------------------------------------------------------------------------ From: naya@tgrs2.gsfc.nasa.gov (Juan E. Naya) Subject: 60Fe Paper \documentstyle[11pt,aaspp4,psfig]{article} %\documentstyle[11pt,aasms4]{article} %\psfigurepath{plots/} \def\etal{{\rm et~al.\ }} \begin{document} \title{Gamma-ray limits on Galactic $^{60}$Fe nucleosynthesis and \\ implications on the Origin of the $^{26}$Al emission} \author{Juan E. Naya\altaffilmark{1}, Scott D. Barthelmy\altaffilmark{1}, Lyle M. Bartlett\altaffilmark{2}, Neil Gehrels, Ann Parsons, Bonnard J. Teegarden and Jack Tueller} \affil{NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA} \author{Marvin Leventhal} \affil{Dept. of Astronomy, University of Maryland, College Park, MD 20742-2421, USA} \altaffiltext{1}{Universities Space Research Association, 7501 Forbes Blvd. \#206, Seabrook, MD 20706-2253, USA} \altaffiltext{2}{NAS/NRC Resident Research Associate, Code 718, NASA/GSFC, Greenbelt, MD 20771, USA} \begin{abstract} The Gamma Ray Imaging Spectrometer (GRIS) recently observed the gamma-ray emission from the Galactic center region. We have detected the 1809 keV Galactic $^{26}$Al emission at a significance level of 6.8$\sigma$ but have found no evidence for emission at 1173 keV and 1332 keV, expected from the decay chain of the nucleosynthetic $^{60}$Fe. The isotopic abundances and fluxes are derived for different source distribution models. The resulting abundances are between $2.6\pm0.4$ and $4.5\pm0.7$ $\, M_\odot$ for $^{26}$Al and a 2$\sigma$ upper limit for $^{60}$Fe between 1.7 and 3.1 $\, M_\odot$. The measured $^{26}$Al emission flux is significantly higher than that derived from the CGRO/COMPTEL 1.8 MeV sky map. This suggests that a fraction of the $^{26}$Al emission may come from extended sources with a low surface brightness that are invisible to COMPTEL. We obtain a $^{60}$Fe to $^{26}$Al flux ratio 2$\sigma$ upper limit of 0.14, which is slightly lower than the 0.16 predicted from current nucleosynthesis models assuming that SNII are the major contributors to the galactic $^{26}$Al. Since the uncertainties in the predicted fluxes are large (up to a factor of 2), our measurement is still compatible with the theoretical expectations. \end{abstract} \keywords{Galaxy : abundances --- gamma rays : observations --- nucleosynthesis --- stars : Wolf-Rayet --- supernovae : general} \section{Introduction} Stellar nucleosynthesis produces a number of radioisotopes which are potentially observable through their gamma-ray emission. Radioisotopes with decay times that are long compared to the intervals between the events that eject them establish steady state abundances in the interstellar medium. Among these radioisotopes, the species $^{26}$Al ($t_{1/2}=3D7.2\cdot10^{5}$ y) and $^{60}$Fe ($t_{1/2}=3D1.5 \cdot10^{6}$ y) are of particular interest since their decay times are short compared to Galactic rotation. The abundances of these species in the interstellar medium therefore serves as an important tracer of the stellar population responsible for their synthesis. The existence of Galactic 1809 keV line radiation was well established after the detections by the HEAO-C and SMM spacecraft and was subsequently confirmed by different balloon-borne instruments (see Prantzos \& Diehl 1996 for a review). In spite of the enormous progress in the understanding of the 1809 keV emission in the last few years, thanks to the sky maps obtained by COMPTEL, the main contributor to this emission is still an issue of discussion. Theory predicts that $^{26}$Al is released into the interstellar medium by nova and supernova explosions, from winds of massive stars in the Wolf-Rayet phase, and from less-massive stars in the very late stages of their evolution (in the Asymptotic Giant Branch phase). However, uncertainties in the models do not allow the contributions of each source to be precisely predicted (\cite{pra96}). An important clue to this puzzle would be provided by the detection of the $^{60}$Fe emission. The isotope $^{60}$Fe ($t_{1/2}=3D1.5\cdot10^{6}$ y) decays to $^{60}$Co ($t_{1/2}=3D5.3 $ y) which then decays to $^{60}$Ni, simultaneously emitting two gamma-ray