Acta Polytechnica doi:10.14311/AP.2013.53.0631 Acta Polytechnica 53(Supplement):631–634, 2013 © Czech Technical University in Prague, 2013 available online at http://ojs.cvut.cz/ojs/index.php/ap THE SEARCH FOR BLAZARS AMONG THE UNIDENTIFIED EGRET γ-RAY SOURCES Pieter J. Meintjesa,∗, Pheneas Nkundabakuraa,b a Department of Physics, University of the Free State, PO Box 339, Bloemfontein, 9300, South Africa b Kigali Institute of Education, P.O. Box 5039, Kigali, Rwanda ∗ corresponding author: MeintjPJ@ufs.ac.za Abstract. In this paper we report the results of a multi-wavelength follow-up study of selected flat spectrum extragalactic radio-optical counterparts within the error boxes of 13 unidentified EGRET sources. Two of these previously unidentified counterparts have been selected for optical photometric and spectroscopic follow-up studies. Spectroscopic observations made with the 4.1 m SOAR telescope at Cerro Pachón, Chile, showed that the spectra of the optical counterparts of 3EG J0821−5814 (PKS J0820−5705) and 3EG J0706−3837 (PMN J0710−3835) correspond to a flat spectrum radio quasar (FSRQ) and LINER-Seyfert I galaxy respectively. Optical photometry of these sources, performed with the 1.0 m telescope at Sutherland (South Africa) shows noticeable intranight variability for PKS J0820−5705, as well as a 5 sigma variation of the mean brightness in the R-filter over a timescale of three nights. Significant variability has been detected in the B-band for PMN J0710−3835 as well. The gamma-ray spectral indices of all 13 candidates range between 2–3, correlating well with the BL Lacs and FSRQs detected with Fermi-LAT in the first 11 months of operation. Keywords: radiation mechanisms: non-thermal, line: identification, techniques: spectroscopic, galaxies: jets, BL Lacertae objects. 1. Introduction The Energetic Gamma Ray Telescope Experiment EGRET (20 MeV ÷ 30 GeV) provided the highest gamma-ray window on board the Compton Gamma- Ray Observatory (CGRO). The main scientific ob- jective was to survey the gamma-ray sky to iden- tify and study possible point sources of gamma-ray emission. EGRET detected 271 gamma-ray sources above 100 MeV, 92 % of which were blazars. Of the 271 sources detected, 131 remained unidentified, i.e. could not be associated with any specific point source of gamma-ray emission [5]. The aim of this study is to search for possible extra-galactic radio loud Active Galactic Nuclei (AGN), i.e. blazars and Flat Spectrum Radio Quasars (FSRQs) within some selected EGRET error boxes. To avoid confusion with possible galactic sources, especially molecular cloud distributions, the search was restricted to some unidentified sources at galactic latitudes |b| > 10 deg. Selection criteria The counterparts should be in- side the error box associated with the detection [5], confirmed as extragalactic in the NASA Extragalactic Database (NED), have radio brightness above 100 mJy at 8.4 GHz [8], exhibiting hard spectra with spectral indices |α| < 0.7 [8] and display variability (e.g. [4]) that may be associated with an inner accretion disc or jet. Based upon these criteria, 13 sources have been selected for further follow-up study. The EGRET (30 MeV÷10 GeV) gamma-ray spectra of these sources observed between April 1991 and Figure 1. Gamma-ray spectral index distribution of selected unidentified sources. October 1995 (cycles 1, 2, 3 and 4 of the mission) have been determined. The spectral index distribution is displayed in Fig. 1. The spectral distribution of these unidentified sources corresponds remarkably well with the gamma- ray blazar spectral index distribution observed by Fermi-LAT in the first 11 months of operation [1]. In the first phase of this study, two of these previ- ously unidentified counterparts, i.e. PKS J0820−5705 and PMN J0710−3850, were selected for further opti- cal spectroscopic and photometric follow-up studies. 