RXTE Image Gallery
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XTE Spacecraft and Team
XTE on the Launch Pad
Photo # KSC-95PC-1790
Photo # KSC-95PC-1791
Photo # KSC-95PC-1792
Credit: NASA.
At Launch Complex 17, Pad A, on Cape Canaveral Air Station a Radio
Frequency Interference (RFI) test of the Delta 230 expendable launch vehicle
is under way. During the prelaunch test, the service tower at the pad is
rolled back and all Radio Frequency (RF) sources that support final
countdown -- such as range tracking and launch control -- are radiated
toward the vehicle to insure that no interference to the signals exists.
Delta 230 is a Delta II 7920-configuration vehicle built by McDonnell
Douglas Aerospace. Encapsulated atop the vehicle is the X-ray Timing
Explorer (XTE), which the Delta will loft into a low-Earth orbit to begin
a two-year mission to gather data about X-ray emitting objects in the
Milky Way galaxy and beyond.
Photo # KSC-95PC-1797
Credit: NASA.
The mobile service tower has been rolled back at Launch Complex 17,
Pad A, to prepare for launch of NASA's X-ray Timing Explorer (XTE)
spacecraft atop the Delta 230 expendable launch vehicle. Once lofted into
orbit, the XTE spacecraft will embark on an approximately two-year mission
to carry out an
in-depth study of X-ray sources in the universe. The Delta 230 is built by
McDonnell Douglas Aerospace and is a Delta II 7920-configuration vehicle.
It features a new advanced technology avionics system that offers improved
reliability at reduced cost. The XTE vehicle and launch services are being
provided by McDonnell Douglas under a Medium Expendable Launch Vehicle
contract with NASA.
Photo # KSC-95PC-1842
Photo # KSC-95PC-1841
Credit: NASA.
A Delta II launch vehicle carrying the X-ray Timing Explorer (XTE)
lights up the sky at 8:48 a.m. EST, December 30, 1995. Liftoff occurred from
Launch Complex 17, Pad A, on Cape Canaveral Air Station under the management
of a combined government/contractor launch team that includes NASA, the Air
Force and McDonnell Douglas. The XTE spacecraft is outfitted with three
scientific instruments that will study X-rays, including their origin and
emission mechanisms, and the physical conditions and evolution of X-ray
sources within the Milky Way galaxy and beyond. The XTE is one is a series
of Explorer missions planned by NASA; it will perform its observations from
a vantage point in low Earth orbit for a mission duration expected to
last two to five years. The Delta II 7920 expendable launch vehicle
carrying the XTE spacecraft into orbit is provided by McDonnell Douglas.
Delta 230, as this vehicle was designated, is the first in the Delta family
to fly outfitted with a new advanced avionics system.
XTE Target Objects
The Large Magellanic Cloud, shown here in visible light, is a
satellite galaxy of the Milky Way, about 160,000 light years distant. It
contains about 10 million solar masses of stars, gas, and dust. Several X-ray
binary star systems residing therein are highly luminous in X-rays. Two or
more of these sources may contain black holes. (North is up).
Credit: David Malin; taken with the UK Schmidt telescope.
© Royal Edinburgh and Anglo-Australian Telescope Board.
Supernova 1987a shortly after its outburst (left) completely
outshines its neighboring stars in the Large Magellanic Cloud. In a
pre-outburst photograph, the progenitor star which exploded is seen (arrow).
A rapidly spinning neutron star may be embedded in the debris of the
explosion. It could be seen first in X-rays as the debris expands and thins
out. (North is up)
Credit: David Malin; taken with the 3.9-m Anglo-Australian telescope.
© Anglo-Australian Telescope Board.
The size of a white dwarf is comparable to the Earth, but its mass
is up to 500,000 times greater. A neutron star has about the same mass as
the white dwarf, but it is extremely compact, only about 1/600 the size of
the Earth.
Credit: C. Jones, C. Stern, & W. Forman.
© Smithsonian Institution Astrophysical Observatory.
The Orion Nebula (In Orion's sword), shown here in visible light,
is the site of star formation and many young stars, some with intense
coronal activity visible in medium-energy X-rays. (North is left)
Credit: David Malin; taken with the UK Schmidt telescope.
© Royal Edinburgh and Anglo-Australian Telescope Board.
This view of the nearby galaxy Centaurus A in visible light shows
its elliptical distribution of starlight and a broad, dark dust lane. It is
roughly 15 million light years distant and has an active nucleus which
exhibits jets of optical, radio, and X-ray emission (see the next image).
(North is up)
Credit: David Malin; taken with the 3.9-m Anglo-Australian telescope.
© Anglo-Australian Telescope Board.
The X-ray jet emanating from the center of Centaurus A illustrates
the active nature of the nucleus (bright spot; lower right). It extends
about ~20,000 light years from the nucleus. The colors represent X-ray
intensity. (North is up)
Credit: C. Jones, C. Stern, & W. Forman; taken with the
Einstein X-ray Observatory
(HEAO 2); adapted from Feigelson et al, Ap. J., 251, 31.
© Smithsonian Institution Astrophysical Observatory.