photons of energies 1173 and 1332 keV. Models predict that $^{60}$Fe is released into the interstellar medium through supernova explosions and calculate the average $^{60}$Fe mass yield from Type II supernovae (SNe II) to be about one-third of that for $^{26}$(\cite{tim95}; \cite{tim97}). This implies that, if the main contributor to the Galactic $^{26}$Al are supernovae, the 1173 and 1332 keV $^{60}$Fe line emissions should be close to the limits of detectability of current gamma-ray telescopes. In this paper we present the observation of $^{26}$Al and $^{60}$Fe line emissions performed by the Gamma-Ray Imaging Spectrometer (GRIS). While $^{26}$Al is clearly detected, we can only derive upper limits for the $^{60}$Fe. We obtain the fluxes and Galactic abundance of these two isotopes assuming various source distribution models and we discuss the implications of the derived values on the current models of Galactic nucleosynthesis. \section{The GRIS Flight} The Gamma Ray Imaging Spectrometer (GRIS) is a balloon-borne high-resolution gamma-ray spectrometer consisting of an array of seven germanium detectors surrounded by a thick (15 cm) active NaI anticoincidence shield. This instrument was reconfigured with a wide field collimator ($100^{\circ}\times 75^{\circ}$ FWHM field-of-view), and a 15 cm thick NaI blocking crystal to optimize its capability for observation of diffuse gamma-ray sources such as the cosmic diffuse background and Galactic line emissions, in particular the $^{26}$Al and the $^{60}$Fe radiations. The measurements reported in this paper were made on a flight from Alice Springs, Australia, on 1995 October 24-26. The total germanium detector area and volume were 237 cm$^{2}$ and 1647.6 cm$^{3}$ respectively. The duration at float altitude was 32 hours at an average atmospheric depth of 3.8 g cm$^{-2}$. The collimator was always pointed at the zenith. From Alice Springs, the Galactic center, south Galactic pole, and Galactic plane (l=3D$240^{\circ}$) transit nearly overhead (see top of fig~\ref{fig1}). GRIS observed alternating 10 minute exposures with the blocking crystal open and closed. \section{Data Analysis and Results} The variation of the 1809, 1332 and 1173 keV line intensities measured during the flight is displayed in fig. 1. These values were calculated by fitting the GRIS data with a Gaussian plus a power law model. The intensity, centroid and width of the Gaussian were set as free parameters for the 1809 keV fits. For the 1332 and 1173 keV fits, the centroids were fixed at the values expected from decay at rest and the widths were fixed at the values for narrow line emission. Since there was no hint of these lines in the spectrum, it was more appropriate to leave the line intensity as the only parameter of the fit. The instrument energy resolution was precisely determined by fitting a line to the measured widths of many intense narrow lines in the background spectrum. The derived instrumental width at 1809, 1332 and 1173 keV were respectively 3.4, 2.8 and, 2.7 keV FWHM. \placefigure{fig1} Notice that the 1809 keV drift scan shows an excess during both Galactic center transits which is a detection of Galactic $^{26}$Al emission. Such modulation is not observed in the 1332 and 1173 keV drift scan data. Due to the lack of imaging capabilities of GRIS, the flux of the emissions was derived by fitting the drift scan data with a given source distribution model and taking into account the instrument response plus the effects of atmospheric absorption. Based on recent nucleosynthesis and stellar evolution models (\cite{tim97}) we have assumed that most of the Galactic $^{26}$Al and $^{60}$Fe is generated by SNII explosions and thus, both isotopes have an identical spatial distribution. This assumption is also supported by the COMPTEL irregular profile that favors massive stars as the source of $^{26}$Al. In order to quantify the influence of the source distribution profile on the derived fluxes, we have extended the study to several models. Fig ~\ref{fig2} shows the latitude integrated flux profile for the models considered. The long-dashed line corresponds to the Galactic high-energy-gamma-ray