Astronomy & Celestial Mechanics
the sky, computed
Concept

Particle acceleration — where it appears

The processes that raise charged particles to relativistic energies in astrophysical plasmas. Shocks and turbulence do it chiefly, by scattering particles back and forth across moving magnetic fields, and they produce the power-law spectra of cosmic rays and synchrotron sources.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

Also named here as spectral ageing — the same set of essays touches all of them, so they are one junction rather than several.

A spectral age with a ceiling, against the time the lobe took to grow. The spectral age of electrons whose spectrum breaks at 2 GHz, against the magnetic field in microgauss (curve), beside the dynamical age of a 400-kpc lobe whose head has advanced at 0.01c, 0.03c, 0.1c (horizontal lines: 130, 43, 13 million years). The spectral age has a maximum, 109 million years, at a field of 1.9 microgauss — the microwave background's equivalent field divided by √3 — because a weaker field leaves the electrons cooling on the background's photons instead. A lobe that advanced at 0.01c is 1.2 times older than any field can make its spectrum, so the disagreement cannot be removed by choosing a different field. Either the advance speed is overestimated, or the electrons at the break are younger than the lobe — re-accelerated, or freshly mixed in from the hotspot.

Two clocks for one lobe

A radio galaxy's lobe can be dated twice: by the break in its spectrum, where the oldest electrons have radiated away their energy, and by its length divided by the speed its head advances. The two clocks disagree, the spectral age usually several times younger, and no choice of magnetic field can make them agree for the slowest lobes. What the disagreement measures is that the electrons are not as old as the lobe.

starlight · Synchrotron radiation Spectra that age, and spectra that stop ageing. Synchrotron spectra in a 3-microgauss field, divided by the straight power law the electrons were injected with so that a spectrum with no break would be a flat line at zero, at ages of 10, 30, 100 million years. Dashed: electrons left alone, whose break falls steadily to lower frequency. Solid: the same electrons stirred throughout, gaining energy at a rate of one e-fold every 30 million years against their losses. The stirred spectra's break moves down at first and then stops, at the frequency radiated by electrons of Lorentz factor 41,763, where gain and loss balance. Read as passive spectra, the stirred ones give ages of 8.5, 18.9, 28.8 million years: the last is 3.5 times younger than the electrons are.

A lobe that reads the time since it was stirred

If a radio lobe's electrons are re-energised in place by the turbulence around them, the break in their spectrum stops falling and its spectral age stops growing, settling at the time the stirring takes to double their energy a few times over. The spectrum's shape does not betray it: a stirred lobe fits an unstirred one exactly. What betrays it is a map that stops ageing with distance, and a bill for energy that grows exponentially with the time the stirring lasts.

starlight · Synchrotron radiation

Named alongside it

The objects these essays reach for when they reach for this one.

Inverse Compton scatteringSpectral ageingSynchrotron radiationCooling breakCosmic microwave backgroundFermi accelerationMagnetic fieldRadio galaxyRadio lobeRelativistic jetTurbulence

All concepts