The Rossi X-ray Timing Explorer (RXTE) satellite was launched on a Delta II
rocket on December 30, 1995. The scientific instruments were developed by
science and engineering teams at the NASA Goddard Space Flight Center (GSFC),
the University of California at San Diego (UCSD), and the Massachusetts
Institute of Technology (MIT). The project is managed by GSFC and is sponsored
by the National Aeronautics and Space Administration Office of Space Science
(OSS). The observing time on RXTE is freely available to the international user
community through peer-reviewed proposals.
The primary purpose of RXTE is to study the structure and dynamics of compact
X-ray sources, including accreting neutron stars, white dwarfs, and black holes
in our galaxy and compact, massive objects thought to be present in the nuclei
of active galaxies. This is accomplished through observations of temporal and
broad-band spectral phenomena. A wide variety of physical processes are
involved. RXTE targets are often characterized by substantial X-ray emission
in the 2-250 keV energy range and vary in X-ray intensity on a wide range of
time scales. RXTE is designed to study the intensity variations of these
objects over times as short as microseconds and as long as years. The
scientific objectives of successful RXTE investigations will address questions
concerning the fundamental physics and astrophysics of such systems, including:
the nature of the central engine of active galactic nuclei.
To achieve these goals, RXTE's science payload allows the determination of
properties of compact objects (masses, moments of inertia, magnetic fields),
properties of the system dynamics (accretion onto different types of compact
objects), and properties of the emission regions. In addition, RXTE can
respond quickly (within 7-24 hours, depending upon the type of trigger) to
X-ray transients and other unpredictable time-critical events. RXTE can also
address objectives concerning several classes of noncompact objects, including
active stars, clusters of galaxies, and diffuse emission regions.
A.2 Science Payload
RXTE carries three scientific instruments: (i) the Proportional Counter Array
(PCA), (ii) the High-Energy X-ray Timing Experiment (HEXTE), and (iii) the
All-Sky Monitor (ASM). The PCA and HEXTE are co-aligned; the ASM scans the sky
over several hours. The data from the PCA and the ASM are fed into the
Experiment Data System (EDS) for preliminary data processing, whereas HEXTE has
its own dedicated processing electronics. The primary mode for utilizing RXTE
for the conduct of scientific investigations is through requests for observing
time using the PCA and HEXTE instruments. Due to its nature, observing time on
the ASM cannot be proposed for.
The PCA is a mechanically-collimated array of five xenon (Xe) proportional
counters with a total effective area of ~7000 cm2. The counters are
sensitive to X-rays in the 2-60 keV band. The collimators restrict the
detector's field-of-view to ~1 degree. Two gas volumes are present in the PCA;
the main volume contains Xe with a small quantity of methane at a pressure of
1.1 atm. A front layer of propane is used to screen out unwanted
electron-induced events. The propane layer has a low absorbing efficiency for
photons with energies above ~3 keV. Each of the five detectors carries a
241Am source that provides calibration lines. The output of the PCA
is routed through the EDS for analysis before transmission to the ground
The EDS has 8 Event Analyzers (EA's) which process the detected events
according to selectable programs. Six EA's are dedicated to handling the PCA
data while the other two EA's are dedicated to the ASM. Two of the EA's
dedicated to the PCA generate data in Standard Modes for the RXTE archive. One
EA builds energy histograms in 16-sec intervals as a standard product. A
second EA builds a time series with 0.125 sec bins for each of the PCA layers
as a standard product. The configurations of the remaining 4 EA's can be
selected by the user, with parameters of temporal and spectral resolution
appropriate to the constraints imposed by the telemetry rates. The PCA data
contains information needed to estimate the background during an observation
from a model based upon observations of off-target fields.
HEXTE is sensitive to photons in the 15 to 250 keV range. NaI(Tl) and CsI(Na)
scintillation crystals, in the phoswich configuration, optically coupled to
photomultiplier tubes, comprise the HEXTE detectors. The thickness of the NaI
crystals was chosen to give high efficiency for stopping hard X-rays while
still maintaining a low intrinsic background. The field-of-view is
mechanically collimated to be ~1o. HEXTE contains two clusters of 4
detectors with a total collecting area of 1600 cm2. The clusters
are co-aligned with the PCA in their rest position; they will usually be
commanded to rock +/-1.5o or +/-3o to provide data for
background subtraction by sampling the on-source and the off-source (i.e.,
background) fluxes. An individual 241Am source is present above
each detector for continuous automatic gain control and in-orbit calibration.
HEXTE has its own data system that processes the data prior to inserting it
into the telemetry stream.
The ASM consists of three scanning shadow cameras (SSC's) on a rotating
assembly. Each SSC is a one-dimensional position-sensitive proportional
counter with a coded mask over a collimator with a 6ox90o
field-of-view. The angular resolution in the 6o direction is 10'.
