The EINSTEIN Observatory Extended Medium-Sensitivity Survey (EMSS)

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1990ApJS...72..567Gioia
1991ApJS...76..813Stocke


IX/15       Einstein EMSS Survey       (Gioia+ 1990, Stocke+ 1991)
============================================================================
The EINSTEIN Observatory Extended Medium-Sensitivity Survey (EMSS)
     Gioia I.M., Maccacaro T., Schild R.E., Wolter A.
    
    =1990ApJS...72..567G
     Stocke J.T., Morris S.L., Gioia I.M., Maccacaro T., Schild R.E.,
     Wolter A., Fleming T.A., Henry J.P.
    
    =1991ApJS...76..813S
============================================================================
ADC_Keywords: X-ray sources ; Active gal. nuclei ; BL Lac objects ;
              Clusters, galaxy ; Galactic plane ;
Mission_Name: Einstein

Description:

The Einstein Extended Medium Sensitivity Survey (EMSS) 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*10-14 to to ~ 3*10-12 erg.cm-2.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. Here we present the table listing all the sources taken from the publication referenced below and the optical identifications. At present over 96% of the 835 X-ray sources have been successfully identified in the following proportions: active galactic nuclei (QSO's, quasars and Seyfert), 51.1%; BL Lacertae objects, 4.3%; clusters of galaxies, 12.2%; normal galaxies, 2.1%; cooling flow galaxies, 0.6%; Galactic stars 25.8%; and unidentified, 3.9%. Most of the individual optical counterparts are previously unknown objects and so constitute large statistical samples independent of previously selection methods.
The contents of the table is described below. The sky coverage computed for a specific assumed source spectrum is also given under "Additional Information" below. For further details please see the published articles: Gioia et al. 1990, Stocke et al. 1991.

1990ApJS...72..567G Abstract

This paper presents the results of the analysis of the X-ray data and the optical identification for the Einstein Observatory Extended Medium-Sensitivity Survey (EMSS). The survey consists of 835 serendipitous sources detected at or above 4 times the rms level in 1435 imaging proportional counter fields with centers located away from the Galactic plane. Their limiting sensitivities are about (5-300) x 10 to the -14th ergs/sq cm sec in the 0.3-3.5-keV energy band. A total area of 778 square deg of the high-Galactic-latitude sky has been covered. The data have been analyzed using the REV1 processing system, which takes into account the nonuniformities of the detector. The resulting EMSS catalog of X-ray sources is a flux-limited and homogeneous sample of astronomical objects that can be used for statistical studies.

1991ApJS...76..813S Abstract

The optical identifications are presented of the Einstein Extended Medium-Sensitivity Survey (EMSS), including the methodology used to optically identify the EMSS sources and the uncertainties involved with that process. The optical properties of the classes of X-ray, optical, and radio data for each of the identified and, as yet, unidentified sources of the survey are described. A new class of X-ray emitters, cooling flow galaxies, is proposed. The criteria used to determine whether the proposed optical counterpart to the X-ray source is a plausible identification are described. Plausibility is based on the optical classification of the counterpart, e.g., AGN, cluster, G star, and the X-ray-to-optical flux ratios previously observed for these classes of X-ray emitters. Two independent schemes of optical classification of the counterparts are used to check the plausibility of these identifications; one is based on moderate-resolution optical spectroscopy, and the other, on inferred X-ray luminosity and the overall energy distribution. Additional Information:

The EMSS sky coverage

This sky coverage has been produced using the counts in the standard detection algorithm and assuming a power law spectrum with energy index = 1 and the measured Galactic hydrogen column density in the direction of each IPC pointing. We caution the user that this sky coverage is not appropriate for computation of functions like logN(>S)-logS, or Luminosity Functions of resolved sources, like clusters of galaxies or "normal" galaxies, nor of stars. The fluxes for these objects have been calculated either using extended counts, when appropriate, and/or different assumptions for the incident spectrum (see below).

