With Ángel Gil Muyor, a PhD student at IFAE (Barcelona), we showed here that a QCD axion cloud around black holes can resonantly convert to radio photons that might be detectable at the LOFAR radio telescope.
The long story…
The QCD axion remains one of the most well-motivated and experimentally testable fundamental particles beyond those in the Standard Model of Particle Physics. In this quest for detection of the QCD axion, it is fruitful to consider different experimental setups and assumptions so that we can robustly scan all the parameter space (and we have good reasons to think that we will do so in the next decades).
This was what I recently did in my recent publication with Ángel Gil Muyor, a PhD student at IFAE (Barcelona). We knew that strong magnetic fields around compact astrophysical objects such as white dwarfs or magnetars can efficiently convert the QCD axion into photons. The question we asked ourselves was whether something similar could happen in the magnetic fields of the plasma that is being accreted onto black holes in our galaxy. And the answer we found was kind of a yes with caveats!
The typical magnetic fields that surround isolated black holes in our galaxy are weak, thus making the conversion inefficient. However, spinning black holes have a special mechanism known as superradiance that allows for the formation of extremely dense clouds of bosonic fields (in particular axions). Therefore, even if the conversion probability is small, we can still have detectable signals because there are so many.
On the other hand, the fact that the density of the accreting plasma increases towards the black hole allows for photons to have an effective mass (known as the plasma mass) that matches up the axion mass thus allowing for a resonant conversion between the two and an enhancement of the signals.
Putting everything together, we found that the mechanism can be used to probe the QCD axion with masses above 10^-9 eV but only if the black holes are primordial (with masses below 10^-3 solar masses)