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\IBVShead{5127}{22 June 2001}
 
\IBVStitle{Short-term Radio Variability of Cygnus X-1}

\IBVSauth{Mart\'i, J.$^1$; Mirabel,  I.F.$^{2,3}$; Rodriguez, L.F.$^4$}

\IBVSinst{Departamento de F\'{\i}sica, Universidad de Ja\'en, C/ Virgen
de la Cabeza 2,  E-23071 Ja{\'e}n, Spain\\ \indent e-mail: jmarti@ujaen.es}

\IBVSinst{CEA/DSM/DAPNIA/Service d'Astrophysique, Centre d'\'Etudes de
Saclay, F-91191 Gif-Sur-Yvette, France}

\IBVSinst{Instituto de Astronom\'{\i}a y F\'{\i}sica del Espacio, C.C.
67, Suc.\ 28, 1428 Buenos Aires, Argentina}

\IBVSinst{Instituto de Astronom\'\i a, UNAM, Campus Morelia, Apdo.
Postal 3-72, 58089 Morelia, Michoac\'an, M\'exico}

\SIMBADobjAlias{Cygnus X-1}{V1357 Cyg}
\IBVStyp{ microquasar }
\IBVSkey{VLA observations}

\begintext

In this note we report high time resolution radio photometry of the
high mass X-ray binary and classical black hole candidate Cygnus X-1.
The results presented here are a by-product of an observational program
carried out several years ago. At that time, our primary goal was to
obtain high sensitivity images of the extended radio emission around
Cygnus X-1 (Mart\'{\i} et al.\ 1997).  In addition to these results,
the observed data can provide a radio light curve of Cygnus X-1 with
time resolution of a few minutes and extending for several hours. The
short-term variability of Cygnus X-1 remains practically unexplored at
radio wavelengths. Therefore, we are confident that the data presented
in this note will help improve this situation.

The radio counterpart of Cygnus X-1 was originally discovered by
Tananbaum et al.\ (1972) and Hjellming (1973). At centimetric
wavelengths, this source has a rather stable radio emission at the
10--20 mJy level with a very flat spectral index (Fender et al.\ 2000).
The radio luminosity of the system displays a $\sim30\%$ amplitude
modulation with the orbital period of 5.6 d, together with a long-term
modulation on time scales of 150 d (Pooley et al.\ 1999).  Here we study
the Cygnus X-1 radio light curve with time resolution much higher than
in most previous studies.

\IBVSfig{9.2cm}{5127-f1.ps}{Radio light curve of Cygnus X-1 at the
wavelength of 6 cm on 11 April 1996.  The data points have been
averaged every 120 s. The source has a variability amplitude of
$\sim30\%$ on time scales of one hour. The horizontal axis is labelled
in International Atomic Time (IAT) whose difference with Universal Time
in 1996 was close to 30 s}

\vskip-11pt
Our observations were carried out on 1996 April 11 (JD 2450185) with
the interferometer Very Large Array (VLA) of the National Radio
Astronomy Observatory (NRAO) in New Mexico (USA)\@.  The array had its 27
antennas in its C configuration and operated at the wavelength of
$\lambda=6$ cm.  In this configuration, the longest baselines extend
over 3.4 km equivalent to $57$ k$\lambda$ thus providing an angular
resolution of about $4^{\prime\prime}$.  The data were processed using
the AIPS package of NRAO following the common procedures for connected
radio interferometry.  The amplitudes of the visibilities were
calibrated by observing 1331+305 for a few minutes at the beginning of
the run. The adopted flux density at 6 cm of this VLA primary
calibrator is close to 7.5 Jy.  We also observed the unresolved source
2007+404, before and after each of the Cygnus X-1 scans, to be used as
the phase calibrator. Cygnus X-1 was found to be bright enough to
self-calibrate its visibility data in phase using a simple point source
model.  This step allows us to get rid of most atmospheric phase
instabilities.  Our final radio light curve has been produced using the
AIPS task DFTPL applied on the self-calibrated data.  This task
performs the direct Fourier transform of the measured visibilities as a
function of time for an arbitrary point in the sky, i.e., the Cygnus
X-1 position in our case.  This Fourier transform is a real quantity
that gives the flux density of any point source at that point.
No significant nearby confusing sources were present in the array field
of view, that is limited by the beam of the individual antennas
(FWHM $\sim10'$).  In fact, Cygnus X-1 was the brightest source
in the field of view.

In Fig.~1, we present the final radio light curve of Cygnus X-1 for
several hours in 11 April 1996.  The radio emission of the system is
clearly variable with several radio flares in time scales of hours. The
variability amplitude observed is close to $\sim30\%$. This is
remarkably similar to that exhibited by Cygnus X-1 during its X-ray
flares, which often last from hours to days.  The only X-ray coverage
of Cygnus X-1 simultaneous with our radio data is that provided by the
All Sky Monitor (ASM) on board the Rossi X-ray Timing Explorer
satellite posted on the web.  Unfortunately, the ASM fluxes are too
scarce to check if there is any correlation or anti-correlation between
radio and X-ray variability at this time resolution.

The plot in Fig.~1 also suggests a possible recurrence period of the
radio flares close to 1 h.  These flaring events are also reminiscent
of the radio and infrared oscillations observed in the microquasar GRS
1915+105 (see e.g.\ Mirabel et al.\ 1998). These events are interpreted
in terms of repeated ejections of pairs of relativistic synchrotron
emitting plasmons every half an hour or so. It is thus very likely that
we are seeing the same phenomenon in Cygnus X-1.  The rise time of an
individual flare is about 20 minutes with an amplitude of $\sim5$ mJy.
From light travel time arguments, this implies an upper limit of $3.6
\times 10^{13}$ cm (2.4 AU) for the size of the radio emitting region.
At a distance of 2.5 kpc and assuming a flat spectral index, the
corresponding brightness temperature is $\geq 4 \times 10^8$ K, i.e.,
consistent with non-thermal synchrotron emission. Further concurrent
radio and X-ray observations with high time resolution are required to
better clarify the tentative suggestions of this paper. The hypothesis
of repeated ejection of synchrotron plasmons will be tested in the
future, when the very sensitive Expanded Very Large Array (EVLA)
becomes available.

\bigskip

{\bf Acknowledgements:} JM acknowledges partial support by DGICYT
(PB97-0903) and by Junta de Andaluc\'{\i}a (Spain).  The National Radio
Astronomy Observatory is a facility of the National Science Foundation
operated under cooperative agreement by Associated Universities, Inc.
We also thank S. Chaty (Open University, UK) for valuable discussions
on this work.

\references

Fender, R.P., Pooley, G.G., Durouchoux, P., Tilanus, R.P.J., Brocksopp,
C., 2000, {\it MNRAS}, {\bf 312}, 853

Hjellming, R.M., 1973, {\it ApJ}, {\bf 182}, L29

Mart\'{\i}, J., Mirabel, I.F., Rodr\'{\i}guez, L.F., Paredes, J.M.,
1997, {\it VA}, {\bf 41}, 33

Mirabel, I.F., Dhawan, V., Chaty, S.,  Rodr\'{\i}guez, L.F., et al.,
1998, {\it A\&A}, {\bf 330}, L9

Pooley, G.G., Fender, R.P., Brocksopp, C., 1999, {\it MNRAS}, {\bf
302}, 1L

Tananbaum, H., Gursky, H., Kellogg, E., Giacconi, R., Jones, C., 1972,
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