EMSS - Einstein Catalog IPC EMSS Survey
This database table contains information from the Einstein Extended Medium
Sensitivity Survey (EMSS) which consists of 835 serendipitous X-ray sources
detected at or above 4 times the rms level in 1435 IPC fields with their
centers located away from the galactic plane. Their limiting sensitivities
range from ~5*10E-14 to ~3*10-12 ergs cm-3 s-1 in the 0.3-3.5keV band. A
total area of 778 square degrees of the high galactic latitude sky (|b|>20)
has been covered. The analysis has been performed using data from the Rev 1
processing system at the CfA. The resulting EMSS catalog is a flux-limited and
homogeneous sample of astronomical objects that can be used for statistical
studies. Additional information is available from the HEASARC.
The published articles: Gioia et al. 1990, Stocke et al. 1991.
The background counts.
The Galactic Longitude of the object.
The BROWSE object classification. The classification is based on the 'Type'
parameter, if one is available.
This parameter contains miscellaneous information about the object.
The corrected IPC count rate in units of counts/second. The count rate is
derived from the net counts given in Net Counts (which are then corrected for
vignetting, mirror scattering, and point response function scattering) and the
livetime given in the Exposure field, which has already been corrected for
instrumental dead time.
The Declination of the object. The listed position is that of the centroid of
the X-ray source.
The offset in position the X-ray centroid and the proposed optical counterpart
in arcseconds of Dec. Negative offsets indicate directions west and south of
the X-ray centroids. Most of the offsets were measured automatically on the
POSS or SRC J plates. They are accurate to +/- 5". For the SAO stars we have
used the equinox 1950, epoch 1980 positions from the SAO catalog. When the
optical counterpart is a radio source (Column <fr>), the VLA radio position is
used (+/-1"). If more than one optical object is visible on the POSS within
10" of the offset position listed in this column, the identity of the optical
counterpart is clarified with a note in the Note field. When the optical
counterpart is a cluster of galaxies, the optical offset refers to the
brightest cluster member.
The Declination, given in radians.
Also known as the live-time, the parameter contains the exposure time of the
IPC observation, corrected for instrumental dead time. It is given in seconds.
The Ext_Flag column is flagged with an asterisk ('*') if extended counts from
Net Ext Count were used to calculate the X-ray flux.
The radio flux or 5 sigma upper limit in mJy at 5 GHz for the optical
counterpart and mostly comes from VLA observations. When a cluster of galaxies
is the X-ray counterpart, radio emission from any cluster galaxy within the
cluster is listed here.
The hydrogen column density along the line of sight to the IPC field target
determined using the HI survey of Stark et al. (1989), in units of 10^21
atoms/cm^2. For regions of sky not surveyed by Stark et al. (south of
declination -42 degrees), the surveys of Heiles and Cleary (1979) and Cleary,
Heiles, and Haslam (1979) have been used.
The Galactic Latitude of the object.
The source name; denoted by MS, followed by right ascension in hours, minutes,
and truncated fraction of minutes, then declination in degrees and arcminutes
The error on the net counts, computed as the square root of the total observed
counts in the detection cell.
The uncorrected net counts in the 0.2-3.5 keV band. No correction has been
applied to the counts.
The net extended counts. For IPC sources, counts were computed manually to
include all counts belonging to the source. For sources resolved by the IPC,
the observed counts have been computed manually within a region centered on the
source and with a size evaluated case by case so as to contain all the counts
belonging to the source itself. Background counts for these sources have been
computed within this same area from the background map produced by the REV.1
processing. In these cases only the vignetting and mirror scattering
corrections have been applied.
This column is flagged with an 'n' if a note on the source is present at the
end of the table. Please request the references from the HEASARC, due to the
size of the list.
The positional uncertainty, in arcseconds, the error associated with the
position (90% confidence error circle radius). A positional uncertainty of 4"
indicates sources detected also by the HRI. In these cases the coordinates of
the source come from the HRI.
The Right Ascension of the object. The listed position is that of the
centroid of the X-ray source.
The offset in position between the X-ray centroid and the proposed optical
counterpart in arcseconds of RA. Negative offsets indicate directions west and
south of the X-ray centroids. Most of the offsets were measured automatically
on the POSS or SRC J plates. They are accurate to +/- 5". For the SAO stars
we have used the equinox 1950, epoch 1980 positions from the SAO catalog. When
the optical counterpart is a radio source (Column <fr>), the VLA radio
position is used (+/-1"). If more than one optical object is visible on the
POSS within 10" of the offset position listed in this column, the identity of
the optical counterpart is clarified with a note in the Note field. When the
optical counterpart is a cluster of galaxies, the optical offset refers to the
brightest cluster member.
The Right Ascension, given in radians.
Marked with a less than sign (<) if the radio flux is an upper limit.
If the counterpart is extragalactic the redshift is listed (+/- 0.003).
The sequence number of the observation, a unique numeric identifier for each
observation which was allocated sequentially at the time of proposal
submission. It is an internal index used to key on all references to an
This parameter gives the signal-to-noise ratio.
The proposed identification or classification of the X-ray source.
This parameter gives the reference for identification or classification.
Identifications come from either our own spectroscopic work (EMSS in the 2nd
line) or from other authors' work as indicated in the id_ref column.
References to other authors are given when the proposed identification has
been published even if additional spectroscopic observations may have been
obtained. Please request the references from the HEASARC, due to the volume of
Photographic magnitude. Some entries are from photoelectric aperture
photometry or from CCD photometry with the Whipple Observatory 24 inch. These
are typically accurate to 0.01 mag. Other entries are from the literature or
are estimated magnitudes (+/- 0.5 mag) from the STScI digitized sky survey
plates. A value of 0.0 indicates that the source is still unidentified, so
there is no magnitude listed. Please consult the HEASARC for more information.
This parameter contains the X-ray flux in the 0.3-3.5keV band, in units of
This parameter contains the one-sigma error on the X-ray flux in the 0.3-3.5keV
band. The error on the x-ray flux is from photon counting statistics only
and is computed as the square root of the total observed counts in the
The logarithm of the ratio of the X-ray flux to optical flux, calculated from
the observed X-ray and visible fluxes using the expression:
log(fx_fv) = log fx + V/2.5 +5.37 (Maccacaro et al. 1988).
These values are used to determine whether the optical counterpart is
plausible. For this reason the X-ray flux used for this computation is
not the value in the X-ray flux field, but rather the X-ray flux computed
prior to assigning an optical identification class to each source.
Questions regarding the EMSS database table can be addressed to the
HEASARC User Hotline.
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