Supernova 1987A from 1 keV to 10 MeV: The status as of 1990

Adv. Space Res. Vol. 11, No.8, pp. (8)207—(8)216, 1991

0273—1177/91 $0.00 + .50 1991 COSPAR

Printed in Great Britain.

SUPERNOVA 1987A FROM 1 keV TO 10 MeV: THE STATUS AS OF 1990 Mark D. Leising U.S. Naval Research Laboratory, Washington DC 20375, U.S.A. ABSTRACT Satellite, rocket, and balloon borne instruments have obtained a wealth of high-energy data from SN 1987A. These have added significantly to our understanding of Type II supernovae. Besides the exciting direct detection of just-synthesised radioactivity via ~-ray line observations, the light curves of the lines and the extremely hard continuum which results from them have provided clues to the structure of the newly disrupted star. Mysteries still remain. We do not yet understand how the radioactive matter reached low depths in the envelope at such early times. We have not yet identified the mechanism which produced the softest observed X-ray emission. We do not know how much of other radioactive species were produced. This could help us begin to understand in detail the nuclear burning and mass ejection processes. We have not yet seen the signature of the compact remnant of the collapse. At this point we have great hope that some of these questions will be answered with the state-of-the-art experiments already flying and scheduled for launch soon. INTRODUCTION Astronomers have long anticipated the opportunity to observe a nearby supernova, and have often reasoned that a Galactic supernova is overdue. The occurrence on 23 February 1987 of the supernova SN 1987A in the Large Magellanic Cloud provided a unique opportunity to observe a supernova over the entire electromagnetic spectrum, and even in neutrino emission. In some respects a supernova in the LMC is better than a Galactic supernova, namely that the distance is more accurately known and interstellar absorption is relatively small. The timing of the arrival of light from SN 1987A was fortuitous in that modern instrumentation allowed a more detailed view of the supernova event than would have been possible a few years earlier, and in that the precursor star had been previously studied. For X-ray and -i-ray astronomy, which can only be conducted above the Earth’s atmosphere, the timing was extremely fortuitous. The only orbiting -y-ray detector sensitive enough to detect the supernova had already overreached its expected lifetime by six times and was steadily falling back to Earth. Two satellites carrying hard X-ray experiments were being readied for launch in 1987. Also, a new generation of balloon-borne hard X-ray and ~-ray telescopes were being developed at that time. First we review the observations of emission from a few keY to a few MeV. Then we will review some of the framework for interpreting these observations. We try to be inclusive in referencing relevant paper. from the refereed literature, but the volume of work dedicated to SN 1987A almost certainly guarantees that some are missed. The author apologizes for any oversight. There are now several good, more general reviews of observations and theory of SN 1987A, e.g., /1/, for the interested reader. The first two experiments briefly discussed here (GINGA, and MIR/Kvant) are reviewed in detail in this issue /2,3/. REVIEW OF THE OBSERVATIONS The Japanese X-ray astronomy satellite GINGA, launched on sensitivity 5 Februaryfrom 1987,2 has of 2, and to a 40 set keV. proportional counters with effective area 4000 cm GINGA began observing SN 1987A just two days after outburst. No X-ray excess above the contributions of known sources was detected in the first few months, but by July 1987 (130 days after outburst) there was a hint of additional emission from the location of SN 1987A /4/. Because the bright and variable X-ray source LMC X-1 is only 0.6° from SN 1987A, great care had to be taken in observations with the 20 x 40 field of view proportional counters. Two methods were used to prove that the emission did indeed come from SN 1987A. One method was to scan the region and fit the collimator response functions at the

