\documentstyle[twoside,epsfig,CJK]{carticle}
%\documentclass[twoside]{carticle}
%\usepackage{epsfig}
\pagestyle{empty}
\headsep=0.6cm %%%%%%%

\setcounter{page}{310}

%\input cchead.sty
%\input Vatola.sty

\def\thefootnoot{}

\textwidth=140mm \textheight=210mm \topmargin=0pt
\oddsidemargin=0pt \evensidemargin=0pt \footskip 0.2 truecm

%%...................
%\vspace*{1\baselineskip}
\doublerulesep=0pt
%%..................
% \renewcommand{\baselinestretch}{1.4} %%cc
\parindent=7.5mm

\setlength{\parskip}{0 true pt}
    \def\hang{\hangindent\parindent}
    \def\RE{\par\hang\textindent}
    \catcode`@=11
%%%双页
    \def\@evenhead{\underline{\parbox{\textwidth}
    {\small\rm\zihao{6}\songti\thepage\hfill
    \quad\quad ACTA ASTRONOMICA SINICA \hfill
    \hspace{2mm} Vol.44\vspace{2mm}}}} %%%%%%%
%%%单页
    \def\@oddhead{\underline{\parbox{\textwidth}
    {\small\rm \zihao{6}\songti suppl.\hfill X. Liu: Detection of Intrahour Variabilities in Quasar 3c273?  %%（作者：题目名称）
    \vspace{1.5mm}
    \hfill\thepage}}} %%%%%%%
    \normalsize\rm
\zihao{5}\songti

  %  \begin{CJK}{GB}{song}
    \begin{document}


%%篇首
    \thispagestyle{empty}
    \vspace*{-1.5 true cm}
    \small\rm
    \zihao{6}\songti
    \noindent
    \\%%%[-0.5 true mm]
    Vol.44~ Suppl.
    \hfill\hspace{8mm} %%% if ``October/January'' then \hspace{8/10 mm} July
    \small ACTA ASTRONOMICA SINICA
    \hfill
      Feb.,~2003 \\[-1.5 true mm] %%January
    \rule {\textwidth}{0.12 true mm}\\[-3.2 true mm]
    \rule {\textwidth}{0.12 true mm}
%%%%%%%

\input{../newcom.ctx}

\renewcommand{\baselinestretch}{1}

\setcounter{footnote}{1}

\vs\vs\vs \bc {\Large\bf
Detection of Intrahour Variabilities in Quasar 3c273?    %%（文题）
%\\[1ex]
%$\!\!^*$
}

\vs\vs{\normalsize Xiang Liu}  %%（本文作者）

\vs
({\footnotesize {\it Urumqi Astronomical Observatory, NAO, CAS, Urumqi}~ 830011}) \\%%（本文作者单位）

\vs\vs \ec

\normalsize
\begin{quotation}

 \no{\bf A{\scriptsize\bf BSTRACT }}~~~\small
We detected a kind of intrahour variations in the blazar 3c273
both at 6 and 3.6cm. We explain that the variability is most
likely due to shocked relativistic jet or instabilities of matter
orbiting near the horizon of central black hole.

 \vn
 \no{\bf Key words}~~   blazar, 3c273, variability
\end{quotation}

\para{\large\bf{1}~~ Observations and Results}

\vs\vs A long term monitoring campaign was carried out for a set
of strong radio active galactic nuclei at 18, 6 and 3.6cm
wavelength using Urumqi 25 meter radio telescope during 2000, to
detect possible intraday variations in these sources. The
observations were scheduled month by month at one or two
frequencies when the antenna is free. The observing is using a
source on-off technique and standard noise calibration. The signal
is digitized and sampled in the MKIV total power device, and
recorded in log file. A calibrator source is observed when time is
available.

