Γ-ray Blazars Research Articles

Statistics of Cosmological Black Hole Jet Sources: Blazar Predictions for theGamma-Ray Large Area Space Telescope

A study of the statistics of cosmological black hole jet sources is applied to EGRET blazar data and predictions are made for GLAST. Black hole jet sources are modeled as collimated relativistic plasma outflows with radiation beamed along the jet axis due to strong Doppler boosting. The comoving rate density of blazar flares is assumed to follow a blazar formation rate (BFR), modeled by analytic functions based on astronomical observations and fits to EGRET data. The redshift and size distributions of γ-ray blazars observed with EGRET, separated into BL Lac objects (BLs) and flat spectrum radio quasar (FSRQ) distributions, are fit with monoparametric functions for the distributions of the jet Lorentz factor Γ, comoving directional power l, and spectral slope. A BFR factor ≈10× greater at z ≳ 1 than at present is found to fit the FSRQ data. A smaller comoving rate density and greater luminosity of BL flares at early times compared to the present epoch fits the BL data. Based on the EGRET observations, ≈1000 blazars consisting of ≈800 FSRQs and FR 2 radio galaxies and ≈200 BL Lac objects and FR 1 radio galaxies will be detected with GLAST during the first year of the mission. Additional AGN classes, such as hard-spectrum BL Lac objects that were mostly missed with EGRET, could add more GLAST sources. The FSRQ and BL contributions to the EGRET γ-ray background at 1 GeV are estimated at the level of ≈10%-15% and ≈2%-4%, respectively. EGRET and GLAST sensitivities to blazar flares are considered in the optimal case, and a GLAST analysis method for blazar detection is outlined.

Multi-frequency monitoring of γ-ray loud blazars

Multi-frequency monitoring of γ-ray loud blazars

Analysis of the correlation between γ-ray and radio emissions from γ-ray loud Blazar using the discrete correlation function

In this paper, we collect 119 γ-ray-loud Blazar (97 flat spectrum radio quasars (FSRQs) and 22 BL Lacertae objects (BL Lac)),and investigate respectively the correlation between the γ-ray emission (maximum, minimum, and average data) at 1GeV and the radio emission at 8.4GHz by discrete correlation function (DCF) method. Our main results are as follows: there is good correlation between the γ-ray in high state and average state and radio emissions for the whole 119 Blazar and 97 FSRQs. And there are no correlation between γ-ray emission and radio emission in low state. Our result shows that the γ-rays are associated with the radio emission from the jet, and that the γ-ray emission is likely to have come from the synchrotron self-Compton model (SSC) process in this case.

The Correlation between Gamma-ray and Radio Emissions in γ-ray Loud Blazars

We collect 119 γ-ray-loud blazars (97 flat spectrum radio quasars (FSRQs) and 22 BL Lacertae objects (BL Lacs)), and investigate possible correlations between their γ-ray emission (maximum, minimum and average values) at 1 GeV and the radio emission at 8.4 GHz. Our main results are as follows. For the lower state γ-ray data, there is no correlation between the γ-ray and radio flux density; For the high state γ-ray data, there are good correlations for the whole 119 blazars and 97 FSRQs, and a weak correlation for the 22 BL Lac objects; For the average γ-ray data, there are good correlations. According to our analysis, we propose that the γ-rays are associated with the radio emission from the jet, and that the γ-ray emission is likely from the SSC process in this case.

On the duty-cycle of γ-ray blazars

We study several properties of blazars detected in the γ-ray energy range by comparing the Energetic Gamma Ray Experiment Telescope (EGRET) sources with a sample of radio blazars which can be considered possible γ-ray candidates. We define three classes: non-γ-ray blazars, blazars with quasi-steady γ-ray emission and γ-ray blazars with substantial activity level. We find that, on average, BL Lacertae objects show a relatively steady γ-ray emission, when detected. On the other hand, most flat-spectrum radio quasars (FSRQs) show substantial γ-ray variability. We attribute a γ-ray activity index ψ = ψ -7 x 10 -7 cm -2 s -1 to all EGRET blazars, and show that FSRQs dominate the sample with non-zero ψ in the range 0 < ψ -7 < 0.035. By combining the information of detected and candidate active galactic nuclei, we characterize the blazar activity, including the discovery of a region of consistency between the γ-ray flaring duty-cycle and the recurrence time between flares. We also find a possible relation between the activity index of FSRQs and their black hole mass. More optical and γ-ray data are crucially important to test this relation.

