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EGRET2: Help

EGRET2 - Second EGRET Source Catalog

EGRET2 - Second EGRET Source Catalog


Overview

The Second EGRET Catalog of High-Energy Gamma-Ray Sources is based on data obtained by the Energetic Gamma-Ray Experiment Telescope (EGRET) onboard the Compton Gamma-Ray Observatory (CGRO) during CGRO Phase 1 (1991 April - 1992 November) and Phase 2 (1992 November - 1993 September). EGRET is sensitive to photons in the energy range from about 30 MeV to over 20 GeV, the highest energies accessible by the CGRO instruments, and, like COMPTEL, is an imaging instrument. In addition to including more data than the First EGRET Catalog, this catalog uses an improved model of the diffuse Galactic gamma radiation. There are 129 distinct sources in this catalog: since there are multiple measurements for these sources, there are 1041 entries in the HEASARC's version of the 2nd EGRET Catalog corresponding to the 1041 lines in its published version. Thus, there are an average of about 8 entries for every distinct source. The 129 sources include one solar flare bright enough to have been detected as a source, the Large Magellanic Cloud (LMC), 5 pulsars, 40 high-confidence identifications of active galactic nuclei (AGNs), 11 AGN identifications with lower confidence, and 71 sources not yet identified with known objects. Notice that 31 sources reported in the 1st EGRET Catalog do not appear in this 2nd EGRET Catalog: 5 of these are sources that were identified as AGNs, and all 5 of them now have statistical significances just below the threshold for the present catalog.


Parameters

Name
A Name based on the J2000 coordinates for the best position of the source, following the IAU naming convention (cf. PASP, 102, 1231). It should be emphasized that the measured positions are generally not as precise as the name might suggest.

RA
Right Ascension in the default equinox. The original data was provided in J2000 coordinates.

Dec
Declination in the default equinox. The original data was provided in J2000 coordinates.

LII
Galactic longitude.

BII
Galactic latitude.

Shape_Flag
A value of `Y` indicates that the error box is not well represented by an ellipse.

A_Axis
The semimajor axis of an ellipse fitted to the 95% confidence error contour in arcmin.

B_Axis
The semiminor axis of an ellipse fitted to the 95% confidence error contour in arcmin.

Phi_A_Axis
The position angle of the major axis, measured eastward from north in celestial coordinates, in degrees.

Morphology
Morphology flag, using the following system:

--------------------------------------------------------------------------
   E= extended source (applies only to the Large Magellanic Cloud);
   EM=possibly extended source or multiple sources (based on source location
        maps inconsistent with a single point source or poor fit to the
         calibrated PSF)
   C= source confusion may affect flux, significance, or position.
--------------------------------------------------------------------------
Sources with no entry in this column are consistent with the EGRET point spread function for a single source.

Flux_Limit
A value of `<` implies that the Flux value quoted is only an upper limit.

Flux
The flux (E> 100 MeV) in units of 10^{-8} photons/cm^2 s, or upper limit thereof (for entries where Flux_Limit = '<', i.e., for observations in which the source was not detected) in the same units.

Flux_Error
The 1 sigma statistical uncertainty in the flux.

Spect_Index
The photon number spectral index F(E) ~ E^{-gamma} in the energy range 100-2000 MeV. Spectra have not been determined for all sources. The Flux for most sources was determined assuming an energy spectrum with a photon number index of 2.

Spect_Index_Error
The 1 sigma uncertainty in the spectral index.

Counts_Limit
A value of `<` implies that the Counts value quoted is only an upper limit.

Counts
The number of E>100 MeV photons represented by the flux or upper limit. The uncertainty in the counts is proportional to (Flux_Error/Flux).

Sqrt_TS
The statistical significance of the E>100 MeV detection. It is approximately equal to the statistical sigma for a single measurement at a fixed position.

Viewing_Period
The viewing period of the Observation. There are the following special cases:

--------------------------------------------------------------------------
     P1  = Phase 1 summed map 4/91 - 11/92
     P2  = Phase 2 summed map 11/92 - 9/93
     P12 = Phase 1 + 2 summed map 4/91 - 9/93
     0.2+ is the sum of viewing periods 0.2 - 0.5, all of which covered the
          same region of the sky during the verification phase of the mission
     36.+ is the sum of 36.0 and 36.5, which cover the same region of the sky.
--------------------------------------------------------------------------

ID_Code
An identifier code, as follows:

--------------------------------------------------------------------------
     P = pulsar (indicates detection of pulsed gamma radiation)
     G = galaxy (the LMC is the only such object in the catalog)
     A = active galactic nucleus
     A:= active galactic nucleus (marginal).  In this catalog, a marginal
       identification is defined as one in which the candidate object lies
       near but outside the 95% uncertainty contour. Note that this is a
       different notation from that used in the first EGRET catalog, which
       distinguished identifications by their statistical significance.
--------------------------------------------------------------------------

Alt_name
The source name from the first EGRET catalog (Fichtel et al. 1994, ApJS, 94, 551) or from other gamma-ray references. In some cases, the source position has changed from that given in earlier references. For identified sources, the name (or names) of the object is(are) given in this column.

