Currently, solar physics has no clear answer on the source of solar X-rays. We know two familiar sources used in hospitals and experimental laboratories: bremsstrahlung from X-ray tubes and characteristic X-rays or XRF (X-ray fluorescence). However,solar physicists did not believe presence of any ionizing radiation sources available in our laboratories on Sun's core surface. The question remained what causes the X-rays measured in solar spectrum up to 12.87 nm. Most interestingly, these range of wavelengths exactly coincided with the range of an XRF source (Rb XRF source). That does not mean solar X-rays are from Rb XRF source alone. Most probably, they originate from 235-Uranium fission products (radioisotopes) on Sun's core surface that can emit characteristic X-rays. 

References:

M.A.Padmanabha Rao,

UV dominant optical emission newly detected from radioisotopes and XRF sources,

Braz. J. Phy., 40, no 1, 38-46,2010.

http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-97332010000100007

M.A. Padmanabha Rao,

Discovery of Sun’s Bharat Radiation emission causing Extreme Ultraviolet (EUV) and UV dominant optical radiation,

IOSR Journal of Applied Physics (IOSR-JAP), Volume 3, Issue 2 (Mar. – Apr. 2013), PP 56-60, DOI: 10.9790/4861-0325660

http://www.iosrjournals.org/iosr-jap/papers/Vol3-issue2/H0325660.pdf

M.A. Padmanabha Rao,

Discovery of Self-Sustained 235-U Fission Causing Sunlight by Padmanabha Rao Effect,

IOSR Journal of Applied Physics (IOSR-JAP), Volume 4, Issue 2 (Jul. – Aug. 2013), PP 06-24, DOI: 10.9790/4861-0420624

http://www.iosrjournals.org/iosr-jap/papers/Vol4-issue2/B0420624.pdf