Q0906+6930: The Highest Redshift Blazar

We report the discovery of a radio-loud flat-spectrum QSO at z = 5.47 with properties similar to those of the EGRET γ-ray blazars. This source is the brightest radio QSO at z > 5, with a parsec-scale radio jet and a black hole mass estimate ≳1010 M☉. It appears to be the most distant blazar discovered to date. High-energy observations of this source can provide powerful probes of the background radiation in the early universe.

CCD photometry of blazars at Abastumani Observatory

Abstract We give a brief summary of the ongoing Abastumani Active Galactic Nuclei Monitoring Program, started in May of 1997. More than 75000 frames were obtained during more than 900 nights of observation for about 50 target objects, among them X-ray, γ-ray and optical blazars. All observations were done in the BVRI bands using the ST-6 CCD based Photometer attached to the Newtonian focus of the 70-cm meniscus telescope. Image reductions have been done using DaophotII. The lightcurves illustrating the activity of selected blazars in R band, namely AO 0235+164 and S4 0954+658 are presented. Most of the objects under study show light variations in optical bands over one magnitude. The largest variation was observed for AO 0235+116 - 4 m . 0 in R band.

The Gamma‐Ray Blazar PKS 0208−512 from MeV to GeV Energies

We present a comprehensive report on the high-energy properties of the γ-ray blazar PKS 0208-512 as observed with EGRET and the Imaging Compton Telescope (COMPTEL) of the Compton Gamma Ray Observatory (CGRO), spanning the entire nine-year mission (1991-2000). More high-significance detections were recorded with EGRET of PKS 0208-512 (nine at greater than 6 σ confidence) than for any other γ-ray blazar. On timescales of weeks to months, PKS 0208-512 is one of the five most variable of the γ-ray blazars. We find a statistically significant correlation between intensity and spectral index for PKS 0208-512, and this source constitutes the best example of spectral hardening with intensity within the EGRET database of blazar observations. Under the assumption of isotropic Eddington-limited emission, we infer a black hole mass of approximately 6 × 107 M☉ at the nucleus of PKS 0208-512. Gamma-ray transparency arguments, however, clearly support the hypothesis of relativistic beaming in PKS 0208-512, with a minimum relativistic Doppler factor for the jet of δ 3.2 at GeV energies. We have also carried out a comprehensive reanalysis of the COMPTEL data associated with PKS 0208-512 at MeV energies and reexamined the earlier reports of excess 1-3 MeV emission from this source. For individual CGRO viewing periods, we find upper limits only for MeV emission from PKS 0208-512. Our reanalysis of the original COMPTEL data for the period 1993 May-June leads us to conclude that the significance of the original reported detection is marginal at best and that there is no compelling evidence at present for the existence of a distinct flaring state for PKS 0208-512 at MeV energies.

On the Cross-Correlation between the Arrival Direction of Ultra-High-Energy Cosmic Rays, BL Lacertae Objects, and EGRET Detections: A New Way to Identify EGRET Sources?

With the aim of testing recent claims for a particularly strong correlation between ultra-high-energy cosmic rays (UHECRs), observed with the AGASA and Yakutsk experiments, and a sample of BL Lacertae objects, we conduct a blind statistical assessment. We search for associations between the same set of BL Lac objects and the arrival directions of 33 relevant UHECRs observed with the Haverah Park and Volcano Ranch experiments. Within the accuracy of the angle determination, there are no positional coincidences. The probability that this null result arises as a statistical fluctuation from the strongly correlated case is less than 5%. This implies that the possible correlation between the arrival directions of UHECRs and BL Lac objects is not statistically sustained. We discuss the impact of our findings on the proposed additional connection among UHECRs, BL Lac objects, and EGRET γ-ray blazars. Recently, such an association was used as a classification technique for EGRET sources. Here we show that its main underlying hypothesis (i.e., the EGRET angular uncertainty is twice that quoted in the third EGRET catalog) grossly underestimates the goodness of existing γ-ray data.

