5 GHz observations of intraday variability in some AGNs

1. 5 GHz observations of intraday variability in some AGNs • Huagang Song & Xiang Liu • Urumqi Observation, NAOCAS

2. Abstract • We present the results of intraday variability (IDV) in compact extragalactic radio sources with the 25m radio telescope of Urumqi Astronomical Observatory at 5GHz. We present here a summary table of the results, as well as light curves and structure functions of some sources. The results cannot rule out either interstellar scintillation or intrinsic explanations. Both could be a possible cause of the variability for these IDV sources.

3. Observation and data reduction • We selected the source that have flat-spectrum and compact QSO as targets. • Flux densities were determined with “ cross – scans ” in azimuth and elevation, twice in each coordinate. This enables us to check the position offsets in both coordinates. The integration time varied between 30 and 50 seconds depending on the declination of the sources.

4. Subsequently, a Gaussian fit was performed on every subscan. The amplitude of the Gaussian is a measure of the flux density of each source. After applying a correction for small pointing offsets, the amplitudes of both AZ and EL in one cross – scan are averaged. Secondly, we correct the measurements for the antenna gain (the elevation-dependent effects), using secondary calibrators which are known to show no variations on short timescales. Finally, we link our observations to an absolute flux density scale by using the primary calibrator 3C286.

5. Statistical analysis • We use several quantities to do a statistical analysis. Here we briefly summarize them: • The modulation index m • (1) • denotes the standard deviation of flux density, <S> denotes its average in time. It provides a measure of the strength of observed variation.

6. We perform a test on whether a source is variable or not with (2) where N is the number of measurements, the Si are the individual flux densities and Si their errors. This tests whether a light curve could be modeled by a constant value or not.

7. Relative variability amplitudeY is define as (3) where m0is the modulation index of the secondary calibrator source observed in the same experiment

8. Structure functions can be used for the analysis of the characteristics of the variability, and to search for typical • timescales and periodicities. The first order structure function D( ) is defined as (see Simonetti et al.1985): (4)

9. with denoting averaging in time. For any given time lag , the value of S(t+ ) is calculated by linear interpolation of two adjacent data points. The range of is between the minimum time of two adjacent data points and the whole observation time. A source whose structure function reaches a maximum within the observing period is called type II. In the case of a monotonically increasing structure function, it is assigned type I. Nonvariable sources are assigned type 0.

10. Results and discussion • Table 1 shows the properties of some sources. It lists source name , the optical identification , redshift , the date of observation, the mean value of flux density, modulation index, the reduced • , variability amplitude, and variability type. • Our observation reveal that half of these sources show IDV in total flux density. 3 sources show type II variation, and 6 sources exhibit type I variation.

11. Some results of observations

12. specific properties of some sources • 0109+224: This quasar was observed in December 2004. There are some indications of variability in our observation. Fig.1 shows the structure function and light curve of 0109+224.

13. 0954+658 • 0954+658: This quasar was observed in December 2004. It displays variability on a level of 5 percent. Fig.2 show the light curve and structure function of 0954+658. Both intrinsic variations and propagation effects in the interstellar medium (Rickett et al.1995 ) have been explored as potential explanations of IDV in this source. The timescale is almost 2 hours.

14. 1739+522 • 1739+522: We observed this source on December 2004. Besides variations at about the 10 percent level, a rapid flux density dip has been seen. This source has shown a dip one time before in May 1991(Kraus et al. 2003). It could be explained as an extreme event. The light curve and structure function of 1739+522 are shown in Fig.3 .

15. 2351+456 • 2351+456: The quasar was observed in December 2004. This source display strong variation. The timescale is about 4 hours. Fig.4 shows the light curve and structure function of 2351+456.

16. Assuming an intrinsic origin of the variability, the size of a variable source can be derived from the variability time-scale using the light travel time argument. In this case, the linear size cannot be much larger than c*t (A.Kraus et al 2003). Intraday variation with time-scales shorter than 2 days may imply minimum brightness temperatures of 10^16-10^19K(e.g. Wagner & Witzel 1995). Therefore, IDV would cause a severe violation of the inverse Compton limit of 10^12K( Kellermann & Plauliny-Toth 1969) .

17. Intrinsic explanations have been used in e.g. the motion of compact structure (shock) in an underlying relativistic jet or the reconnection of the magnetic field lines and coherent emission processes (A.Quirrenbach 2000). But there are difficulties in these intrinsic explanation. Alternatively, IDV could be caused by extrinsic effect (Rickett 1995). It is likely that both intrinsic and extrinsic effect cause intraday variation together.

18. References • Dennett-Thorpe,J., & de Bruyn, A. G. 2000, APJ, 443, 209 • Kraus,A., Witzel,A., Krichbaum,T.P. astro-ph/9902328 vl, 23 Feb 1999 • Kraus,A.,st al 2003, A & A, 401, 161 • Krichbaum,T.P., et al 2000 aprs. conf. . 133 • Liu,X., 2003, AcASn, 44S, 310 • Quirrenbach,A., et al 1992, A & A, 258, 279 • Quirrenbach,A., et al 2000, A & ASS, 141, 221 • Rickett,B.J. 1990, ARA & A, 28, 561 • Rickett,B.J., Witzel,A., Kraus,A.,Krichbaum, T.P., & Qian,S.J. 2001, APJ, 550, L11 • Wagner S.J., Witzel A., 1995, ARA & A 33.163