Radioactive nuclei, especially those created in SN explosion, have long been
suggested to be important contributors of galactic positrons. In this paper we
describe the findings of three independent OSSE/SMM/TGRS studies of positron
annihilation radiation, demonstrating that the three studies are largely in
agreement as to the distribution of galactic annihilation radiation. We then
assess the predicted yields and distributions of SN-synthesized radionuclei,
determining that they are marginally compatible with the findings of the
annihilation radiation studies.Comment: 7 pages, accepted for publication in New Astronomy Reviews (Astronomy
  with Radioactivites III

Chan

Cheng

Dermer

Harris

Hatano

Howell

J.D Kurfess

Johnson

Jose

Kinzer

Leising

M.D Leising

Milne

P.A Milne

Purcell

R.L Kinzer

Share

Timmes

Woosley

English

arXiv

Radioactive nuclei, especially those created in SN explosion, have long been sug-gested to be important contributors of galactic positrons. In this paper we describe the ﬁndings of three independent OSSE/SMM/TGRS studies of positron annihi-lation radiation, demonstrating that the three studies are largely in agreement as to the distribution of galactic annihilation radiation. We then assess the predicted yields and distributions of SN-synthesized radionuclei, determining that they are marginally compatible with the ﬁndings of the annihilation radiation studies

