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XTE on the Launch Pad

    blue ball Photo # KSC-95PC-1790
    blue ball Photo # KSC-95PC-1791
    blue ball 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.

    blue ball 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.

    blue ball Photo # KSC-95PC-1842
    blue ball 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

    blue ball 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.

    blue ball 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.

    blue ball 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.

    blue ball 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.

    blue ball 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.

    blue ball 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.

    blue ball 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.

    blue ball 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

    blue ball 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.).

    blue ball 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.

    blue ball 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.

    blue ball 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.

    blue ball 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.

    blue ball 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.

    blue ball 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

    blue ball 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.

    blue ball 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

    blue ball 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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