James C.A. Miller-Jones (Oxford University)
Katherine M. Blundell (Oxford University)
Michael P. Rupen (NRAO AOC, Socorro)
Amy J. Mioduszewski (NRAO AOC, Socorro)
Peter Duffy (University College Dublin)
Anthony J. Beasley (OVRO, Caltech)
VLBA project code: BM126G through BM126L
Observation Epoch: 2001 September 18-23
X-ray binaries are double-star systems which we can observe within our own Milky Way Galaxy. In such systems, one of the stars has used up all of its nuclear fuel, and has evolved to become a compact object (a neutron star or a black hole), while its companion remains relatively unevolved. The compact object accretes matter from its companion, which, to conserve angular momentum, forms an accretion disc around the compact object. Temperatures in the disc are sufficiently high for the disc material to emit X-rays, hence the name "X-ray binary system".
Microquasars are a subset of X-ray binary systems in which material is ejected from the compact object in opposite directions (thought to be perpendicular to the plane of the accretion disc). This material is moving outwards with a bulk flow velocity that is a significant fraction of the speed of light. It is well-collimated, forming a relativistic jet.
The electrons in these oppositely-directed jets emit so-called synchrotron radiation due to their acceleration as they spiral around magnetic field lines, and this radiation is visible in the radio part of the spectrum. Since the mass of the compact object in a microquasar is so low (1-10 solar masses, compared with a billion solar masses in the case of quasars), these systems evolve on human timescales (hours and days, rather than thousands of years). Coupled with their proximity relative to the extragalactic quasars, this means that we can observe the evolution of the jets with high-angular resolution telescopes such as the VLBA.
Cygnus X-3 is a microquasar at a distance of about 10 kiloparsecs (about 30000 light years), which undergoes periodic radio flares. During these flares the radio brightness increases by a factor of between 10 and 100 from a quiescent level of about 0.1 mJy to several Jy. This enhanced radio emission originates from relativistic jets moving outwards from the centre of the system. We have used the VLBA to monitor the radio jets at three different frequencies (5, 15 and 43 GHz) over a period of six days following the peak of an outburst in 2001 September.
There had been some discrepancy as to whether the radio jets were one-sided or two-sided in Cygnus X-3. From these images, we were able to locate the centre of the system (labelled "C"), and detected a northern extension (visible above the core in the figure). This shows that the jet is indeed two-sided, although the northern jet is foreshortened by relativistic effects, which imply that the southern jet is moving towards us and the northern jet is receding from us. The curvature of the jet is a sign that the it is precessing (the jet is rotating about a fixed axis), and a detailed modelling analysis allowed us to constrain the precession parameters of the system. The black dots superimposed on the radio image denote the predicted positions of jet material ejected every tenth of a day with the derived jet parameters, and appear to trace the observed path of the jet very well. We found the jet to be moving at 63% of the speed of light, at an angle of 10.5 degrees to the line of sight, and that it is precessing round this axis with a period of 5.3 days. Extrapolation of the jet motion back to the centre of the system suggested that the jets were ejected about 2.5 days after the radio brightness of Cygnus X-3 began to increase.
In addition, we monitored the source brightness between 74 MHz and 100 GHz with the VLA and the OVRO Millimeter Array, and the spectra we measured allowed us to constrain some of the physical parameters in the jet. We thus derived a magnetic field of <0.15 gauss, and found the electrons to be moving with Lorentz factors (the ratio of their energies to their rest-mass energies) of betwen 50 and 320. We constructed a model invoking light travel time effects to explain the evolution of the spectrum of Cygnus X-3 over the course of the outburst.
This work is due to be published in The Astrophysical Journal on 2004 January 1. It can be found as astro-ph/0311277 at http://xxx.soton.ac.uk/abs/astro-ph/0311277
A mosaic of all our VLBA images can be seen at http://www-astro.physics.ox.ac.uk/~jcamj/f2.gif