Superluminal Motion of Gamma-Ray Blazars

High Energy Blazar Astronomy ASP Conference Series, Vol. 299, 2003 L.O. Takalo and E. Valtaoja

S.G. Jorstad Institute for Astrophysical Research, Boston University, 725 Commonwealth Ave., Boston, MA 02215-1401 Astronomical Institute of St. Petersburg State University, Universitetskij pr. 28, 198504 St. Petersburg, Russia

A.P. Marscher Institute for Astrophysical Research, Boston University, 725 Commonwealth Ave., Boston, MA 02215-1401

Abstract. We have completed a program of monitoring of a sample of γ-ray blazars with the VLBA at 22 and 43 GHz. Analysis of the data allows us to conclude that the population of bright γ-ray blazars can be classiﬁed as highly superluminal, with apparent speeds as high as ∼40c. However, many of the blazars exhibit a wide range of apparent speeds from ∼4c to ∼20c in the same source. Comparison of brightness and polarization parameters with the jet velocities of moving components does not show signiﬁcant correlation between these parameters. Intensive multifrequency VLBI monitoring is needed to reveal any patterns that might exist in the distribution of apparent speeds observed in the same object.

1. Introduction

Apparent speed is one of the most important parameters for determining the physics of radio jets. The largest class of identiﬁed γ-ray sources (Hartman et al. 1999) consists of violently variable AGNs (blazars) characterized by one-sided superluminal radio jets. A popular model for the production of high-energy radiation in AGNs involves inverse-Compton scattering of low-energy photons by highly relativistic electrons in the jet. However, the seed photons for this process are uncertain: they might be from the accretion disk and/or nearby region (ECS-model; Sikora et al. 1994) or they could originate in the jet itself (SSC-model; Bloom & Marscher 1996). For both models Lister & Marscher (1999) predict a high range of Lorentz factors (18−20) and small viewing angles (1◦−3◦) and high apparent speeds (7−8)h−1c obtained from multiple Monte Carlo runs using a simulated γ-ray blazar population. The observational results are controversial: Vermeulen & Cohen (1994) list proper motions of 13 EGRET sources, of which four show no motion, four have low apparent speeds, and the remainder have high superluminal velocities, up to 30c; Kellermann et al.(1999) ﬁnd that for a sample of 15 observed γ-ray sources, the mean apparent speed is (5.7±0.6)c, which is slightly higher than the mean jet velocity (3.9±0.4)c for 48 111 112 S.G. Jorstad & A.P. Marscher non-γ-ray sources with comparable radio luminosity and structure. Our sample consists of 42 EGRET blazars that we have observed with the VLBA to study the proper motions in the radio jets.

2. Observations and Data Reduction

The sample contains 31 quasars and 11 BL Lac objects, roughly 60% of known γ-ray blazars. The observations were carried out from November 1993 to August 1997, contemporaneous with the EGRET mission. Monitoring was performed with the VLBA at 22 and 43 GHz at a resolution of about 0.15 mas. On average, every source was observed at 5-6 epochs. For a given source the uv-coverage did not change signiﬁcantly from epoch to epoch. The data were calibrated, imaged, and modelled in the same manner (Jorstad et al. 2001a). Images obtained at all epochs may be found at our web site www.bu.edu/blazars. A standard −1 −1 Friedmann cosmology with H◦=100h◦(km s Mpc ), q◦=0.1, and h◦=0.65 is used to calculate the apparent transverse velocities. Our results have already been published (Marscher 1999, Mattox et al. 2001, Jorstad et al. 2001a, Jorstad et al. 2001b, Marscher et al. 2002). Here we summarize the main ﬁndings on the apparent speeds and polarization features of moving jet components.

