The Compton imaging telescope (COMPTEL) on the Gamma Ray Observatory (GRO) is a wide field of view instrument. The coincidence measurement technique in two scintillation detector layers requires specific analysis methods. Straightforward event projection into the sky is impossible. Therefore, detector events are analyzed in a multi-dimensional dataspace using a gamma ray sky hypothesis convolved with the point spread function of the instrument in this dataspace. Background suppression and analysis techniques have important implications on the gamma ray source results for this background limited telescope. The COMPTEL collaboration applies a software system of analysis utilities, organized around a database management system. The use of this system for the assistance of guest investigators at the various collaboration sites and external sites is foreseen and allows different detail levels of cooperation with the COMPTEL institutes, dependent on the type of data to be studied

Bennett, K.

Collmar, W.

Connors, A.

Denherder, J. W.

Diehl, R.

Hermsen, W.

Lichti, G. G.

Lockwood, J. A.

Macri, J.

Mcconnell, M.

English

NASA Technical Reports Server

N92-2i886
Data Analysis of the COMPTEL Instrument
on the NASA Gamma Ray Observatory
R. Diehl 1'*, K. Bennett 4, W. Collmar 1, A. Connors 3, J.W. den Herder 2, W. Hermsen 2, G.G.
Lichti 1, J.A. Lockwood 3, J. Macri 3, M. McConnell 3, D. Morris 3, J. Ryan 3, V. SchSnfelder 1, H.
Steinle I , A.W. Strong I , B.N. Swanenburg 2, C. de Vries _, C. Winkler 4,
1Max Planck Institut fiir Extraterrestrische Physik D-80g6 Catching, FRG
2Space Research Leiden, Huygens Laboratorium, NL-_333 Leiden, The Netherlands
3Space Science Center of University of New Hampshire Durham, NH , USA
4ESA/ESTEC Space Science Depar_ement NL-2200 Noordw(ik, The Netherlands
(*) on behalf of the COMPTEL Collaboration
ABSTRACT
The imaging Compton telescope COMPTEL on the NASA Gamma Ray Ob-
servatory is a wide field-of-view instrument. The coincidence measurement
technique in two scintillation detector layers requires specific analysis meth-
ods. Straightforward event projection onto the sky is impossible. Therefore
detector events are analyzed in a multi-dimensional dataspace using a gamma-ray
sky hypothesis convolved with the point spread function of the instrument in
this dataspace. Background suppression and analysis techniques have impor-
tant implications on the gamma ray source results for this background limited
telescope.
The COMPTEL collaboration applies a software system of analysis utilities,
organized around a database management system. The use of this system for
assistance of guest investigators at the various collaboration sites and exter-
nal sites is foreseen and allows different detail levels of cooperation with the
COMPTEL institutes, dependent on the type of data to be studied.
INTRODUCTION
The COMPTEL instrument as part of the NASA Gamma Ray Observatory, and
its instrumental characteristics, have been described in detail by SchSnfelder et
at., 1984. Instrumental performance and characteristics are presented by den
Herder et al. (this volume). For details of calibrations and application of the
instrumental response see Diehl et at., 1991. The processing and analysis software
environment COMPASS is described by den Herder et al., 1991.
COMPTEL data analysis proceeds in three major steps:
1. Calibration processing of raw data
2. Response and background determination
3. Scientific Analysis
The raw data processing translates the raw telemetry data into normalized
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and sorted databases(e.g. attitude data, housekeepingrates, event messages
with calibrated and normalized parameters). After this step, the resultant data
products (called 'level I data products') can be interpreted without particular
km_wledgeof the data recording characteristicsand stability performanceof the
instrument.
The secondstep is the analysis of the normalized event messagesin terms
of the instrumental response. Here a study of the signatures in tile data is
performed, the results are compared to prelaunch calibration data and other
prior instrumental knowledge. Correlation studiesof features in the dataspace
spannedby all measuredparametersof the events (e.g. signal pulse shapeand
time-of-flight) and instrument backgroundenvironment (e.g. veto detector rates,
cutoff rigidity) are performed on the flight data to establish models for the
instrumental background. Here symmetry characteristicsof the instrument are
exploited to extract and smooth the backgroundmodel. During this analysisthe
selectioncriteria for event messagesare optimized. The resulting data products
(response,background) constitute the secondset of 'low level' data ('level IIa')
neededfor analysis.
The third step of analysis then combinesthe eventdata with theseresponse
and background matrices. Initially, a 'generic' skymap without specific astro-
physical model assumptionsis generated(via the Maximum Entropy Method).
Hopefully this map givesindications of interesting regions,and the real in-depth
scientific analysisbegins: astrophysicalmodelsand their specificparametersare
tested against the COMPTEL data, and parameter significancelimits are de-
rived. Maximum Likelihood based testing of the source models is applied in
