The characteristics of crystal diffraction telescopes (the fact that
one observes in a narrow energy band of typically a few keV with a field-of-view
of typically 15-30 arc seconds and with virtually no background) can be
exploited for a variety of observational aims: precise source localization,
two-dimensional intensity mapping of sources with arc minute extent, the
observation of narrow spectral lines, measurement of pulsar light curves
in a narrow energy band...
While the first focused cosmic gamma-rays may originate from well studied
compact continuum sources like the Crab nebual and pulsar [see
CLAIRE], the ultimate potential of a crystal diffraction telescope
is in gamma-ray lines. The concept of a broad bandpass crystal telescope
is ideally matched to the gamma-ray lines in the domain of nuclear transitions
- the sites of explosive nucleosynthesis are thus a natural target : The
primary scientific objective of MAX is the study of type Ia
supernovae by measuring intensities, shifts and shapes of their nuclear
gamma-ray lines. When finally understood and calibrated, these profoundly
radioactive events will be determining in measuring the size, shape, and
age of the Universe.
Observing the radioactivities from individual classical novae and core
collapse supernovae will significantly improve our understanding of explosive
nucleosynthesis. Sensitive gamma-ray measurements also hold out the prospect
of observing SNe in optically obscured regions and resolving problems in
understanding SN rates. Gamma-ray line spectroscopy is expected to clarify
the nature of galactic microquasars (e+e- annihilation radiation from the
jets), neutrons stars and pulsars, X-ray Binaries, AGN, solar flares and,
last but not least, gamma-ray afterglow from gamma-burst counterparts.
Accomplishing all these objectives will require significant improvement
in sensitivity over current and planned missions.
While the evidence for point like sources of narrow gamma-ray line emission
has been mostly implicit at this point - besides the supernovae 1987A
and 1991T - various objects like galactic novae and extragalactic
supernovae are predicted to emit detectable gamma-ray lines. These
sources should have small angular diameters but very low fluxes - mostly
because such objects are relatively rare and therefore are more likely
to occur at large distances. The instrumental requirements for exploring
this kind of sources match with the anticipated performance of a crystal
diffraction telescope:
| Principal scientific objectives for a tunable gamma-ray lens |
| class of object | process | line energy [ keV ] | potential sources |
| ‘broad class annihilators’ | e+e- | 511 | 1E1740.7-2942
GRS1758-258 Cyg X-1 ... |
| classical novae | 7Be(EC,g)7Li
22Na (ß+)22mNe |
478
511 1275 |
GC novae
(N Cyg 1992, N Aqi 1982 ...) |
| Supernovae | 57Co(EC,g)57Fe
56Co(EC,g)56Fe 44Ti(b+,g)44Sc |
122
847 1238 1157 |
SN1987A, SN1991T |
| NS, Pulsars | e+e-
1H+n -> 2H+g |
<511
<2220 |
Crab ...
(1.8 MeV lens ?) |
| X-ray Binaries | e+e- | 511 | Nova Musca , Persei |
| AGN | e+e-
56Fe(p,p',g) 24Mg(p,p',g) 20Ne(p,p',g) 28Si(p,p',g) 12C(p,p',g) |
< 511
<847 <1369 <1634 <1779 <4439
|
NGC4151, 3C273 ...
redshifts ! for z~3.5 => Eg~0.5 MeV => Eg~1.2 MeV |
| solar flares | e+e-
56Fe(p,p',g) 24Mg(p,p',g) |
511
847 1369 |
|
| g-ray afterglow | e+e- | < 511 | g-burst counterpart |
| "Broad class annihilators" |
The recent discovery of broad annihilation features in several compact
sources (Bouchet et al., 1991; Goldwurm et al, 1992; Briggs, 1991) has
shown that there is one or several types of objects that obviously can
produce intense eruptions of positrons. The question is now whether these
“broad class annihilators” also generate the positrons that produce the
narrow 511 keV line.
The galactic center source 1E1740-29 has been observed by the SIGMA
telescope to emit a strong spectral features in the energy interval 300-700
keV that emanated and vanished within days. Radio observations of this
object reveal the presence of an AGN like structure with double sided radio
jets emanating from a compact and variable core. If the “broad class annihilator”
indeed is associated with the radio source, the origin of its high energy
emission becomes a key question for gamma-ray astronomy.
