²⁶Al Radioactivity in the Galaxy

(Version in deutscher Sprache hier)

Nucleosynthesis in cosmic sources can be observed through a variety of, mostly indirect, measurements; examples are stellar photospheric absorption lines, or mass spectrometry of meteoritic inclusions. Gamma-rays from radioactive by-products of nucleosynthesis ejecta provide a rather direct measurement, in comparison, as their decay gamma-ray measurements with satellite-borne telescopes provide direct isotopic constraints to the physics of nuclear burning regions inside these sources. Yet, the technique of gamma-ray telescopes is complex, and less precise than the alternatives for cosmic abundance measurements, mainly from two reasons: Spatial resolutions of ~degrees and signal-to-background ratios of ~% restrict contributions from gamma-ray astronomy to nearby sources in the Galaxy. There are, however, advantages to gamma-ray astronomical data: They provide isotopic information, are unaffected by physical conditions in/around the source such as temperature or density, and gamma-rays are nearly un-attenuated along the line-of-sight due to their penetrating nature (attenuation length ~few g/cm²). Longer-lived isotopes such as ²⁶Al (t~1.04 Myrs) will reflect the kinematic properties in an interstellar medium around massive star sources, which is otherwise hard to study due to its hot and diluted nature. The MPE high-energy astrophysics group studies the gamma-ray sky in ²⁶Al emission since the developement of the Compton telescope technique in the 1970ies. First results from the MPE balloone-borne instrument (1982) were followed by the all-sky studies with the COMPTEL telescope aboard the NASA space observatory 'Compton' (1991-2000), spectrosopic studies followed with the SPI spectrometer aboard ESA's INTEGRAL space mission (2002-current).

²⁶Al Science Results: Imaging from COMPTEL aboard the Compton Gamma-Ray Observatory and Spectroscopy from SPI aboard the INTEGRAL Observatory

assembled by Roland Diehl

26Al Map and spectrum

This Figure shows the COMPTEL Map of the 26Al Sky, plus the INTEGRAL/SPI Spectrum of the 26Al Line from the Inner Galaxy. (click for larger version in jpg or pdf)
If you use & show figures from here, please give credits as advised (specific references given with the respective figure) or to MPE Garching/Roland Diehl .
Thank you.

Update May 2009: 26Al along Galactic Star Formation Regions
Spectral and Intensity Variations along the Plane of the Galaxy

Using the imaging spectrometer on ESA’s Gamma-Ray Observatory INTEGRAL, measurements of the gamma-ray line from radioactive decay of ²⁶Al (T_1/2~720000 years) can now be distinguished from different regions which host young stars in the Galaxy.

This spatially-resolved spectroscopy shows that the line centroid indeed traces the bulk motion imposed by large-scale rotation within the Galaxy, and that the line in general is narrow. A suggested broadening in the Aquila region hints at recent young-star activity, possibly embedded within interstellar clouds and hence unnoticed in other tracers.
imaging spectroscopy 26Al

imaging spectroscopy 26Al

Update January 2006: 26Al and the Galactic Supernova and Star Formation Rates

Determining the Galactic Supernova Rate through Radioactivity

Using the spectrometer on ESA’s Gamma-Ray Observatory INTEGRAL, a precision measurement of the gamma-ray line from radioactive decay of ²⁶Al (T_1/2~720000 years) demonstrates that this radioactivity reflects the entire population of massive, young stars in the Galaxy.

Radioactive ²⁶Al is ejected into interstellar space together with other new elements, when massive stars reach the terminal phases of their evolution and finally explode as supernovae. Using the gamma-ray spectrometer SPI on INTEGRAL, an international team of researchers led by MPE scientists obtained a unpredecented precision measurement of the ²⁶Al decay gamma-ray line . From the otherwise heavily occulted inner region of the Galaxy, they recognized the signatur of galactic rotation intheir data. This led the team to conclude that observed ²⁶Al gamma-rays represent the massive star population over the full extent of the Galaxy. From the total amount of observed ²⁶Al, this corresponds to a rate of supernovae from massive stars (i.e. "Types Ib/c and II") of two per century.

26Al gamma-rays

(illustration of 26Al astronomy and Galaxy science: radioactive decay of the 26Al isotope produces the excited 26Mg nucleus, which emits a gamma-ray photon of specific energy upon its transition into its stable ground state. The INTEGRAL satellite measures those gamma-rays since its launch in October 2002. Precision spectroscopy revealed the signature of the Galaxy's differential rotation, which shifts the line centroids from regions of the inner Galalx through the Doppler effect in acharacteristic way. The background image shows the Milky way, as it would appear at visible wavelengths, if projected into a Galactic coordinate system where the plane of the Galaxy is a horizontal straight line; superimposed is the 26Al sky image from COMPTEL, which shows bright emission from regions with massive young stars. pdf pdf2, and jpg versions.)