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locations of known sources and SN l987A (separating the sources much as one separates blended lines.) The second method was to point away from the supernova just enough to exclude LMC X-1 from the collimator. Both methods gave similar results, namely that the SN 1987A flux increased steadily over the period from July to September 1987 and then leveled off. (However, see /2/ for discussion of possible systematic errors.) The spectrum in September 1987 was very unusual, consisting of a soft component (consistent with a thermal bremsstrahlung spectrum with kT 4 keY), a hard component (essentially flat to 30 keY), and an emission line, presumably of iron. (See 1’ erg cur2 s1. Subsequent GINGA Figure of /2/.) The flux (10-30 keY) was -~ 5x10 observations revealed a nearly constant flux through day 300. However, a striking increase in flux was detected during the period from 26 December 1987 to 12 January 1988 /5/. Over that time the fluxe, in the bands 6-16 keY and 16-28 keY increased by factors of 2 and 4, respectively. After this “January flare” episode, the fluxes measured by GINGA declined slowly and steadily until they reached marginal detectability by day 700 /6/. (See Figure of /2/.) It is important to note that the fluxes in two aforementioned energy bands were generally well correlated (although early variability of the lower energy band was not obvious in the higher band), even during the rapid variation which occurred near day 320. ThIs Is significant because it was presumed that the two bands had their origins in different phenomena, as discussed below. The Roentgen Observatory in the Kvant module of the Soviet space station Mir became operational in 1987 also. Three X-ray instruments, the TTM coded mask imaging xenon gas proportional counter (2 - 32 keY), the HEXE phoewich (15 - 250 keY), and the Pulsar X- 1 scintillators (50 keY - 1 MeY) began observing SN 1987A starting in June 1987. The HEXE experiment first detected emission from SN 1987A in early August 1987 (beginning on day 168) /7,8/. Only upper limits were obtained at this time with the TTM imager /9/ so a series of pointing. just off the position of SN 1987A, combined with rocking of the HEXE collimator away from the sources to measure bacJiground, were used to locate the emission at SN 19$7A with the HEXE experiment. The Pulsar X-1 instrument was used to extend the detected continuum up to 300 keV and to confirm the HEXE results at lower energy. This spectrum measured above 20 keY was extremely hard, with a power law photon spectral index of — -1.4. The flux at 100 keY was ~ 2x105 cur2 s1 keV1. (See Figure of /3/.) The HEXE observations were consistent with the GINGA observations in the region of overlap, but the TTM upper limits were somewhat inconsistent with the GINGA measured flux, although the observations were not strictly concurrent. Subsequent observations of the hard continuum by the ffvant Roentgen Observatory showed perhaps a slight Increase over the remainder of 1987 (~ day 300) and a significant Increase between day 300 and the time after day 330 /10/. This increase was particularly significant in the harder band. > 45 keY. After day 400 the fluxes in all bands detected by Kvanl Instrument. dedilned steadily, until they reached the limit of detectability by day 700 /11/. (See Figure of /3/.) On 24 August 1987 a sounding rocket was launched from Australia carrying a position sensitive proportional counter to measure 0.2 - 2.1 keY X-ray emission from SN 1987A. An upper limit of 10-12 erg cur2 s1 keY4 at 1 keY was obtained /12/. Perhaps the most remarkable feature of this effort was that the entire international program, from conception to flight took only five months. On two subsequent rocket flights, In November 1987 and February 1988 (thus before and after the GINGA January flare), a CCD X-ray imaging spectrometer was flown to observe X-ray. in the range 0.65 to 2.0 keY /13/. Upper limits near 10-11 erg cm~2 s1 over that range were set for both observation.. All three of these rocket observations indicated the soft component of the GINGA spectrum turned over somewhere in the range 2 - 4 keY. Launched In 1990, ROSAT was used to set extremely low limits on the flux at 1 keY from SN 1987A (see /14/). Thus the X-ray spectrum of SN 1987A must have had a precipitous turnover at a few keY, if all of these observations were correct. NASA’s Solar Maximum Mission (SMM), a dedicated solar observatory containing seven experiments, was launched in 1980. Among those experiments was the Gamma Ray Spectrometer, a broad field of view Nal scintillator for detecting transient solar y-ray emission in the energy range 0.3 - 8.5 MeY. The experiment was always pointed at the Sun and made no direct background measurements. It had operated for seven years with one short interruption, and had been successfully used in several ways to detect ‘y-ray emission from outside the solar system /15/ as well as many solar flares. The Instrument could not be pointed off the Sun for more than a very short time, so the search for y-rays from SN