THE OPTICAL-X-RAY–GAMMA-RAY SPECTRAL ENERGY DISTRIBUTIONS OF GEV GAMMA-RAY LOUD BLAZARS AND CONNECTION AMONG LBLs, HBLs AND FSRQs

In this paper, we introduce a new composite spectral indices αγxγ = αxγ - αγ, and prove [Formula: see text], that means αγxγ is intrinsic. We plot a αxox - αγxγ diagram for 25 Gev γ-ray blazars for which αx and αγ have been provided in the literature, where αxox = αox - αx which was introduced by Sambruna et al. (1996) and proved that it is intrinsic by our previous paper (Xie et al. 2001). Using this new composite color–color (αxox - αγxγ) diagram, we investigated the nature of the HBLs–LBLs relationship, and the BL Lacs–FSRQs relationship, in high-energy emission. The results show that the spectral energy distributions of three subclasses of Gev γ-ray loud blazars are different, but essentially continuous: HBLs and FSRQs occupy separated regions while LBLs bridge the gap between HBLs and FSRQs. The results are consistent with that derived from a low energy color–color(αxox - αoro) diagram by Sambruna et al. (1996) and Xie et al. (2001). However, on the αox - αxγ diagram, FSRQs, LBLs and HBLs occupy same region. Because both αγxγ and αxox are intrinsic, thus, the new connection among HBLs, LBLs and FSRQs obtained by us is intrinsic.

The X-ray Spectral Properties for Gamma-ray Loud Blazars

We selected a sample of bright γ-ray (E&gt;100 MeV) blazars whose X-rays in 0.1–10.0 keV band are observed by ROSAT and other X-ray satellites, and make a detailed spectral analysis between the soft (0.1–2.0 keV) and hard band (2.0–10.0keV) as well as between X-ray and γ-ray properties using both un-weighted and weighted analysis methods. Our results indicate: (i) there are significant anti-correlations of the spectral indices between soft X-rays and γ-rays, and between hard X-ray and γ-ray of flat-spectrum radio quasars (FSRQs) and BL Lac objects; (ii) in the X-ray band, the correlation is confusing, there is positive correlation of the spectral indices between soft and hard X-ray band using weighted correlation analysis, but this correlation is negative using un-weighted correlation analysis.

High‐Frequency VLBA Total and Polarized Intensity Images of Gamma‐Ray Bright Blazars

We present Very Long Baseline Array images of 42 γ-ray bright blazars, including 36 with polarization vectors, obtained during the course of a multiepoch monitoring program. Each object was observed at either 43 or 22 GHz, with some objects observed at both frequencies and/or at 15 or 8.4 GHz. The morphologies are varied, with some of the blazars displaying long, thin jets, others short, broad jets, and still others containing cores with only very weak features that are probably knots in faint jets. The polarization of the cores ranges from less than 1% to 8.6%, with electric vector position angles (EVPAs) that are split between nearly parallel and nearly transverse to the jet axis. The polarization of knots in the jets covers a much broader range, from less than 2% to tens of percent. The EVPA of the brightest compact feature in each jet ranges from 0° to 80° from the jet position angle, with roughly half measuring less than 20°. The distribution is consistent with intrinsically oblique magnetic fields whose observed directions are altered by relativistic aberration.

Stellar Velocity Dispersion and Black Hole Mass in the Blazar Markarian 501

The recently discovered correlation between black hole mass and stellar velocity dispersion provides a new method to determine the masses of black holes in active galaxies. We have obtained optical spectra of Mrk 501, a nearby γ-ray blazar with emission extending to TeV energies. The stellar velocity dispersion of the host galaxy, measured from the calcium triplet lines in a 2'' × 37 aperture, is 372 ± 18 km s-1. If Mrk 501 follows the MBH-σ* correlation defined for local galaxies, then its central black hole has a mass of (0.9-3.4) × 109 M☉. This is significantly larger than some previous estimates for the central mass in Mrk 501 that were based on models for its nonthermal emission. The host galaxy luminosity implies a black hole of ~6 × 108 M☉, but this is not in severe conflict with the mass derived from σ* because the MBH-Lbulge correlation has a large intrinsic scatter. Using the emission-line luminosity to estimate the bolometric luminosity of the central engine, we find that Mrk 501 radiates at an extremely sub-Eddington level of L/LEdd ≈ 10-4. Further applications of the MBH-σ* relation to radio-loud active galactic nuclei may be useful for interpreting unified models and understanding the relationship between radio galaxies and BL Lac objects.

Cutoff in the T[CLC]e[/CLC]V Energy Spectrum of Markarian 421 during Strong Flares in 2001

Exceptionally strong and long-lasting flaring activity of the blazar Mrk 421 occurred between 2001 January and March. Based on the excellent signal-to-noise ratio of the data, we derive the energy spectrum between 260 GeV and 17 TeV with unprecedented statistical precision. The spectrum is not well described by a simple power law even with a curvature term. Instead, the data can be described by a power law with exponential cutoff: dN/dE ∝ Ee m-2 s-1 TeV-1 with E0 = 4.3 ± 0.3stat TeV. Mrk 421 is the second γ-ray blazar that unambiguously exhibits an absorption-like feature in its spectral energy distribution at 3-6 TeV.

