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Thermal Bremsstrahlung Spectrum

What you are seeing: bremsstrahlung (free-free) emissivity of a thermal plasma at temperature TT. The spectrum is flat at low frequencies (modulo ln⁡\ln Gaunt factor) and cuts off exponentially at hν∼kTh\nu \sim k T.

Figure 1. Log-log thermal bremsstrahlung emissivity.
log10(T / K)7.00
log10(n / cm⁻³)0.0

WHAT TO TRY

  • Raise the temperature: the free-free spectrum stays flat at low frequency but its exponential cutoff near h nu = kT slides to higher energy, the thermal signature of a hotter plasma.
  • Increase the density: emissivity scales as n squared, since every photon needs an electron deflected by an ion, the dependence that makes dense plasmas blaze in X-rays.
  • This free-free emission is how galaxy-cluster gas shines in X-rays and how HII regions glow, a thermometer for any ionized cloud.