EGRET Blazars Research Articles

Unusual long-term low-activity states of EGRET blazars in the Fermi era

We examine the long-term (∼ 10 years) γ-ray variability of blazars observed by EGRET and Fermi and find that 10 EGRET-detected flat spectrum radio quasars (FSRQs) were not present in any of the Fermi catalogue (during the first four years of Fermi observation). Six of these 10 EGRET sources show possible Fermi counterparts based on overlap with error circles given in 3 r d Fermi catalogue. A seventh EGRET source is positionally consistent with a newly discovered Fermi source from our analysis. Unlike the high luminosity state exhibited during 1991 to 1995, our analysis using seven years of Fermi data showed that five of these are at its low luminosity state in the GeV band. This result indicates that FSRQs exhibit a long-term quiescent state (more than 7 years). In addition, 37 new γ-ray sources are also detected during the course of this analysis. On the basis of AGN being the most dominant source class in γ-rays, many of these new sources could be associated with AGN and may exhibit long-term variabilities in γ-rays.

γ-RAY LUMINOSITY FUNCTION AND THE CONTRIBUTION TO EXTRAGALACTIC γ-RAY BACKGROUND FORFERMI-DETECTED BLAZARS

We study γ-ray luminosity functions (GLFs) and the contribution to the extragalactic diffuse γ-rays for blazars. After using a clean blazar sample in which 485 blazars are observed at both the 8.4 GHz and ∼ GeV bands, we investigate the radio/γ-ray luminosity correlation and find out that a strong correlation exists between the radio and γ-ray luminosities using the partial correlation analysis method. Based on such a correlation, we obtain the GLF, while also considering the GLF based on studies of the X-ray luminosity function of active galactic nuclei and the GLF of the EGRET blazars. K-S tests indicate that these three kinds of GLFs are consistent with the observed data of our sample in a reasonable parameter range. Using these GLFs, we further investigate the blazar contribution to the extragalactic diffuse γ-ray background (EGRB). Our results show that (1) the contribution of blazars are ∼21.3% ±2.7%, ∼12.9% ±1.7%, and ∼37.9% ±4.8% of unresolved Fermi EGRB flux for the three GLFs and (2) the contribution of flat-spectrum radio quasars are ∼11.2% ±1.6%, ∼6.9% ±1.0%, and ∼9.0% ±1.2% of unresolved Fermi EGRB flux for the three GLFs.

The radio-γ-ray connection in Fermi blazars

We study the correlation between the gamma-ray flux F_g, averaged over the first 11 months of Fermi survey and integrated above 100 MeV, and the radio flux density (F_r at 20 GHz) of Fermi sources associated with a radio counterpart in the AT20G survey. Considering the blazars detected in both bands, the correlation is highly significant and it is F_g~F_r^0.85+-0.04, similar for BL Lac and FSRQ sources. However, only a small fraction (~1/15) of the AT20G radio sources with flat radio spectrum, are detected by Fermi. To understand if this correlation is real, we examine the selection effects introduced by the flux limits of the radio and gamma-ray surveys, and the importance of variability of the gamma-ray flux. We find that the radio/gamma-ray flux correlation is real, but its slope is steeper than the observed one, i.e. F_g~F_r^delta with delta in the range 1.25-1.5. The observed F_g-F_r correlation and the fraction of radio sources detected by Fermi is reproduced assuming a long term gamma-ray flux variability following a log-normal probability distribution with standard deviation sigma>0.5 (corresponding to F_g varying by at least a factor 3). Such a variability is compatible with what observed when comparing, for the sources in common, the EGRET and the Fermi gamma-ray fluxes (even if the Fermi fluxes are averaged over ~1 year). Another indication of variability is the non detection of 12 out of 66 EGRET blazars by Fermi, despite its higher sensitivity.We also study the strong linear correlation between the gamma-ray and the radio luminosity of the 144 AT20G-Fermi associations with known redshift and show that it is statistically robust. Two possible implications of these correlations are discussed: the contribution of blazars to the extragalactic gamma-ray background and the prediction of blazars that might undergo extremely high states of gamma-ray emission in the next years.

