Interplanetary scintillation observations of meter-wavelength radio emission from galaxies

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V. S. Artyukh

Abstract. We present an explanation of the interplanetary scintillation method. Observations of various types of galaxies indicated that scintillating sources with flat or inverted spectra generally occur in the nuclei of giant elliptical galaxies, while the steep-spectrum sources are found in spiral galaxies. This effect can be explained if the magnetic field is weaker and the relativistic plasma density is higher in the nuclei spiral galaxies than in the nuclei of elliptical galaxies. The measured integrated flux densities of M 31 and M 33 at 102 MHz indicated that the density of supernova remnants is an order of magnitude higher in M 33 than in M 31. No halo was observed around these galaxies.

  1. Introduction

Observing the radio emission from galaxies is one of the most pressing problems of current years. In recent years, a special interest has been taken in active galaxies (which show excess emission above that of nearby normal galaxies in some region of the electromagnetic spectrum). This excess emission frequently originates in very small regions located at the center of the galaxy, which suggests that active galactic nuclei are the reason -behina the activity of galaxies. Broad, strong emission lines of hydrogen and forbidden lines of several other elements, strong infrared and X-ray emission (which is variable in many objects), and optical and radio jets are typical signs of activity in galactic nuclei.

The first research on active galaxies dates back to 1943, when Seyfert's paper on spectroscopic observations of 6 spiral galaxies with bright stellar nuclei [l] appeared. Unusually broad emission lines of high excitation were observed in the spectra of these objects. The linewidths correspond to gas velocities between 300 and 3000 km/sec. After the war, the development of radio astronomy led us to observe galaxies with abnormally high radio emission—radio galaxies [2]. Optical identifications of radio galaxies indicated that some of them could be identified with N galaxies [3]. The development of a relay interferometer with a 122-km baseline [4] enabled us to observe several compact sources with angular sizes less than one arcsecond, such as 3C 48. The development of lunar occultation methods [5] also led to an increase in resolving power and enabled us to determine the precise coordinates for sources. In 1963, this method was used to detect a compact component in 3C 273 and measure its precise coordinates [б]. The optical identification revealed a 13'" star with an unusual spectrum con­taining broad emission lines was located at this position. It was shown in the same year that these lines could be identified with the hydrogen Balmer series and Mg II if the object was assumed to have a redshift z = 0.158 [7]. Thus this quasi-stellar radio source turned out to be an extremely bright extragalactic object — and so quasars were discovered.

Many other compact blue objects with all the properties of quasars, but with­out any radio emission were observed [8]. Radio-quiet quasars are sometimes called quasags or simply quasistellar sources in the literature. Some quasistellar objects were later identified as BL Lac objects [9]. These are highly variable ob­jects whose continuous spectra bear a very close resemblance to those of quasars, but the spectral lines are so weak that they can only be observed when the source is at minimum brightness. The idea of an interferometer with independent local oscillators [10] then enabled us to construct very-long-baseline interferometers which were then used to discover superluminal motion of the radio emitting clouds [11, 12].

The violent processes in Seyfert galaxies, radio galaxies, N galaxies, BL Lac objects and the unusually high rate of energy production in quasars—these all indicate that the physical conditions are unusual compared to those found in normal galaxies and are of special interest to astrophysicists. These galaxy types are not very well-defined, so reports periodically appear in the literature claiming that a certain galaxy that was originally classified as a certain type should actually be classified as a different type. As the observational mate­rial accumulates, proposals for expanding the existing classification scheme to include classifications for galaxies with narrow emission lines, roarers, blazars, etc. These classifications are not based on any physical theory, and it is unclear whether galaxies in the different classes are actually different physical systems or the same objects at different evolutionary stages.

Active galaxies are observed throughout the electromagnetic sp.ectrum. For example, galaxies with ultraviolet continuum (Markaryan galaxies) have been observed [13, 14]. With the advent of space-based platforms, observations have begun at infrared and X-ray wavelengths, where galaxies with active nuclei are also strong sources of radiation. Detailed reviews of this research are presented in [15, 16].