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.

The ARISE space VLBI mission

Abstract ARISE (Advanced Radio Interferometry between Space and Earth) is a proposed future space VLBI mission currently under consideration by NASA. The primary mission goal is to orbit a radio telescope whose performance for high-frequency VLBI is similar to a 25-meter antenna of the Very Long Baseline Array (VLBA). A VLBA antenna has a mass greater than 200 tons, but inflatable antenna technologies may enable launch of an orbiting antenna with a similar diameter, but a mass of only a few hundred kilograms. This would permit key scientific questions to be investigated, such as the nature of the innermost relativistic jets in γ-ray blazars, and the physics associated with extragalactic H2O megamasers. If ARISE is selected by NASA, a launch date of 2008 is feasible.

Ultraviolet and Multiwavelength Variability of the Blazar 3C 279: Evidence for Thermal Emission

The γ-ray blazar 3C 279 was monitored on a nearly daily basis with IUE, ROSAT, and EGRET for 3 weeks between 1992 December and 1993 January. During this period, the blazar was at a historical minimum at all wavelengths. Here we present the UV data obtained during this multiwavelength campaign. A maximum UV variation of ~50% is detected, while during the same period the X-ray flux varied by no more than 13%. At the lowest UV flux level, the average spectrum in the 1230-2700 Å interval is unusually flat for this object (⟨αUV⟩ ~ 1). The flattening could represent the lowest energy tail of the inverse Compton component responsible for the X-ray emission, or it could be due to the presence of a thermal component at ~20,000 K, possibly associated with an accretion disk. The presence of an accretion disk in this blazar object, likely observable only in very low states and otherwise hidden by the beamed, variable synchrotron component, would be consistent with the scenario in which the seed photons for the inverse Compton mechanism producing the γ-rays are external to the relativistic jet. We further discuss the long-term correlation of the UV flux with the X-ray and γ-ray fluxes obtained at various epochs. All UV archival data are included in the analysis. Both the X-ray and γ-ray fluxes are generally well correlated with the UV flux, with approximately square root and quadratic dependences, respectively.

EGRET gamma-ray blazars: luminosity function and contribution to the extragalactic gamma-ray background

We describe the properties of the blazars detected by EGRET and summarize the results on the calculations of the evolution and luminosty function of these sources. Of the large number of possible origins of extragalactic diffuse γ-ray emission, it has been postulated that active galaxies might be one of the most likely candidates. However, some of our recent analysis indicate that only 25% of the diffuse extragalactic emission measured by SAS-2 and EGRET can be attributed to unresolved γ-ray blazars. Therefore, other sources of diffuse extragalactic γ-ray emission must exist. We present a summary of these results in this article.

Predictions of ECS and SSC models for flux-limited samples of γ-ray blazars

The external Compton scattering (ECS) and synchrotron self-Compton (SSC) models make distinct predictions for the amount of Doppler boosting of high-energy γ-rays emitted by blazars. We examine how these differences affect the predicted properties of AGN samples selected on the basis of γ-ray emission. We create simulated flux-limited samples based on the ECS and SSC models, and compare their properties to those of identified EGRET blazars. We find that for small γ-ray-selected samples, the two models make very similar predictions, and cannot be reliably distinguished. This is primarily due to the fact that not only the Doppler factor, but also the cosmological distance and intrinsic luminosity play a role in determining whether an AGN is included in a flux-limited γ-ray sample.

GeV emission from the nearby radio galaxy Centaurus A

EGRET has detected 67 sources associated with active galactic nuclei. With the exception of radio galaxy Cen A, all are classified as belonging to the blazar class of active galactic nuclei. The cumulative exposure from multiple EGRET observations has provided the first clear detection of Centaurus A. Unlike the γ-ray blazars seen by EGRET which are believed to exhibit near-alignment of the central jet along the line-of-sight, Cen A provides the first evidence for > 100 MeV emission from a source with a confirmed large-inclination jet. Although the high-energy emission represents a lower luminosity than most EGRET blazars, with the advent of new more sensitive instruments such as GLAST and VERITAS, the detection of off-axis high-energy emission from more distant radio galaxies (space density of radio galaxies being ∼10 3 times the blazar density) is an exciting possibility.