Supplement to the Second EGRET Catalog of High-Energy Gamma-Ray Sources Appeared in ApJS, 107, 227-237 (1996 December) D. J.Thompson {1}, D. L. Bertsch, B. L. Dingus {2}, J. A. Esposito {2}, A. Etienne {3}, C. E. Fichtel, D. P. Friedlander {3}, R. C. Hartman, S. D. Hunter, D. J. Kendig {3}, J. R. Mattox {4}, L. M. McDonald {3}, C. v. Montigny {5}, R. Mukherjee {2}, P. V. Ramanamurthy {6}, P. Sreekumar {2} Code 661, NASA Goddard Space Flight Center, Greenbelt, MD 20771 USA J. M. Fierro, B.B. Jones, Y. C. Lin, P. F. Michelson, P. L. Nolan, W. Tompkins, T. D. Willis W.W. Hansen Experimental Physics Laboratory and Department of Physics, Stanford University, Stanford CA 94305 USA G. Kanbach, H. A. Mayer-Hasselwander, M. Merck, M. Pohl Max-Planck-Institut fur Extraterrestrische Physik, Giessenbachstr D-85748 Garching FRG D. A. Kniffen Department of Physics, Hampden-Sydney College, Hampden-Sydney, VA 23943 USA E. J. Schneid Northrop Grumman Corporation, Bethpage, NY 11714 USA 1 e-mail:djt@egret.gsfc.nasa.gov 2 USRA Research Associate 3 Hughes STX 4 Compton Observatory Science Support Center, operated by USRA. Present address: Astronomy Department, University of Maryland, College Park, MD 5 NAS-NRC Research Associate 6 NAS-NRC Senior Research Associate Abstract This supplement extends the second EGRET catalog of high-energy gamma-ray sources (Thompson et al. 1995), incorporating data from 1993 September - 1994 October. The second catalog contained 129 sources, based on data from 1991 April -- 1993 September. An additional 28 sources are found, including three new identifications of Active Galactic Nuclei. Introduction The Energetic Gamma Ray Experiment Telescope (EGRET) is the high-energy gamma-ray telescope on the Compton Observatory. Descriptions and capabilities of the instrument are given by Thompson et al. (1993) and references therein. The telescope covers the energy range from about 30 MeV to over 20 GeV. EGRET records gamma-ray photons individually as electron-positron pair production events, which are processed automatically (with manual verification) to provide the arrival direction and energy of each photon. The point spread function is energy-dependent, having a FWHM of approximately 5 deg at 100 MeV and smaller values at higher energies. The arrival time of each gamma ray is recorded in Universal Coordinated Time (UTC) to an accuracy of better than 100 mu s. The field of view of EGRET extends to more than 30 deg from the instrument axis, although the sensitivity at 30 deg is less than 15% of the on-axis sensitivity. Because of the low flux level of the high energy gamma rays, observing periods are typically 2-3 weeks. The second EGRET catalog (Thompson et al. 1995) covered Phases 1 and 2 of the Compton Gamma Ray Observatory (CGRO) viewing program, which began shortly after launch of the observatory on 1991 April 5 and ended on 1993 September 7. Because many of the 129 sources in this catalog show evidence of time variability, and because the catalog did not reach the source confusion limit of EGRET, additional observations have yielded new source detections. In this supplement to the EGRET catalog, data are included from 1993 September 7 - 1994 October 4, Phase 3 of the viewing program. There have been no major changes to the EGRET data analysis procedures since the construction of the second catalog; therefore, the additional data do not justify a complete re-analysis of the source information. This supplement contains only sources which are new as a result of the Phase 3 data analysis. This includes sources seen only in one of the Phase 3 viewing periods, those seen in the Phase 3 summed map, and those which are added from analysis of the Phases 1 + 2 + 3 summed map. Observations and Analysis Fig. 1 shows the sky coverage during Phase 3 observations. Fig. 2 shows the total coverage during Phases 