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.
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.
References
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)
(rpf@phys.soton.ac.uk)
