The Energetic X-ray Imaging Telescope Experiment (EXITE) was developed to a flight-ready status and conducted two flights (May 18, 1988, and May 8-10, 1989) from Alice Springs, Australia, as part of the campaigns to observe the supernova SN1987A. The basic operation of the detector and gondola systems in the laboratory was tested on the first flight and found to meet expected performance values. A bizarre 'balloon tape' insulation problem, however, prevented normal telescope pointing on the first flight so no data on SN1987A or other targets were obtained. Following a successful second EXITE flight from Ft. Sumner, NM, in October 1988, the experiment was flown again on a successful 30 hour flight as part of the final 1989 supernova campaign. A second x-ray imaging experiment from MSFC was also flown (piggy-back) for this third flight. Good data were obtained on the supernova and a variety of high priority galactic targets, and final analysis is still in progress. Preliminary results from this flight are presented

Grindlay, Jonathan E.

English

NASA Technical Reports Server

NASA-CR-192133
/
Final Report on
EXITE/IPC Observations of SN1987A and Southern Targets
• NASA Grant NAGW-1721
-_- The Energetic X-ray Imaging Telescope Experiment (EXITE) was developed to a
flight-ready status and conducted two flights (May 18, 1988, and May 8-10, 1989) from
Alice Springs, Australia, as part of the campaigns to obse_e the supernova SN1987A. The
basic operation of the detector and gondola systems in the laboratory was tested on the first
flight and found to meet expected performance values. A bizarre "balloon tape" insulation
problem, however, prevented normal telescope pointing on the first flight so no data on
SN1987A or other targets were obtained. Following a successful second EXITE flight from
Ft. Sumner, NM, in October 1988, the experiment was flown again on a successful 30 hour
flight as part of the final 1989 supernova campaign. A second x-ray imaging experiment
from MSFC was also flown (piggy-back) for this .third flight. Good data were obtained on
the supernova and a variety of high priority galactic targets, and final analysis is still in
progress. Preliminary results from this flight_which was partially supported by this grant,
are presente_ere.
//v -S-7-cP
1 Introduction
The Energetic X-ray Imaging Telescope Experiment (EXITE) (Grindlay et al 1986, Garci'a
et al 1986) conducted its first flight on May 18, 1988, from Alice Springs, Australia, as
part of a recent campaign to observe SN1987A. Because of a thermal-induced "ball6on
tape" failure, the telescope was unable to point on the supernova and only engineering
data were obtained, although an important verification of basic instrument performance
and background levels was carried out. After payload modifications and improvements, a
second flight was conducted on October 8-9, 1988, from Ft. Sumner, NM, on a variety
of northern hemisphere targets. This flight was a success, and showed that the the basic
science objectives could be achieved on a second SN1987A flight. Because of the declining
Compton scattered gamma ray continuum and the possible detection of the suspected
pulsar, it was important that the March-May 1989 campaign include an instrument with
good sensitivity and imaging resolution for continuum observations in the 30-200 keV band.
EXITE provided this capability, and offered the highest angular resolution of any of the
five investigations scheduled to fly during this final campaign to observe SN1987A.
During the final(April-May 1989) SN1987A campaign, the EXITE payload also in-
cluded the high pressure Xe imaging proportional counter (IPC) hard x-ray detector de-
veloped by Ramsey and Weisskopf (1986) together with its own coded mask to provide
complementary sensitivity and angular resolution at energies below 80 keV. The accomo-
dation of this IPC detector with EXITE, and the supernova science as well as galactic
center source observations, were the primary motivation for this grant activity.
(NASA-CR-192133) EXITE/IPC
OBSERVATIONS OF SNI9BTA AND
SOUTHERN TARGETS Fina| Report
(Harvard Coil. Observatory) 8 P
N93-18607
Unclas
G3189 0145535
https://ntrs.nasa.gov/search.jsp?R=19930009418 2020-03-17T07:54:55+00:00Z
2 Scientific Objectives
The hard x-ray continuum emission of the supernova SN1987A is an important comple-
mentary measurement to that of the SSCo line emission, since it arises in part due to the
Compton downscattering of the line in the expanding shell of the remnant. As the remnant
becomes increasingly optically thin to the 847 and 1238 keV photons from decay of SSCo,
the continuum should decrease. The relative slope, as well as flux, of the continuum at
low energies (i.e. below 100 keV) can help to further determine the detmity profile of the
expanding ejecta as well as the degree of mixing of synthesized SeCo. If the line flux decays
faster than the continuum flux, for example, mixing or density parameters are affected.
