| re .0
Some info from a mate of mine who works on AXAF but has contacts in the
XTE team. A brochure with more info is on its way to me so stay tuned...
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Each photon is time tagged when it arrives at the detector. Many
astronomical phenomenon vary with time, e.g., pulsars, flickering from
compact objects acreting matter (e.g., white dwarf in a binary system
with a star), the central engine of quasars, highly variable active
galaxies called BL Lacertae objects. There are also transient events such as
Super Novae, X-ray bursters and large scale departures from some object's
quiescent states.
XTE will monitor these kinds of events in X-rays with a number of
instruments (details of which I do not have in my head) optimized for
particular kinds of detections, e.g. very bright very fast variations
require a different instrument than fainter but slower variation
(there is a trade off of course between sensitivity, time resolution and
position resolution). There will be lots of simultaneous observations
at other wavelengths with ground and space based observatories, mostly
of sources that are known to vary and for which large scale observing
programs are deemed potentially useful.
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| I now have a 40+ page glossy on XTE. Here's a selection of the
material. I wouldn't claim to understand it all...
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Foreword
========
This document describes the X-ray Timing Explorer (XTE), a mission sponsored by
the Office of Space Science and Applications of NASA and managed by NASA's
Goddard Space Flight Centre. This X-ray astronomy observatory is scheduled to
be launched in 1996 on a Delta II rocket. The primary objective of the mission
is the study of temporal and broad-band spectral phenomena associated with
stellar and galactic systems containing compact objects. These systems include
white dwarfs, neutron stars, and possibly black holes, and they involve a
variety of physical processes. The scientific instruments span the energy range
2-200 keV, and time scales from microseconds to years can be studied. The
instruments are provided by science and engineering teams at Goddard Space
Flight Centre, the University of California at San Diego, and the Massachusetts
Institute of Technology. All of the observing time will be available to the
international community through the peer-review process. The capabilities of
the instruments and spacecraft described herein reflect current designs. This
brochure was prepared by the XTE Science Working Group for distribution at the
January 1992 meeting of the American Astronomical Society and for distribution
to interested members of the general public.
Astronomy with X-rays
=====================
X-rays are one of the several types of radiation that comprise the electro-
magnetic spectrum (Fig. 5). They are characterised by a short wavelength and a
high photon energy of 0.2 to about 200 keV. These photons can be emitted by
gases of very high temperature (~10 million K) or by very energetic non-thermal
particles. The high penetrating power of X-rays allows them to escape from the
hot gaseous environs of an object and to travel through the diffuse gases of
interstellar space so that astronomers on earth may observe them directly.
X-rays are an ideal probe of the innermost regions near compact objects.
Studies of the character of X-ray sources by past space missions have led to a
broad understanding of the emitting systems. Temporal and broad-band spectral
studies in X-ray astronomy in the medium to hard X-ray energy band (1-200 keV)
have developed through a series of satellites including the U.S. HEAO-1, the
European EXOSAT, the Japanese Ginga, and the Soviet/European Granat missions.
Imaging and high-resolution spectral studies at lower X-ray energies (0.1-4 keV)
have been carried out with the U.S. Einstein and the German ROSAT missions.
Each of these missions has made vital new contributions to our knowledge of the
emitting systems.
Overview of the Instruments
===========================
XTE will carry out its studies with the 3 instruments listed in Table 1.
Instrument Detector Area Band Field Time Telem.
width of view Res. Rate
---------------------- -------- ---- ------ ------- ---- --------
(cm�) (keV) (kb/s)
PCA Proportional 5 Xe Prop. 6250 2-60 1�*1� 1 �s 18;
Counter Array Counters 256
HEXTE High-Energy X- NaI/CsI 1600 20-200 " 10 �s 5
ray Timing (2 clusters) (rocking)
Experiment
ASM All-Sky 1-dim PSPC 90 2-10 0.2*1� 1.5h 3
Monitor
[DG: PSPC = position sensitive proportional counter?]
