Microquasar XTE J1550-564

Science@NASA





Although XTE J1550-564
is not visible to the naked
eye for observers in the
Southern Hemisphere, it
can become very bright
in x-rays (more).

The binary is composed of a highly evolved star and a black hole in close proximity so that the star's gases overflow its Roche lobe and form an accretion disk around the compact companion. Although XTE J1550-564 is normally almost invisible at x-ray wavelengths, in 1998 it became 1.5 times brighter than the Crab Nebula, surpassing the Crab as the brightest hard x-ray source in Earth's night sky (more). XTE J1550-564 was discovered on September 7, 1998 as the brightest x-ray nova then observed by the Rossi X-Ray Timing Explorer (RXTE) and All-Sky Monitor (ASM) teams at MIT and NASA's Goddard Space Flight Center (Remillard et al, 1999; and Smith et al, 1998).

CTIO, ROE/SERC J Survey
(Used with permission)

Larger image.

XTE J1550-564 has
optically brightened
by a factor of 25
from a previous
quiescent state (see
XTE J1550-564 and
Jain el al, 1999).

As an x-ray nova, XTE J1550-564 sometimes erupts to appear exceptionally bright in x-rays despite its great distance from Sol. Such x-ray novae have been found to be binary systems whose luminous x-ray and optical outbursts can last for as long as a month or more. They serve as compelling evidence for the existence of stellar-mass black holes because the compact companions in these binaries are too massive (exceeding three Solar-masses according to radial-velocity studies) to be stable neutron stars.

A. Hobart, CXC


Larger animation still.



Gases in XTE J1550-564's
primary star overflow its
Roche lobe to form an
accretion disk around the
compact companion (see
animations and more images).

Bright x-ray sources (and their optical emissions) within the Milky Way such as XTE J1550-564 are powered by accretion disks of material swirling into orbit around compact central objects that can be a white dwarf star, neutron star, or black hole in the case of XTE J1550-564. Such disks form in close binary systems that consist of a donor star, supplying the accreting material, and a compact object whose intense gravity eventually draws the material towards its surface. According to astronomer Jerome Orosz, accretion disks can be quite large, and in binary systems such as GRS 1915+105 they are almost certainly several times larger than Sol, since if the companion star fills its Roche lobe, then everything scales to the orbital period (where the longer-period systems have the larger accretion disks). Friction slows the orbital motion of the accreting matter, which causes it to spiral through the disk down to the compact object. The falling and spiraling of the matter toward the compact object releases large amounts of gravitational energy and heats the accretion disk. Three-dimensional simulations indicate that spiral shock waves can form in accretion disks, but they also suggest that the disks develop instabilities that tend to smear the shocks (more discussion and images and animations of spiral shocks and of a binary system undergoing mass transfer while rotating around a common center of mass).

Michael P. Owen, John M. Blondin,
North Carolina State University
(Used with permission)


Larger illustration.


Simulations of accretion
disks in three dimensions
suggest that spiral shock
waves can develop but
that disk instabilities
tend to smear them out
(more discussion and
images and an animation).

XTE J1550-564 belong to an exceptional class of x-ray novae that eject plasma through bi-polar jets at relativistic velocities called microquasars. Because the spin of XTE J1550-564's progenitor star was concentrated into an extremely compact black hole during the collapse of the star's core (and rebound into a supernova), the hole is spinning very rapidly. The hole also causes the gaseous matter in the accretion disk to spin and heat through friction to temperatures of millions of degrees, and to generate an intense magnetic field (more discussion of black hole electromagnetism). As the gaseous material fall onto the blackhole's accretion disk and corona, intense electromagnetic forces can expel a large portion of it as jets of high-energy particles, but astrophysicists do not as yet agree on the exact mechanism (see discussion on the possible role of a "warm" x-ray plasma layer in producing bi-polar jets from a black hole).

CXC, NASA

Larger image.

Travelling at relativistic
velocities, polar jets from
the black hole, at center,
are slowed by interstellar
gas after travelling two
light-years over four
years (more from CXC
and Corbel el al, 2002).

XTE J1550-564's x-ray jets require a continuous source of trillion-volt electrons to remain bright. They have been observed to be moving at about half the speed of light. As XTE J1550-564's jets moves through interstellar gas, however, the gas slows down the jets like air resistance slows down moving objects on Earth. Although all jets (including those of quasars, neutron stars, and young stars with Herbig-Haro objects) are believed to decelerate in this manner, the observations of XTE J1550-564 mark the first time such jets have been observed to be slowing down. The broadband spectrum of the such jets is consistent with synchrotron emission from high-energy (up to 10 tera-electron volts) particles that were accelerated in the shock waves formed within the relativistic ejecta or by the interaction of the jets with the interstellar medium (some recent references: Corbel el al, 2002; Belloni et al, 2002; Orosz et al, 2002; Wu et al, 2002; Corbel et al, 2001; Jain et al, 2001; Hannikainen et al, 2001; and Sobczak et al, 2000). As a stellar-mass object, however, the XTE J1550-564 microquasar is a few million times smaller than the "quasars" formed by similar relativistic ejections of matter accreted into the central, supermassive black holes of many distant, active galaxies (more discussion and illustrations are available at Variable Star News and Alerts, Optical Spectroscopy on Microquasar GRO J1655-40, and Wu et al, 2002). Another interesting microquasar is GRO J1655-40.

The Black Hole

A. Hobart, CXC,
SAO, NASA


Larger illustration.


While black holes are invisible,
they may have detectable accretion
disks and bi-polar jets (more at
Astronomy Picture of the Day and
Chandra X-Ray Observatory).

XTE J1550-564's rapidly spinning, compact object has an estimated mass of around 10.6 Solar, which is well above the maximum mass of a stable neutron star and so it appears to be a black hole (Orosz et al, 2002). Indeed, another study estimated a mass between 12 and 15 Solar (Titarchuk and Schrader, 2002). The hole appears to be separated from its companion star by only around 12 to 13 Solar radii (Orosz et al, 2002).

© Jerome A. Orosz
(Used with permission)




Larger illustration.




Comparison of relative sizes
and orbital separations of
black holes and their binary
companions (more at Jerome
Orosz's homepage).

The progenitor star of the black hole was once a supermassive star that evolved into a supergiant. Subsequently, most of its substance was expelled during a supernova explosion. However, the star was so massive that when its core accumulated so much iron "ash" that fusion shut down, the loss of radiative pressure allowed the mass of the star to crush around 11 Solar-masses of its core into a rapidly spinning black hole.

NASA Observatorium

XTE J1550-564's progenitor
star was once a supergiant
with an interior of onion-like
layers fusing ever heavier
elements inwards, until an
inert iron core developed.

See a discussion of the
"Burning of Elements
Heavier than Helium" and
"Supernova Explosions"
as part of stellar
evolution and death.

Because the spin of a stellar object was concentrated into a city-sized object (e.g., maybe 10 miles wide) during the collapse of the progenitor star's core, the black hole created by a supernova is spinning very rapidly. Human ice skaters obtain the same effect while twirling by pulling their arms to spin faster. The rapid spin of the black hole causes its accretion disk to spin and so creates a very strong magnetic field, so that the disk acts like a gigantic rotating magnet that generates many quadrillion volts of electricity (more discussion of black hole electromagnetism).

The Stellar Companion

The star that is donating its gases to the black hole's accretion disk is probably a highly evolved, subgiant or giant of spectral and luminosity type G8 IV to K4 III (Orosz et al, 2002). An earlier study suggested that it may be a low-mass K0-K5 main-sequence dwarf star (Sánchez-Fernández et al, 1999).