ERRATUM: “THE BLAZAR SEQUENCE AND THE COSMIC GAMMA-RAY BACKGROUND RADIATION IN THEFERMIERA” (2009, ApJ, 702, 523)

(Abridged) We present a new model of the blazar gamma-ray luminosity function (GLF) and the spectrum of the extragalactic gamma-ray background (EGRB), which is consistent with the observed distributions of EGRET blazars. The unified sequence of blazar spectral energy distribution (SED) is taken into account to make a non-trivial prediction for the EGRB spectrum and more realistic comparison with the data than previous studies. We then try to explain the EGRB data by the two AGN populations: one is blazars, and the other is non-blazar AGNs that are responsible for the EGRB in the MeV band. We find that ~80% of the EGRB photon flux at > 100 MeV can be explained by the sum of the two populations, while ~45% can be accounted for only by blazars. The predicted EGRB spectrum is in agreement with a wide range of the observed data from X-ray to GeV, within the systematic uncertainties in the EGRB determination by EGRET. These results indicate that AGNs including blazars are the primary source of EGRB. Blazars are dominant in EGRB at higher energy bands of >100 MeV, while non-blazar AGNs dominate at <100 MeV. Almost all of the EGRB flux from blazars will be resolved into discrete sources by the Fermi Gamma-ray Space Telescope, while that from non-blazar AGNs will largely remain unresolved. Therefore, comparison between the integrated source counts and diffuse EGRB flux as a function of photon energy will give a simple and clear test of our model. Various quantitative predictions for Fermi observations are also made. Especially, our model predicts 600-1200 blazars in all sky down to 2e-9 photons/cm^-2/s (>100 MeV), which is considerably smaller than most of previous studies.

Millimeter-VLBI observations of blazars

High angular resolution observations of blazars are a key step to understand the properties of relativistic jets emerging from Active Galactic Nuclei (AGN). Space-VLBI and mm-VLBI are complementary approaches to obtain an extraordinary resolution, with the latter being more suitable in the case of the most compact self-absorbed regions at the base of the jet. In this contribution, I present the results of mm-VLBI observations of a TeV active blazar (Mrk501), successfully detected with a flux density of ∼ 110 mJy and a resolution of 110 μas ×40 μas. A limb-brightened structure seems to be present even on the smallest scales. The need for new observations of Mrk 501 and of four more EGRET blazars is discussed, with an outlook to the main questions in the era of gamma-ray satellites.

THE RADIO AND GAMMA-RAY CONNECTION OF EGRET BLAZARS: CORRELATION, REGRESSION, AND MONTE CARLO ANALYSIS

A comprehensive statistical analysis of the broadband properties of Energetic Gamma-Ray Experiment Telescope blazars is presented. This analysis includes sources identified as blazars in the Sowards-Emmerd publications. Using this sample of 122 sources, we find that there is a relationship L γ Lr 0.77±0.03, as well as a correlation between α og and α ro , and a correlation between radio luminosity and α og . Through the use of Monte Carlo simulations, we can replicate the observed luminosity relationship if a synchrotron self-Compton model is assumed. However, this relationship cannot be replicated if an external Compton scattering model is assumed. These differences are primarily due to beaming effects. In addition, it has been determined that the intrinsic radio luminosity of the parent sample falls in the range 1021 < L < 1030 W Hz-1 and that the bulk of the Lorentz factors of the source are in the range 1 < Γ < 30, in agreement with very long baseline interferometry observations. Finally, we discuss implications for the Gamma Ray Large Area Space Telescope, successfully launched in 2008 June.