631 http://dx.doi.org/10.14311/AP.2013.53.0631 http://ojs.cvut.cz/ojs/index.php/ap Pieter J. Meintjes, Pheneas Nkundabakura Acta Polytechnica EGRET Counterpart Dec(J2000) RA(J2000) 3EG J0159−3603 J0156−3616 −36 16 14 01 56 47 3EG J0500+2502 J0502+2516 +25 16 24 05 02 59 3EG J0702−6212 J0657−6139 −61 39 26 06 57 02 3EG J0706−3837 J0710−3850 −38 50 36 07 10 43 3EG J0724−4713 J0728−4745 −47 45 14 07 28 22 3EG J0821−5814 J0820−5705 −57 05 35 08 20 58 3EG J1300−4406 J1302−4446 −44 46 52 13 02 31 3EG J1659−6251 J1703−6212 −62 12 38 17 03 37 3EG J1709−0828 J1713−0817 −08 17 01 17 13 06 3EG J1800−0146 J1802−0207 −02 07 44 18 02 50 3EG J1813−6419 J1807−6413 −64 13 50 18 07 54 3EG J1822+1641 J1822+1600 +16 00 12 18 22 11 3EG J1824+3441 J1827+3431 +34 31 05 18 27 00 Table 1. Some unidentified high galactic latitude unidentified sources and counterparts. 5e−17 1e−16 1.5e−16 2e−16 2.5e−16 3e−16 3.5e−16 4e−16 4.5e−16 5e−16 5.5e−16 4000 4500 5000 5500 6000 6500 7000 7500 8000 8500 F lu x (e rg s − 1 cm − 2 Å − 1 ) Wavelength (Å) Hα+[NII] MgIb NaD B−band Ca II H and K A−band PKS J0820−5705 z=0.06 Figure 2. Spectrum of PKS J0820−5705 (FSRQ). 2. Optical follow-up studies Optical spectroscopy of the two selected counterparts was performed using the 4.1 m SOAR telescope in Chile on the night of January 16–17 2009, utilizing the Goodman spectrograph. The spectra of both these sources are shown in Fig. 2 (PKS J0820−5705) and Fig. 3 (PMN J0710−3850), respectively. The spec- trum for PKS J0820−5705 resembles that of an FSRQ at redshift z = 0.06, while the spectrum of PMN J0710−3850 shows broad and narrow lines resembling the spectrum of a LINER or Seyfert I galaxy at redshift z = 0.129. What distinguishes the spectrum of PKS J0820−5705 from that of a normal radio galaxy is the shallow K4000 depression of only 8.8 % ± 2.5 %, indi- cating substantial non-thermal activity, while the cor- responding value for PMN J0710−3850 is 80 % ± 1 %, in agreement with the value expected for a LINER- Seyfert I galaxy (e.g. [2]). Optical photometry in the B and R filters (Fig. 4) 1e−16 2e−16 3e−16 4e−16 5e−16 6e−16 7e−16 4000 4500 5000 5500 6000 6500 7000 7500 8000 8500 F lu x (e rg s − 1 cm − 2 Å − 1 ) Wavelength (Å) Hα+[NII] [OI] NaD B−band [OIII] Hβ A−band PMN J0710−3850 z=0.129 Hγ Figure 3. Spectrum of PMN J0710−3850 (LINER- Seyfert I). of these systems during January 2009 shows pecu- liar intranight variability, as well as variability at the 5 σ level over timescales of a few nights for PKS J0820−5705. This level of intranight variability from PKS J0820−5705 has also been observed in a few other sources e.g. blazars AO 0235+164 and PKS J0736+17 (e.g. [4]) respectively. The variability observed in PMN J0710−3850 corresponds to that observed from another Seyfert I galaxy, i.e. NGC 4395 (e.g. [3]). 3. SED modelling The multi-wavelength data from the two counterparts from radio to gamma-rays have been combined to cre- ate the Spectral Energy Distribution (SED) over more than 15 decades in energy (Fig. 5). The data is fitted with a single zone Synchrotron self-Compton (SSC) model (e.g. [6]) and External Compton (EC) model (e.g. [7]), i.e. where relativistic jet electrons up-scatter 632 vol. 53 supplement/2013 The Search for Blazars among the Unidentified EGRET γ-Ray Sources Figure 4. Optical photometry (B-band and R-band) of PKS J0820−5705 and PMN J0710−3850. Also shown are the lightcurves of constant brightness comparison stars in the same field. infrared (IR) photons from the disc torus and possi- bly optical photons from the emission line regions to high energies. The model parameters are explained in the footnote of Table 2. The gamma-ray emission for 3EG J0706−3837 can readily be explained by an EC process, where jet electrons upscatter photons from both the disc torus and emission line regions to high energies, while for 3EG J0821−5814 the gamma-ray component is mostly compatible with an SSC process. For 3EG J0821−5814, a higher energy component of the EGRET spectrum below the indicated upper lim- its, could possibly be associated with an EC process. 