Leising, Mark D

Milne, P A

Kurfess, J D

Kinzer, R L

Clemson University: TigerPrints

Clemson UniversityTigerPrintsPublications Physics and Astronomy10-18-2001Supernovae and Positron Annihilation RadiationMark D. LeisingClemson University, lmark@clemson.eduP A. MilneNaval Research LabJ D. KurfessNaval Research LabR L. KinzerNaval Research LabFollow this and additional works at: https://tigerprints.clemson.edu/physastro_pubsThis Article is brought to you for free and open access by the Physics and Astronomy at TigerPrints. It has been accepted for inclusion in Publicationsby an authorized administrator of TigerPrints. For more information, please contact kokeefe@clemson.edu.Recommended CitationPlease use publisher's recommended citation.arXiv:astro-ph/0110442v1  18 Oct 2001Supernovae and Positron Annihilation RadiationP.A. Milne a J.D. Kurfess, R.L. Kinzer b M.D. Leising caNRC/NRL Resident Research Associate, Naval Research Lab, Code 7650,Washington DC 20375bNaval Research Lab, Code 7650, Washington DC 20375cClemson University, Clemson, SC 29631AbstractRadioactive nuclei, especially those created in SN explosion, have long been sug-gested to be important contributors of galactic positrons. In this paper we describethe findings of three independent OSSE/SMM/TGRS studies of positron annihi-lation radiation, demonstrating that the three studies are largely in agreement asto the distribution of galactic annihilation radiation. We then assess the predictedyields and distributions of SN-synthesized radionuclei, determining that they aremarginally compatible with the findings of the annihilation radiation studies.Key words: Supernovae, ISM1 OSSE/SMM/TGRS ObservationsNine years of observations made with the Oriented Scintillation Spectrom-eter Experiment (OSSE) on-board NASA’s COMPTON observatory (1991-2000)[1], eight years of observations made with the Gamma-Ray Spectrometeron-board the Solar Maximum Mission (SMM) (1980-1989)[2], and two yearsof observations made with the Transient Gamma-Ray Spectrometer (TGRS)on-board the WIND mission (1995-1997) [3] have been utilized to study thegalactic distribution of positron annihilation radiation. The OSSE instrumentfeatured a 3.8◦ x 11.4◦ FWHM FoV, a ∼3 x 10−5 photons cm−2 s−1 line sensi-tivity (106 s on-source time), and a 45 keV energy resolution at 511 keV. Thesedetector attributes have permitted the first detailed studies of the distributionof annihilation radiation in the inner radian of the Galaxy. The annihilationof positrons with electrons gives rise to two spectral features, a line emissionat 511 keV and a positronium continuum emission (which increases in inten-sity with energy roughly as a power law up to 511 keV and falls abruptlyto zero above 511 keV)[4]. The TGRS instrument, which featured a germa-nium detector with excellent energy resolution, has demonstrated that theintegrated flux from the inner radian is best described as a narrow 511 keVPreprint submitted to Elsevier Preprint 18 December 2013line (FWHM ≤ 1.8 keV) and a positronium continuum to 511 keV line ratioof ∼ 3.6 (which corresponds to a positronium fraction of fPs=0.94)[3].Purcell et al. (1997) [5] reported results from combined OSSE/SMM/TGRSstudies of the 511 keV line component of annihilation radiation. They foundthe 511 keV emission to be comprised of three components; 1) an intense bulgeemission, 2) a fainter disk emission, and 3) an enhancement of emission at pos-itive latitudes (hereafter called a PLE). The PLE was also reported by Chenget al. (1997)[6], and has been interpreted to be an “annihilation fountain” byDermer & Skibo[7]. Purcell et al. (1997) characterized the emission via map-ping, employing the SVD matrix inversion algorithm, and via model fitting,testing the combination of a spheroidal Gaussian bulge, a disk that is flat inlongitude to ±40◦ and Gaussian in latitude (FWHM = 9◦), and a spheroidalPLE. The two characterizations differ in the thickness of the Gaussian disk(SVD being narrower) and the extension of the PLE. The enhancement of thePLE varied from 1.5 x 10−4 phot cm−2 s−1 for the SVD map to 9 x 10−4 photcm−2 s−1 for the 2D Gauss. PLE (FWHM = 16.4◦). The positron annihilationrates suggested by these two approaches are (4.2±0.5) x 1043 e+ s−1 (SVD)and (3.3±0.50) x 1043 e+ s−1 (2D Gauss.), with B/D ratios of 0.5 and 0.3.Parallel studies were performed by Kinzer et al. (2001). Those studies inves-tigated line and continuum positron annihilation radiation from the galacticplane, finding that the two emissions are similarly distributed within the statis-tical precision of the data. Kinzer et al. (2001) suggest two spatial distributionsto explain the OSSE/SMM observations. The first model is comprised of a 2DGaussian bulge (4.9◦±0.7◦(b) x 6.3◦±1.5◦(l)), and two disks; a 2D Gaussiandisk (12◦(b) x 35◦±10◦(l)) and a widely-distributed CO disk (also with 12◦latitudinal thickness). The second model is comprised of a slightly ellipsoidalbulge which follows the “R1/4” distribution function and an exponential disk(refer to Kinzer et al. (2001) for details of these functions). The positron anni-hilation rates suggested by these models are 3.9±0.4 x 1043 e+ s−1 and 3.1±0.4x 1043 e+ s−1 respectively, with