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l987A was an exercise in data analysis. Because no major operational changes were involved, the data from the long history of 5MM could be analyzed just as the data after February 1987 were, in order to study systematic background effects. Because of its location near the ecliptic pole, photons from SN 1987A had to pas. through 2.5 cm of CsI anticoincidence shielding to reach the delectors. For each orbit, data taken with the instrument exposed to the source were compared with those accumulated with the56Co Earthdecay between supernova and the instrument. evidence line the energies was found in data throughNoJuly 1987. for In -y-ray line emission at the the months that followed the evidence that there was 847 keY and 1238 keY line emission from SN 1987A mounted, and became more convincing with each additional accumulation /161. The fluxes were consistent with being constant from August through October 1987, near 10 cm2 s~in the 847 keV line and 6x104 cm2 s-i at 1238 keY. it is interesting to note that four ten-day spectral summations with some hints of 847 keY emission were shown at a workshop at George Mason University in October 1987, but theoretical prejudice against any such large early fluxe. was strong and little notice was taken. Further analysis of the SMIvf data revealed that line emission at 2.6 and 3.2 MeY (weaker lines of 56Co decay) were also present /17/. The light curves of all four lines were similar: they rose rather rapidly in the period 150 to 200 days, peaked shortly thereafter, and declined steadily until they were no Longer detectable by SMM by day 600. Figure 1 shows a long accumulation of the ~-ray line spectrum at the times of peak emission. Figure 2 shows the light curves of the two strongest 56Co lines from 9MM and the experiments described below. 0.8

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Energy (MeV) Fig. 1. The mean spectrum Observed by the 5MM GRS over the period from 1987 August 1 through 1988 May 28, from /17/. The offset solid line shows the approximate expected signal in the instrument from an embedded 56Co source. A CsI scintillation detector was carried by the satellite Cosmos 1686 to the Salyut-7 space station where it was employed from February to October 1987 to set upper limits on the 1.5 to 4.4 MeV -p.ray emission of 1.5x104 cur2 s1 keVt /18/. A major campaign was mounted to observe SN 1987A with high altitude balloons each spring and fall (when the high-altitude winds change direction) between May 1987 and May 1989 from Australia. There were many successful flights and several positive detections of both continuum and lines. An experiment resulting from an international collaboration, carrying a hard X-ray detector and a high-energy ~-ray spark chamber, was flown on days 55 and 407

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/19,20/. Only upper limits were obtained on the earlier flight, but a very hard continuum from 17 - 165 keV was observed on day 407. The shape and intensity of the emission was very similar to the Mi, results discussed above and other measurements described below. A hard X-ray/i-ray imaging system, GRIP, was flown on days 86, 209, 414, and 771 /21,22/. During the second and third flights hard emission was detected to over 1 MeV. 56Co lines, There was forwere excess counts The in the vicinity of the allowed two lower although no evidence line fluxes extracted. imaging capability theseenergy experimenters to rule out the possibility that the hard emission came from LMC X-1 and to definitely attribute it to SN l987A. A payload with a hard X-ray experiment and a high spectral resolution Ge ‘v-ray detector was flown on days 95, 248, 411, and 619 (actually flown without the hard X-ray detector on the first flight and without the -y-ray detector on the last.) On day 248 the 847 keV line /23/ and continuum /24/ (45 - 200 keV) were detected. The continuum was more intense and somewhat harder than previously reported. The 847 keY flux was consistent with other measurements, depending somewhat on the assumed width and possible contamination from background lines. No evidence was found for other 56Co decay lines. Although the 847 keY line can be interpreted as being broad in this observation, there was no evidence for flux on the high-energy side of the rest energy. This will be further discussed below. On day 411 the continuum was detected, but at a somewhat lower level, and only upper limits were obtained on day 619 /25/. Another high resolution Ge -y-ray spectrometer was flown on day 287. It detected continuum emission above 200 keY and a line at 1238 keY /26/. The continuum spectrum was similar to others measured near that time, but the line flux was quite high. No significant feature was seen at 847 keY. The 1238 keY line was found to be slightly blueshifted and significantly broadened. The width corresponded to a velocity of 2000 km s1. On day 320 a high resolution Ge spectrometer, known as GRAD, was launched In Antarctica. Very high fluxes, near 2x10-3 cm~2~ were measured In the 847 and 1238 keY lines /27/. Both lines were found to be near laboratory energies and broadened, width widths also corresponding to — 2000 km r1 velocities. A large volume high resolution spectrometer, CR19, was flown on days 433 and 013. Both 847 and 1238 keY lines were detected in both flights /28,29/, although the 847 keY line was of marginal significance In the first. The fluxes were generally consistent with other measurements. The line centroids were found to be near rest energies, and in all cases the lines were quite broad (2 3000 kin r~FWHM).

