1. Searching the X-ray Sun for Solar Axions H. S. Hudson (SSL Berkeley) L. W. Acton (MSU) E.E. DeLuca (CfA) I.G. Hannah (U. Glasgow) G.J. Hurford (SSL Berkeley) R.P. Lin (SSL Berkeley) K.P. Reardon (INAF/Arcetri) K. Van Bibber (LBNL)

2. St. Louis AAS, May 2008 What is this about? Axions are elementary particles theorized by Peccei and Quinn to “solve the strong CP problem” The name, due to F. Wilczek, refers to the name of a brand of detergent Axions, being charge-neutral and weakly interacting, are a prime candidate to explain dark matter in the Universe The solar core should be a copious source of axions via the Primakoff effect

3. St. Louis AAS, May 2008 Axion conversion in a magnetic field Bottom line: the X-ray intensity will increase as 2, but a dense medium induces oscillations that suppress the conversion:

4. St. Louis AAS, May 2008 Geometries for solar axion detection Solar core Photosphere Coronal B-Field Earth 123 1: RHESSI, Yohkoh, Hinode 2: CAST * or 57 Fe resonance 3: Conversion in the Earth’s field * Geomagnetic Field X-ray/axion flux * Davoudiasl & Huber, 2005 * Andriamonje et al. 2007

5. St. Louis AAS, May 2008 Merits of the different sites for observing solar axions Ground-based observations (e.g., CAST): low background, low signal, good laboratory control Sunlit space (e.g., RHESSI): one gets a huge advantage in 2, but varying backgrounds and other uncertainties Earth’s shadow, using geomagnetic field: low 2, but free of solar background X-rays Figure of merit ( 2 A  t/R  , where R is the background counting rate, strongly favors using the solar field

6. St. Louis AAS, May 2008 The solar axion spatial-spectral source distribution function Serpico & Raffelt (2007)

7. St. Louis AAS, May 2008 Predicted solar fluxes Carlson & Tseng (1996) 10 mCrab X-ray spectrum Moriyama (1996) The 14.4 keV line is the  -ray used in Mössbauer studies, here photonuclear The main ~5 keV emission, due to the Primakoff effect, is shown here for a specific value of the coupling constant g

8. St. Louis AAS, May 2008 Using the Schrijver-DeRosa PFSS solar magnetic model Line-of-sight B component derived from photospheric Zeeman observations Potential-field extrapolation used to approximate the coronal field Integration across theoretical axion source distribution => 2

9. St. Louis AAS, May 2008 PFSS Prediction for solar 2 Strengths and Weaknesses The solar 2 product is much larger than that achievable in laboratories The field is strongly variable in both space and time, and not well known quantitatively Strong fields drive solar activity, potentially confused with the axion signal or a source of background

10. St. Louis AAS, May 2008 Studying the Yohkoh and Hinode soft X-ray data Prediction: increasing levels of theoretical source intensity Yohkoh observation: selection for no disk center active regions at solar minimum (1996) Hinode observation via histogram analysis of 400 images

11. St. Louis AAS, May 2008 Studying the RHESSI hard X-ray data Churazov et al. (2008), cosmic X-ray albedo component Hannah et al. (2007), quiet- Sun upper limits The 57 Fe  -ray line at 14.4 keV may have less competition from solar sources, but at extreme sensitivity other effects may compete

12. St. Louis AAS, May 2008 Conclusions Axions are interesting and solar X-ray observations allow for sensitive searches We have studied data from Yohkoh/SXT, RHESSI, and Hinode/XRT thus far, with comparable sensitivity to ground-based searches such as CAST A dedicated observation from space could make use of the large solar 2 value to make improvements of 10 3-4 in these limits