CGRaBS: An All‐Sky Survey of Gamma‐Ray Blazar Candidates

We describe a uniform all-sky survey of bright blazars, selected primarily by their flat radio spectra, that is designed to provide a large catalog of likely γ-ray active galactic nuclei (AGNs). The defined sample has 1625 targets with radio and X-ray properties similar to those of the EGRET blazars, spread uniformly across the |b| > 10° sky. We also report progress toward optical characterization of the sample; of objects with known R < 23, 85% have been classified and 81% have measured redshifts. One goal of this program is to focus attention on the most interesting (e.g., high-redshift, high-luminosity,...) sources for intensive multiwavelength study during the observations by the Large Area Telescope (LAT) on GLAST.

Characteristics of EGRET Blazars in the VLBA Imaging and Polarimetry Survey (VIPS)

We examine the radio properties of EGRET-detected blazars observed as part of the VLBA Imaging and Polarimetry Survey (VIPS). VIPS has a flux limit roughly an order of magnitude below the MOJAVE survey and most other samples that have been used to study the properties of EGRET blazars. At lower flux levels, radio flux density does not directly correlate with gamma-ray flux density. We do find that the EGRET-detected blazars tend to have higher brightness temperatures, greater core fractions, and possibly larger than average jet opening angles. A weak correlation is also found with jet length and with polarization. All of the well-established trends can be explained by systematically larger Doppler factors in the gamma-ray-loud blazars, consistent with the measurements of higher apparent velocities found in monitoring programs carried out at radio frequencies above 10 GHz.

The Spectral Shape of the Gamma‐Ray Background from Blazars

The spectral shape of the unresolved emission from different classes of gamma-ray emitters can be used to disentangle the contributions from these populations to the extragalactic gamma-ray background (EGRB). We present a calculation of the unabsorbed spectral shape of the unresolved blazar contribution to the EGRB starting from the spectral index distribution (SID) of resolved EGRET blazars derived through a maximum-likelihood analysis accounting for measurement errors. In addition, we explicitly calculate the uncertainty in this theoretically predicted spectral shape, which enters through the spectral index distribution parameters. We find that: (a) the unresolved blazar emission spectrum is only mildly convex, and thus, even if blazars are shown by GLAST to be a dominant contribution to the ERGB at lower energies, they may be insufficient to explain the EGRB at higher energies; (b) the theoretically predicted unresolved spectral shape involves significant uncertainties due to the limited constraints provided by EGRET data on the SID parameters, which are comparable to the statistical uncertainties of the observed EGRET EGRB at high energies; (c) the increased number statistics which will be provided by GLAST will be sufficient to reduce this uncertainty by at least a factor of three.

The Spectral Index Distribution of EGRET Blazars: Prospects forGLAST

The intrinsic distribution of spectral indices in GeV energies of gamma-ray-loud blazars is a critical input in determining the spectral shape of the unresolved blazar contribution to the diffuse extragalactic gamma-ray background, as well as an important test of blazar emission theories. We present a maximum likelihood method of determining the intrinsic spectral index distribution (ISID) of a population of γ-ray emitters that accounts for error in the measurements of individual spectral indices, and we apply it to EGRET blazars. We find that the most likely Gaussian ISID for EGRET blazars has a mean of 2.27 and a standard deviation of 0.20. In addition, we find some indication that FSRQs and BL Lac objects may have different ISIDs (with BL Lac objects being harder). We also test for spectral index hardening associated with blazar variability, for which we find no evidence. Finally, we produce simulated GLAST spectral index data sets and perform the same analyses. With improved statistics due to the much larger number of resolvable blazars, GLAST data will help us determine the ISIDs with much improved accuracy. Should any difference exist between the ISIDs of BL Lac objects and FSRQs or between the ISIDs of blazars in the quiescent and flaring states, GLAST data will be adequate to separate these ISIDs at a significance better than 3 σ.