Highly effective observations are also being carried out at radio wavelengths, where quasars, BL Lac objects, N galaxies, and radio galaxies have been ob­served. Most of these objects are faint and of small angular size due to the fact that they are at great distances, so that high sensitivity and resolution are required in order to study them. Almost all of the data on galactic nu­clei at radio wavelengths has been obtained at decimeter and centimeter wave­lengths, where both the resolution and sensitivity are much higher than at meter wavelengths. For example, the giant VLA array (which enables one to observe compact sources with flux densities of order a few millijanskys) operates at cen­timeter wavelengths, and very-long-baseline interferometry has yielded a record resolution of ~ 0.0001". On the other hand, high-resolution, high-sensitivity low-frequency observations are also required to determine the physical conditions within galactic nuclei. Here, the most effective technique involves the observa­tion of interplanetary scintillations, which have a limiting resolution of 0.01".

Nearby normal galaxies are also of interest. Their radio luminosities are sev­eral orders of magnitude less than those of radio galaxies, so we can only observe those objects which are relatively close to us. Just as for the active galaxies, almost all of the research on radio galaxies has been carried out at centimeter wavelengths, where the thermal component of the radio emission makes a notice­able contribution in normal galaxies. A detailed review of this work is contained in [17].

At meter wavelengths, the radio emission from galaxies can be assumed to be completely nonthermal at meter wavelengths, and the generally accepted mechanism is synchrotron radiation. Supernovae and pulsars are the most likely sources of relativistic electrons, although young evolving stars and the cores of galactic nuclei may also provide some relativistic particles. The question of whether supernovae can provide all of the nonthermal emission in normal galaxies is still open, and it would be interesting to examine the relationship between supernova remnants and the nonthermal emission of galaxies in this connection.

Cosmic rays formed in the galactic disk will move away from the plane of the galaxy by diffusion, and should form a galactic halo after leaving the disk [18]. The presence of such a halo can only be detected by its nonthermal radio emis­sion, and the search for radio halos around normal galaxies is one of the problems currently under study by radio astronomers.

Another current problem in radio astronomy is the search for supernova rem­nants (SNRs) in external galaxies. Until now, all SNRs in external galaxies have been found by optical methods and the radio emission was observed later, unlike our Galaxy, where the situation was the reverse. The radio emission from SNRs is nonthermal, while H II regions (which are frequently confused with SNRs) have thermal radio emission. It is therefore appropriate to search for SNRs at meter wavelengths; however, this requires high sensitivity and high resolu­tion. Interplanetary scintillations provide a quite suitable method for solving this problem.

The present review paper describes the interplanetary scintillation observa­tions of the radio emission from galaxies at 102 MHz currently under way at the Pushchino Radio Astronomy Station of the Lebedev Physics Institute (USSR Academy of Sciences).

  1. Interplanetary Scintillation

The scintillation effect was first observed in 1964 [19], and has been widely used in studying the properties of the interplanetary plasma (IPP). In particular, a great deal of the research (both experimental and theoretical) on the proper­ties of the interplanetary plasma has been carried out at the Lebedev Physics Institute Radio Astronomy Laboratory. The results of this research have been presented in review paper and book form [20, 21]. In addition research on the IPP itself, scintillation observations have been used to obtain information on the angular sizes of compact sources. This is precisely the aspect of scintillation that we will be interested in.

Scintillations of radio sources are due to the diffraction of radio waves on inhomogeneities in the IPP. This process is described by the theory of wave propagation in random media. The modern theory [21-23] is based on a fairly complex set of mathematical tools; this makes it difficult to explain. In the phase-screen approximation (which provides a completely satisfactory descrip­tion of the interplanetary scintillations at small elongations), however, the theory is relatively straightforward. Indeed, since the density of the IPP decreases with increasing distance from the Sun as 1/r2 [24], the distribution of electron den­sity along the line of sight will be as shown in Fig. la, which indicates that one can identify a region with thickness L much less than 1 AU at small elongations (e < 50°) where the density of the IPP is largest. This is precisely the layer that introduces the largest perturbations in the radio waves as they pass through the layer. Since only the phase of the wave is modulated while the amplitude of the wave remains virtually unchanged within the layer we have isolated, we can use a phase-screen model to describe the propagation of the radio waves in the IPP.

