AN ULTRASOFT EINSTEIN X-RAY SURVEY

Abstract

We describe preliminary results of a survey of ultra-soft sources using the data base acquired with the Imaging Proportional Counter on the Einstein Observatory. Of the /approximately/7700 detections of sources in this data base with signal-to-noise ratios greater than or equal to 3 in the energy band 0.16--3.5 keV, /approximately/3% fit the selection criteria for sources that may have extremely soft x-ray components; i.e., they have a very low ratio of medium energy band (0.56--1.08 keV) to soft energy band (0.16--0.56 keV) counts. Preliminary results of our searches for counterparts of the ultra-soft sources, done with catalogs and optical and radio observations, reveal that the Ultra-Soft Survey is populated by, among other objects, isolated degenerate dwarf stars, particular subclasses of cataclysmic variable binaries, low-mass x-ray binaries in the LMC, some SAO stars of late spectral type, and several Hyades and dMe stars. With observations utilizing the Very Large Array, we identified one bright, Ultra-Soft Survey object with the radio pulsar PSR 0656 + 14. A particularly exciting result is the finding, thus far, of numerous active galactic nuclei among the Ultra-Soft Survey sources. Two-thirds of these AGN do not have any detectable x-ray emission above 1 keV. The remainder appearmore » to have both an ultra-soft and a hard component. The soft component can be characterized by a power law with an energy spectral index, ..cap alpha../sub E/, > 2. The soft AGN are mostly low-redshift sources with V magnitudes ranging from 17 to 20. All of these AGN are viewed along lines of sight in which the equivalent hydrogen column density is less than 6 /times/ 10/sup 20/ cm/sup /minus/2/. We present a table of the Ultra-Soft Survey sources for which we have identified probable optical counterparts. These objects presage the classes of source that EUVE and the Wide-Field Camera on ROSAT are likely to find in their all-sky, extreme ultraviolet surveys. 21 refs., 6 figs., 1 tab.« less