September 26, 2011

1982: first gamma-ray map of the Milky way galaxy

Logo 50 ans de résultats scientifiques
Logo 50 ans de résultats scientifiques

In 1982, the first scientific satellite of ESA provided the first gamma-ray map of the Milky Way.

When COS-B was launch in August 9th, 1975, the high energy astronomy was taking its first steps.
If sky charts with thousands of sources are being studied since antiquity in the visible range, only a few gamma sources were detected in particular with the SAS-2 (NASA) and TD-1 (ESRO - ancestor of ESA) satellites, both launched in... 1972!

During more than 6 years (instead of the 2 previously planned), the 278kg satellite observed the celestial sphere for energy ranges between 70MeV and 5GeV to gather scientific information.

Surrounding the a anti-coincidence plastic scintillator guard counter permits to reject triggers due to the interactions between the cosmic rays and the satellite. It was provided by the CEA.

The first catalogue of gamma sources, which listed almost 30 objects, was thus published in 1981.

In 1982, the first entire gamma-ray map of the Milky Way Galaxy was available to the science community.

From our position in the Milky Way Galaxy (the solar system is in one of the spiral arms of the galaxy, which is thus seen from the side), the map showed a very important and structured diffuse emission as well as punctual bright sources already identified in the 1981 catalogue.

The great correlation established thanks to observations in the radio and visible ranges between the diffuse radiation and the gas partial mapping in the Galaxy is amazing.

This confirms that the emission is galactic and produced by the interstellar gas under the influence of the sustained bombardment of cosmic rays it experiences. This information sheds a new light on the Milky Way Galaxy and its gaseous content.

It is found that the scale height of the gamma ray disk within the solar circle is about 130 pc. In this area to the estimated atomic hydrogen concentration (detected at 21 cm), a too important gamma emission attributed to the molecular hydrogen had already been shown thanks to SAS-2. However, many estimates of the multiplication factor were very different. The accuracy of the COS-B's data allowed us to announce a gaseous density near the galaxy center almost three times higher than near the Sun. It also gave us the opportunity to estimate the total luminosity of the Galaxy for energy ranges between 70MeV and 5GeV at 1039 erg.s-1.

The map shows that the radiation spectrum index (the "colour") is the same in the whole Galaxy, telling us that the cosmic rays completely enter the gaseous clouds, even the weakest ones.

The richness of the COS-B's data paves to way for the large gamma space missions. The very massive CGRO satellite (17 t.) and then the current Fermi mission confirmed the COS-B's discoveries and revealed a bestiary of extremely diverse sources.


See also