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Preliminary results have been reported for analysis of data from: a detector on the satellite COSMOS 1870 /30/ - a positive detection of both 847 and 1238 keV lines over the course of the second half of 1987; a balloon-borne Compton telescope /31/ - a significant detection of the 1238 keY line on day 417; and a high resolution spectrometer flown on a balloon on day 819 /32/ - quite low upper limits on 122, 847, and 1238 keY line fluxes. In general it can be said that all of the above measurements are amazingly consistent with each other and consistent with reasonably simple models of the hard emission expected from 56Ni in the supernova ejecta. We now discuss the theoretical framework for decay of understanding these observations. UNDERSTANDING THE HARD EMISSION The possibility of observing 56Co -v-ray lines to verify ideas about the nature of supernovae was discussed twenty years before SN 1987A /33/. Other nuclei were added to the list of prospects and analytic treatments of the time variations of the fluxes were developed /34,35/. Prior to SN 1987A, somewhat more detailed studies of the line and continuum emission were performed /36,37/. Since the discovery of SN 1987A calculations of hard emissions have been performed in spades, with increasingLy detailed multi-zone spherically symmetric models. However complicated the models of the disrupted stars may be, the physics describing the hard photon emission and escape is particularly simple. The evolution of the -v-ray line fluxes is determined by the competition between the decay of the emitting nuclei and the progressive free escape of the 1-ray photons due the thinning of the expanding ejecta. Therefore the relevant variables which determine the time variation of the flux from any given parcel of radioactive material is the total number of electrons per unit column area outside the parcel and the velocity of those electrons. The continuum arises from Compton scattering of the line photons one or more times. Those continuum photons which escape are produced, on average, external to the regions where their line photon progenitors were emitted, so the peak continuum fluxes will precede time freely escaping line flux maxima somewhat. When the continuum photons have scattered enough times to reach the hard X-ray regime (< 100 keV) they can be lost to photoelectric absorption. That crosssection becomes significant compared to that of Compton scattering at lower energy, particularly if heavy elements are present. This produces a low-energy cutoff to the continuum. As the ejecta become very thin and photons can scatter only a few times before escaping, the continuum becomes truncated above the energies where photoelectric absorption Is important. Soon after the outburst of SN l987A calculations of the X-ray and ~y-ray emission were undertaken. Simple models of the envelope structure were employed. All radioactivity was assumed to be at the inner edge of the ejecta, and for preferred values of 56N1 mass and envelope mass and velocity the calculations predicted marginally detectable fluxes of both lines and continua which would peak roughly two years after outburst (when the 1-ray depth of the entire ejected envelope reaches T — 1) /38-43/. More detailed models of the star, its composition and expansion gave very similar results for the escape of hard photons /44,45/. Meanwhile the presence of 0.07 M~ of 56Co in the ejecta was inferred from observations of the exponential decline of the optical light curve /46,47/. As described in detail above, just five months after the explosion a very hard X-ray continuum was detected and only one month later -v-ray lines were detected. This could be explained If the 1-ray depth of the ejecta were much less than anticipated, i.e., if the envelope were either very low mass or moving very fast. In this case the high energy fluxes would continue to increase, as only a very small fraction of the energy was then emerging in hard photons. Because this fraction was small, another possibility was that only a small amount of the radioactive 56Co was at very low -v-ray depth. Although several mechanisms could achieve this situation, it has been generically referred to as ~mixing”. With a simple treatment of electron scattering it was shown that the ratio of -v-ray line to continuum fluxes near 200 days implied that some 56Co was already found at very low 1-ray depth (of order unity) /48/. Several groups continued to refine models of the entire supernova phenomenon (not including explosion mechanism itself). They found that models with the SeCo initially distributed in depth within the envelope could describe well many aspects of SN l987A. Such models with 56Co at low enough depths could also describe reasonably well the evolution of the -v-ray line and continuum fluxes /50-51,11/. This was also shown by fitting simple two and three parameter models to the full light curves of the four strongest 56Co -i-ray lines / 17/. Clearly some 56Co (a few percent of 0.07 Me) was already at 1-ray depths of order ~r = 1 by just 200 days after outburst. But neither the -v-ray line or continuum light curves (given the precision of the measurements) tell us how the radioactivity was distributed in the envelope

