A new neutron star relativistic jet source in the Galaxy

The relation between accretion and outflow is a key topic in modern high energy astrophysics and offers us a unique opportunity to understand the physics of distant, supermassive black holes in active galactic nuclei (AGN) by studying nearby, rapidly varying objects such as X-ray binaries. Much of the focus in recent years has been on black holes; however, neutron star X-ray binaries represent an extremely valuable "control sample" of objects which also produce relativistic jets without requiring an event horizon, ergosphere etc.

The brightest set of neutron star X-ray binaries are the so-called "Z sources", of which there are currently six or seven identified, all of which have been known since the early days of X-ray astronomy in the 1960s and 1970s. They are all accreting at, or close to, the Eddington limit, and their integrated, steady X-ray emission contributes a significant fraction of the X-ray luminosity of the entire galaxy. Furthermore, they are all radio sources and in two cases, Sco X-1 and Cir X-1, the radio emission has been resolved into relativistic jets.

Colour-colour diagram for Z sources

Figure 1: X-ray colour-colour diagram for the newly-discovered source XTE J1701-462, displaying the three branches characteristic of a "Z source". Each branch corresponds to a different configuration of the accretion flow in the X-ray binary, and we have found a strong relation between the radio emission and accretion branch. During the radio "on" phases it is likely that this source - which harbours a neutron star, not a black hole - is producing a transient, relativistic radio jet (click on image for larger version).

In January 2006, for the first time, a new, transient, Z source was discovered: XTE J1701-462 was rapidly identified as a member of this exclusive class by its characteristic X-ray timing behaviour (Homan et al. 2006). Following a Director's time request to the ATNF, we observed this source 15 times with the Compact Array between 22 January and 20 March 2006. During this period the X-ray emission properties varied in the pattern characteristic of a Z source (see Figure 1; hence the name), moving along three distinct spectral branches. Over this period the radio source was also observed to switch "on" and "off", in a characteristic relation to the X-ray emission (Figure 1). This strongly indicates that XTE J1701-462 is following the same pattern of disc vs jet coupling as the other Z sources, alternating between phases of relativistic jet production and phases of "jet-less" accretion. The most variable of the previously-known Z sources is Circinus X-1, which has been discovered with the Compact Array to harbour the most relativistic jets in the Galaxy (Fender et al. 2004), which power a parsec-scale radio nebula (Stewart et al. 1993). It is not clear how XTE J1701-462 will evolve in the future, but we do know that using the Compact Array together with X-ray observatories we have observed the activation of a powerful new jet source within our Galaxy.


Fender R., et al. 2004, Nature, 427, 222

Homan J., et al. 2006, Astronomer's Telegram 725

Stewart R., et al. 1993, MNRAS, 261, 593

R Fender (Southampton), M Dahlem (ATNF), J Homan (MIT), T Belloni (Brera), R Sault (ATNF)