           Limiting Sensitivity            Area Covered
              (erg/cm**2/s)                 (sq. deg)

                 5.08E-14                     0.09
                 6.09E-14                     0.72
                 7.31E-14                     2.54
                 8.78E-14                     6.37
                 1.05E-13                    15.1
                 1.26E-13                    29.4
                 1.52E-13                    55.2
                 1.82E-13                    94.2
                 2.18E-13                   139.4
                 2.62E-13                   191.6
                 3.14E-13                   249.5
                 3.77E-13                   319.1
                 4.53E-13                   402.0
                 5.43E-13                   497.0
                 6.52E-13                   582.9
                 7.83E-13                   657.7
                 9.39E-13                   711.8
                 1.13E-12                   743.7
                 1.35E-12                   762.7
                 1.62E-12                   771.9
                 1.95E-12                   775.7
                 2.34E-12                   777.4
                 2.80E-12                   777.9
                 3.36E-12                   778.1

File Summary:
--------------------------------------------------------------------------------
 File Name        Lrecl       Records    Explanations
--------------------------------------------------------------------------------
ReadMe              80             .    This file
catalog.dat        194           835    EMSS Catalogue
notes.dat           80           412    Notes to EMSS sources
--------------------------------------------------------------------------------

See also:
    IX/18 : EINSTEIN extended source survey (EXSS) (Oppenheimer+ 1997)

Byte-by-byte Description of file: catalog.dat
--------------------------------------------------------------------------------
   Bytes Format  Units     Label    Explanations
--------------------------------------------------------------------------------
   2- 14  A13    ---       name     *Name of EMSS source
  17-18   I2     h         RAh      *Right Ascension (B1950.0) hours
  19-20   I2     min       RAm      *RA minutes
  21-24   F4.1   s         RAs      *RA seconds
  25      A1     ---       DE-      *Declination sign (B1950.0)
  26-27   I2     deg       DEd      *Dec degrees
  28-29   I2     arcmin    DEm      *Dec arcmin
  30-33   F4.1   arcsec    DEs      *Dec arcsec
  35- 36  I2     arcsec    e_pos    *Positional Uncertainty
  37- 40  I4     arcsec    x        *? Position offset between optical and X-ray
  41- 44  I4     arcsec    y        *? Position offset between optical and X-ray
  46- 51  F6.2  10-16W/m2  fX       *X-ray flux (*1.E-13 ergs/cm**2/s or
                                     *1.E-16 W/m**2) in the 0.3-3.5 keV band
      52  A1     ---       ext_flag *Flagged '*' if source is extended
  53- 57  F5.2  10-16W/m2  e_fX     *1 sigma error on X-ray flux
  59- 64  F6.2  ct/ks      ctrate   *Corrected IPC count rate
  66- 69  F4.1   ---       sn       *Signal to Noise Ratio
  71- 76  F6.1   ---       cts      *Uncorrected net counts in 0.2-3.5 keV
  78- 81  F4.1   ---       e_cts    *Error on net counts
  83- 87  I5     s         live     *Corrected exposure time
  89- 93  F5.1   ---       bkgcts   *Total background counts
  95- 99  I5     ---       seqno    *IPC sequence number of the image used
 101-104  I4     ---       cts_ext  *? Net extended counts
 106-111  F6.4  10+25m-2   nH       *Hydrogen column density
                                     (*E+25 m^-2 or *E+21 cm^-2)
 113-117  F5.2   mag       Vmag     *? Visual magnitude
     119  A1     ---       n_fR      '<' for upper limit in fR
 120-125  F6.1   mJy       fR       *? Radio flux at 5 GHZ
 127-131  F5.2   ---       fX/fv    *? Logarithm of X-ray to visual flux ratio
 133-136  A4     ---       class    *Proposed identification class of the
                                     X-ray source (see below)
 138-141  A4     ---       r_class  *Reference for class
 143-147  F5.3   ---       z        *? Redshift
     149  A1     ---       n        'n' if a note on the source is in file notes
 151-194  A44    ---       comment  *Miscellaneous information on the source

--------------------------------------------------------------------------------

Note on name:

This column gives the source name; denoted by MS, followed by right ascension in hours, minutes, and truncated fraction of minutes, then declination in degrees and arcminutes (i.e. MS0013.4+1558).

Note on RAh, RAm, RAs, DE-, DEd, DEm, DEs:

The right ascension and declination (B1950) correspond to the centroid of the X-ray source.

Note on e_pos:

The e_pos column gives 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.

Note on x, y:

The offset in position between the X-ray centroid and the proposed optical counterpart in arcseconds of R.A. (x) and dec (y). 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 ), 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 column. When the optical counterpart is a cluster of galaxies, the optical offset refers to the brightest cluster member.