B/D ratios of 0.7 and 3.We report here updates from our continuing analysis which extends the studyof Purcell et al. (1997) (see also Milne et al. (1998,1999) [9,13]). The primarydifferences between current studies and Purcell et al. (1997) are; 1) the inclu-sion of more observations, both archival and data collected after Purcell etal. (1997), and 2) reporting maps of the positronium continuum emission inaddition to the 511 keV line. To extract the positronium continuum compo-nent from the underlying galactic continuum emission, we widened the spec-tral modeling to include thermal bremsstrahlung and exponentially-truncatedpower-law models. We also removed high-energy diffuse continuum emissionfollowing a prescription from Kinzer et al. (1999), distributing the emissionspectrally according to a power-law (α = -1.65) and spatially according to a90◦ x 5◦ 2D Gaussian[11].2Fig. 1. Four characterizations of positron annihilation radiation from mapping algo-rithms. The upper two panels are Richardson-Lucy and SVD maps of 511 keV lineemission. The lower two panels are Richardson-Lucy and SVD maps of positroniumcontinuum emission. The SVD mapping algorithm was used in Purcell et al. (1997).3Maps made from mapping algorithms applied to the resulting data-set areshown in Figure 1. The top two panels show Richardson-Lucy and SVD mapsof 511 keV line emission, the lower two panels show Richardson-Lucy and SVDmaps of positronium continuum emission. Although not identical, the fourmaps are similar in appearance, with all exhibiting an intense bulge emissionand a fainter planar emission. 1 Pairings of bulge and disk components suggestthe same families of solutions for both the line and positronium continuumemissions. Both suggest that the bulge-to-disk ratio can vary from 0.2 -3.3depending upon whether the bulge component features a halo (which leadsto a large B/D). All of these comparisons support the Kinzer et al. (2001)suggestion that the two annihilation components are similarly distributed.Two bulge-disk combinations that span the range of favored solutions areshown in Figure 2. The upper panel shows a “bulge-dominated” solution,where a halo bulge plus a thin disk combine to annihilate 4.1 x 1043 e+ s−1with a B/D = 3.3. The lower panel shows a “disk-dominated” solution, wherea 2D Gaussian bulge (without a halo) plus a thick disk combine to annihilate4.1 x 1043 e+ s−1 (B/D =0.2).2 Supernovae and Galactic PositronsThe B/D ratios favored by the current work bracket those found by Purcellet al. (1997) and Kinzer et al. (2001), allowing a range of 0.2 ≤ B/D ≤3.3. The positron annihilation rate is suggested to lie within the range of(3.1 -4.2) x 1043 e+ s−1. To compare suggested SN positron production rateswith observations, a number of ad-hoc assumptions must be made. First, weassume that positrons do not travel large distances from their site of origin, sothat source B/D ratios are assumed to equal observed annihilation radiationB/D ratios. Second, we assume that there is no leakage of positrons from theGalaxy nor enrichment of positrons from extra-galactic sources. Third, weassume that the number of galactic positrons is not changing, that there isa steady-state population of positrons. These three assumptions allow us tocompare annihilation radiation fluxes with positron production rates. Fourth,we assume that the SNe in the Galaxy scale with B-band luminosity the sameway as SNe in other late spiral galaxies. This permits the use of extra-galacticSN rates as an indicator of the recent SN activity in the Galaxy.SNe Ia produce positrons primarily from the decay of 56Ni → 56Co → 56Fe,although the decay of 44Ti → 44Sc → 44Ca also contributes positrons. Chan &Lingenfelter (1993) simulated positron transport through SN Ia models andsuggested that ∼ 5% of 56Co-decay positrons escape the ejecta for favorablemagnetic field scenarios. Milne et al. (1999,2001a) followed up that study bycomparing SN Ia light curves with simulations. That study found that the1 We refer the reader to Milne et al. 2001 for a discussion of current investigationsof the PLE.4Fig. 2. Two characterizations of positron annihilation radiation from model-fitting.The upper panel shows a “bulge-dominated” solution. The lower panel showsa “disk-dominated” solution. The “bulge-dominated” solution is similar to thenova-spheriod model of Kinzer et al. (2001), the “disk-dominated” solution is similarto the three 2D Gaussian model of Purcell et al. (1997) and to the two 2D Gaussian+ CO model of Kinzer et al. (2001).light curves suggest that ∼8 x 1052 positrons escape the ejecta from a normaltype Ia supernova. Assuming the typical 44Ti yield in a SN Ia to be 2 x 10−5M⊙, and 100% escape of those positrons, only 5 x 1050 44Ti positrons would beproduced in a single SN Ia. Assuming the SN Ia rate in the Galaxy to be 0.4SN Ia per century, this translates to 1.0 x 1043 and 6 x 1040 positrons s−1 from56Co and 44Ti decays. The SN Ia contribution to galactic positrons principallydepends upon whether positrons escape in quantity from the SN ejecta.Estimating the massive star (SNII/Ib and WR star) contribution to galacticpositrons is more complicated. They produce positrons through the