Energetic Gamma Ray Experiment Telescope Source Research Articles

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 LX ∼ 9 × 1043 erg s−1 (PKS J0820−5705) and LX ∼ 9 × 1042 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.

PSR J1410-6132: a young, energetic pulsar associated with the EGRET source 3EG J1410-6147

Abstract We present the discovery of PSR J1410-6132, a 50-ms pulsar found during a high-frequency survey of the Galactic plane, using a seven-beam 6.3-GHz receiver on the 64-m Parkes radio telescope. The pulsar lies within the error box of the unidentified Energetic Gamma Ray Experiment Telescope (EGRET) source 3EG J1410-6147, has a characteristic age of 26 kyr and a spin-down energy of 1037 erg s-1. It has a very high dispersion measure of 960 ± 10 cm-3 pc and the largest rotation measure of any pulsar, RM =+2400 ± 30 rad m-2. The pulsar is very scatter-broadened at frequencies of 1.4 GHz and below, making pulsed emission almost impossible to detect. Assuming a distance of 15 kpc, the pulsar's spin-down energy and a γ-ray efficiency factor of ∼10 per cent is sufficient to power the γ-ray source. We therefore believe that we have identified the nature of 3EG J1410-6147. This new discovery suggests that deep targeted high-frequency surveys of inner-galaxy EGRET sources could uncover further young, energetic pulsars.

MAGIC J0616+225 as delayed TeV emission of cosmic rays diffusing from the supernova remnant IC 443

Abstract We present a theoretical model that explains the high-energy phenomenology of the neighbourhood of the supernova remnant IC 443, as observed with the Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescope and the Energetic Gamma-Ray Experiment Telescope (EGRET). We interpret MAGIC J0616+225 as delayed TeV emission of cosmic rays diffusing from IC 443 and interacting with a known cloud located at a distance of about 20 pc in the foreground of the remnant. This scenario naturally explains the displacement between EGRET and MAGIC sources, their fluxes, and their spectra. We compare this model with others recently presented, and discuss how it can be tested with observations by the Gamma-ray Large Area Space Telescope.

Radio detections towards unidentified variable EGRET sources

Context. A considerable fraction of theγ-ray sources discovered with the Energetic Gamma-Ray Experiment Telescope (EGRET) remain unidentified. The EGRET sources that have been proper ly identified are either pulsars or variable sources at both r adio and gamma-ray wavelengths. Most of the variable sources are strong radio blazars. However, some low galactic-latitude EGRET sources, with highly variableγ-ray emission, lack any evident counterpart according to the radio data available until now. Aims. The primary goal of this paper is to identify and characterise the potential radio counterparts of four highly variable γ-ray sources in the galactic plane through mapping the radio surroundings of the EGRET confidence contours and determining th e variable radio sources in the field whenever possible. Methods. We have carried out a radio exploration of the fields of the sel ected EGRET sources using the Giant Metrewave Radio Telescope (GMRT) interferometer at 21 cm wavelength, with pointings being separated by months. Results. We detected a total of 151 radio sources. Among them, we identified a few radio sources whose flux density has apparently changed on timescales of months. Despite the limitations of our search, their possible variability makes these objects a top-priority target for multiwavelength studies of the potential counte rparts of highly variable, unidentified gamma-ray sources.

A possible EGRET source with a two-sided radio jet structure on a parsec scale - 3EG J0808+5114

We report on the European VLBI Network observations of the radio source 0803+5126 in the field of the Energetic Gamma Ray Experiment Telescope (EGRET) source 3EG J0808+5114. High-resolution images of 0803+5126 at 6 cm showed a core emission with an extremely low brightness temperature plus an asymmetric two-sided jet structure on both parsec and tens of parsec scales, suggesting a relatively weak beaming effect in radio emission. The radio structures on different scales exhibit quite different orientations of extension, indicating the occurrence of jet bending. Based on the positional uncertainty of 3EG J0808+5114, its separations to various possible radio counterparts, and very low gamma-ray variability, it is reasonable to believe that 0803+5126 is the most probable radio counterpart of 3EG J0808+5114, making it the first EGRET-detected active galactic nucleus to be associated with a two-sided jet structure on a parsec scale. The central engine is estimated to have a mass of similar to 5.0 x 10(8) M-circle dot by using Mg II emission line features, for which the Eddington limit is approximately two times lower than the integrated gamma-ray luminosity. The minimum Doppler factor in gamma-ray emission is similar to 2.5, but the true one should not be much larger than this value on account of its quiet stable gamma-ray emission. This is consistent with our very long baseline interferometry results of the two-sided radio jet structure. The relativistic beaming effect and downstream jet bending may play an important role in the paradox between the large gamma-ray luminosity far above its Eddington limit and the observed asymmetric two-sided radio structure.