Unfortunately the predicted Compton continuum flux for April 1989 (e.g. Kumagai et al
1989) was expected to be only ,_10-6photons/cm2-sec, or below the detection threshold
of most instruments. The Compton continuun from S_Co (below 122 keV) is also likely to
be weak but the EXITE/IPC telescopes have peak sensitivity at these energies.
A second, and now more compelling, reason for conducting continuum measurements
at hard x-ray energies is the search for the pulsar at energies where it may be detectable.
The calculations of Kumagai et al (1989) indicated that the most likely band for the
detection of the pulsar, either an isolated pulsar or an accreting pulsar, is ,_30-50 keV. Thus
the departure of the spectrum shape (from that expected from continued downscattering
of the S6Co continuum) is a likely indicator that the pulsar exists.
As pointed out by Covault, Braga and Grindlay (1988), EXITE is the only instrument
now developed which has sufficient angular resolution to clearly and unambiguously isolate
the supernova from the neighboring source LMC X-1 at energies above 30 keV. With a
separation of only 0.6 degree, resolution of ,,_0.3 ° is needed to cleanly isolate the two
objects, which may have similar hard x-ray spectra (both in slope and amplitude) and
similar soft x-ray (i.e. below 20 keV) spectra as well (cf. discussion by Covault, Braga
and Grindlay 1988). The EXITE angular resolution is 20 arcmin, and source positions can
therefore be centroided to a fraction of this value which is dependent on signal to noise
An additional objective of the EXITE flight on SN1987A, then, was to measure the hard
x-ray spectrum of the black hole candidate LMC X-1.
The major secondary objective of the flight was to observe the galactic center region
for a higher angular resolution view of the puzzling hard x-ray source 1E1740-2942 that
had been detected by Skinner et al (1987) as the dominant hard x-ray source in the galactic
center region at 2-20 keV x-ray energies with the Spacelab experiment. This source had also
been observed with the Caltech GRIP imaging detector, but at lower angular resolution.
The third major objective of the EXIT]_ flight was to observe the black hole candidate
GX339-4 for the first time at hard x-ray energies. This source had been reported as a hard
x-ray source by the HEAO-A4 experiment (Nolan et aI 1982) but has not been confirmed
(for its identification) by an imaging hard x-ray experiment.
2
3 Performance of EXITE on its May 1989 Alice Springs Flight
The third flight of EXITE, together with the MSFC IPC detector as mentioned above, was
launched on May 8, 1989, from Alice Springs, Australia. The flight was very successful,
with a total of more than 24 hours of float data obtained. The gondola pointing systems and
absolute aspect systems were improved with the addition of a newly designed sunsensor
and integrating CCD TV cameras for nighttime aspect. Sources were acquired within
about 0.5 ° of the center of the field of view (post-flight measurements showed that the
collimators on EXITE were misaligned with the optical axis of the telescope by 0.5 ° ) and
tracked with rms deviations of K 5 _cmin. Good images were obtained on the Crab which
allowed absolute calibrations of flux and spectral sensitivities to greater accuracy than had
been possible with the Ft. Sumner flight. The image and spectrum of the Crab _e shown
in Figures I and 2 below.
This second EXITE flight from Alice Springs ("Alice II") was carried out with the
MSFC hard x-ray IPC detector "piggy-backed" on the EXITE telescope and gondola. The
two telescopes were co-aligned and viewed the same target. Power and telemetry, as well as
commands, for the MSFC instrument were independent of EXITE, however, so integration
was conceptually simple. The MSFC instrument, on its first flight, performed well also
and yielded Crab images of comparable quality to those from EXITE shown below.
Right Ascension
5h 22rn §h 26rn 5h 30rn _h .*_ rn
_, • _ .
_......e=°..,*o_**- 20
2 : :
-1 .: .............. :.............. "..............................
: 1
: :.
.i..............i ..........i ...................