decaysof 44Ti → 44Sc → 44Ca, and 26Al → 26Mg, the latter from both hydrostaticand explosive phases. The predicted yields of these isotopes vary dramaticallyin individual events. Woosley & Weaver (1995) suggest that they range from≤ 10−6 up to 2.3 x 10−4 M⊙ for44Ti, and 1.7 x 10−5 –3.6 x 10−4 M⊙ for26Al[16]. SN II/Ib rates have been reported to range between 1.5 –3.3 SNe II/Ibper century[15,21]. To help to constrain our estimates, we use observations of1.809 MeV line emission, which is produced in 100% of 26Al decays. Thesesuggest about 3 x 1042 26Al decays s−1 occur entirely in the galactic disk (see5Pluschke in these proceedings) supplying 2.4 x 1042 positrons s−1.The positron contribution of 44Ti is best constrained by galactic chemicalevolution and the solar abundance of the daughter 44Ca. This requires that2–4 x10−6 M⊙ of44Ti are now ejected into the galactic disk [17]. We note,however, that the production obtained by multiplying the above rates andyields falls short of this. The implied positron production rate is 1.6–3.2 x1042 s−1, and we estimate the total massive star contribution to the disk to be4–6 x 1042 positrons s−1.Combining these estimates, 30% -50% of galactic positrons may be explainedby thermonuclear SNe and massive stars. This appears promising, but thismodel must also agree with the observed B/D ratio of annihilation radiation.SNe Ia occur in both bulges and disks of spiral galaxies, with suggested B/Dratios ranging from 1/2 to 1/7 [20,21]. Our estimates suggest that the B/Dratio ranges from 0.1 to 0.6. These values are at the low extreme of the fit-ted B/D ratios from the annihilation data (which are realized when halo-lessbulges are combined with thick disks). If 56Co positrons do not escape fromthe ejecta of SNe Ia, in contrast to the studies of Milne et al. (1999,2001) ,then the massive star contribution is at most 20% of galactic positrons, andthe bulge component is entirely due to a different type of source.3 DiscussionAfter accepting a couple of ad-hoc assumptions, we have shown that SNeare likely to be a dominant contributor to galactic positrons. We find thatthe maximum SN + 26Al contribution occurs with low B/D ratios, requiringthat the remaining 45% of positrons be produced by a wholly-bulge sourcetype. Lowering the SN II/Ib contribution somewhat relaxes this requirement,but the remaining sources must collectively have a large B/D ratio. Potentialsources include; 1) a recent galactic center starburst that produced a popu-lation of bulge positrons through massive star nucleosynthesis and SNe II, 2)positrons produced by a galactic center compact source, 3) nucleosynthesisin novae. All three of these sources are contrained by theory or observations.The starburst is constrained by the absence of a bulge component in the 1.809MeV maps and the failure to detect 1.173 MeV emission from the decay ofSN-produced 60Fe. OSSE observations of 511 keV line emission from the galac-tic center limit the contribution of a compact source to less than 1.5 x 10−4photons cm−2 s−1[10]. This is less than 6% of the total annihilation radia-tion. However, if the positrons diffuse throughout the bulge, a GC compactsource could contribute at a higher level. Novae production of positrons fromradionuclei (other than 26Al) are constrained by simulations that suggest thatfew positrons produced in CO novae would escape [22], and low 22Na yieldsin ONe novae[23].6There are too many uncertainties involved in these comparisons to determinethe validity of any of the assumptions that were used. Advances in the studiesof the gamma-ray and optical emission from SNe will eventually resolve thesequestions. At present, we must be content with interpreting hints as to theSN contribution to positrons in the Galaxy.References[1] Johnson, W.N., et al. ApJS, 86, 693 (1993).[2] Share, G.H., et al., ApJ 326, 717 (1988).[3] Harris, M.J., et al., ApJ 501, L55 (1998).[4] Ore, A. & Powell, J.L., Phys Rev, 75, 11 (1949).[5] Purcell, W.R., et al., ApJ 491, 725 (1997).[6] Cheng, L.-X., et al., ApJ 481, L43 (1997).[7] Dermer, C.D. & Skibo, J.G., ApJ 487, L57 (1997).[8] Kinzer, R.L., et al., ApJ, 559, 705 (2001).[9] Milne, P.A. et al., Astro. Lett. & Comm. 38, 441 (1998).[10] Milne, P.A. et al., in Proceedings of the 6th Compton Symposium, in press(astro-ph 0106157) (2001a).[11] Kinzer, R.L., Purcell W.R., & Kurfess, J.D., ApJ 515, 215 (1999).[12] Chan, K.-W. & Lingenfelter, R., ApJ, 405, 614 (1993).[13] Milne, P.A., The, L.S., Leising, M.D., ApJS, 124, 503 (1999).[14] Milne, P.A., The, L.S., Leising, M.D., ApJ-accepted, (astro-ph 0104185) (2001b).[15] Cappellaro, E., Turatto, M., astro-ph, 0012455 (2000).[16] Woosley, S.E., Weaver, T.A., ApJS, 101, 181 (1995).[17] Timmes, F. X., Woosley, S. E., Hartmann, D. H., & Hoffman, R. D., ApJ, 464,332 (1996).[18] Vink, J., et al. astro-ph, 0107468 (2001).[19] Mahoney, W., Ling, J.C., Jacobson, A., ApJ, 286, 578 (1984).[20] Howell, A., Wang, L., Wheeler, J.C., ApJ, 530, 166 (2000).[21] Hatano, K., Fisher, A., Branch, D., MNRAS, 290, 360 (1997).[22] Leising, M.D., & Clayton, D.D., ApJ, 323, 159 (1987).[23] Jose, J., Coc, A., & Hernanz, M., ApJ, 520, 347 (1999).7

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