Session Abstract:

The Rossi X-ray Timing Explorer (RXTE), launched in December 1995, is a NASA astrophysics satellite designed to study the brightest X-ray sources in the sky. These range from the most luminous sources in our own Galaxy, X-ray binaries, to the active galactic nuclei and quasars that are the most luminous sources known in the Universe. The enormous X-ray luminosities observed from these systems are due to energy released within a very small volume by infall of matter through the intense gravitational field of a ``compact object''---such as a neutron star or a black hole. Fine temporal resolution (as low as ten microseconds) over the broad energy range 2-250 keV enables RXTE to probe temperatures, magnetic fields and other fundamental physical processes close to the compact objects, as well as the geometry of the systems and the nature and characteristics of the compact object itself (e.g. masses, spin periods, and internal structure of neutron stars). This session will present the highlights of new scientific results from the first year of the mission. There will also be a workstation set up with a direct link to RXTE to allow session participants to see realtime scientific data from the satellite.

Pulses, Bursts, Flares, and Dips: Understanding the Enigmatic X-ray Binaries.

X-ray binaries (XRB) are the brightest and most dynamic objects in the sky, and display a wide and perhaps bewildering variety of phenomena. XRB consist of a neutron star or black hole accreting matter from a ``normal'' close companion star, generally through an accretion disk, but this accretion is anything but a gentle process; X-ray light curves show flares, bright super-Eddington bursts, eclipses, dips and a wealth of other regular and irregular variability on timescales ranging from a few milliseconds to several years.

The Rossi X-ray Timing Explorer (RXTE) is ideal for the study of XRB: it is the only X-ray mission capable of studying the entire crucial 2-200 keV energy range with a large collecting area, high time resolution, and rapid response to changes in the X-ray sky. I will briefly summarize the capabilities of RXTE, review the properties and behavior of the X-ray binaries (concentrating mainly on the powerful Galactic bulge sources and bursters), and conclude by showing how the fast timing results of RXTE and previous X-ray missions provide the key to a unified model of X-ray binary behavior.

Ultra-fast QPOs: RXTE and the Study of Neutron Stars.

Neutron stars are among the most exotic astronomical objects known. Formed in the supernova explosion of a dying star, they contain bulk matter at the highest densities known in the universe. When present with a normal star in a binary system, mass accretion onto the neutron star can generate a luminosity in X-rays 50,000 times greater than the total luminosity of the Sun. NASA's Rossi X-ray Timing Explorer (RXTE), launched in late December, 1995, was specifically designed to study with unprecedented sensitivity the X-ray emission from neutron stars. The high density and compact nature of these objects requires measuring the X-ray emission on very short timescales. The variability of the X-ray emission from neutron stars on millisecond timescales conveys information about the physical properties of neutron stars and their environs. I will discuss recent observations of neutron star binary systems by RXTE and what they are telling us about the physics of neutron stars.

RXTE and the Most Luminous Objects in Our Universe.

Galaxies are aggregates of billions of stars, and the light we see from any given galaxy contains a contribution from each star. For some galaxies, the starlight is weak in comparison to a bright source of light coming from the nucleus of the galaxy. This time-variable extra component is distinctly different in nature and has been hypothesized to originate from matter accreting onto a massive black hole. The power emitted in this process allow us to see the most distant quasars, which are thought to harbor such objects at their cores, and to study the environment around massive black holes, as well as the black holes themselves. As with the center of the Milky Way, gas, dust, and other obscuring material limits what can be learned from optical, ultraviolet, and infrared observations. X-ray and gamma-ray astronomy, on the other hand, can penetrate this barrier to the very heart of the galactic nucleus, and give us a view of the primary processes involved. While the spectral and temporal character may vary radically from object to object, they can be explained by having different views of the black hole and the matter surrounding it. The Rossi X-ray Timing Explorer (RXTE) has observed many such active galaxies and quasars over a broad enrgy range in its first year of operation. How they support this unified picture and the questions they raise will be presented.

RXTE and Cataclysmic Variables: The 45 Day Supercycle of V1159 Ori.

Cataclysmic variables are the nearest X-ray sources. They are close binaries (separated by a solar diameter) with a late main sequence star transferring matter to a white dwarf. This material ultimately accretes onto the white dwarf, creating X-rays in the process. The short and long timescale capabilities of XTE provide unique opportunities to study the details of the accretion flow in the cases of normal white dwarfs with accretion disks and magnetic white dwarfs. As the orbital periods are only a few hours, the accretion geometry can be mapped out in a short time. Several examples of XTE results on systems of these types will be discussed, including a 40 day multiwavelength campaign on the dwarf nova V1159 Ori that encompassed XTE data combined with 3 other satellites and ground-based observations.

Realtime RXTE Science Data Display.

The RXTE Science Operations Facility (SOF) allows Guest Observers to do remote observing using any computer attached to the Internet and running the X Window System. Such a terminal will be set up here at the session to display live RXTE science data. Science planners have made a special effort to schedule exciting observations: public targets-of-opportunity and peer reviewed science, with permission of the Principal Investigator. Dramatic X-ray variability, such as bursts, dips, coherent and quasi-periodic oscillations, and other fast variability are often observed directly in the realtime data stream.

ABSTRACT FOR TOPICAL LECTURE FEB 15, 1997, SEATTLE, 1 PM

Milliseconds to Months: X-ray Timing of Neutron Stars and Black Holes.

Superman with his X-ray vision would see a sky that consists of many point-like and extended objects that emit X-rays. The sky he sees would be very different than the visible sky (at optical wavelengths); objects faint in the visible would be bright in X-rays and vice versa, and many of the point-like sources would exhibit extreme variability of intensity on time scales from milliseconds to months and years. The X-rays in most cases arise from matter that is extremely hot, of order 10 million degrees or more. Such matter, often in the form of ionized gases, is found in the deep gravitational potential wells of "compact objects", neutron stars, stellar black holes, or the massive black-holes believed to be at the active cores of some galaxies. The extreme conditions of gravity, temperature, and magnetic field near these objects lead to the extreme observed variability. Studies of the temporal variability thus probe the regions and conditions very close to these objects. Recent timing results from the Rossi X-ray Timing Explorer are providing new insight into these compact objects, their environs, their very nature, and even their life histories.