First limits on the 3-200 keV X-ray spectrum of the quiet Sun using RHESSI

Abstract

We present the first results using the Reuven Ramaty High-Energy Solar Spectroscopic Imager, RHESSI, to observe solar X-ray emission not associated with active regions, sunspots or flares (the quiet Sun). Using a newly developed chopping technique (fan-beam modulation) during seven periods of offpointing between June 2005 to October 2006, we obtained upper limits over 3-200 keV for the quietest times when the GOES12 1-8A flux fell below $10^{-8}$ Wm${}^{-2}$. These values are smaller than previous limits in the 17-120 keV range and extend them to both lower and higher energies. The limit in 3-6 keV is consistent with a coronal temperature $\le 6$ MK. For quiet Sun periods when the GOES12 1-8A background flux was between $10^{-8}$ Wm${}^{-2}$ and $10^{-7}$ Wm${}^{-2}$, the RHESSI 3-6 keV flux correlates to this as a power-law, with an index of $1.08\pm 0.13$. The power-law correlation for microflares has a steeper index of $1.29\pm 0.06$. We also discuss the possibility of observing quiet Sun X-rays due to solar axions and use the RHESSI quiet Sun limits to estimate the axion-to-photon coupling constant for two different axion emission scenarios.

4 pages, 3 figures, Accepted by ApJ letters

References in corpus (2)

• Quiet Sun X-rays as Signature for New Particles
• A new method of observing weak extended x-ray sources with RHESSI

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• NuSTAR as an Axion Helioscope
• Signatures of Type III Solar Radio Bursts from Nanoflares: Modeling
• Quiet Sun impulsive events observed with NuSTAR during solar minimum