1 through 3. Table 1 shows the viewing periods for Phase 3. Primary Catalog The entries in the catalog (Table 2) are given in order of increasing right ascension. For each source, multiple entries are given. For each source, the first entry is the one from which the source position was derived. In almost all cases, this is the detection with the highest statistical significance. Other entries in the table give the results for the summed maps for Phase 3, Phase 1+2, and Phase 1+2+3 and for all viewing periods meeting the following criteria: (1) the source was within 30 degrees of the EGRET pointing direction (2) the exposure was large enough to derive a meaningful flux or upper limit. For each observation with sqrt{TS} > 3, the excess is presented as a flux with its uncertainty. For lower values of the sqrt{TS}, the result is presented as an upper limit. Each analysis was carried out independently; therefore, the sum of counts for a source from individual viewing periods does not necessarily match precisely the counts from the summed map. The primary catalog is Table 2 The columns in the table are: Name -- based on the J2000 coordinates for the best position of the source, following the IAU naming convention (PASP 102, 1231). It should be emphasized that the measured positions are generally not as precise as the name might suggest. The 2EGS designation is used to distinguish these sources from those of the second catalog, which were designated 2EG. RA and Dec -- the J2000 coordinates measured by EGRET, in degrees. l and b -- the Galactic coordinates measured by EGRET A and B are the semimajor and semiminor axes respectively of an ellipse fitted to the 95 % confidence error contour, in arcmin. Phi is the position angle of the major axis, measured eastward from north in celestial coordinates, in degrees. Morphology: em=possibly extended source or multiple sources (based on source location maps inconsistent with a single point source) C=source confusion may affect flux, significance, or position. Sources with no entry in this column are consistent with the EGRET point spread function. F -- the flux (E > 100 MeV) in 10^{-8} photons/cm^2 s, or upper limit in the same units for observations in which the source was not detected Delta F -- the 1 sigma statistical uncertainty in the flux g -- the photon number spectral index F(E) ~E ^{-gamma} in the energy range 100-2000 MeV. Spectra have not been determined for most sources. Dg -- the 1 sigma uncertainty in the spectral index Counts -- the number of E > 100 MeV photons represented by the flux or upper limit. The uncertainty in the counts is proportional to Delta F/F. 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. VP -- the viewing period of the Observation. P12 = Phase 1 + 2 summed map 4/91 - 9/93; P3 = Phase 3 summed map 9/93 - 10/94; P123 = Phase 1 + 2 + 3 summed map 4/91 - 10/94. 321+ is the sum of viewing periods 321.0 and 321.5, which cover the same region of the sky. 331+ is the sum of viewing periods 331.0 and 331.5, which cover the same region of the sky. ID A=Active Galactic Nucleus; a?=possible Active Galactic Nucleus. This applies only to source 2EGS J1800 - 4005, which is positionally consistent with the bright radio source PMN J1802-3940 (Gregory et al. 1994). Comparing the 4.85 GHz flux to the Molonglo 408 MHz flux for this source (Large et al. 1981) indicates a flat spectrum. The bright, flat-spectrum radio source, coupled with the gamma-ray time variability, suggest that this may be a blazar like others detected by EGRET (Montigny et al. 1995). Other names -- the AGN identification in IAU format, or the source name from the first EGRET catalog (Fichtel et al. 1994). Notes -- positional coincidences with objects of interest which might be the gamma-ray source, or special information about the detection. References -- previous EGRET references