measurement performed by the COS-B instrument, which has been used to fit most previous 1809 keV observations. The dotted line represents the profile derived from the 1.8 MeV COMPTEL map from 3.5 yr of observation integrated for $-10^{\circ}$450 km s$^{-1}$ and it favors models of an origin of the $^{26}$Al in Supernovae or Wolf-Rayet stars rather than from the slower winds in AGB stars. However, it is not well understood how $^{26}$Al can maintain such a high speed for 10$^{6}$ years. Different scenarios that can account for a broad emission have been studied, but none of them seems to provide a satisfactory explanation to the GRIS observation (\cite{chen97}). For the $^{60}$Fe lines (fig~\ref{fig3} (b) and (c)) only the line intensity was left as a free parameter. The best fit is consistent with no $^{60}$Fe detection, which is in good agreement with the drift scan analysis previously shown. \placefigure{fig3} \section{Discussion} The derived $^{26}$Al and $^{60}$Fe abundances and fluxes shown in table ~\ref{tbl-2} represent a valuable piece of information for the understanding of the nucleosynthetic activity in our Galaxy. Notice however, that these values are closely related to the assumed models which, in principle, could differ significantly from reality. The obtained $^{26}$Al abundances are significantly higher than the 1.8 $\, M_\odot$ reported by COMPTEL (\cite{kno98}). This discrepancy results from the $^{26}$Al emission flux derived by GRIS, which is higher than that derived by COMPTEL. Taking the measured COMPTEL longitude distribution as the model the source distribution (see fig~\ref{fig2}) we derive a flux which is $5.4\pm0.7\cdot10^{-4}$ photons s$^{-1}$ cm$^{-2}$ rad$^{-1}$ which is 2.8 times higher than that derived for the inner Galaxy l=3D[328,35] and b=3D[-5,5] in the COMPTEL map (\cite{obe96}). Furthermore, a comparison with the fluxes measured by all other instruments shows that COMPTEL has the lowest flux value reported (\cite{pra96}). This suggests that a significant fraction of the $^{26}$Al emission is not shown in the COMPTEL map. This is not surprising since COMPTEL has little sensitivity to low surface brightness extended emission. In a recent study it has been shown that the existence of dispersed $^{26}$Al from nearby SN or $^{26}$Al confined to fragments located at medium latitude that do not appear clearly in the map cannot be ruled out with the COMPTEL data (\cite{kno97}). A combined analysis of the GRIS and COMPTEL observations is currently being performed in order to refine the comparison of GRIS and COMPTEL results. We have studied the effect that known local sources could have on the results. The Cygnus Loop, one of the local sources identified in the COMPTEL map, should not contribute to the observed emission since this source was well outside the GRIS FOV during the entire flight. Vela, the other local source identified in the COMPTEL map, is within the GRIS FOV during the Galactic plane transit but, is so weak that it has a negligible contribution to the measured 1809 keV count rate. Another potential local source is Loop I in the Sco-Cen association, which has an angular diameter of 116=83 centered at 170 pc from the Sun and has been suggested to be the source of 1809 keV emission (\cite{bd89}). Recent ROSAT and radio observations suggest a mean rate of 2 SNIIs per 10$^{6}$ y in the Sco-Cen association and indicate that the latest SNII may have occurred $\approx 2\cdot10^{5}$ y ago with a progenitor mass of 15-20 $\, M_\odot$ (\cite{egg93}). Assuming that these events ejected about 10$^{-4}$ $\, M_\odot$ of $^{26}$Al, the resulting emission would be more than one order of magnitude too weak to account for the 1809 keV line rates observed by GRIS. Another argument for discarding Loop I as an important source of the observed 1809 keV line is based on the drift scan profile it would produce. The maximum emission from Loop I should be shifted 3 hours with respect to the maximum emission expected from a source located in the Galactic center region. We have calculated the maximum contribution from Loop I that is compatible with the data. For this calculation we fit the drift scan