The Compton Gamma-Ray Observatory's Burst and Transient Source Experiment (BATSE) has a powerful capability to provide nearly uninterrupted monitoring in the 25 keV-10 MeV range of both active galactic nuclei (AGN) and galactic black hole candidates (GBHC) such as Cygnus X-1, using the occultation of cosmic sources by the Earth. Since the Crab is detected by the BATSE Large Area Detectors with roughly 25(sigma) significance in the 15-125 keV range in a single rise or set, a variation by a factor of two of a source having one-tenth the strength of Cygnus X-1 should be detectable within a day. Methods of modeling the background are discussed which will increase the accuracy, sensitivity, and reliability of the results beyond those obtainable from a linear background fit with a single rise or set discontinuity

Fishman, G. J.

Gruber, Duane E.

Harmon, Alan

Ling, James C.

Meegan, C. A.

Paciesas, William S.

Rubin, Brad

Skelton, R. T.

Wheaton, William A.

Wilson, R. B.

English

NASA Technical Reports Server

N92-21916
GAMMA-RAY MONITORING OF A.G.N. AND
GALACTIC BLACK HOLE CANDIDATES BY
THE COMPTON GAMMA-RAY OBSERVATORY
R. T. Skelton, James C. Ling, Wm. A. Wheaton,
Jet Propulsion Laboratory, California Institute of Technology
Alan Harmon, G. J. Fishman, C. A. Meegan, William S. Paciesas 1,
Brad Rubin 2, R. B. Wilson,
Marshall Space Flight Center, NASA
Duane E. Gruber,
Center for Astrophysics and Space Science, UCSD
ABSTRACT
The Compton Gamma-Ray Observatory's Burst and Transient Source Experiment
(BATSE) has a powerful capability to provide nearly uninterrupted monitoring in the 25
keV-10 MeV range of both A.G.N. and galactic black hole candidates such as Cygnus
X-I, using the occultation of cosmic sources by the Earth. Since the Crab is detected
by the BATSE Large Area Detectors with roughly 25a significance in the 15-125keV
range in a single rise or set, a variation by a factor of two of a source having one-tenth
the strength of Cygnus X-1 should be detectable within a day. Methods of modeling the
background are discussed which will increase the accuracy, sensitivity, and reliability of
the results beyond those obtainable from a linear background fit with a single rise or set
discontinuity.
1University of Alabama at Ituntsville
2USRA
Fh_ECEDti'_ PAGE BLANK NOT FILMED
321
https://ntrs.nasa.gov/search.jsp?R=19920012673 2020-03-17T13:15:40+00:00Z
The Burst and Transient Source Experiment (BATSE) aboard the Compton Gamma Ray Observa-
tory (CGRO) offers a capability to provide almost uninterrupted monitoring of gamma-ray sources by
Earth occultation. "Almost" means that one gets a measure of a source whenever it rises or sets in the
96-minute CGRO orbit. Those sources continuously above the horizon ca_,)t be monitored, but the
orbit precesses so that a source can spend only about two weeks continuously up before setting and again
being subject to Earth occultation. BATSE consists of eight modules which are located octahedrally
at tile corners of the CGRO. Each module consists of a Large Area Detector (LAD), which is a NaI
scintillator 50 cm in diameter and 1.25 cm thick, and a Spectroscopy Detector (SD), which is a 12.5 cm
by 7.5 cm NaI scintillator. Data are accumulated in 2.048-second bins for each of 16 energy bands for
the LADs; more sophisticated modes exist but have not been used to date in this investigation. BATSE
is described in detail by Fishman el al. (1989).
The need for continuous monitoring of gamma-ray sources is illustrated by the fragmentary record of
the premiere galactic candidate, Cygnus X-l, compiled by Ling et al. (1987a). It was noted to exhibit a
two-week period of low flux below about 140 keV with a simultaneous high flux in the _ 1 MeV region in
the form of a broad emission hump (Ling et al. 1983; Ling et al. 1987b). Tile hump can be interpreted as
a Doppler-broadened and blue-shifted signature of electron-positron annihilation in an electron-positron
plasma theoretically expected to result from photon-photon interactions near a luminous compact gamma-
ray emitter (Liang 1979; Zdziarski 1980). Evidence, albeit at a 1.9 _r level, has recently been noted (Ling
and Wheaton 1989) of a narrow 511-keV line, present only during the time of the MeV hump, which
could result from positrons being blown away by radiation pressure and annihilating in the primary star
or its wind (Liang and Dermer 1988; Dermer and Liang 1988). For a positron-electron plasma, radiation
pressure exceeds gravity by a factor of 20 in Cygnus X-1 (Kovner 1984). The Cygnus X-1 spectrum is
similar to that observed for the galactic center (Lingenfelter and Ramaty 1983; Lingenfelter and Ramaty
1989; Riegler et al. 1981; Riegler et al. 1985). Variable MeV emission has been reported for NGC 4151,
Cen A, and MCG 8-11-11 (Bazzano et al. 1989; Bassani and Dean 1983). Confirming the tentative
results and establishing that the observed time variability and correlated spectral shifts constitute a
pattern would allow better insight into the physics of compact objects.
The power of BATSE/Earth occultation is illustrated by Figure 1, which shows raw count rates
for two orbits from the LAD most closely facing the Crab; the Crab becomes occulted at 1.9 hr and
3.4 hr and rises 0.5 hr later in each case, producing the prominent steps. A model consisting of a linear
background (vs time) with a step discontinuity at tile rise or set yields a statistical significance (combining
all energy channels and LADs) of around 25cr for a single rise or set of the Crab with a fit window of
-1-262 sec. BATSE Mission Operations uses this simple model (which was quite effective at detecting the
flaring of GX339-4 in June-July 1991, reported by Fishman et al. 1991) with a suitably selected time
