Attempts to DetectX-ray Albedo

Brian DennisNASA GSFC

Albedo GeometryBrown, J. C. , Van Beek, H. F., and McClymont, A. N. Astron. & Astrophys. 41, 395 (1975)

Source SSource height hScattering point PSubsource point QDistance P to Q rQSP θSun center CSun’s radius RDirection to Earth

Albedo GeometrySource at solar disc center (L = 0)

dI(θ) (counts cm-2 arcsec-2) = f I0 F1 F2 F3 F4

dI Albedo flux from point P in albedo patch

θ Angle from vertical to point P (angle QSP in Figure 1)

f Photospheric reflectance (~0.6 at 15 – 20 keV)

I0 Primary source flux (assumed isotropic)

F1 = (cos θ)-2 Inverse-square fall off from S

F2 = (cos θ)-1 Projection onto plane photosphere

F3 ~ 1 Compton scattering directivity

F4 ~ 1 Curvature correction

dI(θ) ~ f I0 / (2 cos3θ)

Albedo Patch (Kontar & Jeffries 2010) • Lower flux density (photons s-1 cm-2 arcsec-2) compared to primary source

– down by factors of >10.– Impossible to image using current version of CLEAN– Should be possible to image albedo patch using pixon– Evidence for extended source using Visibilities (VIS-FF)

• Geometric foreshortening close to limb.• Centroid shifted towards disc center compared to primary source.

Albedo Fraction vs. X-ray Energy

Albedo flux assuming isotropic emission• Peaks between 30 and 50 keV• Greater for flatter spectra

Simulated DataCLEAN Image

Source Alone

Cross-section through single sourceCount-rate vs. roll angle for all 9 detectors

Red: simulated dataBlack: predicted from CLEAN image

Simulated DataCLEAN Image

Source + Albedo

Cross-section through source

Clean doesn’t see the albedo wings Count-rate vs. roll angle for all 9 detectorsRed: simulated data

Black: predicted from CLEAN image

Source Alone

Simulated DataPixon Image

Count-rate vs. roll angle for detectors 1 - 7Red: simulated data

Black: predicted from pixon image

Cross section through source

Pixon does see the albedo wings

Simulated DataPixon – Circular Source + Albedo

Simulated FlareLongitude = 80

Color: simulated flare + albedoContours: Pixon Image

Simulated Flare - Longitude = 80

Color: simulated flare + albedoContours: MEM_NJIT Image

Flux contour of sourceGreen: original sourceRed: MEM_NJIT image

Disc Flare6 Nov. 2004

Possible compact source + albedo patchAltitude = 2 – 3 Mm

Disc Flare – Early Impulsive Emission2 June 2002

Note double HXR footpoint sources. Possible symmetric wings around each source.

Limb Flare20 Feb. 2002

Possible compact source + albedo patchEvidence for foreshortening???

Limb Flare21 April 2002

Note two footpoint HXR sources along TRACE 195Å ribbons and extended coronal HXR source(s) above the limb.

Limb Flare21 April 2002

Note more intense wings closer to the limb.

Albedo DetectionImaging?

Schmahl, E. J. and Hurford, G. J. (2002, 2009)Report detection of extended HXR sources.

RHESSI Observations of the Size Scales of Solar Hard X-ray Sources

Sol. Phys., 210, 273 (2002)

Solar Hard X-ray AlbedoRHESSI Science Nugget #119 (2009)

Schmahl and Hurford (2002)

Schmahl and

Hurford (2002)

Cumulative flux vs. radius (r)

Schmahl, and

Hurford, 2009,

RHESSI Science Nugget

#119

Flare on 10 April 2002Schmahl and Hurford, 2009, RHESSI Science Nugget #119

Schmahland

Hurford 2009

RHESSI Science Nugget

#119Detector # + position angle/180

Schmahl and Hurford (2009) RHESSI Science Nugget #119

But, reduced chi-squared = 7.And, 15 – 20 keV and 12 – 15 keV are low energy ranges to see albedo.

Clean Images10 April 2002 12:30 UT

Detector #6 Effect

Detectors 4, 5, 7, 8, 9Detectors 4, 5, 6, 8, 9

Visibility Correction Factors10 April 2002

Clean Image – det. 3, 4, 5, 7, 8, 9Is there an albedo patch?

1% contour level

UV-smooth Image – det. 345789Correction factors: 0.00,0.00,1.33,1.01,1.06,0.81,0.97,0.93,1.04

10% - lowest contour level

Pixon Image – no stacker10 April 2002 12:30 UT

Pixon Image – stacker on10 April 2002 12:30 UT

Albedo Detection - Spectroscopy?Kontar & Brown (2006) ApJ, 653, L152.Kontar et al. (2006) A&A, 446, 1157.

September 17, 2002 solar flare, 5:50:48-5:51:36 UT.

Mean electron flux spectraPhoton spectra

Solid lines – without albedo correctionDashed lines – with albedo correction

Albedo Detection - Statistical?• Kasparova et al. A&A, 466, 705 (2007)• Kasparova (2008) RHESSI Science Nugget #74

Anisotropy of RHESSI hard X-ray emission

Observed distributions Corrected for albedo

Solid lines – limb eventsDashed lines – disc center events

Spectral index Spectral index

Albedo Imaging?Schmahl and Hurford (2002, 2009) report

detection of extended HXR sources.

Possible Explanations1. Albedo patch2. Extended coronal source(s)3. Extended footpoint(s) along ribbons4. Instrumental Effects

Pulse pile-up Image reconstruction technique (Visibility Forward Fit) Detector sensitivity mismatch

Schmahl, E. J. and Hurford, G., J., 2002, Sol. Phys., 210, 273.Schmahl, E. J. and Hurford, G., J., 2009, RHESSI Science Nugget #119,

Conclusion• Albedo must be present unless primary source is highly asymmetric.• Simulations show that pixon image reconstruction is capable of showing

albedo wings.• Wings detected in pixon images for most flares.

– But origin of wings is uncertain.• No evidence of longitude foreshortening of flare sources.

– Is image reconstruction capable of detecting foreshortening?

• Extended sources evident from visibility amplitudes– Albedo patches?– Coronal sources?

• Not certain that albedo has ever been conclusively detected from RHESSI imaging information.

• Spectroscopy evidence for albedo is more compelling.

Future Work• Improve detector sensitivity calibrations.• Pixon reconstructions to image albedo patches.

– Correct annular sector to XY coordinates problem with compact sources.

• Examine images and spectra for more flares.– Spectral analysis for consistency with imaging.

• Further simulations with more realistic multiple source geometries and background rates.

• Variations with longitude to reveal foreshortening and altitude effects.

• Visibility Forward Fit with assumed albedo patches.– Schmahl and Hurford