23
24
-2 -1.5 -1 _.5 0 0.5 1 1.5 2
degrees
Figure 1: Crab image (20-100 keV) obtained with EXITE/Alice II flight.
Fit to EXITE model
8
0
i
0
2.1
r ! | I | I l" • I ! ! ! !
9 20 30 40 50 70 90 200
energy (keY)
2.1
Figure 2: Crab spectrum obtained with EXITE/Alice II flight.
The Crab observations showed that the telescope achieved its desired sensitivity and
performance levels. Results of the supernova and galactic source observations are summa-
rized in the next section.
4 Results of the Alice II Flight: SN1987A and Galactic Sources
4.1 SN1987A
The supernova field was successfully observed for a total of about 4 hours on the first day
of the flight (UT May 9) and then again for about 2.5 hours on the second day, under
control from our downrange telemetry station in Longreach, Queensland.
We did not detect the supernova SN1987A in our data, though analysis is still contin-
uing since final post-flight calibrations have not been yet included. The actual observed
Crab spectrum (Fig. 2) has been used to update the sensitivity expected for EXITE for
detection of the hard x-ray spectrum reported for SN1987A most recently by Sunyaev et
al (1990). The expected sensitivity includes not only the actual observed backgound spec-
trum (from the Alice Springs flight) but also the explicit effect of the energy-dependent
spatial resolution of the detector as actually measured for the flight configuration of the
detector and electronics; neither of these could be included in the pre-flight estimates of
sensitivity given by Covault et al (1988). Results of this calculation for sensitivity give our
upper limit as Fx(30-110 keY) _ 3 x 10 -s photons/cm2-sec-keV. This limit is for a 4 sigma
detection with a total of 2 x 104 seconds observation time on the supernova. As indicated,
EXITE is most sensitive to detecting the supernova in the 30-150 key range, where its
angular resolution surpasses previous balloon and satellite experiments.
The EXITE upper limit is a factor of ,,_3 above the near-simultaneous HEXE/Mir limit
of Sunyaev et al (1990), which themselves represent a much longer (30 hour) exposure time.
The fact that our upper limit also includes the nearby black hole candidate LMC X-1 also
means that the Mir results for this epoch were probably not signi_cantly contaminated by
LMC X-l, which they otherwise could not resolve.
4.2 Galactic Center and GX339-4
The galactic center region was observed for a total exposure time of about 2 hours. A
planned second exposure on the galactic center, which would have been of slightly longer
duration, was unfortunately not possible to carry out when the balloon package was out of
telemetry range from Alice Springs and not yet in command and control from the down-
range station. Nevertheless, our results on the galactic center region contain a potentially
important discovery: the possible presence of a new transient source in the galactic center
source complex. In Figures 3 and 4 below we show the hard x-ray images obtained by
EXITE in a broad band (Figure 3) and narrow band (Figure 4). The broad band image
shows the persistent source 1E1740.7-2942 as being detected at a flux level very consis-
tent with the results from the lower resolution Caltech gamma-ray imager GRIP the year
before (Cook et al 1991). The spectrum of the EXITE detection of the "IE" source is
also consistent with the GRIP results. In the narrow band from 82-111 keV, however, the
image in Figure 4 shows a new and much brighter source which we have tentatively called
EXS1730-2858, This possible transient source shows a remarkably flat spectrum. When
extrapolated back to the lower x-ray energies detectable with the Ginga all sky monitor,
it would be expected to give a flux of about 200 mCrab in the 6-20 keV "hard" band, or
at just about the confusion limit for the galactic center region. Unfortunately, no ASM
coverage of this region is available for the time of our observation. Before publishing this
possible discovery, we are carrying out further analysis of the images (and spectra) using
5
the new post-flight re-calibration of the detector.
• i i ..........!.............._.........
" "-.,%_¢. +,A, ....LZ:......
t o _ 5..." ............. _,......
I_36m
fBl_ 40m
1Th _lm
l
-0._ " " ......... lrn_m
.,........._..............!.......di ...........i
..... ....
.2 ....... . . . .
i I ! _ ! ; I ! I
-2 -5.5 -1 -0.,5 0 0.5 1 1,5 2
Figure 3. EXITE hard x-ray image _achc center regaon (48-153 keV).