this dataspaceto determine significancesof the detectedfeaturesand their pa-
rameters. In the course of these activities, selection criteria may be further
optimized to improve the signal-to-backgroundratio for a specific topic under
study; this leadsback to the previousstepsof re-generatingapplicableresponse
and background matrices. The COMPTEL collaboration will assessthe overall
consistencyof analysis steps, methods and selection criteria, before releasing
the baselineset of results and data products (skymap; result parameter table;
filtered event set; selectioncriteria; backgroundmatrices; responsematrices) to
the scientific community.
RESPONSECHARACTERISTICS and ANALYSIS METHODS
The COMPTEL instrument operates in the range of MeV photons. Here
the main photon interaction with matter is the Compton scattering process,
thus dominating the responseof the instrument. But this regime is also the
domain of de-excitation radiation of atomic nuclei; this results in substantial
contamination of the measureddata with instrumental backgroundeventswhich
donot originate in a Comptonscatter interaction asquantified by the instrument
response.
The interpretation of the telescopeeventmessagesis complicatedfurther by the
nature of the instrument responseto photons from within the field of view:
For energiesof about 1-2MeV, the interactions in the instrument are mainly
a single Compton scatter in the upper detector plane, followed by absorption
of the scattered photon in the lower detector plane. This results in a fairly
narrow 'source cone' signature of measuredevent parameters (see figure 1).
Howeverthe Compton tail of the responseof the scintillation detector results in
event scatter anglemeasurementslarger than the true scatter angle, forming a
96
'halo' in the inner part of the 'source cone' signature. At higher energies,the
single Compton scattering competeswith pair production as first interaction,
resulting in a halo around the sourceconein all directions. Similar halo events
are produced at higher photon energiesalso by the increasingprobability that
the Compton scattered electron may produce bremsstrahlung photons, or may
escapethe scintillator. These responsecharacteristics of the instrument are
handled appropriately with the a_alysismethods operating in the full dataspace
shownin figure 1; 'eventcircle' methodswhich just explorethe main conefeature
of the responsecan be usedto confirm the more complex deconvolution method
results, but only for strong, widely separatedsources.
Cornp(on Scanm Angle I Commm_ llaamw Angle
Fig.1 (left): Dataspace for imaging analysis of COMPTEL data. The first order signature
of a source is shown
Fig.2 (right): Overlapping measurement signatures from widely separated sources
The azimuthally undetermined arrival directions of photons along a source
cone also imply that source photons from very different directions in the sky
may produce identical measured event parameters (see figure 2). Therefore the
results of analysis of a particular sky region depend on the source assumptions
in sky regions which are tens of degrees away. The standard analysis method
should always involve a complete model for the entire instrument field of view.
ANALYSIS ILLUSTRATIONS
In the initial phase of the GRO mission, several cosmic gamma ray bursts were
observed within the field of view of COMPTEL. Figure 3 displays the events
from a burst (June 1, 1991) as distributed in the measured dataspace; the cone
pattern can be traced in the form of rings of increasing diameter in the different
slices of the (vertical) scatter angle axis. (Note that the large gamma-ray flux
of cosmic bursts provides virtually background-free data, so that instrumental
response features show up in the data with little contamination.) Figure 4 shows
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Fig.3: Measured signatures from a cosmic burst (count diagrams in scatter
direcLions are shown for different scatter angle slices). The cone signature is
visible as a set of (inhomo#eneously ezposed) rings
the projected raw event messages displayed as 'event circles' of possible arrival
directions on the sky. These circles assume a Compton scatter interaction in
the upper detector, and a total absorption of the scattered photon in the lower
COMPTEL detector. Clumping of the event circle intersections can be seen at
1=170, b=10 degrees. The Maximum Entropy deconvolved skymap using the full
response detail (shown in figure 5) clearly shows that the sky is dominated by a
single point source. Note that for these burst data any instrumental background
is negligible.
In the normal case of imaging a complex sky region with the presence of
substantial instrumental background, the analysis procedure is more compli-
cated. Figure 6 shows a skymap generated with the same procedure as the
burst skymap, namely ignoring background suppression and modelling. If no
background knowledge is used at all, the skymap shows a wide range of struc-
tures; specifically a large extended structure is seen at longitudes 165-170 de-
grees. However, if the measurement of the Lime-of-flight is exploited to establish
a background model via selection of events with time-of-flight values outside the
98
.. • .- '
,\
, /
Fig.4: Projected event circles measured from the cosmic burst 3 May 1991. Clumping of
