THE source VRO 42.22.01 (BL Lac) has interesting and unusual properties at both radio and optical wavelengths. The radio object was first discovered during a source survey at 610 MHz (ref. 1). It was studied in further detail at centimetre wavelengths by J. M. M. and B. H. A.2, who delineated the spectrum, and identified the source with an optical object. Schmitt3 noted that the optical object was catalogued as BL Lac and listed as a variable star. But the true nature of the object is uncertain. It is a rapid optical variable and its spectrum shows no emission or absorption lines (unpublished work of T. D. Kinman, E. Harlan and E. Marosan, and of D. DuPuy, J. L. Schmitt, R. McClure, S. van den Bergh and R. Racine). At optical frequencies there is about 10 per cent linear polarization4. Radio observations by Biraud and Veron5 indicated that the source was variable at a wavelength of 11 cm, and was circularly polarized.

Rapid Radio Variations in BL Lac

The degree of circular polarizationpc is calculated for two models of a source of synchrotron radiation:

(1)

A source with an inhomogeneous magnetic field and isotropic angular distribution of the electrons with respect to the magnetic field;




(2)

A source with a homogeneous magnetic field and anisotropic angular distribution of the electrons in which the anisotropy of angular distribution substantially increases with the electron energy.

The circular polarization of sources of synchrotron radiation

While intensity of electromagnetic radiation (radio, infrared, light, or X-ray) gives us primary information about the distribution of the baryonic matter in the Universe, polarisation is a parameter that enables us to investigate many additional details. Polarisation at radio frequencies gives us details of emission processes since the non-thermal synchrotron process dominates at low radio frequencies in emission regions. In addition, polarised radio sources can be used as probes of the intervening interstellar medium through which the radio waves are propagated. Faraday rotation effects are observed and in conjunction with known thermal emission can be used to determine magnetic fields. The Zeeman effect, a direct method of determining magnetic fields, depends on the observation of the circular polarisation components of a spectral line. In this paper I describe the early polarisation observations of radio sources, but in addition I follow the developments through to the present day.

A history of radio astronomy polarisation measurements

THE non-thermal radio source, VRO 42.22.01 (BL Lac), shows linear polarization in both the radio1 and the optical2 regions of its spectrum. The first report of variations in the total flux density of this source included the possible detection of circularly polarized radiation in its emission at 2695 MHz (ref. 3). A recent publication4 describing extensive observations of this object at 2.8 cm and 4.5 cm has shown that the variations in its total flux density are the most rapid detected to date. BL Lac has been monitored since April 1968 at a frequency of 8 GHz (3.75 cm) as part of a programme at the University of Michigan to study the polarization of variable extra -galactic radio sources, and large variations in its linearly polarized emission have been detected.

Rapid Variations in the Radio Polarization of BL Lac

In the past years relativistic (magneto)hydrodynamic simulations have been used extensively to study the time-dependent hydrodynamic properties of extra galactic jets. While these simulations have been very successful in studying the formation, collimation, propagation and termination of relativistic jets, the models used to compute synthetic images from the hydrodynamic properties were relatively simple. On the other hand, there exist several theoretical models which assume a very simple hydrodynamic evolution, but treat the non-thermal particles and their emitted radiation with great detail.
 It was the aim of this work to include a detailed treatment of the non-thermal particles and their synchrotron radiation in high-resolution shock-capturing relativistic hydrodynamic (RHD) simulations. To achieve this goal we have developed a transport scheme for the non-thermal particles by treating them as "tracer" fluids in the RHD equations. Their temporal evolution is calculated using an analytic kinetic equation solver, and their synchrotron radiation is computed in a time-dependent manner taking into account the relevant relativistic effects, (e.g., light travel times to the observer). The energy density of a dynamically negligible magnetic field is assumed to be a fraction of the energy density of the thermal fluid. Two models have been developed for the parameterization of the acceleration of non-thermal particles at relativistic shocks: A type-E model where only the strength of the shock influences the number of accelerated particles and a type-N model where the shock strength only influences their energy distribution.
 We have demonstrated that our numerical method is able to capture the essentials of the temporal and spatial evolution of the non-thermal particles and the observed synchrotron radiation with a reasonable accuracy when applied to subparsec scale relativistic jets.
 Understanding the physical processes connected to the observed X-ray blazar light curves has been the main object of research with our new numerical tool. For the first time, the hydrodynamic evolution and the synchrotron radiation of a blazar jet was simulated consistently. We have simulated collisions of density inhomogeneities (shells) within a blazar jet. The results have shown that the efficiency of the observed synchrotron radiation varies with the relative velocities of the shells as well as with the amount of initially available mass. The surrounding medium plays an important role, because it heats up the shells prior to the collision, a fact which is neglected in simpler models.
 Assuming that the observed radiation results from the interaction of shells within a blazar jet, we have developed an analytic model which enables the determination of the unobservable parameters of the jets (i.e., length and velocity of the shells) from the light curve. The parameters predicted by the model have been compared to results of our simulations and we find that the agreement is surprisingly good, given the simplicity of the model.
 In addition, several long-term simulations of collisions of many shells have shown that a model of an intermittently working central engine seems to produce light curves more similar the observed ones than a model in which the central engine ejects a continuous outflow.

Numerical Simulations of Blazar Jets and their Non-thermal Radiation

THE radio source VRO 42 22 01 was recently identified with a variable stellar object, BL Lac (ref. 1), and we have obtained results which show that the source is radio variable and probably circularly polarized. Since September 1967, a 2,695 MHz receiver has been in operation at the Nancay radiotelescope. It allows simultaneous measurements of flux densities with electric vectors at position angles 0° (V) and 90° (H), and of the products, in phase and through a quarter-wavelength delay, of the two corresponding voltages. It is thus possible to determine the complete polarization, linear or circular, of the most intense sources. One of the programmes in progress with this receiver is the measurement of flux variations for known variable sources, and of possible polarization variations. The radio source VRO 42 22 01 was included in that programme. This source has a flux density at 610.5 MHz of S610 = 4.0 flux units (ref. 2) (1 flux umt = 10−26 W/m−2 Hz−1), but it does not appear in the 4C catalogue3, so S178<2.0 flux units. Its flux density was measured at 1,417 MHz by Williams, and Stewart4, who give S1417 = 7.3 flux units. Its radio spectrum resembles that of the variable components of well known radio sources.

P. Veron

Papers

VRO 42 22 01: a Variable Source with Circular Polarization