Figure 1 Density distribution of the interplanetary plasma along the line of sight (a) and a diagram illustrating the phase screen model (b).
Fig. 1b contains a diagram illustrating the passage of a plane wave through a phase screen in the one-dimensional case. The variations in the index of refraction lead to variations in the velocity of propagation. This is due to the fact that the path length for the wave is has been lengthened due to the refraction of the direction of propagation of the wave front, and means that upon exiting from the screen, the plane wave will suffer a random phase change Ф(а;) relative to the unperturbed wave at the point x. This leads to fluctuations in the intensity of the radiation in the far field, where the observer is located. We shall assume that all of the random processes under discussion are completely described by their autocorrelation functions. For example, the fluctuations in the intensity of the radiation can be characterized by the autocorrelation function M(r) :

, (1)

where and is called the scintillation index. The Fourier transform of this function, , is called the spatial scintillation spectrum. The following simple equation relates the spatial scintillation spectrum to the spectrum of electron density fluctuations in the IPP and the spectrum of the source brightness distribution [25]:

, (2)

where A = 510–25 cm2, is the wavelength of the radiation, k is the wavenumber, z is the line-of-sight coordinate, q is the spatial frequency, Фe(q) is the spatial spectrum of the fluctuations in the electron density of the IPP, and F(qz/k) is the spatial spectrum of the radio source.

The inhomogeneities in the IPP are moving away from the Sun with a mean velocity  400 km/s (at the Earth). This phenomenon is called the solar wind. The diffraction pattern created on the Earth by the inhomogeneities in the IPP moves with the same velocity, and the radio telescope records the time variation in the intensity. The time spectrum of the scintillations is of the following form [26]:


where / is the time frequency, V^(z) the projection of the solar wind velocity onto the plane of the sky at the point z, and qu the component of the spatial frequency parallel to the projection of the solar wind velocity onto the plane of the sky.

Thus, the statistical characteristics of the fluctuations in the intensity of radi­ation are determined by the statistical characteristics of the medium and the brightness distribution of the source. If we know the characteristics of the medium, we can in principle obtain a strip map of the brightness distribution across the source in the direction of motion of the solar wind. This requires solv­ing integral equation (3). The presence of measurement errors in the estimated time spectrum of the scintillations turn this into an ill-posed problem and make the problem more difficult to solve. Because of this, the solution is generally restricted to a single parameter of the brightness distribution—the angular size of the source.

There are two methods used in determining the angular sizes of sources: 1) using the behavior of the scintillation index for the sources as a function of elon­gation; and 2) using the behavior of the time spectra of the scintillations. The latter method is the one used at the Lebedev Physics Institute Radio Astronomy Station, where the angular diameters of several hundred radio sources have been measured to date. Shishov and Shishova [27] have discussed various scintillation modes and carried out numerical calculations of the scintillation time spectra for sources of various angular sizes under the following assumptions:

  1. The spectrum of the turbulence in the IPP is a power law:


  1. The turbulence decreases with distance from the Sun like


3) The solar wind velocity is constant, radial, homogeneous, and spherically symmetric; and

4) The brightness distribution of the source is a spherically symmetric Gaussian:


where 0 is the 1/e radius of the source.

These calculations were repeated in [28], but for a larger number of angular source sizes and higher frequencies. The theoretical results from [28] are shown in Fig. 2; the frequencies are given in units of (where V is the velocity of the solar wind and 1 AU is the astronomical unit). The spectra have been superposed at low frequency in order to emphasize the differ­ences between the scintillation spectra for sources of various angular sizes. The figure clearly indicates that the differences between the spectra are larger at the higher frequencies. Thus, the larger the observed region of the spectrum acces­sible for analysis, the more reliably the angular size can be determined by the method of interplanetary scintillation. The resolution of the method depends on the signal/noise ratio. The weaker the signal, the lower the resolution. Active galactic nuclei are generally faint objects, so the highest resolution obtained in the present observations was 0.01". Artyukh and Shishova [28] showed that saturation is the main effect that limits the resolution. Artyukh and Shishova analyzed the effect of various factors on the accuracy with which the scintillation spectra can be measured, and showed that the method has a limiting resolution of 0.01" at 102 MHz . Note that a
Figure 2 Theoretical scintillation spectra for sources of various angular size (from [28]).
n interferometer of the same resolution oper­ating at the same frequency must have a baseline of 60 000 km, or much larger than the size of the Earth! Thus, interplanetary scintillation has the highest res­olution of any of the ground-based methods of studying compact radio sources at meter wavelengths.

