted;
• The Galactic RM foreground in any direction can be calculated by using the weighted average of RMs of a set radio sources;
• Full dataset and reference can be accessed.