The beginning of photoionization marks the transition between the post-Asymptotic Giant Branch ( post-AGB ) and planetary nebula ( PN ) phases of stars with masses \lesssim 8 M _ { \odot } .
This critical phase is difficult to observe , as it lasts only a few decades .
The combination of jets and magnetic fields , the key agents of PNe shaping , could give rise to synchrotron emission , but this has never been observed before in any PNe , since free-free emission from the ionized gas is expected to dominate its radio spectrum .
In this paper we report radio continuum observations taken with the Australia Telescope Compact Array between 1 and 46 GHz of the young PN IRAS 15103 - 5754 .
Our observations in 2010-2011 show non-thermal emission compatible with synchrotron emission from electrons accelerated at a shock with spectral index \alpha \simeq - 0.54 .
However , in 2012 , the spectral index \alpha \simeq - 0.28 is no longer compatible with synchrotron emission in these types of processes .
Several hypothesis are discussed to explain this change .
The more plausible ones are related to the presence of the newly photoionized region in this young PN : either energy loss of electrons due to Coulomb collisions with the plasma , or selective suppression of synchrotron radiation due to the Razin effect .
We postulate that the observed flattening of non-thermal radio spectra could be a hallmark identifying the beginning of the PN phase .