Gamma-ray luminosity function of blazars and the cosmic gamma-ray background: evidence for the luminosity-dependent density evolution

We present a comprehensive study of the gamma-ray luminosity function (GLF) of blazars and their contribution to the extragalactic diffuse gamma-ray background (EGRB). Radio and gamma-ray luminosity correlation is introduced with a modest dispersion, consistent with observations, to take into account the radio detectability, which is important for blazar identification. Previous studies considered only pure luminosity evolution (PLE) or pure density evolution, but here we introduce the luminosity-dependent density evolution (LDDE) model, which is favored on the basis of the evolution of the X-ray luminosity function (XLF) of AGNs. The model parameters are constrained by likelihood analyses of the observed redshift and gamma-ray flux distributions of the EGRET blazars. Interestingly, we find that the LDDE model gives a better fit to the observed distributions than the PLE model, indicating that the LDDE model is also appropriate for gamma-ray blazars and that the jet activity is universally correlated with the accretion history of AGNs. We then find that only 25–50% of the EGRB can be explained by unresolved blazars with the best-fit LDDE parameters. Unresolved blazars can account for all the EGRB only with a steeper index of the faint-end slope of the GLF, which is marginally consistent with the EGRET data but inconsistent with XLF data. Therefore, unresolved AGNs cannot be the dominant source of the EGRB, unless there is a new population of gamma-ray emitting AGNs that evolves differently from the XLF of AGNs. Predictions for the GLAST mission are made, and we find that the best-fit LDDE model predicts about 3000 blazars in the entire sky, which is considerably fewer (by a factor of more than 3) than a previous estimate.

Gamma‐Ray Luminosity Function of Blazars and the Cosmic Gamma‐Ray Background: Evidence for the Luminosity‐Dependent Density Evolution

We present a comprehensive study for the gamma-ray luminosity function (GLF) of blazars and their contribution to the extragalactic diffuse gamma-ray background (EGRB). Radio and gamma-ray luminosity correlation is introduced to take into account the radio detectability which is important for the blazar identification. Previous studies considered only pure luminosity evolution (PLE) or pure density evolution, but we introduce the luminosity dependent density evolution (LDDE) model, which is favored from the evolution of X-ray luminosity function (XLF) of AGNs. The model parameters are constrained by likelihood analyses about the observed redshift and gamma-ray flux distributions of the EGRET blazars. We find that the LDDE model gives a better fit to the observed distributions than the PLE model, indicating that the LDDE model is also appropriate for gamma-ray blazars, and that the jet activity is universally correlated with the accretion history of AGNs. The normalization between the GLF and XLF is consistent with the unified picture of AGNs, when the beaming and a reasonable duty cycle of jet activity are taken into account. We then find that only 25--50% of the EGRB can be explained by unresolved blazars with the best-fit LDDE parameters. Unresolved blazars can account for all the EGRB only with a steeper index of the faint-end slope of the GLF, which is marginally consistent with the EGRET data but inconsistent with that of the XLF. Therefore unresolved AGNs cannot be the dominant source of the EGRB, unless there is a new population of gamma-ray emitting AGNs that evolves differently from the XLF of AGNs. Predictions for the GLAST mission are made, and we find that the best-fit LDDE model predicts about 3000 blazars in the entire sky, which is considerably fewer than a previous estimate.

The Fastest Relativistic Jets: VLBA Observations of Blazars with Apparent Speeds Exceeding 25c

We have measured peak apparent speeds of 25.6 +/- 7.0c, 25.6 +/- 4.4c, and 28.2 +/- 6.6c in the jets of 0235+164, 0827+243, and 1406-076, respectively, based on six epochs of high-sensitivity VLBA observations at 22 and 43 GHz during 2002 and 2003 (H_{0}=71 km/(s Mpc), Omega_{m}=0.27, and Omega_{Lambda}=0.73). These blazars had been identified as potentially having apparent speeds exceeding 40c in an earlier VLBA survey of EGRET blazars by Jorstad et al. We therefore confirm (with high confidence in 0827+243, and lower confidence in 0235+164 and 1406-076) the presence of highly relativistic pattern speeds in these three jets, although not at the >40c levels reported by Jorstad et al. The lower limit to the bulk Lorentz factor implied by the observed apparent speeds is Gamma>25-30 in these three sources, if the pattern speeds are equal to or slower than the bulk flow speed.