Note on fX, ext_flag, e_fX,:

These columns give the X-ray flux (in units of 10E-13 erg/cm**2/s) and the one sigma error on the X-ray flux in the 0.3-3.5 keV band. The ext_flag column is flagged with an asterisk ('*') if extended counts from the column cts_ext were used to calculate the X-ray flux. 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 detection cell. The flux has been computed in the 0.3-3.5 keV band for consistency with previous work (note that the Rev.1 processing computes it in the 0.2-3.5 keV band). The following assumptions have been adopted:

(a) For the AGN, BL Lac objects and unidentified sources; we have multiplied the corrected count rate given in the ctrate column by a conversion factor appropriate for a power law spectrum with an energy index alpha = 1.0 and with the measured Galactic hydrogen column density in the direction of each IPC pointing.

(b) For unresolved galaxies and clusters of galaxies; the flux has been computed using an identical procedure but with a different conversion factor appropriate for a Raymond-Smith thermal spectrum (Raymond and Smith, 1977) with temperature of about 6 keV.

The above assumptions are justified by the results of the analysis of the X-ray energy distribution of the EMSS sources performed by Maccacaro et al. (1988). A number of sources are resolved even with the moderate angular resolution of the IPC. They are mainly clusters of galaxies but also 3 galaxies, 8 AGN, and 3 BL Lacs.

For the extragalactic population, the X-ray flux listed has been corrected ("de-reddened") for Galactic absorption. For stars we have adopted a constant conversion factor of 1 IPC count/s = 2E-11 erg/cm**2/s corresponding to a Raymond-Smith thermal spectrum with temperatures in the range 8E05 - 3E06 K and no correction for the hydrogen column density. Note that RS CVn and K and M flare stars usually have a second temperature component in the range 1 - 2E07 K which results in nearly identical conversion factors to those derived from the Raymond-Smith models over these temperature ranges (see also Fleming, 1988, for a discussion of flux estimate of X-ray selected stars).

In the case of clusters of galaxies the flux estimate should be evaluated in a region of constant physical size (e.g. 1 Mpc). Such a procedure cannot be applied to the serendipitous EMSS clusters, especially to the nearby ones, given their proximity to the ribs (or edges) of the detector or, in some cases, to the target of the observation. For all the resolved sources identified with clusters of galaxies or galaxies, and flagged as extended in the ext_flag column, we have used the extended counts measured according to the procedure described below the cts_ext column to compute the flux.

Note on ctrate:

This column gives the corrected IPC count rate in units of cts/kilosecond. The count rate is derived from the net counts given in the cts column (which are then corrected for vignetting, mirror scattering, and point response function scattering) and the livetime given in the live column, which has already been corrected for instrumental dead time.

Note on sn:

This column gives the signal-to-noise ratio (s/n) computed as the source counts divided by the square root of the sum of the source counts and the background counts:
Source Counts / (Source Counts + Background Counts)**(1/2)]

Note on cts, e_cts:

The uncorrected net counts in the 0.2-3.5 keV band are given in the cts column. No correction has been applied to the counts. The e_cts column gives the error on the net counts, computed as the square root of the total observed counts in the detection cell. The cts/e_cts ratio is the signal-to-noise given in the previous column. The total background counts are given in the bkgcts column.

Note on live, bkgcts:

This column gives the livetime, the exposure time of the IPC observation, corrected for instrumental dead time. It is given in seconds. The second line gives the background counts.

Note on seqno, cts_ext:

This column gives 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 observation.

If the ext_flag column is flagged with an asterisk ('*') then extended counts from the column cts_ext were used to calculate the X-ray flux. The cts_ext column gives the net extended counts. For IPC sources, this means that 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.

At the edge of the IPC detector the point spread function becomes significantly degraded so that some sources appear to be extended even though they might not be. Observations with a higher resolution instrument (e.g. the HRI or ROSAT) are needed to decide whether the extension is true or an artifact of the degradation of the IPC point spread function. This distortion of the point spread function cannot be modeled easily, so indications that sources are extended when they are near the IPC edge cannot be always trusted. Since these sources include ones which are identified as AGN and BL Lac objects as well as clusters, the indication of extension should be regarded as tentative.

Note on nH:

The hydrogen column density along the line of sight to the IPC field target determined using the HI survey of Stark et al. (1989). For regions of sky not surveyed by Stark et al. (south of declination -42 deg) the surveys of Heiles and Cleary (1979) and Cleary Heiles and Haslam (1979) have been used.

