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Next: ISIS: An Interactive Spectral Interpretation System for High Resolution X-Ray Spectroscopy
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Wilson, C. A., Harmon, B. A., McCollough, M. L., Fishman, G. J., Zhang, S. N., & Paciesas, W. S. 2000, in ASP Conf. Ser., Vol. 216, Astronomical Data
Analysis Software and Systems IX, eds. N. Manset, C. Veillet, D. Crabtree (San Francisco: ASP), 587
The Earth Occultation Technique with the Burst and
Transient Source Experiment
C. A. Wilson, B. A. Harmon, M. L. McCollough 1,
G. J. Fishman, S. N. Zhang2, W. S.
Paciesas3
SD 50 Space Science Department, NASA/MSFC, Huntsville, AL 35812
Abstract:
The Burst and Transient Source Experiment (BATSE) on the
Compton Gamma
Ray Observatory (CGRO) is successfully being used as an all-sky hard X-ray
monitor. The experiment consists of a set of eight uncollimated detectors
sensitive to photons in the 20 keV to 2 MeV range. Since
CGRO orbits the Earth
at an altitude of about 450 km, about 33% of the sky, as viewed with BATSE, is
covered by the Earth at any given time. The entire sky is subject to Earth
occultation for some portion of
CGRO's 52 day precession period. When a source
sets below or rises above the Earth's limb, atmospheric attenuation produces
step-like features in the BATSE data. The observed change in count rate in
several energy bands provides a measurement of the source intensity and spectrum
without sophisticated background models. These occultation features are used to
locate and monitor astrophysical sources with BATSE when the source signal can
be separated from the detector background. Examples of step searches, spectra,
light curves, and transform imaging are presented.
Most all-sky monitors have been limited to the soft X-ray range (1-12 keV),
but high energy emission from objects such as X-ray binaries, pulsars, and
active galaxies extends well above 10 keV due to Compton scattering of soft
photons with energetic particles, synchrotron radiation, pair production, pion
decay, and Bremsstrahlung processes. The Burst and Transient Source Experiment
(BATSE) on the
Compton Gamma Ray Observatory
(CGRO) is being used as an
all-sky monitor of point-like hard X and
ray sources in the 20 keV-1
MeV energy range (Harmon et al. 1993). Since BATSE is not a pointed instrument, Earth occultation
features are used to locate and monitor astrophysical sources. When a source
goes behind (or emerges from behind) the Earth, step-like features are produced
in the BATSE data twice every 90 min orbit. These ``occultation steps'' are used
to measure source fluxes and energy spectra and to locate new sources. Example
occultation steps are shown in Figure 1.
Figure 1:
Example of rising and setting Earth occultation steps from the Crab
Nebula in BATSE CONT Channel 1 (20-30 keV) data, denoted by the
histogram. The solid line is the step model fit and the vertical line is
the predicted step time for the Crab Nebula.
![](img2.gif) |
BATSE consists of eight identical uncollimated detector modules positioned on
the corners of the CGRO spacecraft such that the normal vectors of the
detectors are perpendicular to the faces of a regular octahedron, providing
all-sky coverage. The BATSE analyses presented here use the large-area
detectors (LADs), which are NaI(Tl) scintillation crystals with a geometric
area of 2025 cm
and a thickness of 1.27 cm. The LADs are sensitive to
photons from 20 keV to 2 MeV. Two BATSE data types are used in these analyses,
the CONT (2.048 s, 16 energy channel) data and the DISCLA (1.024 s, 4 energy
channel) data. A more complete description of the instrument and data types
can be found in Fishman et al. (1989).
The intensity of a known source is measured with BATSE by computing the
difference in total count rate in source facing detectors just before and after
Earth occultation.