Featuring a sensitivity of ~10-6 ph.cm-2s-1 at 511 keV and an angular
resolution of 15”, a spaceborne crystal diffraction telescope can test
hypotheses on the intensity and site of the narrow 511 keV line. If the
radio lobes really track twin jets of positrons out to their annihilation
sites in the superposed molecular cloud, a space borne telescope could
localize the annihilation regions within less than a day: the predicted
flux (Ramaty et al., 1992) of 10-4 ph.cm-2s-1 (‘conservative number’) from
the outer lobes of the jets would result in 5 s detections in a few hours.
| Novae |
The detection of nuclear gamma-radiation from classical novae can offer
unique insights into the conditions within the burning regions and the
dynamic processes initiated by the runaway explosion (Leising and Clayton,
1987). The high temperatures during the thermonuclear processes induce
proton captures on most nuclei in the burning region, transforming stable
seed nuclei into unstable proton rich nuclei. The extreme temperature gradient
across the envelope at the peak of the burning produces rapid convective
energy transport which can mix the envelope material. Large numbers of
unstable nuclei with lifetimes longer than the convective time scale could
appear at the surface where they are in principle detectable from their
nuclear decay or positron annihilation gamma rays (table 1). Unstable nuclei
with even longer lifetimes (greater than a few days) could survive the
ejection and thinning of the envelope. Then their decay could be observed
in gamma rays even if their yields are relatively small.
Since the frequency of Nova explosions in our galaxy is about 40 per
year, this kind of object is a very attractive candidate for point source
gamma ray line observations. A few hours after the explosion the emitted
lines will be blue shifted (*E = 0.7%) as the observer would see only the
emission from the approaching ejecta (v = 2000 km/sec) due to the optically
thick medium (
Harris et al., 1991). This is relevant for the profile of the 511 keV
line that is produced mainly during the first day of the explosion. The
evolution in time over the first two hours is dominated by the positron
annihilation produced by the 13N decay while the 18F decay
dominates later.
| line energy | width | mechanism | time scale | mass produced |
| 478 keV | ~ 6 keV | 7Be (EC)7Li (10.4 %) | 53.3 d | 10-8 Mo |
| 511-516 keV | ~ 3 keV | ß+ decays of 13N (862s), 14O (102s), 15O(176s),18F(158m) | ~ 1 day | N/A |
| 1275 keV | 16 keV | 22Na (ß+)22mNe (90.4 %) | 3.75 y | 1.6 10-7 Mo |
Table 1 : observable gamma-ray lines from novae
After the first few days from the explosion the emitting material will
become optically thin to the gamma rays so blue- and red shifted
material will contribute to the observed flux, in which case a broadening
(*E = 1.3 %), but not a net shift of the line is expected.
It has been pointed out that novae are possibly significant contributors
to the Galactic 7Li abundance - this has important cosmological consequences.
The standard model requires that the primordial 7Li abundance must be enhanced
by subsequent nuclear nucleosynthesis, while the non-standard models require
primordial 7Li to be destroyed by some mechanism in Population II dwarfs.
The problem could be clarified if a stellar source of 7Be was identifiable.
| Supernovae |
Deeper insight in the explosive nucleosynthesis using the usual key
isotopic decay chains identified for supernovae might be used to constrain
the models (at this time, detonation or deflagration) and to understand
the dynamics of the explosion through the shape and red (blue) shifts of
the gamma-ray lines. The expected fluxes are highly dependent on the models
of the different types of SN explosions (especially the convection processes
which could remove synthesized materials from the high temperature burning
regions). The study of the explosive nucleosynthesis represents a crucial
input to better understand the chemical history of the Galaxy.
The nuclear gamma-ray lines from a supernovae that could be observed
by a crystal lens are the 847 keV and 1238 keV line from the decay chain
56Ni -> 56Co -> 56Fe, the 1156 keV line from 44Ti, and the 1173 keV line
from 60Fe. The photons produced by the nuclei in the shell have noticeable
Doppler-shifts due to the motion of the expanding supernova ejecta (a few
10000 km/s). A large broadening of the lines - up to 40 keV at 847 keV
is expected for SN type I where the shell gets transparent relatively early.
At this energy the bandwidth of a crystal diffraction telescope is about
16 keV FWHM which corresponds to > 35% of the flux in the SN line. Tuning
parts of the lens to different energy bands or scanning the line profile
in energy will provide a complete coverage of these potentially broad features.
For supernovae of type II (core collapse SN - the gamma-ray flux is
initially obstructed by the massive shell), the broadening is much less
accentuated than for SNI’s as the observations of SN1987A have shown. A
volume of a few Mpc should be accessible to an instrument with a sensitivity
of 10-7-10-6 ph cm-2.s-1 (Tobs 106 seconds) - this will make their detection
possible for events occurring within our local cluster.