The observations whose results will be presented below were carried out using the Lebedev Physics Institute VLPA antenna [29] at a frequency of 102 MHz. The antenna beam is 1° x 0.5° sec z (where z is zenith distance). The antenna system has an effective area 20 000 m2, the bandwidth  = 1.5 MHz , the receiver time constant  = 0.5 s, and the fluctuation sensitivity S = 0.14 Jy . Figure 3 shows a sample analog trace for the scintillating source 3C 48, and Fig. 4 its estimated scintillation spectrum.

The method used to reduce the data has been described in [30, 31]. We shall not go into detail on this subject, but merely note that the following parameters can be determined from the observations: the coordinates and flux densities of sources, their scintillation indices, and the angular sizes of the scintillating components.

  1. Information Content of the Observations

We shall now discuss the information on the physical conditions in galactic nuclei that can be provided by observations of scintillating sources at meter wavelengths. The fact that radio sources have power-law spectra similar to the cosmic-ray power-law spectrum and polarized emission is consistent with the hypothesis that the emission is due to the synchrotron effect, which is also implied by various theoretical considerations {32]. We can therefore assume that this is indeed the mechanism that is operating in the nuclei of active galaxies. The spectra of many sources are observed to have low-frequency cutoffs. There are several possible reasons for these cutoffs. We shall now describe them in brief:

Figure 3 Analog recording of the scintillating source 3C 48
A low-energy cutoff in the electron energy spectrum. If the relativistic electron energy spectrum is of the form N{E) == NoE~^ for E > Ey and N(E) = 0 for E < eq , the emission will have spectral index cc = —1/3 (6' ~ ^-0!) below vq , the critical frequency corresponding to eq [33]. Other, less severe cutoffs in the electron spectrum will naturally lead to an even shallower rate of decrease in the emission at low frequencies. However, since the vast majority of compact radio sources have steeper cutoffs, this mechanism is not appropriate for explaining them.

2. An index of refraction different from unity. The presence of thermal non-relatiyistic plasma in the region where the emission originates leads to sup­pression of the radiation because of the fact that the coefficient of refraction of the plasma is different from unity. This is the so-called Tsytovich-Razin ef­fect [34, 35]. The phase velocity of light in plasma c/n is greater than с , since , where 0 is the plasma frequency. This means that at low frequencies, the electrons become nonrelativistic, and the cone within which the emission is concentrated becomes larger, which leads to a corresponding decrease in the intensity of emission. The frequency at which this occurs,


where ne is the density of thermal electrons and H is the magnetic field strength in the source.

The emission coefficient shows the following dependence on frequency at fre­quencies less than i/p:

. (5)

A sharp, exponential cutoff in the spectrum is a very rare phenomenon (which we did not in fact encounter in our observations), so this mechanism is also illsuited to explaining the cutoffs in the spectra of the compact sources observed at meter wavelengths.

3. Thermal absorption. Hot plasma within the radio source itself or along the line of sight near the source leads to absorption of radiation in electron-ion collisions. The absorption coefficient is given by the classical formula [18]

, (6)

where Те is the electron temperature. The frequency at which the absorption becomes substantial,

where L is the size of the source along the line of sight (in parsecs).

Figure 4 Scintillation time spectrum for 3C 48. 1) Noise spectrum; 2) source spectrum.
he compact radio sources in Seyfert galaxies (and, one might expect, the nuclei of other galaxies as well) have linear sizes between 1 and 10 pc [36]. The electron temperature is of order 107 – 108 К, since appreciable amounts of X-ray emission are observed the majority of active galaxy nuclei [15]. If we assume L = 100 pc and Т = 105 pc for purposes of estimation, the density of -thermal electrons must be of order 105 cm"3 in order to obtain a cutoff in the spectrum at ~ 1 GHz (as we observe). In our Galaxy, the optical depths indicate [37] that n  2103 cm–3 within a 10-pc radius and n  104 cm–3 within a 1-pc radius [38]. This is less than the required electron density; how­ever, there is no reason to presume that such electron densities cannot exist in the nuclei of active galaxies. One possible proof that this was in fact the mech­anism for the cutoff would be the low-frequency spectrum of the source. Once the frequency decreases to the point where the optical depth becomes greater than 1, the frequency should decrease according to the law (where is the optical depth) until the main contribution to the emission comes from the thermal emission of the absorbing cloud itself, with spectral index  = –2. But observation of this effect would require detailed low-frequency observations of compact sources at several frequencies.

