Axions from the Sun?

H. S. Hudson

SSL, UC Berkeley

http://sprg.ssl.berkeley.edu/~hhudson/presentations/berkeley.090220/

SSL Feb. 20, 2009 2/28

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 soap powder

• 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 and we might detect them using X-ray techniques

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Outline• Basic ideas• Solar tutorial

- spectral, spatial, temporal properties- why the chromosphere is important

• Existing instrumentation• Future instrumentation

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Outline• Basic ideas• Solar tutorial

- spectral, spatial, temporal properties- why the chromosphere is important

• Existing instrumentation• Future instrumentation

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Outline• Basic ideas• Solar tutorial

- spectral, spatial, temporal properties- why the chromosphere is important)

• Existing instrumentation• Future instrumentation

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Solar atmosphere

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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

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Geometries for solar axion detection

Solar core

Photosphere

CoronalB-Field

Earth

1 2 3

1: RHESSI, Yohkoh, Hinode2: CAST* or 57Fe resonance3: Conversion in the Earth’s field*

GeomagneticField

X-ray/axion flux

*Davoudiasl & Huber, 2005

*Andriamonje et al. 2007

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Solar atmosphere

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Giant magnet

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CAST at CERN

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Churazov et al. (2008)

Solar X-ray spectra

X-ray background(negative source!)

RHESSI upper limits(Hannah et al. 2007)

Albedo

CR interactions

{11-yearcycle

SSL Feb. 20, 2009 12/28

Signatures• Spectral: main axion X-ray component is 4-5

keV• Spatial: sources should be near disk center

and correlate with Bperp

• Temporal: should be steady modulo Bperp

• Polarization: following B

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X-ray Images 1

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X-ray Images 2

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X-ray Timeseriesfrom GOES

High activity - October 2003 Low activity - January 2009

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Conversion in the solar atmosphere

:

Need <BperpL>Need n < n0(m)

What (n, B) do we have?

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First approximation to solar (B, n): dipole field, spherical symmetry

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Bperp ~ 10-3 T at photosphere

“VAL-C” semi-empirical model

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Second approximation: complex field and dynamic plasma

Druckmuller eclipse imagery*

Gary (2001)* http://www.zam.fme.vutbr.cz/~druck/Eclipse/index.htm

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Simulations of the chromosphere

MHD simulations by O. Steiner (Kiepenheuer Institut für Sonnenphysik, Freiburg)

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The solar magnetic field 1• We only really know the line-of-sight component of

the photospheric field (Zeeman splitting, Hanle effect)• The coronal field is force-free and currents circulate

in it (xB = B)• Some of the field is “open,” ie linking to the solar wind• We estimate the coronal field by extrapolation • All knowledge is also limited by telescope resolution

to scales of 100 km or so• A potential representation (no coronal currents) is

often used as a first approximation (“PFSS”)

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The solar magnetic field 2• The “seething field” (Harvey et al., 2007)• Hidden magnetism (Trujillo Bueno et al., 2004)• The Hinode 40-gauss field (Lites et al., 2008)

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Using the Schrijver-DeRosaPFSS 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 => <BL>2

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PFSS Prediction for solar <BL>2

Strengths and Weaknesses

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• The solar <BL>2 productis much larger than that achievablein laboratories• The field is strongly variable in both space and time, and not wellknown quantitatively• Strong fields drive solar activity,potentially confused with the axionsignal or a source of background

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Solar X-ray telescopes

Yohkoh (focusing)

RHESSI (image modulation)

Hinode (focusing)

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RHESSI “tail dragging mode”

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Studying the Yohkoh and Hinode soft X-ray data

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Prediction: increasinglevels of theoreticalsource intensity

Yohkoh observation:selection for no disk center active regions at solar minimum (1996)

Hinode observation viahistogram analysis of400 images

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Churazov et al. (2008)

Solar X-ray spectra

X-ray background(negative source!)

RHESSI upper limits (Hannah et al. 2007)

Albedo

CR interactions

{11-yearcycle

SSL Feb. 20, 2009 28/28

Signatures• Spectral: main axion X-ray component is 4-5

keV• Spatial: sources should be near disk center

and correlate with Bperp

• Temporal: should be steady modulo Bperp

• Polarization: following B

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Figure of Merit for X-rayand -ray observations

= efficiencyA = detector areat = integration timeB = background rateE = energy range

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Figure of Merit results

CAST 1.0RHESSI 0.005 57Fe* 2 x 103

X-ray** 7 x 104

*14.4 kev photonuclear -ray**1000 cm2, B = 2x10-4 (cm2.sec.keV)-1

n.b. X-ray and -ray estimates based on a SMEX-level satellite plan - one could do better! This model is better than a “cubesat”

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Conclusions

• Conversion in the solar magnetic field should be the best way to see solar thermal axions of low mass

• The solar atmosphere is not well understood, so any interpretation of an axion signal will be fuzzy (we should be so lucky!)

• Calculations of resonance conditions in solar (B, n) distribution need to be done

• Future instrumentation could greatly improve on the sensitivity

Axions from the Sun?

H. S. Hudson

SSL, UC Berkeley

http://sprg.ssl.berkeley.edu/~hhudson/presentations/berkeley.090220/