National Aeronautics and Space Administration

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 station.

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 ~1^o. 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 +/-1.5^o or +/-3^o 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 6^ox90^o field-of-view. The angular resolution in the 6^o 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 24^o with respect to each other. The ASM gives an error box of ~0^o.2 by ~1^o 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 http://heasarc.gsfc.nasa.gov/docs/xte/.

A.3 Operation

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 Observer Facility).

A.5 Funding

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