4. Synchrotron self-absorption. In the simplest case, the solution to the equation of transport takes the following form:

where (v) is the emission coefficient, () is the absorption coefficient, is the optical depth, , and L is the size of the source along the line of sight. From the theory of synchrotron emission [39], we have the following for an ensemble of relativistic electrons distributed according to a power law:

for <1: , (7)

for >1: , (8)

where H is the component of the magnetic field strength perpendicular to the line of sight, C1 is a constant, and C() and b() are functions tabulated in [33].

At frequencies where the source is optically thick, it should have spectral index  (8). If, however, the source is not homogeneous and different parts of the source become optically thick at different frequencies, the low-frequency spectrum of the source should be flatter. If  > 1, a maximum should be observed in the spectrum of the source. From the condition (с) = 1, we obtain the following simple expression for с, which is very close to the frequency at which the source spectrum reaches its maximum:


( С6() is also tabulated in [33]).

The observed parameters of the compact radio sources—their angular sizes, flux densities, and expected magnetic field values are such that the frequency Vc. should fall at radio wavelengths and synchrotron radiation should be observed. This implies that this mechanism is the most likely explanation for the low-frequency cutoffs in the spectra of the compact sources.

We shall now discuss what information can be extracted from the observations if we are indeed observing synchrotron self-absorption. Slysh [40] has suggested that it is possible to estimate the angular sizes of compact radio sources from their spectra. According to [40],

, (10)

where v is the frequency at which  > 1, S is the flux density of the source at this frequency, and z is the redshift of the source.

The magnetic field was assumed to have a value H = 10–4 G. However, this same formula can obviously be used to determine the strength of the magnetic field in the source once the angular size of the source and its flux density have been measured. If the redshift z is unknown, it can be assumed equal to unity. When the redshift of the galaxy containing the compact radio source in question is embedded is known, the linear size L of the source can be determined from its angular size, and, substituting the resulting H and L into (7), we can obtain an estimate of the relativistic electron density N0 .

We shall now estimate the accuracy of this method of determining H and N0 . To do so, we rewrite (9) in the following form:

. (11)

The frequency v makes practically no contribution to the error in the esti­mated H (even though it appears to the fifth power), since the frequency of observation is always known to high accuracy (with an error equal to the receive bandwidth ~ 1%). At meter wavelengths, the flux density can be measured to within 10-20%, which leads to a factor of 1.5 error in H. If the redshift is not known, and is assumed to be equal to 1, this can lead to an error of no more than a factor of two in H , since z is probably between 0 and 3. The largest, error is that due to the error in the measured angular diameter of the source. Interplanetary scintillations yield an error of ~ 30% in the estimated angular diameters of sources for strong sources and an error of as much as 100% for weak sources [28], depending on the signal/noise ratio, i. e., the measured may be in error by a factor of two. The corresponding error in the estimated value of H is then a factor of 16. Thus, in the worst case, we can obtain a value for the magnetic field strength which is accurate to an order of magnitude, or in the best case, a factor of 2-3. Substituting L in terms of z into (7), we have

, (12)

where с is the speed of light, h is the Hubble constant, and and S are the angular size and flux density of the source, measured at frequency v . At high frequencies, interferometer measurements of angular sizes lead to an error of ~ 10%, and the largest error in No is that due to the error in H . Most radio sources have   3, with No ~ H–2, so that the error in n0 will range from a factor of four to a factor of 100. The situation is better for flat-spectrum sources ( = 1), since N0 ~ H–1 and the error in the estimated N0 is the same as that in H.

Thus, observations of compact radio sources whose low-frequency spectrum is determined by synchrotron self-absorption enable us to obtain information on the magnetic fields and the density of relativistic plasma in the nuclei of the galaxies occupied by these sources. Note that this method of measuring H and N0 does not require one to assume equipartition of energy within the source.

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