Supernova Remnants

In Chap. 11 we have seen that massive stars end their evolution in a supernova explosion. The collapse of the stellar core leads to the violent ejection of the outer layers, which then remain as an expanding gas cloud.

About 120 supernova remnants (SNR's) have been discovered in the Milky Way. Some of them are optically visible as a ring or an irregular nebula (e. g. the Crab nebula; see Fig. 15.23), but most are detectable only

100MHz 1 GHz 10GHz Igv

Fig. 15.24. The radio spectra of typical HII regions and supernova remnants. The radiation of HII regions is thermal and obeys the Rayleigh-Jeans law, I a v2, at wavelengths larger than 1 m. In supernova remnants the intensity decreases with increasing frequency. (After Scheffler, H., Elsasser, H. (1987): Physics of the Galaxy and the Interstellar Matter (Springer, Berlin, Heidelberg, New York))

100MHz 1 GHz 10GHz Igv

Fig. 15.24. The radio spectra of typical HII regions and supernova remnants. The radiation of HII regions is thermal and obeys the Rayleigh-Jeans law, I a v2, at wavelengths larger than 1 m. In supernova remnants the intensity decreases with increasing frequency. (After Scheffler, H., Elsasser, H. (1987): Physics of the Galaxy and the Interstellar Matter (Springer, Berlin, Heidelberg, New York))

in the radio region (because radio emission suffers no extinction).

In the radio region the SNR's are extended sources similar to HII regions. However, unlike HII regions the radiation from SNR's is often polarized. Another characteristic difference between these two kinds of sources is that whereas the radio brightness of HII regions grows or remains constant as the frequency increases, that of SNR's falls off almost linearly (in a log Iv — log v diagram) with increasing frequency (Fig. 15.24).

These differences are due to the different emission processes in HII regions and in SNR's. In an HII region, the radio emission is free-free radiation from the hot plasma. In a SNR it is synchrotron radiation from relativistic electrons moving in spiral orbits around the magnetic field lines. The synchrotron process gives rise to a continuous spectrum extending over all wave-

Fig. 15.25. The Veil nebula (NGC 6960 at the right, NGC 6992 ► at the left) in Cygnus is the remnant of a supernova explosion which occurred several ten thousand years ago. (Mt. Wilson Observatory)

length regions. For example, the Crab nebula looks blue or green in colour photographs because of optical synchrotron radiation.

In the Crab nebula red filaments are also visible against the bright background. Their emission is principally in the hydrogen Ha line. The hydrogen in a SNR is not ionized by a central star as in the H II regions, but by the ultraviolet synchrotron radiation.

The supernova remnants in the Milky Way fall into two classes. One type has a clearly ring-like structure (e.g. Cassiopeia A or the Veil nebula in Cygnus; see Fig. 15.25); another is irregular and bright at the middle (like the Crab nebula). In the remnants of the Crab nebula type there is always a rapidly rotating pulsar at the centre. This pulsar provides most of the energy of the remnant by continuously injecting relativistic electrons into the cloud. The evolution of this type of SNR reflects that of the pulsar and for this reason has a time scale of a few ten thousand years.

Ring-like SNR's do not contain an energetic pulsar; their energy comes from the actual supernova explosion. After the explosion, the cloud expands at a speed of 10,000-20,000 km/s. About 50-100 years after the explosion the remnant begins to form a spherical shell as the ejected gas starts to sweep up interstellar gas and to slow down in its outer parts. The swept-up shell expands with a decreasing velocity and cools until, after about 100, 000 years, it merges into the interstellar medium. The two types of supernova remnants may be related to the two types (I and II) of supernovae.

* Synchrotron Radiation

Synchrotron radiation was first observed in 1948 by Frank Elder, Robert Langmuir and Herbert Pollack, who were experimenting with an electron synchrotron, in which electrons were accelerated to relativistic energies in a magnetic field. It was observed that the electrons radiated visible light in a narrow cone along their momentary direction of motion. In astrophysics synchrotron radiation was first invoked as an explanation of the radio emission of the Milky Way, discovered by Karl Jansky in 1931. This radiation had a spectrum and a large metre-wave brightness temperature (more than 105 K) which were inconsistent with ordinary thermal free-free emission from ionized gas. In 1950 Hannes Alfven and Nicolai Herlofson as well as

Karl-Otto Kiepenheuer proposed that the galactic radio background was due to synchrotron radiation. According to Kiepenheuer the high-energy cosmic ray electrons would emit radio radiation in the weak galactic magnetic field. This explanation has turned out to be correct. Synchrotron radiation is also an important emission process in supernova remnants, radio galaxies and quasars. It is a non-thermal radiation process, i. e. the energy of the radiating electrons is not due to thermal motions.

Synchrotron Radiation
The emission of synchrotron radiation. A charged particle (electron) propagating in a magnetic field moves in a spiral. Because of the centripetal acceleration, the particle emits electromagnetic radiation

The origin of synchrotron radiation is schematically shown in the figure. The magnetic field forces the electron to move in a spiral orbit. The electron is thus constantly accelerated and will emit electromagnetic radiation. According to the special theory of relativity, the emission from a relativistic electron will be concentrated in a narrow cone. Like the beam from a lighthouse, this cone sweeps across the observer's field of vision once for each revolution. Thus the observer sees a sequence of radiation flashes of very short duration compared with their interval. (In the total emission of a large number of electrons, separate flashes cannot be distinguished.) When this series of pulses is represented as a sum of different frequency components (Fourier transform), a broad spectrum is obtained with a maximum at

15.7 The Hot Corona of the Milky Way where B± is the magnetic field component perpendicular to the velocity of the electron, and E its energy, a is a constant of proportionality.

The table gives the frequency and wavelength of the maximum as functions of the electron energy for the typical galactic field strength 0.5 nT:

^max

vmax [Hz]

E [eV]

300 nm

1015

6.6 x 1012

30 ^m

1013

6.6 x 1011

3 mm

1011

6.6 x 1010

30 cm

109

6.6 x 109

30 m

107

6.6 x 108

To produce even radio synchrotron radiation, very energetic electrons are required, but these are known to be present in the cosmic radiation. In the optical galactic background radiation, the contribution from synchrotron radiation is negligible, but, for example, in the Crab nebula, a significant part of the optical emission is due to this mechanism.

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