The X-ray jet emanating from the center of Centaurus A illustrates
the active nature of the nucleus (bright spot; lower right). It extends
about ~20,000 light years from the nucleus. The colors represent X-ray
intensity. (North is up)
Credit: C. Jones, C. Stern, & W. Forman; taken with the
Einstein X-ray Observatory
(HEAO 2); adapted from Feigelson et al, Ap. J., 251, 31.
© Smithsonian Institution Astrophysical Observatory.
Close-up view of an accreting neutron star or white dwarf. The accreting
gas can be guided to the magnetic pole of the compact object where it creates a
hot spot that rotates into and out of sight as the compact star rotates. The
radiation from the spot can depart only in certain direction because of the
in-falling matter and strong magnetic fields. The results is an X-ray
"pulsar". Sketch courtesy of H. Bradt, M. Halverson, and students.
XTE Timing Studies
The spin rate of the Vela X-1 pulsar over a period of 6000 days is
seen to be quite irregular. First it is seen to be spinning up (the period
shortens) and later it irregularly slows down. The changes of period are
due to the torques exerted on the neutron star by the gas that accretes onto
it. Studies of the fluctuations of the spin period give a measure of the
degree to which the crust of the neutron star is coupled to its interior.
The data shown is a compilation from many satellites (Nagase et al 1989,
PASJ, 41, 1.).
The sudden commencement of accretion of gas from a low-mass star onto a
companion compact objects leads to a huge outpouring of relatively soft
X-ray emission, i.e. an X-ray nova.
This X-ray light curve from the
Ginga satellite show such an event. The flux can exceed that of the
brightest persistent X-ray sources. The study of the early phases of these
events with XTE will probe the cause of the sudden accretion. Optical
identifications should lead to new candidate black holes.
Be stars are luminous stars that occasionally eject clouds of plasma.
A neutron star in wide orbit about the
Be star will sometimes encounter the plasma and accrete some of it. The
accretion leads to transient X-ray emission which can be used to diagnose the
geometry and variability of the plasma ejections.
As the accretion rate decreased after flare maximum,
EXOSAT observers found that the rate of
the spin change in source EXO 2030+375 decreased systematically. (The
luminosity is a measure of the accretion rate.) This is as expected if the
torque applied to the neutron star is due to the accreting matter. It is
possible that the matter actually started slowing the spin rate (spindown) at
the end of the observations.
Rapid aperiodic pulsing
(quasi-periodic oscillations; QPO) in low-mass X-ray binaries is
probably due to interactions between the circulating matter in the accretion
disk and the magnetosphere of the neutron star. These pulsations are
directly seen with the Ginga
satellite during a burst of the "Rapid Burster" source. The
mechanism that gives rise to QPO pulsations is not yet known. They may be
symptomatic of a heretofore undetected pulsar spinning almost 1000 times a
second.
The character (intensity and frequency) of the oscillations of QPO
sources appears to depend on where the
sources is located on a two-color plot, as we see here in
EXOSAT data for Cyg
X-2. At any instant, the X-ray spectrum can be represented as a
point on this plot. Cyg X-2 is called a 'Z' source because of the shape of
the plot. The position at a given time is probably a measure of the
accretion rate. Such systematic behavior leads to the hope that the QPO
phenomenon will prove to be a powerful probe of the conditions in the outer
magnetosphere.
X-ray bursts are due to the explosive thermonuclear burning of gas on the
surface of neutron stars.
This is the discovery event found with the
Astronomische Nederlandse
Satelliet (ANS) satellite. The evolving spectrum of these burst
indicates that they come from an object about the size of a neutron star.
Spectral Studies with XTE
Spectral features found at relatively high X-ray energies are believed to
be due to electrons spiraling in the very high magnetic field of neutron
stars. The absorption feature in the spectra
of the X-ray pulsar 1538-52 observed by
Ginga is seen to vary
with the (numbered) phase of the pulsing. This cyclotron radiation is
a direct diagnostic of the hot plasmas and strong magnetic fields in the polar
regions of the neutron star.
A diffuse background of
X-rays is known to emanate from the whole sky. The origin of this is not
known although it is clear that at least some of it comes from many distant
active galactic nuclei (AGNs) which are known to emit X-rays. The possible
power-law spectra from AGN (straight lines) is generally flatter than the
background (data points and bars). If the AGN spectrum continues with the
same power law to energies above ~100 keV, the AGN emission would exceed the
background! It thus becomes imperative to measure the spectra of AGN at
energies beyond ~30 keV.
Multifrequency Studies with XTE
The cross correlation of the
X-ray and optical fluxes of BY Cam show the rapid flickering in both
bands to be highly correlated on ~1 minute time scales. BY Cam
(= H0538+608) is a magnetic white-dwarf cataclysmic-variable system (AM Her
type). The radiation may originate in a shock front just above the
white-dwarf surface. This detection required the large aperture of the
Ginga satellite and
simultaneous optical and X-ray observations.
Credit: Adapted from Silber et al 1992,
ApJ, 389, 704.
High Energy X-ray Timing Experiment (HEXTE)
Proportional Counter Array (PCA)
PCA diagrams are courtesy of Hale Bradt. M. Halverson, and
students.
The All Sky Monitor (ASM)
ASM diagrams are courtesy of Hale Bradt. M. Halverson, and
students.
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