data with a model based on the COS-B distribution plus Loop I, leaving the intensity of both components as free parameters. The resulting best fit is compatible with no contribution from Loop I and the contribution to the total intensity should be under 20\% at the 2$\sigma$ confidence level. Another factor that could alter the results is the width of the $^{60}$Fe emission. We have shown that the $^{26}$Al line was detected with an intrinsic width of 0.3\% FWHM. For the present analysis, we have assumed the $^{60}$Fe emission to be narrow but, in principle, it may present a broadening like the $^{26}$Al line does. It is reasonable to assume that any broadening should not be more than 0.3\%, since $^{60}$Fe has a longer lifetime than $^{26}$Al and therefore has more time to slow down before decaying. Such a broadening would make the continuum background under the lines to be 1.7 times more intense than for the corresponding in a narrow line. The derived upper limits would be therefore $\sqrt{1.7}=3D1.3$ times higher than those presented in this work. The comparison of the measured fluxes with those predicted by theoretical models (\cite{tim97}) has interesting implications. The measured 2$\sigma$ upper limit for the $^{60}$Fe to $^{26}$Al flux ratio is 0.14 and is relatively independent of the assumed source distribution. This ratio is slightly lower than the 0.16 ratio predicted in a recent work by Timmes \& Woosley (1997). The predicted $^{60}$Fe and $^{26}$Al fluxes were calculated by averaging the mass of ejected $^{60}$Fe and $^{26}$Al from Woosley \& Weaver (1995) star models over a Salpeter initial mass function (\cite{tim97}). The flux uncertainties claimed for these calculations are up to a factor of 2. These uncertainties induce an error on the predicted ratio of $\pm0.1$ (\cite{die97}) which makes our measured upper limit to be compatible with the expectations. On the other hand, Meynet \etal\ (1997) have recently estimated that 20\%-70\% of the Galactic $^{26}$Al could be made in the hydrogen envelope of Wolf-Rayet stars. Since these sources do not release significant amounts of $^{60}$Fe, they could well account for a lower $^{60}$Fe to $^{26}$Al flux ratio. The measurement of the $^{60}$Fe emission remains a crucial objective for future generations of gamma-ray telescopes. Its detection would be very valuable to determine the contribution of the different sources to the production of $^{26}$Al, which would provide the most accurate information about the current nucleosynthetic activity in our Galaxy. Instruments such as the spectrometer of the ESA's INTEGRAL project (\cite{pvb95}), the High Energy Solar Spectroscopic Imager (\cite{den96}) and, the Long Duration Balloon GRIS (Marvin Leventhal private communication) should be able to address this issue at the beginning of the next decade. \acknowledgments Our thanks to the GRIS experiment development team, Stephen Deredyn, Stephen Snodgrass, Chris Miller and Kiran Patel. Launch and recovery support was provided by the crew of the National Scientific Ballooning Facility. We also thank an anonymous referee for his helpful comments. \clearpage \begin{table*} \caption{ $^{26}$Al and $^{60}$Fe Galactic Flux and Abundance Derived From GRIS. \label{tbl-2}} \begin{center} \begin{tabular}{lccccc} \hline \hline \multicolumn{1}{c|}{} &\multicolumn{2}{c|}{Flux} &\multicolumn{1}{c|}{$^{60}$Fe/$^{26}$Al} &\multicolumn{2}{c}{Abundance}\\ \multicolumn{1}{c|}{} &\multicolumn{2}{c|}{$10^{-4}$ ph s$^{-1}$cm$^{-2}$rad$^{-1}$} &\multicolumn{1}{c|}{Flux Ratio} &\multicolumn{2}{c}{$M_\odot$}\\ \hline \multicolumn{1}{l|}{Model} &\multicolumn{1}{c}{$^{26}$Al} &\multicolumn{1}{c|}{$^{60}$Fe \tablenotemark{a}} &\multicolumn{1}{c|}{} &\multicolumn{1}{c}{$^{26}$Al} &\multicolumn{1}{c}{$^{60}$Fe\tablenotemark{a}}\\ \hline COS-B\tablenotemark{b}=09 & $4.87\pm0.72$=09& $<$0.68 & $<$0.14 &= -=09 & - \\ COMPTEL\tablenotemark{b} & $5.48\pm0.78$=09& $<$0.72 & $<$0.13 & = - & - \\ T\&C=09 & $3.97\pm0.59$=09& $<$0.54 & $<$0.14 & $2.61\pm0.39$=09& $<$1.69 \\ Exponential=09 & $4.97\pm0.74$=09& $<$0.71 & $<$0.14 = =20 & $4.52\pm0.67$=09& $<$3.11 \\ \hline \hline \end{tabular} \end{center} \tablenotetext{a}{2$\sigma$ upper limits} \tablenotetext{b}{Abundances can not be derived from 2-dimensional models} \tablenum{1A} \end{table*} \clearpage \begin{thebibliography}{} \bibitem[Blake \& Dearborn 1989] {bd89} Blake, J.B., \& Dearborn, D.S.P. 1989, { \apj}, {\rm 338}, L17 \bibitem[Chen \etal 1997] {chen97} Chen, W., Diehl, R., Gehrels, N., Hartmann, D., Leising, M., Naya, J.E., Prantzos, N., Tueller, J., von Ballmoos, P. 1997, Proceedings 2nd INTEGRAL Workshop, 105 \bibitem[Chen \etal 1996] {chen96} Chen, W., Gehrels, N., Diehl, R., Hartmann, D. 1996, { \aaps}, {\rm 120}, 315 \bibitem[Clayton 1971] {cly71} Clayton, D.D. 1971, { \nat}, {\rm 234}, 291 \bibitem[Dennis \etal 1996] {den96} Dennis, B.R., \etal\ 1996, Bull. 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Astron., {\rm 6}, 85 \bibitem[Woosley \& Weaver 1995] {wos95} Woosley, S.E. \& Weaver, T.A. 1995= , { \apjs}, {\rm 101}, 181 \end{thebibliography} \clearpage \begin{figure} \epsscale{.6} \plotone{60Fefig1.eps} \caption {Variation of the 1809, 1332 and 1173 keV line intensities during the flight. These values include the instrumental background lines due to interactions of cosmic-ray induced neutrons with Al and Cu in the instrument. Notice that while the 1809 keV line clearly shows a modulation due to the Galactic center transits, there is not a significant modulation for the $^{60}$Fe lines. The solid line shows the predicted scan profile assuming a COS-B source model distribution. The derived fluxes are $4.8\pm0.7\cdot10^{-4}$ photons s$^{-1}$ cm$^{-2}$ rad$^{-1}$ for the $^{26}$Fe emission and a combined 2-sigma upper limit of $6.9\cdot10^{-4}$ photons s$^{-1}$ cm$^{-2}$ rad$^{-1}$ for the $^{60}$Fe emission.\label{fig1}} \end{figure} \clearpage \begin{figure} \epsscale{.7} \plotone{60Fefig2.eps} \caption {Longitude flux distributions considered for the flux and abundance studies presented herein. The curves are normalized to the value that fits the GRIS 1809 keV drift scan data. The long-dashed line is based on the high-energy gamma ray measurement performed by the COS-B instrument (\cite{may82}). The dotted line corresponds to the longitude profile derived from the COMPTEL map (\cite{obe96}). The solid and short-dashed lines have been derived from the sum of SN events generated by Monte-Carlo technique following 3-dimensional models that are a reasonable fit to the COMPTEL map such as the Taylor \& Cordes distribution (1993) and an exponential distribution with 4.5 kpc scale radius respectively. \label{fig2}} \end{figure} \clearpage \begin{figure} \epsscale{.5} \plotone{60Fefig3.eps} \caption {Net Galactic Center count rate spectrum around the 1809, 1332 and 1173 keV energies. Notice the clear detection of the Galactic $^{26}$Al emission at 1809 keV and the absence of a significant excess for the $^{60}$Fe lines. The solid curve is the best fit of the data to a Gaussian line shape. The derived intrinsic width for the astrophysical 1809 keV line is 5.4 [+1.4,-1.3] keV FWHM, which is more than three times the value expected from previous theories (see a more detailed discussion in Naya \etal\ 1996 ).\label{fig3}} \end{figure} \end{document} \clearpage \begin{table*} \caption{Instrument and Flight Characteristics. \label{tbl-1}} \begin{center} \begin{tabular}{ll} \hline \hline Detector Array & \\ \hspace{0.5 cm} Detector type=09 &Type-n High Purity Ge \\ \hspace{0.5 cm}Configuration=09 &7 coaxial detectors (1 enriched $^{70}$Ge) \\ \hspace{0.5 cm}Active Volume=09 &1647.6 cm$^{3}$ \\ \hspace{0.5 cm}Active Area=09 &237 cm$^{2}$ \\ \hspace{0.5 cm}Effective Area (@ 1.1 MeV)=09 &50 cm$^{2}$ \\ \hspace{0.5 cm}Thickness=09 &7 cm (avg.) \\ \hspace{0.5 cm}Energy Resolution (@ 1.1 MeV)=09&2.7 keV \\ Anti-coincidence Shield & \\ \hspace{0.5 cm}Detector type=09 &NaI \\ \hspace{0.5 cm}Energy Threshold=09 &80 keV \\ \hspace{0.5 cm}Thickness=09 &15 cm \\ \hspace{0.5 cm}Transmission=09 &0.4\% @ 1.1 MeV \\ Collimator Field-of-View=09 &$100^{\circ}\times 75^{\circ}$ FWHM \\ Flight Date=09=09=09 &October 24-26 1995=20 \\ Flight Duration=09 &32 hours \\ Average Altitude=09 &3.8 g cm$^{-2}$ \\ \hline \hline \end{tabular} \end{center} \end{table*} \end{document}