4. Conclusions We report the discovery of 13 flat spectrum extra- galactic sources within the error boxes of some high galactic latitude unidentified EGRET sources. Four of these EGRET sources have been detected with Fermi- LAT within the first 11 months of operation. Optical spectroscopy of PKS J0820−5705 shows a feature- less spectrum, shallow K4000 depression with broad absorption lines at z = 0.06 resembling an FSRQ, while the spectrum of PMN J0710−3850 shows broad and narrow emission lines at z = 0.129, resembling a LINER-Seyfert I galaxy. The optical spectrum of PKS J0820−5705 shows a very shallow K4000 depression of 8.8 %±2.5 %, implying the non-thermal emission asso- ciated with an FSRQ, while the K4000 depression for PMN J0710−3850 is significantly deeper at 80 %±1 %, in accordance with that expected of a LINER-Seyfert I galaxy. Photometry of both these sources shows ∼ 1 magnitude intranight variability in the B-band, with an additional 5 σ variability seen over a 3-day period in the R-band from PKS J0820−5705. The SED of these sources have been fitted with a combination of SSC and EC models. 633 Pieter J. Meintjes, Pheneas Nkundabakura Acta Polytechnica 1e-18 1e-17 1e-16 1e-15 1e-14 1e-13 1e-12 1e+10 1e+15 1e+20 1e+25 ν F ν (W m -2 ) ν (Hz) Radio Opt. NIR ROSAT EGRET 3EG J0706-3837 (PMN J0710-3850) EC (IR) SSC Sync EC (BEL) 1e-18 1e-17 1e-16 1e-15 1e-14 1e-13 1e-12 1e-11 1e+10 1e+15 1e+20 1e+25 ν F ν (W m -2 ) ν (Hz) Radio Opt. NIR ROSAT EGRET 3EG J0821-5814 (PKS J0820-5705) XMM EC SSC-opt Figure 5. SED model fits for 3EG J0706−3837 (PMN J0710−3850) and 3EG J0821−5814 (PKS J0820−5705). SSC Parameter 3EG J0821−5814 3EG J0706−3837 SSC (opt) SSC (X-ray) r (m) 1.00 × 1013 1.00 × 1012 3.50 × 1013 B (T) 2.50 × 10−4 2.50 × 10−4 2.50 × 10−4 δ 3.8 15 12 γmax 3.10 × 103 7.80 × 104 2.00 × 103 EC Parameter 3EG J0821−5814 3EG J0706−3837 r (m) 1.0 × 1015 1.0 × 1015 B (T) 2.50 × 10−4 2.50 × 10−4 Γ 10 10 θobs (rad) 0.2 0.52 Ke 3.0 × 1050 2.5 × 1053 uIR (in erg cm−3) 0.08 25 νIR (in eV) 0.01 0.1 Table 2. SSC and EC model parameters for 3EG J0821−5814 and 3EG J0706−3837 . The SSC (opt) refers to the SSC model fit obtained using the optical data as part of the synchrotron emission while the SSC (X-ray) refers to the model fit obtained using the X-ray data as part of the synchrotron emission. Here r and B are the radius and the magnetic field intensity of the emitting region respectively; δ and γmax are the Doppler factor and the maximum Lorentz factor of the electrons. The main parameters for the EC model are θ, uIR, νIR, Γ and Kel representing the viewing angle, the energy density of dust IR radiation, the IR radiation characteristic frequency, the bulk jet Lorentz factor and the normalization constant in the electron density distribution respectively. References [1] Abdo A.A. et al., 2010, ApJ, 710, 1271 [2] Caccianiga A., della Ceca R., Gioi I.M., Maccacaro T & Wolter T., 1999, ApJ, 513, 51 [3] Desroches L-B et al., 2006, ApJ, 650, 88 [4] Fan J-H., 2005, Chin. J. Astron. Astrophys. Suppl., 5, 213 [5] Hartman R., et al., 1999, APJS, 123, 79 [6] Katarzynski K., Sol H & Kus A., 2001, A&A, 367, 809 [7] Sikora M., Begelman M.C. & Rees M.J., 1994, ApJ, 421, 153 [8] Sowards-Emmerd D., Romani R.W. & Michelson P.F.: 2003, ApJ, 590 Discussion James Beall — Does the variability you mentioned correlate with that observed in blazars? Thank you. Pieter Meintjes — The level of intranight variability seen in in B and R-filter from PKS J0820−5705 has been seen a few other blazars, for example AO 0235+164 and PKS J0736+17. However, it must be stressed that in most cases the intranight (microvariability) is at a much lower level than has been observed from PKS J0820−5705. The intranight variability seen from PMN J0710−3850 resembles that of another Seyfert I galaxy NGC 4395. So the level of variability is not normally observed, especially in PKS J0820−5705, but it could represent an active phase from these sources. Regular optical monitoring will be performed to verify this. 634 Acta Polytechnica 53(Supplement):631–634, 2013 1 Introduction 2 Optical follow-up studies 3 SED modelling 4 Conclusions References