Solar X-rays from axions

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Number: 50
1st Author: Hugh Hudson
2nd Author:
Published: 3 January 2007
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A hypothetical elementary particle - the axion - may be flooding out of the solar core in great profusion. This particle, invented for an entirely different theoretical purpose, is a leading candidate to supply the dark matter inferred to compose the bulk of the mass in the Universe. Alas, theory does not predict the mass of the axion, but if axions do exist and if their mass is not too small, then the dark matter mystery may be solved and our understanding of the Universe greatly expanded. Accordingly many searches for this weakly-interacting particle are underway in laboratories on Earth and via astrophysical inference. The Feynman diagram in Figure 1 below shows the Primakoff effect by which we can hope to detect axions with RHESSI. In this simple Primakoff interaction, the photon continues in the same direction as the original axion, and has the same energy. Figure 1: A Feynman diagram showing the Primakoff effect: an axion enters from the left, meets a magnetic field, and converts directly into a photon. We've already described our general approach to observing the quiet Sun in a previous nugget. In this one, we go back and describe the basic properties of solar axion generation, and (hopefully after we've discovered them and become famous) we will have another Nugget describing the results.

How does the Sun make axions?

The solar production of axions is simple as it is the inverse of the above diagram: a photon in the solar core meets a magnetic field and becomes an axion, which then escapes by virtue of its feeble interaction with ordinary matter. At the center of the Sun the photons have a blackbody spectrum with a high enough temperature to encourage the nuclear reactions that provide the Sun's energy. So, the axions would have a very similar blackbody spectrum, which peaks at a few keV as shown in Figure 2.

Figure 2: The solar axion source intensity as a function of energy (Y-axis) and distance from Sun center (X-axis). This calculation was done by P. Serpico and G. Raffelt of the CERN Axion Solar Telescope project. Once the axion escapes from the Sun, then it can convert back into a photon when it meets another magnetic field perpendicular to its flight. Thankfully, the corona is full of magnetic field, especially above sunspots which may have fields thousands of times stronger than the Earth's (or even a fridge magnet. Even outside the spots, the whole solar corona has a relatively strong magnetic field (tens of Gauss). It is possible to create still stronger fields in terrestrial laboratories and other experiments are using this method. Our "sister" terrestrial experiment, CAST, uses a large laboratory magnet to generate a field in which solar axions may be converted to X-rays. The X-rays should then be observed through the use of X-ray focusing optics (more on this in a later nugget!). We hope to take advantage of the already-present magnetic fields on the Sun (and their very large size scales) in order to convert some axions and detect the generated X-rays before they encounter a barrier such as our atmosphere. It would not be the first time, the Sun is used to discover a new particle (e.g., helium)!

Figure 3: The CERN Axion Solar Telescope.

Two non-optimal searches continue

The RHESSI search is continuing, and this Nugget introduces an analysis of the Yohkoh soft X-ray telescope (SXT) data. Yohkoh operated from September 1991 until December 2001, and with the masterful help of Loren Acton of Montana State University, we have been searching for an axion signal with SXT as well as with RHESSI. Neither search is particularly optimal, but by very different imaging techniques each has a chance to detect the axions.

Figure 4: Model soft X-ray images (the base image coming from the Hinode satellite, but it could as well have been Yohkoh), showing a larger and larger axion signal. The axions, for a uniform coronal magnetic field, would give a ghostly image of the nuclear-burning core of the Sun. This is a negative image for clarity. Because solar X-ray telescopes are designed mainly to observe relatively bright solar sources such as solar flares, instead of the rather faint axion-related X-ray source, they are not ideal. But we are trying, both with RHESSI and with SXT (1991-2001), and perhaps also with Hinode in the future (see Figure 4).


At present the axion searches has not produced any Nobel-prize-winning results. If this situation persists, we will still have interesting upper limits on the axion source brightness, which can be interpreted in terms of the basic physics of the axion (e.g., its mass). In the next RHESSI nugget on this subject (perhaps in a few months) we will describe how well we have done this.

Biographical note: Hugh Hudson is a RHESSI team member in Berkeley. The solar axion group at Berkeley consists of Iain Hannah, Gordon Hurford, and Bob Lin, with key consultation via Karl van Bibber. Loren Acton of MSU is the key person for SXT axion searches.

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