to the source. F{nu} -- approximate flux density at 400 MeV ( ~ 1 x 10 ^{23} Hz) in units of picoJy. The statistical uncertainty in the flux density is proportional to Delta F/F. For each source, this entry is given only for the detection with the highest statistical significance and is rounded to the nearest 5 picoJy to emphasize the approximate nature of the result. The specific approximations used in deriving the flux density are discussed in the Appendix. Summary The supplement to the Second EGRET catalog contains 28 sources. Locations of these sources are shown in Fig. 3. Three of these are identified with blazar-class Active Galactic Nuclei, and one is positionally consistent with a radio source which may be an AGN. The remainder are unidentified. Time variability is indicated for the 19 of the sources which were seen primarily in Phase 3. Locations of all 157 sources seen in the second EGRET catalog plus this supplement are shown in Fig. 4. On-Line Catalog Information The catalog tables are available by anonymous ftp from gamma.gsfc.nasa.gov, subdirectory pub. The format is plain ASCII. The source location figures are also available in PostScript format at this same location. Acknowledgments The EGRET team gratefully acknowledges support from the following: Bundesministerium fur Forschung und Technologie, Grant 50 QV 9095 (MPE authors); NASA Grant NAG5-1742 (HSC); NASA Grant NAG5-1605 (SU); and NASA Contract NAS5-31210 (GAC). J. Mattox acknowledges support from NASA Grants NAG5-2833 and NAGW-4761. This work has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. Appendix -- Other Units for Flux Values The source flux values in the catalog are given in units of photons (E > 100 MeV) photons/cm ^2 s, which has been a standard in gamma-ray astronomy. Such units are less useful to astronomers at other wavelengths, and this appendix presents approximate conversions to different units. Although the flux values are given as integral photon fluxes above 100 MeV, all sources which have measured photon number spectra show a differential form which can be approximated by dN/dE(E) = A E^{-\g } photons cm^{-2} s^{-1} GeV^{-1} where A is the normalization, E is the energy in GeV, and g is the power law index. These indices vary from approximately 1.0 to 3.0. Most are, however, clustered around 2.0. For this reason, the integral fluxes have been calculated (with few exceptions) in the EGRET catalog under the assumption of a 2.0 power law. A second important feature in interpreting the flux values is the fact that the upper end of the EGRET energy range is determined not by the measurement capability of the instrument (EGRET has seen a few gamma rays with energies measured above 100 GeV) but rather by the small number of photons detected at high energies. For all but the brightest sources, the number of photons above a few GeV is negligible. Effectively, photons above 5 GeV contribute little to the integral fluxes in the catalog. The catalog fluxes can, therefore, be thought of as spanning 100 MeV -- 5 GeV, with little loss of accuracy. Under these assumptions, the integral flux F can be used to approximate the normalization A in equation 1: so that A = F/9.8 photons cm^{-2}s^{-1}GeV ^{-1} . Note that the catalog table removes a factor of 10 ^{-8} from the complete flux value. This value of A can then be used in equation (1) to approximate the photon flux at any energy E. Similarly, the energy flux F_E can be approximated as F_E = E x dN/dE(E) = A E^{-g + 1} GeV cm^{-2} s^{-1} GeV^{-1} and the integral of the energy spectrum can be used to derive the observed energy flux S( E) in the 0.1 -- 5 GeV band (again assuming g = 2): and the mean energy < E > = S( E)/F for the E ^{-2} photon spectrum is 0.4 GeV (= 6.4 x 10^{-4} erg ~ 1 x 10^{23} Hz). The flux density at < E > from equation (3) is approximately 0.25 