Searching Solar X-ray Observations for Axions

Searching Solar X‐ray
Observations for Axions
Iain G. Hannah
University of Glasgow, UK
H. S. Hudson, G. J. Hurford & R. P. Lin
Space Science Lab, UC Berkeley, USA
K. van Bibber
Naval Postgraduate School, Monterey, CA, USA
Email: iain@astro.gla.ac.uk

Hannah–Durham, July 2009
Motivation & Outline
Can we observe the X‐ray signal from axions
converting directly in the solar atmosphere?
•Summary of basic ideas & properties of expected axions signal
•Some solar physics
– Density & magnetic fields of the solar atmosphere
– Competing Solar X‐ray emission
•Current Solar Observations
– Hard X‐ray (HXR) with RHESSI
– Soft X‐ray (SXR) with Yohkoh/SXT and Hinode/XRT
•Future Observations
– NuSTAR, NEXT, FOXSI........
•Conclusions
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• Predicted axion spectrum from core will feature
Hannah–Durham, July 2009
Axions from Solar Core
– Continuum emission (Sikivie 1983, van Bibber et al 1989)
– 57 Fe spectral line at 14.4 keV (Moriyama 1995)
Moriyama 1995
Data from Raffelt
– Although likely to be faint, unique spectral and spatial distribution
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Hannah–Durham, July 2009
Geometry for Coronal Conversion
• Conversion of axion to X‐ray in coronal magnetic field (Carlson & Tseng
1996)
• Detect X‐rays from appropriate spacecraft
Axions
• Or conversion in Earth’s night side magnetic field (Davoudiasl & Huber 2006)
– No competing Solar X‐rays
B
X‐rays
*not to scale
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– Carlson & Tseng 1996
Hannah–Durham, July 2009
Probability of Axion Conversion
• This depends on the density and perpendicular magnetic field structure in
the solar atmosphere the axion encounters
• So to estimate the detectability of X‐rays from solar axions
– Need to know n,B as a function of (x,y,z) in the solar atmosphere
– Then need to compare this to the other competing sources of X‐ray
emission from the Sun.
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• Simple Dipole Field
– B perp ~ 10 ‐3 T at photosphere
Hannah–Durham, July 2009
"Simple" B & Density Model
Solar Wind
Corona
Chromosphere
Photosphere
• Gary 2001
• n, T and B vary with height
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• Do not accurately know B
Hannah–Durham, July 2009
Problems Determining Magnetic Field
– Can measure the photospheric line‐of‐sight B from Zeeman splitting
– Estimate coronal field by extrapolation (�xB = αB)
• Schrijver‐DeRosa PFSS method (takes α=0)
– But this is a current free extrapolation
• So no magnetic energy available for the magnetic phenomena we observe
– So this model is a start but nowhere near the complete picture
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Hannah–Durham, July 2009
Further Issues
• Even if using more accurate extrapolation depends on resolution of
magnetograms
– With higher spatial resolution instruments (like Hinode/SOT) observing
"ubiquitous presence of horizontal magnetic field" in very quiet Sun
regions ‐ Lites et al 2008
– This is an active field of research since all phenomena we observe in the
Sun's atmosphere are magnetic
• If we had quantitative knowledge of the magnetic field we could determine
which of our models/theories are correct
• Solar atmosphere is highly dynamic on a variety of timescales
– From seconds to 11‐year activity cycle to even longer!
• So difficult to convolve Axion flux with realistic n,B to get Axion X‐ray
spectrum
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• The solar 2 product is much larger
than that achievable in laboratories
– But the field is strongly variable in
both space and time, and not well
known quantitatively
• Magnetic fields drive solar
activity/phenomena,
– potentially confused with the axion
signal or a source of background
• "Figure of Merit" calculation
– ε= efficiency, A= detector area, Δt=
integration time, B = background rate, ΔE=
energy range
Hannah–Durham, July 2009
Merit of Space Observation
>4 orders of
magnitude bigger
PFSS Magnetic Fields (Hudson)
Detector FOM
CAST 1.0
RHESSI 0.005
57 Fe* 2x10 3
X‐ray** 7x10 4
Both assume SMEX level spacecraft
*14.4 kev photonuclear γ‐ray
**1000 cm 2 , B = 2x10 ‐4 (cm 2 .sec.keV) ‐1
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Hannah–Durham, July 2009
Soft X‐ray Solar Images
• Yohkoh SXT images from maximum of solar cycle (1991) to minimum