window to monitor the gamma-ray sky. The purpose of this paper is to report progress for an improved
analysis which will allow monitoring to greater precision. Cutting measurement errors by, say, a factor
of two would be of some value for a strong source like the Crab, but the greater motivation is to improve
accuracy and sensitivity for weaker sources.
The linear-plus-step model is limited by the time window in which the background flux can be treated
as linear. We have considered improvements along the lines of more polynomial terms, but the risk of
introducing systematic errors outweighs the additional formal precision. A further fundamental problem
is that polynomials are complete, and that the signal, a step function, can be represented by a sum of
polynomials. Any complete mathematical model (Fourier series, etc.) shares the same shortcoming. Our
approach will therefore be to model the sources of background with parameters that have a physical
basis. In this manner we expect to increase the fit window substantially without introducing comparable
systematic errors.
322
To illustratetheelementsthat mustgo intothismodel,weconsiderthegeneralcharacteristicsof
therawdata.Figure2showsthefirst 12hoursof 17April 1991in the1100-1800keVbandforeachof
theeightLADs.Thecountratesoscillatetwiceperorbit,andall LADsoscillateinphase.Thedominant
sourceof countsis cosmic-raysecondaries;theoscillationsresultfromv:_rinlionsin the geomagnetic
latitude. VariationsfromoneLADto anotheresultprimarilyfromEartha,lglos;at this relatively
highenergy,the Earth is bright from cosmic-ray secondaries compared to the diffuse cosmic background.
Figure 3 shows the same time frame in the 24-32 keV band. The count rates oscillate once per orbit,
with a narrow minimum compared to the broad maximum; oscillation amplitudes and phases differ. The
dominant cause of variation is Earth blockage of the diffuse cosmic background. The narrow minimum
corresponds to detector nadir. Two LADs whose numbers sum to seven face in opposite directions, and
it is clear that count rates are 180 ° out of phase for such pairs.
Not shown are data from orbits wherein the CGRO passes through the South Atlantic Anomaly
(SAA); the high voltage is switched off during actual passage, and activation decays are an obvious
additional feature once HV is turned back on.
In light of this discussion, the dominant background contributors which must be modeled are diffuse
cosmic gamma rays, cosmic-ray secondaries produced at the CGRO, cosmic-ray secondaries produced in
the atmosphere, and activation. An enhanced model also needs to take account of the source confusion
problem; combining set and rise data from the source into a single fit should alleviate this problem,
because sources which are widely separated on the sky but which happen, for example, to set together
will usually rise well separated.
Diffuse cosmic gamma rays refers to the aggregate output of sources too weak to catalogue in-
dividually, as well as truly diffuse sources. Since this contribution is approximately isotropic (with a
nonisotropic galactic component.), the primary modeling involved will be to define its spectral parameters
and to account for blockage of it by the Earth.
Background due to prompt effects of cosmic rays is expected to be proportional to the SD upper
level discriminators, based on experience with HEAO-3 (Mahoney el al. 1984). Work is in progress to
extend the simple IIEAO-3 model to make use of the 8 BATSE Spectroscopy Detector ULD rates to
characterize the cosmic-ray environment and also the spectral dependence of the response.
In principle, background due to cosmic-ray secondaries produced from interactions with the Earth
atmosphere presents the most formidable challenge. To be very precise, one must model the atmospheric
response as a function of energies, each depending on the zenith angle and on the angle from tile detector
normal; the total LAD response would then obtained by integrating over the Earth disk. Again, expe-
rience with IIEAO-3 leads us to believe that this extremely computationally intensive approach can be
circumvented by suitably parametrizing the problem.
Activation occurs throughout the orbit, but is most significant during SAA passages. In principle,
one simply integrates the activation rate vs time, weighting with the appropriate exponential for each
radioactive species, and sums such exponential terms. In practice, there is difficulty in determining
activation rates, which will differ among detectors, and the sum-of-exponentials problem has a notorious
tendency to be ill-conditioned. Experience with IIEAO-1 (Gruber et al. 1989) has shown how these
problems can be overcome.
There exists a considerable body of experience in dealing with these matters, both by the authors and
by others in the community, on spacecraft such as ttEAO-1, ItEAO-3, and the Solar Maximum Mission
Gamma Ray Spectrometer. We believe that the challenges of increasing the fit window to improve
the data analysis can be met and that considerable scientific value will result from the unprecedented
continuous monitoring capability afforded by the CGRO and BATSE.
323
REFERENCES
Bassini, L. and Dean, A. J. 1983, Space Sci. Rev., 35, 367.
Bazzano, A. et al. 1989, in Proceedings of the GRO Science Workshop, ed. W. N. Johnson, p.
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Definer, C. D. and Liang, E. P. 1988, in Nuclear Spectroscopy of Astrol_hysicai Sources, AIP
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324
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Gamma ray monitoring of a AGN and galactic black hole candidates by the Compton Gamma Ray Observatory

Gamma ray monitoring of a AGN and galactic black hole candidates by the Compton Gamma Ray Observatory

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