Radio Brightness Temperature Jansky

The Jansky: flux density unit of radio astronomy

Radio astronomers measure spectral flux density in Jansky, an SI-derived unit with the symbol Jy, where 1 Jy = 10⁻²⁶ W·m⁻²·Hz⁻¹. The name comes from Karl Guthe Jansky. Back in 1932 he pointed a rotating Bell Labs antenna at 20.5 MHz and picked up the first radio signal from space, traced to the centre of the Milky Way, and the field was born.

Brightness temperature ties a flux density to the temperature of an equivalent black body through the Rayleigh-Jeans law, T_b = (S λ²) / (2 k_B Ω). Here S is the flux density, λ the wavelength, k_B Boltzmann's constant, and Ω the solid angle the source subtends.

Applications

Cygnus A, the brightest extragalactic source, peaks near 2300 Jy at 1.4 GHz. Quiet quasars sit down at the milliJansky level, and deep surveys with the VLA (Very Large Array), MeerKAT and ALMA reach all the way to microJansky sources. When the SKA (Square Kilometre Array) comes online it will push past sub-μJy sensitivity, bringing the cosmological 21 cm signal of the Epoch of Reionization within reach.

FAQ

Why such a tiny unit? By the time cosmic radio sources reach Earth they are incredibly faint. Quoting them in watts per square metre per hertz would mean writing scientific notation in every line.

Is brightness temperature a real temperature? Only when the source is thermal and sits in the Rayleigh-Jeans regime. Synchrotron sources can show T_b of 10¹² K or more, and at that point the number is a handy label for brightness rather than anything physically hot.

What flux is 1 Jy on a household scale? Take a 25 m dish, integrate over 100 MHz of bandwidth, and 1 Jy works out to roughly 5×10⁻¹&sup5; W. That is less than a trillionth of the Wi-Fi signal from your phone.