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56Co at or what the mechanism for distributing It was. Possible scenarios for locating some low depths Include: I) Fragmentation of the overlying envelope, leaving some low-columndepth lines of sight into the thin, slow moving shell of material which had experienced temperature 2 5x109 K. 2) Acceleration, by dynamic instability or other mechanism, of some of the radioactivity to higher velocities and well into the thick expanding ejecta.

The y-ray line energies and widths could distinguish between these two possibilities, because of the differences in the velocities of the emitting nuclei (see /62/ for a discussion of this). As discussed above, the high energy resolution measurements indicated that the emitting

material was indeed accelerated to high velocities (~ 2000 km s-i), effectively ruling out scenario 1 in its simplest form. However, the lines were nearly symmetric about the rest energies which is also inconsistent with scenario 2. In that picture the receding material should be shielded by significantly more of the ejecta. Perhaps both of these mechanisms are involved, and only some combination of acceleration and fragmentation is required to explain

all the observations. Considerable theoretical effort is being expended to understand the dynamics of the simulations show in velocity space only a matter of

supernova ejecta following the explosion. Multidimensional hydrodynamic that dynamic instabilities can mix the ejected core material macroscopically /63,64,65,66/. These studies already show us the physics involved, and it is time until they begin to confront the observations directly.

So far, no other radioactivity has been seen in SN 1987A, but there are interesting prospects.

Nickel-57 should be co-produced with 56N1, and in the subsequent decay to 57Fe from 57Co 1-ray lines at 122 and 136 keY are emitted and are 5~Fe to 5~Fe,the 122 /34/. keY line flux 2otentially detectable If their could be detected experiments SIGMA GRANA radioactive parentsbyarethe produced in the solar on ratio of T /67/ and CR0 OSSE /68/ within

the next year /60,61,17/. Some calculations indicate that a somewhat higher ratio of mass 57 to mass 56 nuclei obtains in SN 1987A, depending on the cut between ejected and accreted material /69/, so there is hope that this important measurement will be made soon. At times 3-4 years after outburst there is still substantial scattering of the line photons into a hard continuum, although the ejecta have thinned enough that photoelectric absorption of continuum photons is no longer important. Therefore the number of hard photons Is very nearly conserved and a measure of the total flux from — 40 140 keY, even if the lines are not resolved, gives a very precise measure of the mass of 57Co ejected independent of a model of the ejecta /17/. Typical calculated spectra at late times are shown is FIgure 3. 0.1

