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TWOSIGMA - Einstein Two-Sigma Catalog



The X-ray sources from observations made with the Einstein Observatory (HEAO-2) Imaging Proportional Counter (IPC) that have intensities of 2-sigma or more above the background are compiled in this catalog. This catalog covers more sky at fainter flux levels than the Einstein Medium Sensitivity Survey. Fields with diffuse emission sources such as bright Abell clusters of galaxies and supernova remnants were excluded. Thus, data within 10 degrees of the galactic plane as well as fields within the boundaries of the Magellanic Clouds were excluded. Regions crowded with galactic sources such as the Orion and Pleiades fiels were also excluded. Excluding redundant fields, this catalog covers 1850 sq. degrees of the sky.

The generation of the Einstein Two-Sigma Catalog was described in detail by Moran et al. (1996). Please read this article carefully to ensure responsible use of the Catalog. Detailed scientific and technical questions on the contents and methodology of this catalog should be addressed to the first author, Ed Moran ( In particular, it should be noted that, by design, this catalog contains a significant number of spurious sources: only 28%, or about 13,000 sources, out of the 46,000 source in the 2-sigma catalog are `real` astrophysical sources, with the remainder of the sources being spurious ones. Moran et al. show in their paper that performing cross-correlations of 2-sigma sources with other catalogs is an effective way of selecting sources in this catalog that are probably real.

Catalog Bibcode



Moran, E.C., Helfand, D.J., Becker, R.H., White, R.L. 1996, ApJ, 461, 127.


Name derived from B1950 coordinates and the prefix ETS (for Einstein Two Sigma), using the standard IAU position-based nomenclature.

The Right Ascension of the object.

The Declination of the object.

The galactic longitude of the object.

The galactic latitude of the object.

The signal-to-noise ratio of the source, based on the counts collected within a 1.25 arcmin radius circular aperture.

The IPC Sequence number of the observation containing the source.

The number of background-subtracted counts contained in the source, collected within a circular aperture of radius 1.25 arcmin.

The number of background-subtracted counts contained in the source, collected within a circular aperture of radius 2.0 arcmin.

Flatfield-weighted Exposure Time, Radii 1.25

Flatfield-weighted Exposure Time, Radii 2.00

Number of "Good" Pixels, Radii 1.25 arcmin

Number of "Good" Pixels, Radii 2.00 arcmin

For the Background in the Concentrici, Radii 3-6 arcmin

Contact Person

Questions regarding the TWOSIGMA database table can be addressed to the HEASARC User Hotline.
Page Author: Browse Software Development Team
Last Modified: Wednesday, 10-Nov-2004 16:06:22 EST