The PCA is supported by a powerful microprocessor-driven flight data system
with multiple analysis channels capable of processing high rates (up to about
500000 X-rays per second) with a minimum loss of information.
Together the PCA and HEXTE are a single powerful "telescope". The large areas
and low backgrounds provide high sensitivity to weak sources. They can view a
single source in their common 1 degree field of view (FOV). The third
instrument is an All Sky Monitor that scans most of the sky every 1.5 hours in
order to monitor the intensities and spectra of the brightest approximately 75
sources in the sky. It will explore the long-term behaviour of these sources
and will provide timely information on any large changes of intensity and
spectral shape. This allows the powerful main instruments (PCA and HEXTE) to be
pointed rapidly (within a few hours) at the subject for studies with great
sensitivity.
[DG: Table 2 gives expected counts from various sources and expected background
count. I have omitted some sources.]
2-10 keV 10-30 keV >30 keV
PCA
Background(1s) 20 24 16
AGN(10s) 113 42 4
Crab Nebula(1s) 8700 1205 80
Cyg X-1(1ms flare) 23 9 1
Sco X-1(1s) 160000 4600 4
[DG: AGN = active galactic nuclei]
HEXTE
Background(1s) - 6 29
AGN(10s) - 9 10
Crab Nebula(1s) - 170 130
Cyg X-1(1ms flare) - 1 3
Sco X-1(1s) - 1670 40
ASM
Background(1s) 40
Crab Nebula(1s) 90
Sco X-1(1s) 1400
Proportional Counter Array
==========================
The PCA instrument consists of 5 large proportional counters with anti-
coincidence features which provide a very low background. A mechanical
hexagonal collimator provides 1 degree (FWHM) collimation. Sources as faint as
1/1000 of the Crab Nebula can be detected in a few seconds.
The PCA consists of 5 large detectors with total net area of 6250 cm�. Each
detector is a version (50% larger) of the HEAO-1 A2 HED sealed detector. They
are filled with Xenon gas and achieve low background through effective anti-
coincidence schemes including side and rear chambers and a propane top layer.
The Xenon of the 3 signal detection layers is 3.6 cm thick at 1.0 atmosphere.
Methane is used as a quench gas. The front window and a window separating the
propane and the xenon/methane chambers are both aluminised mylar of thickness
25 �m. The propane layer may also be used as a signal layer in the energy range
1-3 keV.
The PCA is effective over the range 2-60 keV with 18% energy resolution at 6
keV and 255-channel pulse-height discrimination. The gain of the counter is
monitored continuously with an americium radioactive source for which detection
of the alpha particle identifies the calibration X-rays.
The PCA electronics provide digital pulse-height data to the flight experiment
data system (EDS) for binning and on board analyses. The PCA is being provided
by NASA's Goddard Space Flight Centre.
[DG: Diagram shows dimensions of the PCA as 1.8m wide, 1.4m deep and 0.8m high
(including sunshade).]
Experiment Data System
======================
The EDS serves to pre-analyse and compress the PCA X-ray data prior to its
transmission to the ground via the relatively limited telemetry bandwidth. The
EDS bins and analyses data according to flexible criteria that can be defined
for each observation. Its multiple Event Analysers can process the data from a
given source simultaneously in a variety of modes. It also controls the ASM
rotation and processes ASM data.
The system will process count rates from the PCA up to 500000 counts per second
and will be able to time photon arrivals to approximately 1 �s.
The PCA data stream can be binned and telemetered in 6 different modes
simultaneously by 6 independent Event Analysers (EA) which operate in parallel,
each analysing the total PCA data stream.
Each EA includes a Digital Signal Processor (DSP) chip that will rapidly bin
the individual events according to highly flexible criteria (e.g., non uniform
pulse height bin widths and arbitrarily chosen timing bin widths) which may be
specified for each observation. The DSP is used in conjunction with a table-
lookup scheme to provide the required speed of classification for each event.