Images

All-sky image in 26Al gamma-rays (from COMPTEL): pdf, jpg
Instruments of Gamma-ray Astronomy

INTEGRAL and its Spectrometer SPI: INTEGRAL( jpg), SPI (jpg)
The Compton Gamma-ray Observatory and its COMPTEL imaging telescope: CGRO(gif), COMPTEL( jpg)

Spectroscopy

High-resolution spectrum from the inner Galaxy: pdf, jpg
Signature of Galactic Rotation and Gamma-Ray Line Centroids:

Three spectra towards the inner Galaxy: pdf, jpg
Line positions along the Galactic Plane: pdf, jpg

Publications

General Descriptions:

Using Radioactivity in Astronomy (in german)
The Milky Way Galaxy and ²⁶Al (from MPG Research Journal 01/06): pdf

Scientific Papers

²⁶Al Spectroscopy with SPI (Paper in 'Astronomy & Astrophysics', Jan 2006): pdf
²⁶Al and Massive Stars (Paper in 'Nature', Jan 2006): complete paper(pdf), pdf
²⁶Al: Observations versus Theory (Paper in Physics Reports, 1996): pdf

ESA/INTEGRAL Pictures-of-the-month: Jul 2003, Jan 2006
MPG Journal Article MaxPlanck Forschung 4/2005 (pdf)
Press Releases Jan 2006: MPG (pdf) / ESA (pdf)
Press Echo and Web Links:

PhysicsNews (PhysicsToday)(pdf)
Astronews (html)
Sky&Telescope (www story), USA_Today (pdf), SpaceFlightNow (html), UniverseToday (html), Space.Com (html), CERNCourier (html), BrightSurf ScieneInfo (html), ChineseAstroNews (html!), NewScientist (html), PhysOrg (html), SciTechNews (html)
German Newspaper Articles Jan 2006: Süddeutsche Zeitung(jpg)(pdf), Frankfurter Allgemeine Zeitung (pdf)

Citations: citebase,

Below, you find more graphs and images...

All-Sky Image (from COMPTEL)

the Galactic 26Al emission regions

(... for education/overview; by Roland Diehl; credits: MPE 2001)

Maximum Entropy Map, 7 years of Data (ref.: Plüschke et al., AIP Conf. Proc. 510, ed. M.L. McConnell & J.M. Ryan, p 35-39 (2000))
Multi-Resolution Maximum Expectation Map, 7 years of Data (ref.: Plüschke et al., AIP Conf. Proc. 510, ed. M.L. McConnell & J.M. Ryan, p 35-39 (2000))
Model-Fitted Maximum Likelihood Map, 5 years of Data (ref.: Bloemen et al., Astroph. Letters and Communications, 38, 391-394 (1999))
Maximum Entropy Map, 9 years of Data (Reference&Credits: Plüschke S., Diehl R., Schönfelder V., et al., in: Exploring the Gamma-Ray Universe (4th INTEGRAL Workshop). (Eds.) A. Gimenez, V. Reglero and C. Winkler. ESA SP-459, Noordwijk, The Netherlands, 55-58 (2001), and Plüschke et al., PhD Thesis MPE (2001)) (This is the best-available and recommended 26Al Image from the COMPTEL Mission!)

Profiles

1.8 MeV Emission Profile Along the Plane of the Galaxy (ref. Diehl et al., A&A in preparation)

INTEGRAL/SPI Results:

Spectroscopy Results

SPI and 26Al: schematic

A scheme of the science goals: Radioactive decay as measured by INTEGRAL from 26Al decay, and the signature of Galactic rotation. The background image of the milky way is overlaid with the COMPTEL map of 26Al emission. Reference: MPE, Oct 2005

rotation-signature

rotation-signature

spectra

The signature of galactic rotation, as discovered with SPI. Reference: Diehl et al., Nature, 2006

inner-Galaxy spectrum

inner-Galaxy spectrum

26Al line shape as measured from the inner Galaxy with INTEGRAL. Reference: Diehl et al., A&A, 2006

Results on Specific Sources/Regions:
Vela Region:
WR11

WR11

COMPTEL Limits on 1.809 MeV Gamma-Rays from GammaVelorum, 9 Years of Data (ref.: Diehl et al., unpublished)

wrlimits

wrlimits

COMPTEL Limits on 26Al 1.809 MeV Line Emission from Gamma2 Velorum, 5 Years of Data (ref.: Oberlack et al., , A&A 353, 715-721 (2000))
vela SNR

vela SNR

COMPTEL Observation of 1.809 MeV Gamma-Rays from the Vela Region, 3 Years of Data (ref.: Diehl et al., A&A 298, L25-L28 (1995))
COMPTEL Observation of 1.809 MeV Gamma-Rays from the Vela Region, 7 Years of Data (ref.: Diehl et al., Astroph. and.Lett.Comm. 38, 357 (1999))

Orion Region:
orion map

orion map

COMPTEL Map of Orion Region (ref. Diehl et al., IAU212, 706 (2002))
eridanus

eridanus

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copyright Roland Diehl, MPE Garching
26Jun2009