3. Distributions of Superluminal Speeds

We have obtained the jet velocities in 23 quasars and 10 BL Lac objects. Fig. 1 presents the distribution for quasars (18 knots, light shade) and BL Lac objects (9 knots, intensive shade) when only a single well-determined apparent speed is included for each source. (For sources with multiple components, the one with the best-determined speed is used.) In this case the mean transverse apparent speed equals (8.9±5.1)c. There is no significant difference in the apparent motion of EGRET quasars and BL Lac objects and in 62% of the sample the superluminal velocities are higher than 10 c. Therefore, the average superluminal speed of jet components is significantly higher than that of jet components in the general population of strong compact radio sources (Kellermann et al. 1999). We detected multiple moving components in 16 quasars and 8 BL Lac objects. Fig. 2 (left panel) shows the distribution of the apparent speed for these 24 sources when each source is characterized by the minimum apparent speed. In this case the mean apparent speed equals (6.6±3.5)c. Fig. 2 (right panel) shows the distribution of the apparent speed for the same 24 sources when each source is characterized by the maximum apparent speed. In this case the mean apparent speed equals (13.2±5.7)c. However, a large scatter in the apparent speeds in the same object seems to be a common feature of γ-ray blazars. One can expect that the fastest radio jet components are connected with the production of γ-ray emission. Although the current γ-ray light curves do not allow one to draw a solid conclusion, statisti- cally we found 10 cases when the ejection of superluminal component coincides to within 1 σ uncertanty with the time of a γ-ray flare (Jorstad et al. 2001b). The average apparent speed for these 10 cases equals (11.4±6.2)c. This follows Gamma-ray Blazars 113

Figure 1. Distribution of the jet velocities for γ-ray blazars with a single well-determined apparent speed for each source. The intensive shade corresponds to BL Lac objects.

Figure 2. Distributions of the jet velocities for γ-ray blazars with multiple moving components: (left panel) for the minimum and (right panel) for the maximum apparent speed for each source. The intensive shade corresponds to BL Lac objects. 114 S.G. Jorstad & A.P. Marscher the expectations of inverse Compton models in which the γ-rays are more highly beamed than is the radio synchrotron radiation.

4. Comparison between Superluminal Speed and Brightness of Jet Components

In the sample of γ-ray blazars with multiple moving components many of sources reveal an increase of apparent speed farther from the core. We do not have suffi- cient time coverage to determine whether the motions of individual components accelerate. Most likely, the dependence is caused by the selection effect. How- ever, 5 quasars (0528+134, 3C 273, 3C 279, CTA 102, and 3C 454.3) and 2 BL Lac objects (3C 66A and OJ 287) display knots with different apparent speeds at the same distance from the core. To analyse the patterns in the distribution of apparent speeds in the same object, we estimate jet parameters by adopting the following approximations. 1) The highest apparent velocity corre- 2 sponds to the optimal apparent speed in the source (Γ ∼ 1+βapp,Θ∼1/Γ, where Γ is the Lorentz factor and Θ is the viewing angle). 2) Different observed fluxes (S) of components at the same distance from the core correspond to the different Doppler-boosting factors (δ), where S∝ δ3+α,S∝ ν−α,andα ∼0.7. That is, the luminosities of successive components are assumed to be the same. Under this ansatz, we have calculated the following jet parameters: Lorentz and Doppler factors, jet velocity, and viewing angle. The results are listed in Table 1, whose columns are as follows: source name, designation of component according to Jorstad et al.(2001a), apparent speed, flux, distance from the core, Lorentz factor, velocity of the jet flow, viewing angle, and Doppler factor.

Table 1. Parameters of Components with Diﬀerent Apparent Speeds in the Same Source

Source Comp. βapp(c) S(Jy) R(mas) Γ β(c) Θ(deg.) δ 0219+428 B2 19.3±3.5 0.06±0.02 0.81±0.05 19.3 0.9987 2.97 19.3 B4 9.9±2.4 0.04±0.02 0.86±0.07 11.2 0.9960 2.95 17.3 0528+134 B2 28.8±7.8 0.74±0.03 0.22±0.04 28.8 0.9994 1.99 28.8 B3 16.6±1.7 1.77±0.02 0.28±0.04 21.7 0.9989 1.20 36.5 B4 13.1±1.7 1.08±0.03 0.21±0.02 18.5 0.9985 1.27 31.9 0851+201 B1 8.4±2.0 0.08±0.02 0.61±0.06 8.6 0.9930 6.78 8.5 B2 6.8±0.7 0.07±0.03 0.72±0.05 6.4 0.9876 7.61 8.2 1226+023 B4 3.5±1.8 6.05±0.25 0.69±0.06 5.6 0.9838 3.69 9.9 B5 9.1±1.8 4.53±0.15 0.51±0.09 9.2 0.9940 6.27 9.2 1253−055 B1 8.5±0.6 2.42±0.15 0.45±0.06 8.6 0.9932 6.71 8.6 B3 5.7±0.6 7.55±0.21 0.35±0.06 7.1 0.9900 3.98 11.6 2230+114 B1 17.4±0.5 0.16±0.04 0.27±0.04 17.4 0.9984 3.29 17.4 B3 13.2±0.5 0.13±0.04 0.20±0.03 12.5 0.9968 3.69 16.5 2251+158 B1 24.2±0.9 1.32±0.06 0.30±0.07 24.2 0.9992 2.37 24.2 B2 15.5±1.8 2.66±0.02 0.24±0.03 18.3 0.9985 1.66 32.9