Possible TeV source candidates among the unidentified EGRET sources

We study the γ -ray emission from the pulsar magnetosphere based on outer gap models, and the TeV radiation from pulsar wind nebulae through inverse Compton scattering using a onezone model. We have shown previously that GeV radiation from the magnetosphere of mature pulsars with ages of ∼10 5 ‐10 6 yr can contribute to the high-latitude unidentified Energetic Gamma Ray Experiment Telescope (EGRET) sources. We carry out Monte Carlo simulations of γ -ray pulsars in the Galaxy and the Gould Belt, assuming values for the pulsar birth rate, initial position, proper motion velocity, period, and magnetic field distribution and evolution based on observational statistics. We select from the simulation a sample of mature pulsars in the Galactic plane (|b| 5 ◦ ) and a sample at high latitudes (|b| > 5 ◦ ) which could be detected by EGRET. We calculate the TeV fluxes from the pulsar wind nebulae of our simulated sample produced through inverse Compton scattering by relativistic electrons on the cosmic microwave background and synchrotron seed photons. The predicted fluxes are consistent with the present observational constraints. We suggest that strong EGRET sources may be potential TeV source candidates for present and future ground-based TeV telescopes.

On the nature of the unidentified EGRET sources: Are they Geminga-like pulsars?

Observations with the Energetic Gamma Ray Experiment Telescope (EGRET) aboard the Compton Gamma Ray Observatory (CGRO) show more than 30 unidentified gamma-ray sources concentrated along the Galactic plane. Based on their spatial distribution, the typical distances of the sources are found to be between 1.2 and 6 kpc. Luminosity estimates made using the estimated distances of the sources and their observed gamma-ray fluxes lie in the range (0.7-16.7) x 10(exp 35) ergs/s. These values, on the average, appear to be higher than the luminosities of the gamma-ray pulsars observed by EGRET. The luminosity estimates indicate that the low-latitude unidentified EGRET sources cannot be explained as a group of older, low-luminosity pulsars like Geminga, although the high-latitude sources may be candidates.

High-Energy Gamma-Ray Emission from Active Galaxies: EGRET Observations and Their Implications

view Abstract Citations (455) References (122) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS High-Energy Gamma-Ray Emission from Active Galaxies: EGRET Observations and Their Implications von Montigny, C. ; Bertsch, D. L. ; Chiang, J. ; Dingus, B. L. ; Esposito, J. A. ; Fichtel, C. E. ; Fierro, J. M. ; Hartman, R. C. ; Hunter, S. D. ; Kanbach, G. ; Kniffen, D. A. ; Lin, Y. C. ; Mattox, J. R. ; Mayer-Hasselwander, H. A. ; Michelson, P. F. ; Nolan, P. L. ; Radecke, H. D. ; Schneid, E. ; Sreekumar, P. ; Thompson, D. J. ; Willis, T. Abstract The energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton Gamma-Ray Observatory has so far detected, with a high degree of certainty, 33 active galactic nuclei (AGNs) in high-energy gamma rays (E > 100 MeV) during Phase 1(91/16/05-92/17/11) and Phase 11(92/17/11-93/07/09). There are detections of 11 other active galaxies with lesser significance. Of the sources detected, the majority are quasars and six are usually classified as BL Lacertae (BL Lac) objects. They seem to be all members of the blazar class a rather well defined subclass of AGN. To aid attempts at relating the observed properties of the EGRET sources to physical models of AGNs, a summary of EGRET AGN observations is presented. This summary includes gamma-ray spectra and time-dependent gamma-ray fluxes of detected sources. For sources where the information is available in the literature, multifrequency spectra covering the range from radio to gamma rays are also included. Constraints are discussed which the EGRET observations place upon various emission models. Finally, the contribution of AGNs to the diffuse extragalactic gamma-ray emission is reexamined in light of the EGRET observations. Publication: The Astrophysical Journal Pub Date: February 1995 DOI: 10.1086/175294 Bibcode: 1995ApJ...440..525V Keywords: GALAXIES: BL LACERTAE OBJECTS: GENERAL; GALAXIES: ACTIVE; GAMMA RAYS: OBSERVATIONS; GALAXIES: QUASARS: GENERAL full text sources ADS | data products NED (44) SIMBAD (42) HEASARC (1)

Outburst of TeV photons from Markarian 421

Markarian 421, an active galactic nucleus (AGN) of the BL Lacertae type, is the closest Energetic Gamma Ray Experiment Telescope (EGRET)-detected AGN. It has been monitored by the Whipple Observatory very high energy gamma-ray telescope since its discovery at TeV energies, for approximately 90 nights, totaling 130 hours of observation. Observations from 1993 December 23 to 1994 May 10 showed an average sourve flux only half that of its 1992 discovery level. However, observations on 1994 May 14 and 15 show an increase in flux above this quiescent level by a factor of approximately 10. The timescale of this increase provides the best geometric constraint on the extent of TeV photon emission regions within AGNs. The observation of the high TeV flux occurred 1 day prior to the observation by ASCA of a very high 2-10 keV X-ray flux. This strong TeV outburst is reminiscent of the behavior seen for the stronger 100 MeV-GeV EGRET sources (e.g. 3C 279) but was not anticipated in view of the fact that the six EGRET observations of Mrk 421 from 1991 June to 1993 July showed no evidence for variability.