Two SSC's view the sky perpendicular to the ASM rotation axis; the third SSC
views the direction parallel to the rotation axis. The two perpendicular SSC's
are canted by 24o with respect to each other. The ASM gives an
error box of ~0o.2 by ~1o for weak sources and ~3' by
~15' for strong sources. About 70% of the sky is covered during the course of
an orbit. The remainder of the sky can be covered within a day by spacecraft
maneuvers. The SSC detectors are proportional counters with Xe-CO2 at 1.2 atm,
sensitive in the 2-10 keV band.
Updates on the current status of the RXTE science payload can be found on the
RXTE Guest Observer Facility homepage, at
RXTE was launched into a low-earth orbit on a Delta II launch vehicle
on December 30, 1995. The orbit is circular, with a period of 96
minutes, an original altitude of 580 km, and an inclination of 23 degrees.
The expected orbital lifetime of RXTE is 10 years.
Communications with the spacecraft occur through the Tracking and Data Relay
Satellite System (TDRSS). RXTE has a low-gain antenna system for emergency
operations and a high-gain antenna system for normal operations. Multiple
access (MA) and single access (SA) TDRSS command and telemetry links are
supported. An approximately 10-minute command period is scheduled each orbit.
The data used in monitoring observations and in determining the presence of
targets of opportunity (TOO) are delivered each orbit through a TDRSS contact
to the ground via the MA link. Telemetry comes to GSFC via the White Sands
Ground Terminal (WSGT). The telemetry stream goes to the Mission Operations
Center, to the Data Capture Facility, and to the Science Operations Center
(SOC). The SOC is responsible for generating the science timeline which
incorporates user-imposed constraints as well as the instrument- or
satellite-imposed constraints. Also, in cases where it will enhance the
scientific return from an investigation, real-time monitoring of the progress
of an observation is possible at the SOC.
A.4 Proposal Policy Summary
After the 30-day In Orbit Check-out (IOC) phase, observing time on RXTE was
opened to observations selected from competitive proposals submitted
in response to the Cycle 1 NRA. The current Cycle 7 Announcement covers
observations for the 12 months
beginning approximately March 1, 2002. Note that 100% of the observing
time is available to the scientific community; i.e., there is no guaranteed
time for the instrument teams. Investigations to be carried out with RXTE are
selected by a competitive peer review process directed by the cognizant
discipline scientist in the Office of Space Science, NASA Headquarters.
Prospective investigators may propose for observing
time using either or both the PCA and HEXTE instruments. Both instruments will
view the target field and obtain data with independent telemetry allocations;
the use of one instrument does not preclude use of the other to view the same
source. For operations purposes, information is required about the expected
count rate and telemetry rate of both instruments. Successful proposers will
receive the data from both instruments in a form suitable for analysis.
It is anticipated that programs
involving a range of observing times will be recommended for implementation
(see C.3.1.2), with a continuing trend towards longer observation times.
Results of the routine quick-look and more comprehensive analyses of the ASM
data are determined by the RXTE ASM team and placed in a public archive as soon
as possible after the data are obtained. All ASM data is nonproprietary.
Proposals for specific configurations or observing sequences using the ASM will
not be accepted.
All proposals submitted in response to this Announcement must be for
new pointed observations using RXTE. Proposals for analysis of ASM
data, proposals for analysis of archival RXTE data, proposals for
complementary observations or theoretical work that do not require new
pointed observations, and any other proposal that does not require
new pointed RXTE observations will not be accepted.
For more details about the science payload and operations plan, please see
Appendix C and the RXTE Technical Description (available from the RXTE Guest
No funding will be provided directly through this Announcement, which
is purely for the awarding of RXTE observing time and the subsequent
acquisition of data. Observers with proposals selected through this
Announcement may submit a proposal for funding as part of the NASA
Astrophysics Data Program (ADP).
A.6 The RXTE Science Operations Center
The RXTE Science Operations Center (SOC) consists of two entities: the Science
Operations Facility (SOF), responsible for the planning and conduct of RXTE
observations, and the Guest Observer Facility (GOF), which supports the
proposal process and post-observation data analysis. All raw telemetry,
containing both instrument and spacecraft data, are sent to the SOC, which is
located at the NASA Goddard Space Flight Center in Greenbelt, Maryland. After
conversion to FITS format, the data are distributed electronically
to the Principal Investigators of each specific observation.
The GOF supports observers with their analysis of these data. The GOF is part
of the Office of Guest Investigator Programs at the Laboratory for High Energy
Astrophysics, NASA Goddard Space Flight Center. The GOF provides technical
information, if needed, for the preparation of proposals, validates and
distributes calibration data, produces software for data analysis, provides
expert help and documentation, and assists in the creation of the RXTE
archive, which is maintained by the High Energy Astrophysics Science
Archive Research Center (HEASARC) at NASA/GSFC.
Announcement for the RXTE Cycle
6 GO Program
Appendix A: RXTE Guest Observer Program Mission Description
Appendix B: (not required for this Announcement.)
Appendix C: Information for Proposers Regarding
Proposal Submission, Evaluation, Selection, and Implementation
Appendix D: Filling out RXTE Proposal Forms