Tong-Xu Hua

Radio variability properties for radio sources

In this paper, we used the database of the university of Michigan Radio Astronomy Observatory (UMRAO) at three (4.8 GHz, 8.0 GHZ, and 14.5 GHz) radio frequency to analyze the radio light curves by the power spectral analysis method in search of possible periodicity. The analysis results showed that the radio sources display astrophysically meaningful periodicity ranging from 2.2 to 20.8 years in their light curves at the three frequencies. We also calculated the variability parameters and investigated the correlations between the variability parameter and the flux density. For the variability parameters, we found that the parameters at higher frequency are higher than those in the lower frequency. In addition, the variability parameters of BL Lacertae objects are larger than those of flat-spectrum radio quasars. suggesting that they are more variable than flat spectrum radio quasars.

PHOTOMETRIC MONITORING OF OJ 287 FROM 2002 TO 2007

OJ 287 is one of the best studied blazars. It is also a prime target in our long-term blazar monitoring program carried out with the 1.56 m telescope at the Sheshan Station of the Shanghai Astronomical Observatory, China. Here, we present optical photometry results of V, R, and I passbands of the BL Lacertae object OJ 287 (0851+202) carried out from 2002 January to 2007 February. The largest variations of ΔV = 1.96 mag (14.01-15.97 mag), ΔR = 2.36 mag (13.39-15.75 mag), and ΔI = 1.95 mag (12.79-14.74 mag) were detected in our observations. Intraday variation timescales of ~10 minutes to 2 hr were detected in optical passbands.

The lower limit of the Doppler factor for a Fermi blazar sample

We selected 457 blazars (193 flat spectrum radio quasars, 61 low-synchrotron peaked blazars, 69 intermediate-synchrotron peaked blazars and 134 high-synchrotron peaked blazars) from the second Fermi -LAT catalog (2FGL) of γ-ray sources, which have X-ray observations. We calculated the lower limits for their Doppler factors, δ γ , and compared the lower limits with the available Doppler factors and the apparent superluminal velocities in the literature.

Brightness temperature for 166 radio sources

Using the database of the University of Michigan Radio Astronomy Observatory (UMRAO) at three radio frequencies (4.8, 8 and 14.5 GHz), we determined the short-term variability timescales for 166 radio sources. The timescales are 0.15 d (2007+777) to 176.17 d (0528–250) with an average timescale of Δ t obs = 17 . 1  16 . 5 d for the whole sample. The timescales are used to calculate the brightness temperatures, T B . The value of log T B is in the range of log T B = 10.47 to 19.06 K. In addition, we also estimated the boosting factor for the sources. The correlation between the polarization and the Doppler factor is also discussed.

Radio Polarization of BL Lacertae Objects

In this paper, using the database of the university of Michigan Radio Astronomy Observatory (UMRAO) at three (4.8 GHz, 8 GHZ, and 14.5 GHz) radio frequencies, we studied the polarization properties for 47 BL Lacertae objects(38 radio selected BL Lacertae objects, 7 X-ray selected BL Lacertae, and two inter-middle objects (Mkn 421 and Mkn 501), and found that (1) The polarizations at higher radio frequency is higher than those at lower frequency, (2) The variability of polarization at higher radio frequency is higher than those at lower frequency, (3) The polarization is correlated with the radio spectral index, and (4) The polarization is correlated with core-dominance parameter for those objects with known core-dominance parameters suggesting that the relativistic beaming could explain the polarization characteristic of BL Lacs.

Polarization and Beaming for Blazars

Based on the beaming model, a relation between the observed polarization and Doppler factor was obtained for BL Lacertae objects–BLs. If the flat spectral radio quasars—FSRQs fit a similar polarization-Doppler factor relation as BL Lacs, then we can find that the ratio, f , of the de-beamed jet luminosity to the unbeamed luminosity in the source frame in BLs is greater than that in FSRQs. In addition, in a revised polarization and core-dominance parameter plot, they obey to differently linear correlation suggesting that they have some differently intrinsic properties. The difference in f , found here, is consistent with the result by Fan (2003), which perhaps account for the emission line difference between BLs and FSRQs. We proposed that there is no evolution between BLs and FSRQs and their emission line difference is from the difference in their ratio f .