:.. =.
.... :. ...... °..-°o-:
• . _ .......... : ........ 17h40m
' ...%............_:::::._'_i....
o.s _ ,,_,,_,._ +,_,,,_, ..:....... 1_-_
i
_.s (:2".. '.............................._..... ,_,em
-1,5 ............. ,:
-2
- ¢_,.,..'.. ,_
.115 _ ' t I I. I I
-2 -1 ..O.S 0 05 , 1.5
Figure 4. EXITE hard x-ray image of galactic center region (82-111 keY).
The black hole candidate GX339-4 was observed just after the galactic center for a
total exposure of also about 2 hours, it was detected, as shown in Figure 5 which is the first
hard x-ray image of this source. This object had been claimed as a hard x-ray source on
the basis of its detection with the non-imaging HEAO-A4 experiment (Nolan et al 1982).
However possible source confusion effects in the galactic bulge (as made clear recently
with the discovery, with the imaging telescope SIGMA on the Soviet Granat satellite, of
a new hard x-ray source within 1 degree of the bright galactic source GX5-1) had always
made this identification somewhat questionable. The EXITE image, and spectrum (Figure
6), which shows the source to be in a similar state as in the HEAO observations, are an
important new finding madaxebeing prepc_+dfor publication by Covault et al (1991).
-u 49 +0 +,1.. , I_ L-m
I_..:\....-_\ \.............._ l
, ......\ .'.,.. ..........\
,:4..........-,,........\ ...........\ ...........\1
_ !,, -...!,._ o...1,_............ .....1 • -'"'"'" _ i..'"'"
°-'+'-I" \ _":'" _ '"' '_"" 'E
........... ..........\ J,,,.,,,l
+.,,_'\ ..........."_ _ \ .............'i +I
....',_'" _ .........\ ",.21 1,,,,,,_
-'-I _ ..\..... _ . .--'_ /
+ _...... _ .,;.-' "...
"'"" """ " ; m
-l+q ...... _ \ ..... \ ._. ,_,1+
":'+"" k.O .... +'" \+1 \ ...........\ ..!,,... .. ,
7._-.....\ _...........,,. ..1_
II' I I I I: I I I •
-2 -t.S -1 -0.5 0 0.5 1 1.5 2
" Figure 5. EXITE hard x-ray image of _339-4 (36-63 keY).
' 1_ "
%%
1o
1#
I
"" ,m. :
I I I I l lll I
30 40 50 I_0 70OO 100 200
e.e_ {keY)
Figure 6. EXITE hard x-ray spectrum of GX339-4 compared with HEAO results.
5 Conclusions
The EXITE program to observe the supernova SN1987A and various galactic center region
targets as part of the final SN1987A campaign from Alice Springs, Australia, in May 1989
was a success. Although thesupernova was not detected, interesting limits were obtained
and analysis is continuing. Among the galactic source targets observed were the first
hard x-ray image of the black hole candidate GX339-4 and the discovery of a possible
new transient source in the galactic center region. The EXITE instrument and gondola
performed well, and are now in the final process of being upgraded for their next flight
with a new detector system.
6 References
Cook, W.R. et al 1991, Ap. J. LetterJ, 372, L75.
Covault, C., Braga, J. and Grindlay, J. 1988, in Proeeeding_ of Worl_hop on Nuclear
Speetroleopy of A,trophysical Sources, (N. Gehrels and G. Share, eds.), p. 444.
Covault, C., Grindlay, J. and Manandhar, R. 1991, in preparation.
Garcia, M., Grindlay, J., Burg, R., Mur ay, S., and Flanagan, J. 1986, IEEE Trans. Nucl.
Sci., 33, 735.
Grindlay, J., Garcia, M., Burg, R., and Murray, S. 1986, IEEE Trans. Nucl. Sci., 33,
750. ..
Kumagai, S. et al 1989, Ap. J., 345,412.
Nolan, P. et al. 1982, Ap. J., 262, 727.
Ramsey, B. and Weisskopf, M. 1986, Nucl. Instr. Meth., A248, 550.
Skinner, G.K. et al 1987, Nature, 330,544.
Sunyaev, R. et al 1990, Soy. Astron. Lett., 16(3), 171
S


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