Note on Vmag:

The V band magnitude of the counterpart. Some entries are from photoelectric aperture photometry (for the stellar sources, see Fleming, 1988 and the Bright Star Catalog) or from CCD photometry with the Whipple Observatory 24 inch (for the extragalactic sources). 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.

Note on fR:

The radio flux or 5 sigma upper limit in mJy at 5 GHz for the optical counterpart 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.

Note on fX/fv:

The logarithmic X-ray to optical flux ratio calculated from the observed X-ray and visible fluxes by 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 fX column but is rather the X-ray flux computed prior to assigning an optical identification class to each source. These "raw" X-ray fluxes use an assumed spectral index of -0.5 for all sources and a correction for galactic extinction equal to 3 X 10**20 cm-2 for all sources. The V band magnitudes quoted in the mv column were used for this calculation. For sources identified with clusters of galaxies the fX/fv is not given (fX/fv = 0.0) since the fX does not take into account the extended flux where present, and the V magnitude refers to the brightest cluster galaxy.

Note on class:

The class column gives the proposed identification or classification of the X-ray source. The following abbreviations are used:

             AGN     = Active Galactic Nucleus (quasar or Seyfert galaxy)
             CL      = Cluster of galaxies
             *CL*    = Cooling flow galaxy
             BL      = BL Lac object
             GAL     = "normal" galaxy
             STAR    = star
             UNID    = source still unidentified

Note on r_class:

The r_class column 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 r_class column. References to other authors are given when the proposed identification has been published even if additional spectroscopic observations may have been obtained by us.

References in column r_class of emss table: (MSS1) Stocke, J.T., Liebert, J., Gioia, I.M., Griffiths, R.E., Maccacaro, T., Danziger, I.J., Kunth, D., & Lub, J., 1983, ApJ, 273, 458 (1983ApJ...273..458S)

(MSS2) Gioia, I.M., Maccacaro, T., Schild, R.E., Stocke, J.T., Liebert, J.W., Danziger, I.J., Kunth, D., & Lub, J., 1984, ApJ, 283, 495 (1984ApJ...283..495G)

(1) White, S., Silk, J., & Henry, J.P., 1981ApJ...251L..65W

(2) Margon, B., Downes, R., & Chanan, G., 1985ApJS...59...23M

(3) Pravdo, S., & Marshall, F., 1984ApJ...281..570P

(4) Huchra, J., Davis, M., Latham, D., & Tonry, J., 1983ApJS...52...89H

(5) Hewitt, A., & Burbridge, G., 1987ApJS...63....1H

(6) Kriss, G., & Canizares, C., 1982ApJ...261...51K

(7) Wolstencraft, R., Hu, W., Arp, H., & Scarrott, S., 1983,, MNRAS, 1983MNRAS.205...67W

(8) Chanan, G., Margon, B., & Downes, R., 1981ApJ...243L...5C

(9) Mundt, R., Walter, F., Feigelson, E., Finkenzeller, V., Herbig, G., & Odell, A., 1983ApJ...269..229M

(10) Henry, J.P., Soltan, A., Briel, U., & Gunn, J., 1982ApJ...262....1H

(11) Margon, B., Boronson, T., Chanan, G., Thompson, I., & Schneider, D., 1986PASP...98.1129M

(12) Caillault, J., Helfand, D., Nousek, J., & Takalo, L., 1986, 1986ApJ...304..318C

(13) Morris, S., Schmidt, G., Liebert, J., Stocke, J., Gioia, I., & Maccacaro, T., 1987ApJ...314..641M

(14) Biermann, P., Schmidt, G., Liebert, J., Stockman, H., Tapia, S., Strittmatter, P., West, S., & Lamb, D., 1985ApJ...293..303B

(15) Reichert, G., Mason, K., Thorstensen, J., & Bowyer, S., 1982, 1982ApJ...260..437R

(16) J. Huchra & M. Postman (private communication)

(17) de Vaucouleurs, G., de Vaucouleurs, A., & Corwin, H., 1976, Second Reference Catalogue of Bright Galaxies (Austin: University of Texas Press) (Cat. )

(18) I.J. Danziger (private communication)

(19) Maccagni, D., Garilli, B., Gioia, I.M., Maccacaro, T. Vettolani, G., & Wolter, A., 1988ApJ...334L...1M

(20) Katgert, P., Thuan, T., & Windhorst, R., 1983ApJ...271....1K

(21) Mason, K., Spinrad, H., Bowyer, S., Reichert, G., & Stauffer, J., 1981AJ.....86..803M