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44Ti, 22Na, Supernova explosions might also eject other -v-ray line emitting nuclei, including and 60Co /35/ which might be detectable in the future In the ejecta of SN 1987A. Current best estimates of the fluxes of lines emitted by these species indicate that they will not be detected with any instruments yet planned /60,61/, but in the case of 44Ti at least, there is time. This measurement will be an important probe of the nuclear burning regions of the supernova. Because the ratio 44Ti / 56Ni varies with radius, the measurement of this ratio will help to define the location of the mass cut in SN l987A. Non-radioactive sources of X-ray and 1-ray emission which have been considered for SN l987A include the compact remnant of the collapse and the interaction of the ejects with circumstellar material. Neither has been definitely observed, but both have been invoked to explain the X-ray emission at ~ 20 keY. The origin of this emission remains a puzzle. The features of the soft X-ray emission which must be explained are its spectrum, including the low energy cutoff and emission line, and its variation with time, both in the long term and on the time scale of one day. The soft emission was observed to rise precisely with the harder emission which comes from scattering within the ejecta. Thus a natural explanation is that the soft emission also comes from within the ejecta. The rapid variability (e.g., in 1 day there was a significant decrease after the January 1988 flare) is most easily explained then if the source is very small. A pulsar driven synchrotron nebula has been suggested /70,71/, as has an accretion powered compact object in a binary system /72/. The synchrotron model does not produce, directly at least, the iron emission line. Both models predict that as time goes on and the envelope thins the spectrum should steepen and extend to lower energies, and the intensity should increase and asymptotically approach a constant value. None of these characteristics are indicated by subsequent observations. An alternative explanation is that the soft emission results from the interaction of the ejected matter with circuxnstellar matter, with a reverse shock propagating back into the ejecta and heating it /73,74/. This model is severely constrained by the upper limits on radio emission, 1 keY emission, and the rapid variability. These are explained with a very asymmetric circumstellar medium (to give rapid fluctuations) which is close to the supernova only on the far side from Earth (so radio and I keY emission is absorbed). In this model the simultaneous rise of the 20 keY fluxes is simply coincidental. Another proposal is that at least some emission even below 10 keY is due to the y-rays from radioactivity /75/. Although the primary photons are mostly removed by photoelectric absorption, each line photon can produce several — 100 keY electrons via Compton scattering, some significantly further out in the ejects than the location of the radioactivity. In slowing down, these energetic electrons might produce several bremsstrahlung photons. Those produced at low enough optical depth, particularly In the hydrogen envelope, might provide the observed soft emission. Thu model explains the strong correlation of soft and hard photons, but it is difficult to see how it can explain rapid variability of the soft emission. THE PRESENT STATUS The unambiguous detection of a substantial quantity of freshly synthesized radioactive 56Co is a major milestone for -v-ray spectroscopy and a significant success for theories of supernovae and nucleosynthesis. Undoubtedly we would have probed the structure of the supernova ejects in more detail if higher signal-to-noise observations were possible, but we have gained some insight into the structure, and our theories cannot yet explain completely the observations we have. Detection of other radioactive nuclei almost certainly produced either before or during the explosion awaits observations by SIGMA and CR0. In one case, that of 57Co, even an upper limit at the expected sensitivity will be important. No clear detection of a signature, high energy or other, of the compact object resulting from the collapse has been made yet. Over the next few years, as the ejecta become thin even to soft X-rays, SIGMA, R.OSAT, and CR0 will make sensitive searches for steady and pulsed emission. The soft X-rays observed by GINGA are still not understood, but there might be a contribution from a compact object. Another possibility, interaction of the ejects with circumstellar material, can also be confirmed by future observations. The variability at high energies and radio emission are characteristic features. If no such soft X-ray component is seen again, it will be natural to associate the GINGA emission with the decaying radioactivIty. In short, the present status of X-ray and -v-ray observations of SN 1987A in 1990 is very satisfying - we have some observations which agree strikingly well with predictions, some we still do not quite understand, and the promise of more exciting ones to be made.

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