Notes
Notes on positional coincidences with objects of interest which might be the gamma-ray source, or other special information about the detection.

--------------------------------------------------------------------------
    1  2EG J0241+6119 is positionally consistent with the variable radio
         source GT0236+610
    2  2EG J0618+2234 is positionally consistent with SNR IC443
    3  2EG J1324-4317 is positionally consistent with radio galaxy Cen A,
         but the map is not fully consistent with a single point source
    4  2EG J1626-2452 is strongly affected by the Rho Oph cloud.  More
         detailed modeling by Hunter et al. (1994, ApJ, 436, 216) show that
         the source is positionally consistent with 1622-253
    5  2EG J1746-2852 is positionally consistent with the galactic center.
         The diffuse model has larger uncertainties in this direction than
         in most parts of the galactic plane (Hunter et al. 1995, ApJ, ???).
    6  2EG J2023-0836 is positionally consistent with AGN 2022-077 in
         viewing period 7.2, based on E > 1000 MeV analysis.  The Phase 1
         map suggests the presence of a second source with a steeper spectrum.
--------------------------------------------------------------------------

Reference
Previous EGRET references to the source, as identified below.

-----------------------------------------------------------------------------
    A Fichtel, et al., 1994, ApJS, 94, 551  (First EGRET Catalog)
    B Thompson, et al., 1993c, ApJ, 410, 87
    C Lin, et al., 1995b, ApJ, ???
    D Montigny et al., 1995, ???
    E Nolan, et al., 1995, ApJ, ???
    F Bertsch, et al., 1993a, ApJ, 405, L21
    G Dingus, et al., 1995, ApJ, ???
    H Hunter, et al., 1993b, A&A, 272, 59
    I Kniffen, et al., 1995, ApJ, ???
    J Radecke, et al., 1995, ApJ, 438, 659
    K Kanbach, et al., 1993, A&A, 97, 349
    L Lin, et al., 1995b, ApJ, ???
    M Hunter, et al., 1993a, ApJ, 409, 134
    N Sreekumar, et al., 1992, ApJ, 400, L67
    O Nolan, et al., 1993a, ApJ, 409, 697
    P Thompson, et al., 1993b, ApJS, 86, 629
    Q Bertsch, et al., 1992, Nature, 357, 306
    R Mayer-Hasselwander, et al., 1994, ApJ, 421, 276
    S Mukherjee, et al., 1995, ApJ, ???
    T Kanbach, et al., 1994, A&A, 289, 855
    U Kanbach, et al., 1995, ApJ, ???
    V Sreekumar, et al., 1995, ApJ, ???
    W Fierro, et al., 1993, ApJ, 413, L27
    X Lin, et al., 1992, ApJ, 401, L61
    Y Montigny, et al., 1993, ???
    Z Hartman, et al., 1992, ApJ, 385, L1
    AA Kniffen, et al., 1993, ApJ, 411, 133
    BB Wagner, et al., 1995, ApJ, ???
    CC Bertsch, et al., 1995, ApJ, ???
    DD Hunter, et al., 1994, ApJ, 436, 216
    EE Mattox, et al., 1993, ApJ, 410, 609
    FF Thompson, et al., 1992, Nature, 359, 615
    GG Mayer-Hasselwander, et al., 1995, A&A, ???
    HH Mattox, et al., 1995, ApJ, ???
    II Nolan, et al., 1993b, APJ, 414, 82
    JJ Hartman, et al., 1993, ApJ, 407, L41
    LL Ramanamurthy et al., 1995, ApJ, 450, 791
    LL Lin, et al., 1995a, ApJ, 442, 96
-----------------------------------------------------------------------------

Class
The Browse classification of the source. For the EGRET2 database, we have derived the Class value from the ID_Code parameter.


References

"The Second EGRET Catalog of High-Energy Gamma-Ray Sources", D.J. Thompson et al. 1995, ApJS, 101, 259.

Provenance

This database was created by the HEASARC in October 1997 based on a machine readable version of the 2nd EGRET Source Catalog that was obtained from the Compton Science Support Center (COSSC) Web site (accessible via http://heasarc.gsfc.nasa.gov/).

Contact Person

Questions regarding the EGRET2 database table can be addressed to the HEASARC User Hotline.
If you have any problems, please consult the help page or mail ledas-help@star.le.ac.uk
 
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