An additional microprocessor in each EA serves as the EA manager.
The EAs will create data packets for transfer to the spacecraft memory from
which they will be transmitted via the telemetry stream to the ground at a
time average rate of 21 kb/s or at 256 kb/s for approximately 30 minutes a day.
The EDS is being provided by the Massachusetts Institute of Technology.
ASM PCA
+---------+ +-------------------+
| S S S | | P P P P P |
| S S S | | C C C C C |
| C C C | | U U U U U |
| | | | | | | | | | | |
+-+--+--+-+ +-+---+---+---+---+-+
| | | | | | | |
V V V V V V V V
--------- ---------------------------------
| | | | | | | | |
E E E E E E E E E (spare)
A A A A A A A A A
| | | | | | | | |
V V V V V V V V V
+--------------------------------------------------------+
| Spacecraft Memory |
| 1 Gigabit |
+--------------------------------------------------------+
High-Energy X-ray Timing Experiment
===================================
The HEXTE features a large area and low background with a 1 degree field of
view coaligned with the PCA field of view. Eight "phoswich" detectors are
arranged in two clusters, each of which rocks on and off the source. This and
automatic gain control for each of the 8 detectors together yield a well
determined background which permits spectral measurement of a faint source
(1/1000 of the Crab nebula) at 100 keV in about 1 day.
The HEXTE experiment consists of two rocking clusters of NaI/CsI phoswich
detectors that cover the energy range 20-200 keV. The detectors are improved
versions of the HEAO-1 A4 LED detectors which attained the lowest in-orbit
background for large-area scintillators to date. Each detector consists of a
3-mm thick NaI primary detector coupled to a 38-mm thick CsI anti-coincidence
crystal that also serves as a light guide to the photomultiplier tube. Each
detector has 200 cm� net effective area. Each cluster contains 4 detectors; the
total net area of the entire system is 1600 cm�. The field of view is 1� FWHM
and is coaligned with the PCA when on source.
Each cluster will be rotated ("rocked") on or off the source every
approximately 15 s to provide alternate source and background measurements.
Each cluster will sample background positions on two opposing sides of the
source, and the two clusters will rock in mutually perpendicular directions by
either �1.5� or �3.0�. Thus 4 background positions will be monitored. The
rocking will be phased so that the source is continuously viewed by at least
one of the clusters.
The HEXTE flight data system will provide the following modes: binned, event
encoded, pulsar fold, burst trigger, and an optimum high-speed code as well as
a standard output mode. The telemetry rate for HEXTE will be approximately
5 kb/s.
The instrument is being provided by the University of California at San Diego.
All Sky Monitor
===============
The ASM is the watchdog that alerts XTE to flares and changes of state in X-ray
sources. It consists of three rotating Scanning Shadow Cameras (SSC) that can
scan about 80% of the sky in 1.5 hours. The cameras provide measurements of
intensities of about 75 known celestial sources in a day and can measure the
position of a previously unknown source with a precision of about 3 minutes of
arc.
The All-Sky Monitor consists of 3 Scanning Shadow Cameras (SSC) on one rotating
boom with a total net effective area of 90 cm�. Each SSC is a one-dimensional
'Dicke camera' consisting of a 1-dimensional mask and a 1-dimensional position-
sensitive proportional counter. The gross field of view of a single SSC is 6� *
90� FWHM, and the angular resolution in the narrow (imaging) direction is 0.2�.
Two of the units view perpendicular to the rotation axis in nearly the same
direction except that the two detectors are each rotated by �12� about the view
direction so that they serve as 'crossed slat collimators'. The third SSC unit
views along the axis of rotation. It serves in part as a 'rotation modulation
collimator' and surveys one of the 2 poles not scanned by the other two
cameras.