F GeV cm ^{-2} s^{-1} GeV^{-1} = 1.7 x 10^{-4}\: F Jy. As an example, the catalog flux for the Viewing Period 3 flare of 3C279 is (2.87 +/- 0.11) x 10 ^{-6} ph cm^{-2} s^{-1} (Thompson et al. 1995). This represents an energy flux (0.1 -- 5 GeV) of (1.8 =/- 0.1) x 10^{-9} erg cm^{-2} s^{-1} and a flux density of ~ 5 x 10 ^{-10} Jy (500 picoJansky) at 10^{23} Hz. It should be emphasized that the EGRET data cover a wide range of photon energies. The catalog entries are not intended to be comprehensive sources of information about each source, but rather a consistent set of flux values for the central part of the EGRET energy band. Without using the detailed spectral information contained in the references to individual sources, an estimate using this calculation is just that -- an estimate. For comparison with other wavelengths, however, approximate values may prove useful. Bibliography Bignami, G.F. 1996, Science, 271, 1372 Churazov, E., Gilfanov, M., Cordier, B., Schmitz-Fraysse, M.C. 1992, IAU Circ., No. 5623 Esposito, J.A., Hunter, S.D., Kanbach, G., Sreekumar, P. 1996, ApJ, 461, 820 Fichtel, C.E. et al. 1994, ApJs, 94, 551 Gregory, P.C., Vavasour, J.D., Scott, W.K., Condon, J.J. 1994, ApJs, 90, 173 Hunter, S. D. et al. 1996, ApJ, in press Kaaret, P. & Cottam, J. 1996, ApJ, 462, L35 Kanbach, G., et al. 1996, A&Ap, in press Kniffen, D.A. et al. 1996, ApJ, submitted Large, M.I., Mills, B.Y., Little, A.G., Crawford, D.F., Sutton, J.M. 1981, MNRAS, 194, 693 Lundgren, S.C. et al. 1995, IAU Circular 6258 Mattox, J.R. et al. 1996, ApJ, 461, 396 McGlynn, T.A. et al. 1996, in preparation Montigny, C. von et al. 1995, ApJ, 440, 525 Nolan, P.L. et al. 1996, ApJ, 459, 100 Pavlinsky, M.N., Grebenev, S.A., Sunyaev, R.A. 1994, ApJ, 425, 110 Ramanamurthy, P.V. et al. 1995, ApJ, 447, L109 Sreekumar, P. et al. 1996a, ApJ, in press Sreekumar, P. et al. 1996b, in preparation Sturner, S.J. & Dermer, C.D. 1995, A&Ap, 293, L17 Tavani, M. et al. 1996, A&Ap, in press Thompson, D.J.et al. 1993, ApJs, 86, 629 Thompson, D. J. et al. 1995, ApJs, 101, 259 Yadigaroglu, I.-A. \& Romani, R.W. 1995, ApJ, 449, 211 Figure Captions Fig. 1 EGRET sky exposure in units of cm ^2 s (E > 100 MeV) for the Compton Gamma Ray Observatory Phase 3 (1993 September - 1994 October). The contours are 1.33 x 10 ^8 , 2.67 x 10 ^8 , 4.0 x 10 ^8 , 5.33 x 10 ^8 and 6.67 x 10 ^8 . Fig. 2 EGRET sky exposure in units of cm ^2 s (E > 100 MeV) for the Compton Gamma Ray Observatory combined Phase 1 (1991 April - 1992 November) Phase 2 (1992 November - 1993 September) and Phase 3 (1993 September - 1994 October). The contours are 3.33 x 10 ^8 , 6.67 x 10 ^8 , 10.0 x 10 ^8 , 13.3 x 10 ^8 and 16.7 x 10 ^8 . Fig. 3 Locations of the sources in this supplementary catalog, in Galactic coordinates. The size of the symbol represents the highest intensity seen by EGRET for E > 100 MeV for the source. The symbol size scale is relative to the brightest source in the supplementary catalog, and is not the same as that of Fig. 4. Fig. 4 Locations of the sources in the second EGRET catalog plus this supplementary catalog, in Galactic coordinates. The size of the symbol represents the highest intensity seen by EGRET for E > 100 MeV for the source. The symbol size scale is relative to the brightest source in the catalog, and is not the same as that of Fig. 3. Fig. 5 Color version of Fig. 4. PSR B1951+32 is shown in this figure, although it is not in the catalog. It is seen by EGRET only through its pulsed emission (Ramanamurthy et al. 1995). Figures 6-10. Maps of locations of sources in this catalog. The contours represent the statistical probability that a single source lies within the given contour. In each map, the + marks the location of the highest likelihood test statistic. A * marks the location of objects seen at other wavelengths that may be related to the gamma-ray source.