The nature of extragalactic radio sources variability

Gorshkov A., Konnikova Gorshkov A., Konnikova V., V., Mingaliev M.Mingaliev M.

After the discovery of variability of extragalactic radio sources it became clear that variations on time scales longer than a year are due to instabilities in the associated active galactic nuclei [I.S. Shklovsky, H. Van der Laan]. Variations on shorter time scales can be either intrinsic [S.J. Qian et al.] or due to scattering in the turbulent interstellar medium [B.J. Rickett et al.]. Currently, the best agreement with the observed characteristics of the long-term variations is achieved for shock models [for instance, Marscher and W. K.Gear.].

Active variability studies are under way at many

observatories around the world. Many-year

series of observations at decimeter wavelengths

have been obtained for large samples of radio

sources [M. Bondi et al.]. The longest series of

centimeter-wavelength observations have been

carried out at the University of Michigan [H. D.

Aller et al.]. Since 1997 the RATAN-600 radio

telescope has been used to do simultaneous

observations of a sample of ~600 sources with

VLBI fluxes at 13 cm exceeding 100 mJy [Yu.A.

Kovalev et al.].

Compact radio sources have been monitored at millimeter wavelengths for more than 20 years at the Metsähovi Radio Observatory [E. Valtaoja, M. Tornikoski et al.].

We have also been conducting simultaneous observations at 0.97–21.7 GHz with the RATAN for more than 20 years (1984–2006) . Our monitoring differs from the data of Kovalev’s in two aspects. First, we observe samples that are complete to a limiting flux density; second, from 1998 the duration of our continuous daily observations of our samples was 60–100 days. This has enabled us to study variations on time scales of several days along with the longer-term variations.

As a result of more than forty-year studies of variability became clear that the majority of extragalactic radio sources demonstrate variability on time scales from tens years up to tensminutes: variations on time scales longer than a year (long term variability) are due to instabilities in the associated active galactic nuclei;

variations on scales of month can be either intrinsic or due to the scattering in the turbulent interstellar medium;

variations on daily scales (short term variability) most likely has the external reason.

In this report I will talk about all In this report I will talk about all

temporary scales of the variability.temporary scales of the variability.

Here one can see the selection criteria of our samples:

1.1.S S 200 mJy at 3.9 GHz (Zelenchuk 200 mJy at 3.9 GHz (Zelenchuk Survey)Survey)

RA = 00RA = 00hh 24 24hh

Declination = +04Declination = +04 06 06|b| > 15|b| > 15N = 138 objectsN = 138 objects2.2.S S 200 mJy at 4.85 GHz (MGB Survey) 200 mJy at 4.85 GHz (MGB Survey)RA = 00RA = 00hh 24 24hh

Declination = +10Declination = +10 12.5 12.5|b| > 15|b| > 15N = 154 objectsN = 154 objects

Results in this report about long term variability are mostly from the first sample (Decl = +04 06)The sub-sample of sources with the flat spectra consists of 68 objects.Optical identifications were done for 56 of them (82%): 41 – QSO; 5 – Bl Lac; 5 – Galaxies of different types.

For all sources spectra in a range of 0.97-21.7 GHz are measured.

According to their spectra we derived According to their spectra we derived all sources into 4 classes:all sources into 4 classes:

1)1) single-component sources with no extended single-component sources with no extended component and only one compact component (in component and only one compact component (in most cases these are the GPS or HPS sources, with most cases these are the GPS or HPS sources, with maxima at the GHz frequencies);maxima at the GHz frequencies);

2)2) two-component sources with an extended two-component sources with an extended component and a single compact component;component and a single compact component;

3)3) Sources with a dominant extended component and Sources with a dominant extended component and a weak compact component (these are actually a weak compact component (these are actually also two component sources, but the compact also two component sources, but the compact component’s influence is manifest only in the component’s influence is manifest only in the flattening of the spectrum at high frequencies);flattening of the spectrum at high frequencies);

4)4) sources with complex spectra, which can have an sources with complex spectra, which can have an extended component and two or more compact extended component and two or more compact components observed simultaneously.components observed simultaneously.

We divided all obtained in 20 years light curves

into 3 types, independent of object’s

classifications in the optical domain:

1)Slowly variable sources;

2)Sources with the isolated flares;

3)And sources with the chaotic variability.

Practically all single-component sources and

sources with the prevailing extended component

are the slowly variable objects.

The sources, which have 2 and 3 type of The sources, which have 2 and 3 type of variability, as a rule, have two-component or , as a rule, have two-component or complex spectra.complex spectra.

Here are some examples of Here are some examples of light curves:light curves:

And their spectra:And their spectra:0527+03310527+0331

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Relation between the variability indices of individual flares Vf and the variability indices V of the same sources for the whole period of our observations.

VVff = −0.018 + 0.86V = −0.018 + 0.86V

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In its majority the nature of the In its majority the nature of the variability of of

sources with the flare activity more or less sources with the flare activity more or less

corresponds to model in which corresponds to model in which variability is the is the

result of the evolution of the shock which is result of the evolution of the shock which is

propagating along the jet.propagating along the jet.

According to this model in its development According to this model in its development

shock passes the stages shock passes the stages of amplification,

balancebalance (when the energy losses and gains are comparable) and damping. damping. The shape of the shock spectrum corresponds to that for a single source and remains the same in the course of the shock evolution. As the shock develops, the peak frequency moves towards lower frequencies.