At the end of 2000, the data set of sources 3c273, 3c454.3, 3c279,
3c84 and 3c345 were analyzed. The data at 18\,cm, were experienced
even for calibrator e.g. 3c123, had to be rejected because of
strong interference. At 6 and 3.6\,cm no interference, but we
found that receiver instability sometimes occurs. Most of our data
can be analyzed properly. We found no intraday variations for
majority sources except 3c273. Figure 1 and Figure 2 show the
light curves of 3c273 at 6cm and 3.6\,cm respectively; Figure 3
and Figure 4 show that of the calibrator 3c123 and 0836+710 in the
2 bands.
\begin{figure}[b]
\center {\includegraphics [bb=158 605 385 710,scale=1]{fig1.eps}}
%\parbox[tb]{12cm}
   %{\hspace{3.5cm}
   {\center
   \footnotesize
 Fig.\,1~~ 6\,cm light curve of 3c273 on day Sep. 18,
2000}

    \end{figure}


\para{\large\bf{2}~~ Analysis and Discussion}

\vs\vs The source 3C273 is the first quasar ($z=0.158$, $m=13$),
due to
 rapid multifrequency variability has led to its
classification as a blazar. The source clearly shows a radio and
\begin{figure}[t]
\center {\includegraphics [bb=154 652 390 760,scale=1]{fig2.eps}}
%\parbox[tb]{12cm}
   %{\hspace{3.5cm}
   {\center
   \footnotesize
 Fig.\,2~~ 3.6\,cm light curve of 3c273 on day
Oct. 24, 2000}

    \end{figure}
\begin{figure}[h]
\center {\includegraphics [bb=158 615 385 710,scale=1]{fig3.eps}}
%\parbox[tb]{12cm}
   %{\hspace{3.5cm}
   {\center
   \footnotesize
 Fig.\,3~~ 6\,cm light curve of calibrator 3c123 on day
Aug. 25, 2000}

    \end{figure}
\begin{figure}[h]
\center {\includegraphics [bb=158 605 385 710,scale=1]{fig4.eps}}
%\parbox[tb]{12cm}
   %{\hspace{3.5cm}
   {\center
   \footnotesize
 Fig.\,4~~ 3.6\,cm light curve of calibrator 0836+710 on
day Oct. 24, 2000}

    \end{figure}
optical jetlike feature extending to the southwest from nucleus.
It is very bright across the wave bands from radio to gamma-rays.
The large-scale structure from the VLA and MERLIN shows a compact
flat-spectrum core and a single jet extending about 23$''$.
Multifrequency VLBI observations show a bright core and a number
of jet components extending toward the southwest. The strong
component at the eastern end is identified as the core, no
counterjet is visible. The source is a classical superluminal
source and a low polarization quasar (LPQ), although it does not
show a high optical activity (optical variation is less than 1
mag). Optical and radio connections in this source have shown a
clear correlation between the optical and radio events at least
during some flares (e.g. Roland et al. 1994).



If the detected variability is source intrinsic, it will lead to
the Compton catastrophe, i.e. the brightness temperature exceeds
the upper limit of $10^{12}$\,K. This is an open question for most
intraday AGN sources, though some alternative mechanisms have been
proposed to avoid the problem, e.g. the coherent radiation and the
non-spherical geometry at the base of the jet (Krichbaum et al.
2001). It is believed in general that intraday variability is due
to interstellar scintillation, especially at lower frequencies. At
6cm could be a critical wavelength at which both the interstellar
scintillation and the intrinsic variation could occur, while at
3.6cm and higher frequencies the intraday variability could not be
interstellar scintillation (Qian 1994). We consider the intrinsic
explanation for our findings because similar variations were
detected at both 6 and 3.6\,cm.

A Shocked relativistic jet model is successful in reproducing the
qualitative evolution of some flares in 3c273 (Stevens et al.
1998). The hourscale variations that we found could occur at the
base of a conical jet near the accretion disk for the transverse
shock wave. Alternatively, such short time variability probably is
due to Rayleigh-Taylor instability and/or Kelvin-Helmholtz
instability of matter orbiting near the horizon of central black
hole. The variability time scales (nearly one hour) imply an upper
limit on the mass of the central black hole by requiring that the
time scale be longer than the light travel time across 6\,Rs (this
is the innermost stable orbit of a Schwarzschild black hole), it
gives a black hole mass of $<6\times 10^{7}$ solar mass.

This work was supported by the National Nature Science Foundation of China
under Grant No. 19973014.

\bc {\bf References} \ec

\vspace{-4mm} \no \fns

 \no Krichbaum T.P., Kraus A., Fuhrmann L., et al.,  PASA, 2002,
19, 14\\
\no Qian Shan-jie,  Acta Astronomia Sinica, 1994, 35(4)\\
\no Roland J., Teyssier R., Roos N.,  A\&A, 1994, 290, 357\\
\no Stevens J.A., Robson E.I., Gear W.K., et al., ApJ 1998, 502,
182

%\end{CJK}
\end{document}

