Published in The Evolution of X-ray Binaries, eds. S. S. Holt & C. S. Day, AIP Proceedings No. 308, pp. 561-564 (1994).
Abstract
The Unconventional Stellar Aspect (USA) experiment to be launched in September 1995 on the Advanced Research and Global Observations Satellite (ARGOS) is a low-cost, quick --- yet scientifically ambitious --- X-ray timing experiment. It is designed for the dual purpose of scientific research in X-ray timing and time resolved spectroscopy and also for exploration of applications of X-ray sensor technology. Bright galactic X-ray binaries are used simultaneously for both scientific and applied objectives.
1. Scientific Program
The core observing program will involve viewing ~30 bright X-ray sources, predominantly X-ray binaries, for a cumulative total of about a month apiece over a three year flight. Other selected targets will be observed for shorter intervals. USA will create a unique data base characterized by the highest exposure (area x time) and photon yields of any satellite to date. The precise (~microsecond) time available on ARGOS will provide high accuracy for X-ray event times. Sub-millisecond time resolution will permit probing these sources at the timescale of processes near neutron star surfaces or the innermost stable orbit. A high priority goal is the discovery of millisecond binary X-ray pulsars that are likely sources of gravitational radiation.
States in low mass X-ray binaries. Coupled spectral and temporal states in LMXBs, with each state having a unique color--intensity relation, quasiperiodic oscillation (QPO), and low frequency noise (LFN), offer insight into the accretion mechanisms, physical properties, and geometrical configurations of these sources (Hasinger & van der Klis 1989; Lamb 1989). USA will enable detailed studies of Z-sources (e.g., Sco X-1, Cyg X-2) to be made on all three spectral branches. The large telemetry rates will mean that fast phenomena (e.g., QPOs) will always be accessible. The long exposures needed to overcome Poisson noise present in low signal-to-noise QPO and LFN signals will allow detailed tests (Norris et al. 1990) of prevailing QPO theories.
Coherent pulsations in low mass X-ray binaries. Long observations facilitate sensitive searches for the underlying spin periods of the neutron stars in LMXBs. The existence of QPOs, as well as favored models for the evolution of millisecond radio pulsars, imply that the neutron star in a LMXB has a weak magnetic field and a fast (1--10 millisecond) spin period. Accretion onto the magnetized neutron star should make the spin period detectable. A one month observation with USA of a bright LMXB, such as Sco X-1 or GX5-1, will yield sensitivity to pulse fluctuations well below current observation pulse fraction upper limits of 0.5% and well above current observational frequency limits of 500 Hz (Wood et al. 1991)
Orbital periods in low mass X-ray binaries. Long observing runs provide coverage to detect orbital periods in many LMXBs. Knowledge of binary orbit parameters also improves the sensitivity of searches for coherent millisecond pulsations. A one month observation with USA will contain 200--400 25 minute observations and will be sensitive to orbital modulations less than 0.5%.
Pulse period fluctuations in massive X-ray binaries. Binary pulsars such as Her X-1, Cen X-3, and Vel X-1 generate 500--2000 cts per pulse in the USA detectors. The resolution of individual pulses will reveal pulse-to-pulse variability in the sources. The power spectra of frequency fluctuations provide important constraints on the components of the spinning neutron star, e.g., the crust and core, and their coupling. Very small values of the period derivative will be within reach of USA during the initial three year observing program. Since essentially the entire sky is available every USA orbit, pulsars can be monitored at whatever frequency is necessary to preserve pulse count. Over three years, relative spin-up rates of 10^-8/yr are measurable.
Quasiperiodic variability in cataclysmic variables. Cataclysmic variables (CVs) exhibit a wide range of timing phenomena, including QPOs, X-ray transients, and complex light curves. While CVs are typically ~100 times fainter than LMXBs, their dynamical time scales are ~1000 times longer. USA's thin windows and soft X-ray sensitivity gives it the ability to observe more photons per dynamical time scale and may show that many of the phenomena observable in LMXBs are likewise observable in CVs. Highly correlated optical and X-ray luminosity variations are predicted (Wolff et al. 1991) and optical QPOs observed in several AM Her stars should also be accompanied by detectable X-ray QPOs.
Observational signatures of black hole candidates. Traditional observational characteristics of stellar mass black holes include spectral and luminosity states and high frequency flickering . The energy coverage of USA (1--15 keV) is good for studying both the soft and hard components of black hole candidate spectra. USA's fast time resolution capability (1 µs) and large area will allow flickering to be characterized at all relevant timescales, down to the innermost stable Keplerian orbital period, with highly significant amplitude measurements. From month-long observations of black hole candidates, USA will observe spectral state transitions and track the change in observable characteristics during these transitions.
Simultaneous Observations. Simultaneous USA and Compton GRO observations will include targets such as the Crab Pulsar, Cyg X-1, and 3C279. In Cyg X-1 simultaneous USA and OSSE observations can be used for time lag studies to study the inverse Compton scattering region. Two other instruments alongside USA onboard ARGOS are capable of observing celestial targets. These are the far ultraviolet camera GIMI (P.I. G. Carruthers, NRL) and the extreme ultraviolet camera EUVIP (P.I. S. Bowyer, U. C. Berkeley). Coordinated observations of soft X-ray sources are possible and would provide simultaneous observations over a broad ultraviolet and X-ray bandwidth. USA can provide an enhancement of the simultaneous XTE mission in several ways. When USA and XTE simultaneously observe an X-ray source, they will constitute the largest collecting area ever pointed at an X-ray source, with USA substantially incrementing the collecting area below 3 keV. When the ARGOS and XTE orbits are out of phase, USA can provide monitoring observations of an X-ray source during the time it is in Earth occultation for XTE. The onboard GPS receiver will enable USA to make observations with absolute microsecond accuracy which can be used to check the time calibration of XTE.