These are precisely the sort of spectral variations we observe in most cases. When the peak frequency of a flare is above the studied range the spectrum can be approximated well with a parabola. Sometimes the peak frequency is high enough that the spectrum can be described with a power-law function in our frequency range. As the flare develops the peak frequency moves to lower frequencies finally entering our studied range (1-22 GHz).

The observed maximum variation amplitude normalized so that ΔSmax(νmax) = 1

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Now I’ll talk a little bit about QSOs properties of Now I’ll talk a little bit about QSOs properties of

our sample: our sample: 41 sources (from 68). 4 have single-component spectra with Zmean = 1.7 ± 0.3);

13 two component spectra (Zmean = 1.6 ± 0.2);

3 spectra with a dominant extended

component (Zmean = 2.0 ± 0.4);

and 21 complex spectra (Zmean = 1.6 ± 0.2).

The deviations of the mean redshifts of the QSOs with different spectral classes from the value for

all the quasars in our sample (Zmean = 1.64 ±

0.12) are not statistically significant.

We were able to determine timescales for flares detected in 26 quasars. The apparent linear sizes lie within 0.18–2.3 pc, while their true sizes lie within 0.06–1.0 pc (E. Valtaoja).

The flare Tb values exceed the Compton

limit in 14 quasars. With one exception the linear sizes of the emitting regions in these objects are within 0.29 pc. Among objects with smaller angular sizes none has Tb below the Compton limit.

The variability indices for the objects withTb > 1012 K are higher than those for the objects with Tb < 1012 K.

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Relations between the variability indices and Z (Tb < 1012 K):

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But if we do correction to rest frame there is no any dependence

Some conclusions from the long-term Some conclusions from the long-term variabilityvariability

1) For 40 sources it was possible to determine the visible characteristic time of flare which allowed for the objects with known red shifts to determine the visible angular dimensions and Tb ;

2) In 14 sources Tb > 1012 K. For these sources the average value of the Doppler-factor is 2.0;

3) In all QSOs with the linear sizes less than 0.29 pc Tb exceeds Compton limit.

The reason for so sharp boundary is not clear.

4) There exists a small number of radio sources when the shape of spectrum when the shape of spectrum

remains constant with variations in the remains constant with variations in the

flux densityflux density. We suppose that for multi-component sources such variability is due to changes in the angle between the jet and the line of sight. In the case of flat-spectrum sources (a 0) we probably observe a single inhomogeneous compact source.

The characteristics of the compact components of QSO’s with brightness temperatures within the Compton limit are independent of redshift. This follows from the lack of Z-dependence for the following parameters: the linear sizes of the emitting regions;

the both variability indices (V and Vf );

the peak frequencies of compact components.

We suppose that these can be interpreted as evidence for a lack of cosmological evolution of QSOs at least up to Z = 3.(Astronomy Reports, 2008, Vol. 52, No. 4, pp. 278–298)

We already We already reported earlier reported earlier about the about the detection of detection of variations which variations which we named as we named as “weekly”. These “weekly”. These variations were variations were discovered in discovered in 1998 at the 1998 at the RATAN in RATAN in 0527+0331. 0527+0331. Variations were Variations were cyclic with the cyclic with the characteristic characteristic time of 12-16 time of 12-16 days.days.

Since 1998 we observed 112 sources with the flat spectra of two different samples. “Weekly” variability is detected in 38 objects. The main properties are: Variability usually is manifested at two or three frequencies both at low and at high frequencies; Characteristic times are from 6 to 40 days; In the same source characteristic time can change with the frequency and in the different epochs of observations; In the strongest sources the variation is correlated at the adjacent frequencies.

Light curve: 2123+0535Light curve: 2123+0535

The autocorrelation and cross-correlation The autocorrelation and cross-correlation functions functions

There is correlation with long-term variability: most frequently the “weekly” variability is observed near the maximum of the long-term variability.

The spectrum of variable component The spectrum of variable component corresponds to the spectrum of uniform corresponds to the spectrum of uniform source.source.

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But in general spectrum of the “weekly” variable component not as simple as in 0527+0331.For example - 0409+1217: BL Lac, Z=1.020

The most likely that the falling spectra of The most likely that the falling spectra of

“weekly” variations is the strongest “weekly” variations is the strongest

manifestations of scintillations (“manifestations of scintillations (“flicker”) ”)

discovered by Heeschen in 1984 (variations of discovered by Heeschen in 1984 (variations of

flux density of the flat spectra radio sources at flux density of the flat spectra radio sources at

cm-wavelengths on the scales of several cm-wavelengths on the scales of several

days).days).

The variability index is just several The variability index is just several

percentages but they are observed practically percentages but they are observed practically

in all sources with the flat spectra. Variations in all sources with the flat spectra. Variations

have average characteristic time ~4 days.have average characteristic time ~4 days.

For the definition For the definition of temporal scale of temporal scale structure structure function was function was used:used:

- temporal lag- temporal lag

1 2( ) ( )]D f t f t =

In this case we suggest that we observe In this case we suggest that we observe “flicker” as well as intrinsic variability“flicker” as well as intrinsic variability

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0527+03310527+0331:: observations at 32 m dish of observations at 32 m dish of the Zelenchukskaya Observatory of the Zelenchukskaya Observatory of Institute of Applied Astronomy; Institute of Applied Astronomy; = 3.5 = 3.5 cm.cm.

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1819+3845: observations at 32 m dish 1819+3845: observations at 32 m dish of the Zelenchukskaya Observatory of of the Zelenchukskaya Observatory of Institute of Applied Astronomy;Institute of Applied Astronomy;February 5, 2005; February 5, 2005; = 3.5 cm. = 3.5 cm.

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Thank you for Thank you for attention!attention!