(22) Hoessel, J., Gunn, J., & Thuan, T., 1980ApJ...241..486H

(23) F. Walter (private communication)

(24) Morris, S.L., Liebert, J., Stocke, J.T, Gioia, I.M., Maccacaro, T., Schild, R.E., & Wolter, A., 1990, ApJ, 383, 686 (1990ApJ...365..686M)

(25) Chanan, G., Margon, B., Helfand, D., Downes, R., & Chance, D., 1982ApJ...261L..31C

(26) Kowalski, M., Ulmer, M., & Cruddace, R., 1983ApJ...268..540K

(27) Nesci, R., Gioia, I., Maccacaro, T., Morris, S., Perola, G., Schild, R., & Wolter, A., 1989ApJ...344..104N

(28) Maia, M., DeCosta, L., Willmer, C., Pellegrini, P., & Rite, C., 1987AJ.....93..546M

(29) Marschall, L., Stefanik, R., Nations, R., & Karshner, G., 1989, BAAS, 21, 1083 (1989BAAS...21.1083M)

(30) Fleming, T.A., Gioia, I.M., & Maccacaro, T., 1989AJ.....98..692F

(31) Robb, R., 1989, IBUS, Nos. 3346 and 3370

(32) R. Robb, private communication;

(33) Carter, B., Inglis, I., Ellis, R., Efstathiou, G., & Godwin, J., 1985MNRAS.212..471C

(34) Kurtz, M., Huchra, J., Beers, T., Geller, M., Gioia, I.M., Maccacaro, T., Schild, R., & Stauffer, J., 1985AJ.....90.1665K

(35) Silva, D., Liebert, J., Stocke, J.T. & Aaronson M., 1985, 1985PASP...97.1096S

Note on z:

If the counterpart is extragalactic the redshift is listed (+/- 0.003).

Note on comment:

This column gives miscellaneous information on the source (i.e. radio catalogued source, SAO name, X-ray variable source, IRAS source, EXOSAT source, etc.). The EXOSAT CMA position is reported with the positional accuracy in parentheses. EMSS sources already published as part of the MSS samples are labelled as MSS1 (Maccacaro et al. 1982 and Stocke et al. 1983) and MSS2 (Gioia et al. 1984). --------------------------------------------------------------------------------

History and Modifications:
  * 19-Jan-1993: Original version received from Isabella Gioia, table
    adapted by F.Ochsenbein[CDS]
  * 19-Jan-1993. Documentation reformatted to v1.4 of standard by
    Paul Kuin [ADC] 31-Jul-1995.
  * 29-Aug-1997: catalogue number changed from VII/152 to IX/15
    (high energy data)

References:

Cleary, M.N., Heiles, C., and Haslam, C.G.T., 1979A&AS...36...95C

Fleming, T.A. 1988, Ph. D. Thesis, University of Arizona

Gioia, I.M., Maccacaro, T., Schild, R.E., Stocke, J.T., Liebert, J.W., Danziger, I.J., Kunth, D., & Lub, J., 1984ApJ...283..495G

Gioia, I.M., Maccacaro, T., Morris, S.L., Schild, R.E., Stocke, J.T., Wolter, A., & Henry, P.H., 1990ApJS...72..567G

Heiles, C. and Cleary, M.N., 1979, Australian J.Pys. Ap. Suppl, 47, 1.

Maccacaro, T., & al. 1982ApJ...253..504M

Maccacaro, T., Gioia, I.M., Wolter, A., Morris, S.L., & Stocke, J.T., 1988, 1988ApJ...326..680M

Raymond, J.C. & Smith, B.W., 1977ApJS...35..419R

Stark, A.A., Heiles, C., Bally, J., and Linker, R., 1989, Bells Lab, privately distributed magnetic tape

Stocke, J.T., Liebert, J., Gioia, I.M., Griffiths, R.E., Maccacaro, T., Danziger, I.J., Kunth, D., & Lub, J., 1983ApJ...273..458S

Stocke, J.T., Morris, S.L., Gioia, I.M., Maccacaro, T., Schild, R.E., Wolter, A., Fleming, T.A., & Henry, J.P., 1991ApJS...76..813S

Contact: Isabella Gioia, Institute for Astronomy; gioia@galileo.ifa.hawaii.edu

I.Gioia, F.Ochsenbein, P. Kuin 31-Jul-1995