It has been suggested that the observability of SNIa can be expressed
independently of the distance of the host galaxy since the optical peak
magnitude of the SN should be directly correlated to the gamma-ray line
flux (Arnett, 1982). Indeed, the decay of the ejected gamma-ray isotopes
actually is the energy source of the optical light curve.
Here, SN1991T has been used to establish a relation between gamma-ray
flux f847 and optical peak magnitude mv (the COMPTEL detection of SN1991T
6 givesf847 = (5.3±2.0).10-5 ph cm-2.s-1 for an optical
peak magnitude of mv = 11.6)
log(f847/10-4 ph cm-2.s-1) = 0.4.(10.9-mv) (1)
According to eq. 1, a detectable flux of ~ 10-6 ph cm-2.s-1 is expected
from SNIa’s with optical peak magnitudes mv < 16. In recent years (2/1987-6/1996),
events of this magnitude and brighter were observed at a rate of about
three per year (Tsvetkov et al., 1996)
| Mapping of continuum sources with arc minute extent |
For sources that have arc minute extent, the narrow field- of view of
the lens can be exploited to map the emission intensity. Examples are plerion-type
SNRs such as the Crab Nebula (which will make an excellent scientific objective
for a first balloon flight). As the inner regions of the Nebula are governed
by the pulsar, the emission intensity distribution within the nebula is
determined by the interaction between pulsar and nebula. The nebula size
can be understood in the context of magnetohydrodynamic (MHD) bulk-motion
models (Rees and Gunn, 1974). In these models, the energy released by the
spin-down of the pulsar is emitted via three components: a relativistic
stellar wind of charged particles, a low-frequency, large-amplitude electromagnetic
wave, and a toroidal magnetic field originating from the wind-up of the
dipole field of the pulsar. The pulsar is at the center of a cavity that
is empty except for a relativistic wind and magnetic field emanating from
the pulsar. This supersonic wind is expected to terminate at a shock-boundary
Rs, where the ram pressure is balanced by the magnetic and particle density
of the nebula. Beyond the shock, the particle motions become randomized,
leading to intense synchrotron emission. The cavity seen directly around
the pulsar in the optical and soft X-ray band places the shock boundary
Rs at 10 arc seconds. It has therefore been proposed (Aschenbach and Brinkmann,
1975) that the nebula size should not shrink beyond 10 arc seconds with
increasing energy.
| Observation of pulsar light curves |
Multi-band observations show pulsar light profiles to vary drastically
with wavelength. The light curve of the Crab pulsar has been extensively
studied by Compton GRO and is seen to vary even within the gamma-ray domain.
The low background rate of our telescope can allow the determination of
the pulsar light curve profile in a narrow energy band of a few keV. Independent
of the pulsar model, the typical pulsar emission energy varies with the
position within the magnetosphere, the observation angle with respect to
the pulsar magnetic field and the strength of this field. It is thus possible
that the pulsar light curve in a narrow energy range is more structured
than that obtained in the wide energy band that is typically used for light-curve
analysis. If this is the case, the light curves obtained with the crystal
lens telescope would be indispensable for the understanding of the geometry
of the emission zones within the nebula and the different emission mechanisms.
For strong pulsars such as the Crab pulsar (pulsed flux 7.6·10-5
(E/100keV)-2.04 ph/cm2s1keV) or PSR 1509-58, the statistics from one balloon
flight will not be sufficient for the determination of a precise light
curve, but with a space borne telescope this becomes feasible.
references
Bouchet L. et al., ApJ 383, L45, 1991
Goldwurm A. et al,, ApJ 389, L89 1992
Briggs M., Ph.D. Thesis, Univ. California, San Diego, 1991
Ramaty, R., Leventhal, M., Chan, K.W., & Lingenfelter R., ApJ ,
392, L63, 1992
Leising, M. D., Clayton, D. D., ApJ, 323, 159, 1987
Harris, M. J., Leising, M. D., Share, G. H., ApJ, 375, 216, 1991
Arnett W.D., Supernovae : A Survey of current research, p.221, ed M.J
Rees and R.J Stoneham (Boston:Reidel), 1982
Tsvetkov D.Yu., Pavlyuk N.N., Bartunov O.S., Sternberg Astronomical
Institute Supernova Catalogue,
http://www.sai.msu.su/groups/sn/sncat/, 1996
Rees, M.J., and Gunn, J.E. , M.N.R.A.S.,167, 1, 1974
Aschenbach, B., and Brinkmann, W., A&A, 41,147, 1975