X
|
v
+---+ +---+ +---+
/ / | | \ \
/ / | | \ \
/ ^ / | | \ ^ \90�
/ / /| +---+ |\ \ \
/ X / | | | \ X \
/ / | | | \ \
+---+12�| | | +---+
+--------------+---------------+ 6�
|
/ | ^ rotation
| | | axis
\_/
|
Each SSC detector is a sealed proportional counter filled to 1.2 atm with
xenon-CO2, and has a sensitive depth of 13 mm. It has 8 position-sensitive
anodes, a 50-�m beryllium window, a sensitive area of 60 cm� of which only 1/2
can view a given celestial position through the mask at a given time, anti-
coincidence chambers on the sides and rear, and sensitivity to 2-10 keV X-rays
with three energy channels.
A motorised drive will rotate the three SSCs from field to field in 6� steps.
At each resting position a 100 second exposure of the X-ray sky will be made; a
complete rotation is thus completed in approximately 100 minutes. Frequent
spacecraft man�uvres will make it likely that 100% of the sky is surveyed each
day. The drive can be commanded to stop for an extended observation of a given
source, e.g. to obtain a precise position of a nova. The drive has a total
rotation angle of approximately 500�; it can be stepped or moved rapidly in
either direction.
Spacecraft
==========
The XTE will be carried into a low earth orbit on a Delta II rocket scheduled
for 1996 launch. A new spacecraft design has been developed to make possible
flexible operations through rapid pointing, high data rates and nearly
continuous receipt of data at the Science Operations Centre via a Multiple
Access channel of TDRSS.
The XTE instruments and the service hardware (reaction wheels, star trackers,
transmitters, etc.) are all integrated into a common spacecraft structure. The
XTE is highly man�uvrable (>6�/minute), and the PCA/HEXTE field of view can be
pointed to any position on the sky on any day of the year provided the angle to
the sun is >30�. The pointing accuracy is <0.1�. Rotatable solar panels make
possible anti-sun pointing so that coordinated observations with optical
telescopes can be carried on throughout the ground based night.
The command and data links will be through the NASA TDRSS communication
satellites. Both the multiple access and single access channels of TDRSS will
be used to provide nearly continuous telemetry of data. At least one command
link per orbit will be available. The time-averaged data rate will be about
32 kb/s of which 26 kb/s will be available for the scientific instruments. In
addition, 256 kb/s for about 30 min. a day will be available.
XTE will be in a low earth orbit of altitude approximately 600 km and 23�
inclination. XTE should remain operational for 2 to 5 years. The checkout
period will be 30 days.
Unexplored Measurement Phase Space
==================================
XTE is designed to make possible or facilitate studies of important domains of
science not easily accessible to previous missions, specifically sub-millisecond
timing, the study of early phases of X-ray nov�, continuous sensitive spectra
over the entire 2-200 keV band, and long-term variability of faint sources.
The natural time scale for matter near a 10-km neutron star is about 0.1 ms
(dynamical time scale = size / free fall speed).
[DG: Examples of things XTE will be looking for/at and features used.]
Sub-millisecond timing
* X-ray millisecond pulsars (precursors of radio ms pulsars)
* Compton scattering in QPOs (soft vs. hard delays)
* Matter in last orbits around black holes
[DG: QPO = quasi-periodic oscillation]
Man�uvrability
* Episodic accretion in high-mass X-ray binaries
* Sudden accretion flow in low-mass X-ray binaries
* Change of state of Cyg X-1
* Turn-on of Sagittarius gamma-ray repeating burster (GRO alarm)
High sensitivity spectra up to 200 keV
* Turnover of AGN spectra
* Magnetic fields of neutron stars through study of cyclotron features
* Temperatures of the hard components of magnetic cataclysmic variables
* Hard tails in X-ray spectra of low-mass X-ray binaries
* Intracluster magnetic fields through hard-tail observations
Long term observation
* Precession periods of accretion disks
* Variability in AGN, especially BL Lac objects
* Pulsar rotation-rate changes
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