PKS 1622-253: A Weakly Accreting, Powerful Gamma-Ray Source

In this Letter, we discuss new deep radio observations of PKS 1622-253 and their implications for the energetics of the central engine that powers this strong high energy gamma-ray source. Combining archival infrared and optical measurements with new millimeter observations, we show that even though the accretion flow in PKS 1622-253 is under-luminous by quasar standards, a powerful super-luminal jet is launched with a higher kinetic luminosity than most EGRET blazars. Only a few percent of the total jet kinetic luminosity is required to power even the most powerful gamma ray flares that are observed. The implication is that a high accretion system is not required to power the strongest high energy gamma ray sources.

A Northern Survey of Gamma‐Ray Blazar Candidates

In preparation for the Gamma-Ray Large Area Space Telescope (GLAST), we have compiled a sample of blazar candidates to increase the pool of well-studied active galactic nuclei from which GLAST counterparts will be drawn. Sources were selected with our figure-of-merit (FoM) ranking; thus, they have radio and X-ray properties very similar to the EGRET blazars. Spectroscopic confirmation of these candidates is in progress, and more than 70% of these objects have been identified as flat spectrum radio quasars and BL Lac objects. We present ~250 new optical blazar identifications based on McDonald Observatory spectroscopy, 224 with redshifts. Of these, 167 are in our FoM-selected set. To motivate the γ-ray nature of these objects, we analyzed the current release of the EGRET data for possible point sources at their radio positions. We develop two distinct methods to combine multiple EGRET observations of a sky position into a single detection significance. We report a detection of the signal of the set of blazar candidates in the EGRET data at the >3 σ level by both techniques. We predict that the majority of these blazar candidates will be found by GLAST because of its increased sensitivity, duty cycle, and resolving power.

The Parsec‐Scale Structure and Jet Motions of the TeV Blazars 1ES 1959+650, PKS 2155−304, and 1ES 2344+514

As part of our study of the VLBI properties of TeV blazar jets, we present here a series of high-resolution 15 GHz Very Long Baseline Array (VLBA) images of the parsec-scale jets of the TeV blazars 1ES 1959+650, PKS 2155-304, and 1ES 2344+514, with linear resolutions of ~0.5 pc. Each of these sources was observed with the VLBA at three or four epochs during 1999 and 2000. There is a notable lack of any strong moving components on the VLBI images (in contrast to the rapid superluminal motions seen in EGRET blazars), and the structure of the VLBI jet can be modeled either as a series of stationary Gaussian components or as a smooth power law for two of the sources (PKS 2155-304 and 1ES 2344+514). The low apparent speeds, together with beaming indicators such as the brightness temperature of the VLBI core, imply only modest Doppler boosting of the VLBI radio emission and only modest bulk Lorentz factors (δ and Γ ≈ a few), in contrast to the more extreme values of these parameters invoked to explain the high-energy emission. The fact that no moving shocks or plasmoids are seen on the parsec scale suggests that the shocks or plasmoids that are assumed to be responsible for the high-energy flares must dissipate before they separate from the core on the VLBI images. This requires the loss of a substantial amount of bulk kinetic energy on parsec scales and implies a higher efficiency than is typically assumed for internal shock scenarios.

