Unveiling the nature of two unidentified EGRET blazar candidates through spectroscopic observations

Studies using the Energetic Gamma‐Ray Experiment Telescope (EGRET) revealed that blazars [flat‐spectrum radio quasars (FSRQs) and BL Lac objects] emit most of their luminosity in the high‐energy gamma‐ray ( E > 100 MeV) range. From the 271 sources observed by EGRET, 131 are still unidentified. A systematic search is conducted to identify possible high‐energy gamma‐ray blazars among the unidentified EGRET population. Based upon multiwavelength emission properties, 13 extragalactic radio sources were selected in the EGRET error boxes for further investigation. From the above‐mentioned sample, results of a multiwavelength follow‐up of two EGRET sources, 3EG J0821−5814 and 3EG J0706−3837, are presented. These sources are associated with their radio counterparts PKS J0820−5705 and PMN J0710−3850, respectively. Spectroscopic observations utilizing the SOAR/Goodman spectrograph at the Cerro Tololo Inter‐American Observatory in Chile reveal a spectrum of PKS J0820−5705 that corresponds to that of a radio‐loud active galactic nucleus (FSRQ) with redshift z = 0.06 ± 0.01, while the visibility of wide and narrow emission lines in the spectrum of PMN J0710−3850 resembles that of a low‐ionization nuclear emission‐line region (LINER) or type 1 Seyfert galaxy at z = 0.129 ± 0.001. The observed Ca  ii K&H lines depression ratio at 4000 Å showed a shallow depression of 8.8 ± 2.5 per cent for PKS J0820−5705 and 80 ± 1 per cent for PMN J0710−3850, suggesting the presence of a strong non‐thermal optical contribution in PKS J0820−5705, which clearly distinguishes its spectrum from that of a radio galaxy. The weaker optical non‐thermal contribution for PMN J0710−3850 is in accordance with that expected of a LINER. For PMN J0710−3850 the line flux ratios [O  iii ] λ5007/Hβ < 3 and [N  ii ] λ6583/Hα > 0.6 which are in agreement with the expected ratios of LINERs. However, the absence of [O  ii ] λ3727 implies an anomalously low [O  ii ]/[O  iii ] < 0.5 ratio for a LINER, and agrees more with the ratio observed in type 1 Seyfert galaxies. The average velocities inferred from the Balmer lines range between 2300 and 4300 km s −1 , while [O  i ] and [O  iii ] velocities range between 420 and 490 km s −1 , consistent with both LINERs and type 1 Seyfert galaxies. The X‐ray luminosities of these two sources are L X ∼ 9 × 10 43  erg s −1 (PKS J0820−5705) and L X ∼ 9 × 10 42  erg s −1 (PMN J0710−3850), respectively. The X‐ray luminosity of PMN J0710−3850 is an order of magnitude higher than the upper limit detected from LINERs, and correlates well with the typical X‐ray luminosities observed in type 1 Seyfert galaxies. The X‐ray luminosity of PKS J0820−5705 is consistent with the observed luminosity of FSRQs. Optical photometry carried out with the South African Astronomical Observatory 1.0‐m telescope displayed 1–2 mag variability in the B and R bands for PKS J0821−5705, on time‐scales of hours, while a 5σ variability of the average R ‐band magnitude could be discerned over a 3 d time span. A smaller 0.5 mag variability is visible in the B band for PMN J0710−3850 on time‐scales of hours. No variability was detected in the R band for this source.