event circle intersections at the position of the burst is barely visible
30 f ' ' ' i .... , .... 1 .... r • , r - : , ,
20
10
0
,...d
-10
. , • i
158 16[] ]7C _£'J !90
gal.longitude(degrees)
Fig.5: Mazimum Entropy deconvolved skymap from
the cosmic burst 3 May 1991 usin9 full response de-
tails
normal values for forward scattering, this model can be applied in the analysis;
the skymap in figure 6b is determined using this (crude) background informa-
tion; obviously the large extended feature dissappears completely, while the
dominating feature associated with the Crab nebula/pulsar remains.
Some detailed impacts of the response accuracy can also be demonstrated
in the anticenter region (see figure 7). The COMPTEL point spread function
can be determined in 3 different ways: via detailed calibration of the detecor
99
Fig.6: Deconvolved skymaps from the anticenter region, illustrating the effect of background
model inclusion. Without background modelling (a), left), and with inclusion of a coarse
background model only, where the false feature dissappears (b, right)
.H
!1
l
• 7;NF
/-
k N
Fig.7: Deconvolved skymaps from the anticenter region, illustrating the effect of different
point spread functions. (Details see text}
components and an analytical response calculation based on these detector char-
acteristics, via sample calibration within the entire instrument field of view, or
via simulations of the entire instrument. The analysis of the first observations
(Crab at different aspect angles) will help determine which method actually
describes the in-flight r,:_spoase most accurately. Figure 7 shows how the 2 dif-
ferent calibrated responses modify details of the skymap: The PSF used in the
lefthand picture is based on accurate calibrations of the 21 COMPTEL detector
modules, and on an analytical response calculation; this may ignore unknown
secondary features of the true response. The PSF used in the righthand picture
100
is basedoil a few calibrations of the entire COMPTEL telescopefield of view;
this may include calibration contaminations due to the acceleratorradiation en-
vironment and the non-parallel calibration photon beam. Clearly caremust be
applied in astrophysical interpretation of the small scalefeatures: thesecould
possibly be artifacts produced by interferenceof instrumental background and
responseimperfections as compounded in the complex deconvolution process.
The COMPTEL collaboration analyzesdetails of the Crab imagefrom different
aspects,a_ldexploits burst data, in order to optimize responseand background
treatment in the analysis, so that generation of such artifacts will be largely
eliminated.
SUMMARY
The COMPTEL instrument aboard the NASA Compton Gamma Ray Obser-
vatory provides a unique opportunity for astrophysicists to enhance our present
information about the gamma ray sky and about objects traceable via MeV
gamma radiation. The large field of view of the imaging instrument, its opera-
tion in a high background environment, and its multiple-interaction based detec-
tion principle, necessitate sophisticated data analysis methods and procedures.
First analysis results demonstrate the complexity of the analysis approach. Nev-
ertheless the powerful analysis tools and methods established by the COMPTEL
collaboration succeed in imaging the MeV sky, demonstrated with the 3 May
1991 cosmic gamma ray burst, and the Galactic anticenter region.
COMPTEL data products will be made available to the scientific community
via tile GRO Science Support Center. The complexity of the analysis requires
care in the interpretation of these products and suggests that scientists should
work in close association with the COMPTEL collaboration to benefit from
our acquired expertise. Different levels of involvement are proposed and sup-
ported (for details see Diehl et al., 1989 (1. GRO Science Workshop).
REFERENCES
-- Diehl R., Aarts H., Bennett K., Collmar W., deBoer H., Deerenberg A.J.M.,
denHerder J.W., deVries C., Hcrmsen W., Kippen M., KnSdlseder J., Kuiper
L., Lichti G., Lockwood J.A., Macri J., McConnell M., Much R., Morris D.,
Ryan J., Schgnfelder V., Simpson G., Steinle H., Strong A.W., Swanenburg
B.N., vanSant T., Webber W.R., Winkler C., in: Data Analysis in Astronomy
IV, edited by V. diGesu et al. , Plenum Press New York, (1991) (to be published)
-- den Herder J.W., Aarts H., Bennett K., Diem R., Hermsen W., Johnson
M., McConnell M., Ryan J., SchSnfelder V., Simpson G., Steinle H., Strong
A.W., Swanenburg B.N., deVries C., Winkler C., Wood I., in: Data Analysis
in Astronomy IV , edited by V. diGesu et al. , Plenum Press New York,
(1991) (to bepublished)
-- Strong A.W., Diehl R., in: Data Analysis in Astronomy III , edited by V.
diGesu et al., Plenum Press New York, 55-66 (1989)
-- SchSnfelder V., Diehl R., Lichti G., Steinle H., Swanenburg B.N., Deerenberg
A.J.M., Aarts H., Lockwood J., Webber W., Macri J., Ryan J., Simpson G.,
Taylor B.G., Bennett K., Snelling M., IEEE Trans. on Nucl. Sci., NS-31 ,
No. 1, 766-770,(1984)
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Data analysis of the COMPTEL instrument on the NASA gamma ray observatory

Data analysis of the COMPTEL instrument on the NASA gamma ray observatory

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