The adoption of identical luminosities for different components in the same source is unlikely to be strictly true. However, we find that it leads to a rather elegant picture in which the range of observed apparent speeds and flux densities results from quite modest variations in Lorentz factor and jet direction. The Gamma-ray Blazars 115 same is not true if instead either Lorentz factor or jet direction were assumed constant. In the case of Γ=const the observed scatter in apparent speeds implies a significant change (several degrees) in the jet direction which is unexpected for Γ ∼20 based on momentum consideration. In the case of constant luminosity, the change in jet direction does not exceed 1◦ (except 3C 273 and 3C 279, where it is about 2.5◦). In the case of Θ=const, different values of the apparent speed yield a significant scatter in the Doppler factors in a given source, while for constant luminosity every source can be characterized by mean Doppler factor with a standard deviation less than 15% of the average. Altough most likely, all three parameters of jet components (Lorentz factor, jet direction and luminosity) are variable, it appears that luminosities of detected components in the same source experience the modest variability.

5. Comparison between Superluminal Speed and Polarization Prop- erties

We have obtained VLBA images of 36 objects (out of 42 γ-ray blazars) with polarization vectors (Marscher et al. 2002). The data were compiled and plotted to determine whether there are any correlations between fractional polarization and electric vector position angle (EVPA) of the core and between each of these and apparent velocity of features in the jet. We express the EVPA in terms of the angle χa by which it is inclined relative to the jet axis. The direction of the axis is determined for the core by the position angle of separation between the core and the nearest knot in the jet. For a knot in the jet, the axis is defined as the direction between it and the nearest component between the knot and the core. There are two weak apparent trends: (1) the dearth of cores with χa in the ◦ ◦ range 30–50 ; and (2) 15 of the quasar cores have χa in the range 50–85 , while only 9 have values < 40◦. These trends imply that the magnetic field in the core region is either nearly parallel or perpendicular to the jet axis. The distribution of χa values of knots in the jet whose polarization is detectable (at > 1σ)displays ◦ a peak near χa = 0 and a deficit near χa =90. Among the quasars, there is a broad distribution at intermediate angles, while there are no values > 45◦ among the small sample of BL Lac objects. The distribution of χa of features in the jet implies that the magnetic fields in knots are generally oblique to the jet axis. We find no correlation between the fastest reliable apparent velocities of knots in the jet and either percent polarization (Fig. 3, left panel) or χa (Fig. 3, right panel) of the core. In addition, there is no correlation apparent between χa and percent polarization and between χa (knot) and the ratio of flux density of a knot to that of the core. This negative result provides no support to models in which the knots are shocks whose fronts are transverse to the jet axis. On the other hand, if many of the jets are directed at angles to the line of sight much less than Γ−1, as expected for a sample selected for extreme relativistic beaming, then the projected magnetic field will not be strongly related to the direction of the jet, since the shock front would be observed nearly face-on. This leads to the conclusion that γ-ray bright blazars possess no observable signatures in either jet morphology or polarization structure. 116 S.G. Jorstad & A.P. Marscher

Figure 3. Apparent velocity of moving knots vs. (left panel) percent polarization of the core and (right panel) polarization electriv vector direction of the core relative to the direction of the jet immediately downstream of the core. open symbols represent quasars, and ﬁlled symbols correspond to BL Lac objects.

Acknowledgments. This research was funded in part by US National Science Foundation grants AST-9802941 and AST-0098579 and NASA grants NAG5-7323 and NAG5-2508.

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