(1995)
Solar Max : Flares,
Active Regions &
Hot Loops
Solar Min: X‐ray
Bright Points in
the Quiet Sun
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• From December 2006, approaching solar minimum
Hannah–Durham, July 2009
Hinode/XRT Solar Image
Copious features and activity
even in the quiet Sun!
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• Observe flares (large to
micro) in SXR and HXR
– SXR: thermal brems
continuum and spectral
lines
– HXR: non‐thermal e ‐
brems continuum
• Solar Max: Active Regions
during solar max easily
observed in SXR, HXR difficult
• Solar Min: Quiet Sun/X‐ray
Bright Points easily observed
in SXR, HXR difficult
– See RHESSI Quiet Sun work
Hannah–Durham, July 2009
Solar X‐ray Spectra
• Unfortunately for axions the Sun is bright in X‐rays from a variety of
magnetic phenomena
Modified from Churazov et al. 2008 MNRAS
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Hannah–Durham, July 2009
Multi‐wavelength Approach
• Our knowledge of the processes involved in the solar atmosphere is due to
observations at multiple wavelengths/detecting particles in‐situ
8‐Feb‐2001
– Observational solar physics is a multi‐messenger displine
Radio
Electrons
• The problems of solar atmospheric physics is trying to understand this vast array of
highly complex & dynamic multi‐wavelength observations of magnetic phenomena
when we do not quantitatively know what the magnetic field is doing.
21‐Nov‐2006
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Hannah–Durham, July 2009
RHESSI Quiet Sun Upper Limits
• Reuven Ramaty High Energy Solar Spectroscopic Imager
– NASA, operating since 2002
• High spatial, temporal, energy resolution over 3keV to 17 MeV
• Designed for bright compact flare observations
– Not for large faint sources like Quiet Sun
• Offpointing need to chop
"signal"
• Data up to April 2009
• Still just upper limits
• Even if actual signal it would
NOT be dominated by axions
• Thermal emission from hot
XBPs seen in SXR
• Non‐thermal emission from
accelerated electrons in HXR
quiet Sun/nanoflares
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Hannah–Durham, July 2009
Yohkoh Soft X‐ray Telescope (SXT)
• Japanese with UK & NASA, operational 1991 to 2001
• SXT: SXR focusing optics telescope with variety of filters
– 0.25 to 4 keV 1024x1024 CCD detector
• Stack images from during Solar minimum (1996)
• Choose those with no Active Regions near disc centre
• No evidence of axions
• Bands of Active Regions and
features moving across the
disc as the Sun rotates (left
to right)
• Hudson & Acton plan further
work
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• Japanese with UK & NASA, launched 2006
Hannah–Durham, July 2009
Hinode X‐ray Telescope (XRT)
• XRT: SXR focusing optics telescope with variety of filters
– 0.03keV to 2keV with 2048x2048 CCD
• Observing campaign took many images with
– long exposure (11.5s) through thick filter (Be‐Med)
Histogram Analysis/Not "image"
• Histogram Analysis
• Each pixel shows high energy
tail of histogram of that pixel
from all energies
• No evidence of Axions
• Further work needed
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Hannah–Durham, July 2009
Future Solar Focusing HXR Telescopes
• Need hard X‐ray focusing optics solar X‐ray telescope
– Ideally imaging spectrometer from a few to

Hannah–Durham, July 2009
Future Focusing HXR Telescope
• Astrophysical HXR focusing optic telescopes that are being developed
– NASA's NuSTAR – Launch 2011
• Wolter‐I optics on deployable mast, 6 ‐ 79 keV CZT detectors
• Opportunity for sun pointing (hopefully)
• http://www.nustar.caltech.edu/home
– JAXA's Astro‐H or NeXT – Launch 2013
• 5‐80 keV, Si/CST detectors (& other instruments)
• Sun pointing unlikely
• http://astro‐h.isas.jaxa.jp/
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Hannah–Durham, July 2009
Conclusions
• Conversion in the solar magnetic field should be the best way
to see solar thermal axions of low mass
• As the density and magnetic field structure of the solar
atmosphere is not well understood an axion signal would be
difficult to interpret
• Calculations of resonance conditions in realistic solar (B, n)
distribution need to be done
• Future instrumentation could greatly improve sensitivity
– Although still difficult to separate X‐rays from axions and the
copious conventional X‐ray emission
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