The Gamma‐Ray Blazar Content of the Northern Sky

Using survey data, we have re-evaluated the correlation of flat spectrum radio sources with EGRET sources in the Northern sky. A likelihood analysis incorporating the radio and X-ray properties and the Gamma-ray source localization is used to gauge the reliability of associations and to search for counterparts of previously unidentified EGRET sources. Above |b|=10deg, where the classification is complete, we find that 70% of the Northern EGRET sources have counterparts similar to the bright EGRET blazars. For several of these we identify known blazar counterparts more likely than the earlier proposed 3EG association; for ~20 we have new identifications. Spectroscopic confirmation of these candidates is in progress and we have found flat spectrum radio quasars and BL Lac counterparts with redshifts as high as 4. We also find strong evidence for a set of 28 objects with no plausible counterpart like the known EGRET Blazars. These thus represent either a new extragalactic population or a population of Galactic objects with a large scale height. The survey has been extended into the plane, where we find several new blazar candidates; the bulk of the sources are, however, Galactic. Looking ahead to the GLAST era we predict that several of the present 3EG sources are composite and that higher resolution data will break these into multiple Blazar IDs.

A Quantitative Evaluation of Potential Radio Identifications for 3EG EGRET Sources

The method of Mattox et al. for identifying EGRET sources with 5 GHz radio sources has been applied to the 3EG EGRET catalog of Hartman et al. Complete results are tabulated. We tabulate separately 46 EGRET sources with radio which we expect to have a high probability of being correct. We suggest that these sources are appropriate for most studies of the properties of γ-ray blazars as a class. All but one of these 46 sources were classified by Hartman et al. as identifications; and the additional source was classified by Hartman et al. as a identification. We also tabulate separately 37 additional plausible of EGRET sources with radio sources. These less secure possibilities include the remaining 21 high-confidence identifications of Hartman et al., three of the 27 lower confidence potential blazar identifications of Hartman et al., and an additional 15 which have not been suggested previously. Many of these sources require new radio and optical observations to establish them as blazars. We suggest that the 23 lower-confidence potential blazar identifications of Hartman et al. which we do not find should not be used in studies of the properties of the EGRET blazars. For this analysis, we have made elliptical fits to the 95% confidence contours of the position uncertainty regions of the 3EG sources which are tabulated in an Appendix.

Multiepoch Very Long Baseline Array Observations of EGRET‐detected Quasars and BL Lacertae Objects: Connection between Superluminal Ejections and Gamma‐Ray Flares in Blazars

We examine the coincidence of times of high γ-ray flux and ejections of superluminal components from the core in EGRET blazars based on a Very Long Baseline Array (VLBA) monitoring program at 22 and 43 GHz from 1993 November to 1997 July. In 23 cases of γ-ray flares for which sufficient VLBA data exist, 10 of the flares (in eight objects) fall within 1 σ uncertainties of the extrapolated epoch of zero separation from the core of a superluminal radio component. In each of two sources (0528+134 and 1730-130), two successive γ-ray flares were followed by the appearance of new superluminal components. We carried out statistical simulations that show that if the number of coincidences is ≥10, the radio and γ-ray events are associated with each other at greater than 99.999% confidence. Our analysis of the observed behavior, including variability of the polarized radio flux, of the sources before, during, and after the γ-ray flares suggests that the γ-ray events occur in the superluminal radio knots. This implies that the γ-ray flares are caused by inverse Compton scattering by relativistic electrons in the parsec-scale regions of the jet rather than closer to the central engine.

A Gamma‐Ray Flare of Quasar CTA 26

During the first 3 years of the Compton Gamma Ray Observatory mission, the blazar CTA 26 was observed 10 times by EGRET and not significantly detected. We report an observation in 1995 when CTA 26 flared to a peak γ-ray flux of (4.9 ± 1.5) × 10-6 cm-2 s-1 (E > 100 MeV), the third brightest of all EGRET blazars. Following the γ-ray flare, extensive VLBA and single-dish radio observations were obtained. We find two components of a milliarcsecond jet moving with apparent transverse velocities of 12 ± 1 h-1 c, and 5 ± 2 h-1 c (H0 = 100 h km s-1 Mpc-1, q0 = 0.1). The position angle of VLBI components appears to change with time. The slowest VLBI component's motion is consistent with ejection at the time of the 1995 γ-ray flare. A weak radio flare is also seen in Metsahovi millimeter radio monitoring data, peaking within weeks of the γ-ray flare.