2. USA Instrument and ARGOS Mission Characteristics
The USA experiment will be built by the X-ray Astronomy Branch at Naval Research Laboratory and the Stanford Linear Accelerator Center, and the instrument will be integrated at NRL.
Key characteristics of USA include (i) long observing times on the brightest X-ray objects, (ii) large effective area and high time resolution capability (2000 cm^2 open area; 40 kbps telemetry, with 128 kbps available for short times; 1 µs time resolution), (iii) good low energy response (down to 1 keV), and (iv) high throughput computational capabilities onboard (Table 1).
TABLE 1: Recent and Planned X-ray Timing Missions
area telemetry typical exposure Crab
rate observation count rate
GINGA 4000 cm^2 8 kbps < 1 day 10^8 cm^2 s 10^4 cts/s **
XTE 6250 cm^2 26 kbps < 1 day 10^9 cm^2 s 10^4 cts/s *
USA 2000 cm^2 40 kbps 1 month 10^10 cm^2 s 10^4 cts/s **
* 2--30 keV count rate
** 1--30 keV count rate
USA is a reflight of Spartan-1 hardware, which was flown from the Space Shuttle in June 1985. The Spartan-1 detectors are being refurbished with the addition of a 1.2 degree field of view collimator, electronics capable of fast timing, and a two axis pointing system. The relatively large collecting area and thin front window will give USA the best low energy response (E < 2 keV) of any current or planned non-imaging X-ray mission. USA will have the telemetry rates necessary to support observations of bright sources with high count rates. The USA experiment will be mounted to a three-axis-stabilized nadir-pointed spacecraft. The X-ray detectors will be mounted on a 2-axis gimbaled platform to permit inertial pointing at celestial objects (Table 2).
TABLE 2: Operational Features of the USA Detector System
Gas: P-10 at 1.1 atmosphere (baseline)
Flow rate: low enough to allow for 3--6 year life
Window: 2.5 um Mylar
Energy range: 1--15 keV
Field of view: collimation of 1.2 degree circular
Energy resolution: 0.17 (1 keV @ 5.9 keV)
Aperture (effective): 2000 cm^2 @ 3--6 keV
Background rate in orbit: 5-sided cosmic ray veto gives residual rate of
4x10^-3 cts/cm^2/s @ 1--10 keV
Anode voltage: 2750 V
Calibration: on command Fe55 source @ 5.9 keV
Temperature limits: -10 to 50 C
Dimensions: 75x30x75 cm per detector
Mass: 40 kg per detector
Detailed design has begun, fabrication of USA subsystems will take place during early and mid 1993, and experiment integration will begin in the fall of 1993. Testing and calibration will occur in the summer of 1994 and USA will be delivered to Rockwell Industries in September 1994 for integration onto the ARGOS space vehicle. Launch will occur in September 1995 on a Delta II.
USA presents a rare opportunity to fly a scientific payload of considerable capability at a low cost to the astronomy community because it exploits the flight opportunity provided by the ARGOS mission now being prepared under the DoD Space Test Program (STP). The STP provides the space vehicle, including preflight tests and integration, launch, and data collection. NRL provides only the experimental payload. The ARGOS satellite will carry 8 experiments including three UV sensors and a GPS receiver. The orbit will be sun synchronous and circular at an altitude of 450 nautical miles and an inclination of 98.7 degrees.
3. Technological Innovation
ARGOS will be one of the first research satellites to fly a GPS (Global Positioning System) receiver, and USA will have access to the timing (1 &3181;s) and positional (< 100 m) data from the receiver. This makes possible the determination of absolute UTC time with microsecond accuracy in orbit; similarly, barycentric correction is simplified as the spacecraft position is known onboard and this information becomes part of the downlinked data stream.
USA will provide the first flight test of the RH32 and RH3000 computers. These are radiation hardened, 32 bit processors with throughputs of ~20 MIPS. Two RH32 processors, manufactured by TRW and Honeywell, will be provided by Rome Laboratories, and a Harris RH3000 processor will be provided by the Naval Center for Space Technology at NRL. They will be mounted within the USA central electronics box and used for onboard data processing. This processing includes scientific applications, such as Fourier transforms and correlations, as well as applied navigational and fault tolerant algorithms.
4. Community Access
The USA consortium has established a small Science Working Group (SWG) with community representation. The SWG will optimize the scientific potential of USA and determine scientific priorities for observing targets, subject to certain constraints. Target sources must in general be compatible with the applied objectives of USA and its DoD funding agencies. USA will have the flexibility to respond quickly to some targets of opportunity with approximately 1--3 day turnaround after the decision to revise the observing plan; the SWG will establish guidelines for responding to potential targets of opportunity. The SWG will identify instances when coordinated observations with the Compton Observatory, XTE, or other ARGOS instruments are scientifically advantageous.
This work is supported by the Office of Naval Research, the Department of Energy, and the Ballistic Missile Defense Organization.
References
Hasinger, G., & van der Klis, M. 1989, A&A 225:79
Lamb, F. K. 1989, in Proc 23rd ESLAB Symposium, ed. J. Hunt & B. Battrick, ESA SP-296 1:215
Norris, J. P., et al. 1990, ApJ 361:514
Wolff, M. T., Wood, K. S., & Imamura, J. N. 1991, ApJ 375:L31
Wood, K. S., et al. 1991, ApJ 379:295