These jets are linear features originating very close to the super
massive black hole (SMBH) at the center of some quasars and radio
galaxies. The commonly accepted model consists of two oppositely
directed jets, although in many cases only one side is easily
detected. There is still some uncertainty as to their basic
composition; all we really know is that they act as conduits for
transporting energy over intergalactic distances which in some cases
exceeds a million light years. We also are convinced that when we
detect radiation from jets, it means there are relativistic electrons
producing the emission (see below). These jets are called
'Relativistic' because the energy is being transported at a velocity
very close to the velocity of light, and they are 'Jets' because they
are well collimated and we know that power is flowing away from the
SMBH. A picture of the very innermost part of the jet in one of the
nearest radio galaxies is shown here.
Blazars are a subclass of quasars and radio galaxies and we believe
that their extreme brightness and variability are explained by the
notion that one of their jets is pointed almost exactly towards the Earth.
Whenever an emitting volume is moving close to the speed of light, its
radiated energy is squeezed into the forward direction, producing
a narrow cone of emission. We call this 'relativistic beaming', and
this effect not only explains why blazars are so bright, but also explains
why in many cases, only one jet of a radio galaxy or quasar is detected.
The term "Thermal emission" is used for the natural
radiation from any material which can be characterized as having some
temperature. The sun is close to 6000 deg K and emits most of its
energy in the optical band whereas the Earth is close to 300 deg K and
radiates mostly in the far IR or millimeter wavebands. Hot gas in
clusters of galaxies is generally over a million degrees and radiates
mostly in the X-ray. See: Clusters & Galaxies.
Non-thermal Emission is a general term which describes
radiation from a body which can not be assigned any temperature. In
astrophysics, this usually arises from a power law distribution of
relativistic electrons interacting with a magnetic field and
with ambient photons. The two most common sorts of non-thermal
radiation are synchrotron emission and inverse Compton emission.
Synchrotron emission arises from relativistic electrons
spiraling in the ambient magnetic field. It was first observed on
Earth as a bluish light coming from an electron accelerator called a
Synchrotron. Synchrotron emission is responsible for most of the
celestial radio sources we observe. However,
for many jets, we observe synchrotron emission in the
optical and X-ray bands as well.
Inverse Compton emission results when a high energy electron
scatters off a lower energy photon and transfers most of its energy to
the photon, thereby producing a much higher frequency photon than the
Except for a few classes of objects (e.g. pulsars),
we believe that
most of the X-ray emission we observe is either thermal, synchrotron, or
By measuring the characteristics of jets with all available wavebands
we seek to deduce the basic properties of these enigmatic features:
how and why the jet is formed, how it propagates over such large
distances, the composition, and just how close to the speed of light
are the velocities describing the transport of power and the motion of
the emitting regions associated with the jets.
- XJET: X-ray jet catalog
- The Longjet program: 4C19.44
- Deep Chandra observation of PKS 1055+201
- The M87 monitoring project
- Ongoing work on 3c120, 3C273 and PKS 1127.
- The Centaurus A jet
Dan Harris, Ralph Kraft, Aneta Siemiginowska, Dan Schwartz, Dan Evans
There are many more who have been associated with the occasional jet paper...
Martin Elvis, Tom Aldcroft, Steve Murray, .....
The quasar 3C 273 as seen in the radio (left), optical (center) and
X-ray (right) wavebands. The bright point-like emission at the top
comes from the core of the quasar. The apparent size differs only
because of instrumental effects: the finite angular resolution of the
Very Large Array (radio), the scattered light of the Hubble Space
Telescope (optical) and the smoothing function applied to the Chandra
(X-ray) image. The spikes on the optical image are also artifacts.
The jet is difficult to detect close to the quasar, but brightens at
an angular distance of 13 arcsecs from the quasar. The tip of the jet
lies at a projected distance of about 200,000 light years from the
quasar, but the actual distance is likely to be much larger because
the jet is probably coming towards us. The color scale is mapped to
the relative brightness, with red being the brightest. Note how the
X-ray jet is brightest closer to the quasar whereas the radio jet
is brightest at the end of the jet.