Typically about two minutes of data (fitted separately in each energy channel
and in each detector) immediately before and after an occultation
step are fitted with a quadratic background and source terms:
![\begin{displaymath}
\lambda(t) = \sum^2_{j=0} b_j (t-t_{\rm occ})^j + \sum^N_{n=1} s_n T_n(t)
\end{displaymath}](img4.gif) |
(1) |
where
is the time-dependent model count rate,
is a background
coefficient, and
is the predicted occultation time when the
line-of-sight vector from the spacecraft to the source reaches an altitude of 70
km above the Earth's surface, which corresponds to 50% attenuation of 100 keV
photons. For each of
fitted sources,
is the
source intensity in counts s
and
is the
atmospheric transmission function, a time-dependent step-like
function defined by the atmospheric attenuation along the line of
sight to that source, i.e.,
where
is the energy dependent mass attenuation coefficient and
is the air mass at altitude
at time
. Interfering sources are
defined as known sources expected
to exceed 60-100 mCrab (1 mCrab = 0.1% of the intensity of the Crab Nebula,
the standard candle in X and
ray astronomy), that are occulted within the fitting window. In
practice, usable steps are not obtained twice per spacecraft orbit due to
interfering sources occulting within 10 seconds of the measured source,
ray bursts, solar flares, electron precipitation events, and South
Atlantic Anomaly passages. For most sources, 10-20 clean steps per day are
available. Typically, several occultation steps are averaged to obtain an
intensity measurement. An average 1-day 20-100 keV sensitivity of about
100 mCrab is achieved for most source locations. Daily flux measurements
are generated by convolving a simple spectral model (a power law or thermal
Bremsstrahlung model) with the detector response matrices and fitting it to the
observed count spectra. Detailed spectral fitting is done using BATSE
software or XSPEC. Examples of a light curve and a count spectrum derived from
occultation step fits are shown in Figure 2 for the black hole
candidate GX 339-4.
Figure 2:
( Left):Average daily photon flux from GX 339-4 measured with BATSE
Earth occultation in the 20-100 keV band from April 1991 to July 1998.
( Right):Count spectrum and fit residuals measured with BATSE for GX 339-4.
![](img17.gif) |
A search for occultation steps from new sources is routinely performed on each
day's BATSE data. Data in each LAD are first summed over the 30-200 keV range to
obtain the best signal to noise for Crab-like spectra. Then a series of tests
are performed on a contiguous data segment within a four minute fitting window
to determine if an occultation step is present within the window. If a step is
present, a fit is made to estimate the time of the occultation and the
significance of the step. Once a data segment has been tested, results are
logged and the window moves to the next data segment. Results are then plotted
for a full day. If steps from one source are present, they will cluster in time
(measured since orbit start). Clusters from known sources are identified,
allowing quick localization of new transient sources.
Figure 3:
Example of an Earth occultation transform image centered near the
Crab Nebula.
![](img18.gif) |
Occultation steps in the time domain can be transformed into spatial information
for image construction (See Figure 3). This has enhanced BATSE's
ability to locate and identify weaker sources and increased BATSE's
effectiveness as an all-sky monitor (Zhang et al. 1995). BATSE
measures the change in count rate along the arc of the Earth's limb projected
onto the sky at the time of an Earth occultation. The location of the spacecraft
is well known, resulting in accurate knowledge of the position and orientation
of the Earth's limb projection. This effectively allows us to image one
dimensional strips of the sky. As the spacecraft orbit precesses, the position
and orientation of the Earth's limb projection changes, allowing us to combine
one dimensional strips into a two dimensional image. A curved Radon
transformation (Deans 1983) is used to describe the projection of the Earth's
limb on the sky. The image is reconstructed using the Maximum Entropy Method
(Huesman et al. 1977).
References
Deans, S. R. 1983, The Radon Transform and Some of Its Applications,
(New York: Wiley)
Fishman, G. J., et al. 1989, in Proc. GRO Science Workshop, ed.
W. N. Johnson
(Greenbelt: NASA/GSFC), 2
Harmon, B. A. et al. 1993, in Compton Gamma Ray Observatory, AIP
Conf. Proc. 280, eds. M. Friedlander, N. Gehrels, D. J. Macomb (New
York: AIP),
314
Huesman, R. H. et al. 1977, Donner Algorithms for Reconstruction
Tomography, LBL Publ. 214, 42
Zhang, S. N. et al. 1995, Exp. Astron. 6, 57
Footnotes
- ... McCollough1
- Universities Space Research Association
- ... Zhang2
- University of Alabama in Huntsville
- ...
Paciesas3
- University of Alabama in Huntsville
© Copyright 2000 Astronomical Society of the Pacific, 390 Ashton Avenue, San Francisco, California 94112, USA
Next: ISIS: An Interactive Spectral Interpretation System for High Resolution X-Ray Spectroscopy
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