How did blazar TXS 0506+056 emit neutrinos without gamma rays?
In plain words
In 2017 a neutrino was traced to a flaring blazar (a galaxy whose jet points at us), but an earlier and larger neutrino outburst from the same object in 2014-2015 came with no gamma-ray flare.
Precise statement
IceCube-170922A coincided with a gamma-ray flare of TXS 0506+056 (about $3\sigma$), while archival data show an excess of $13 \pm 5$ neutrinos in 2014-2015 ($3.5\sigma$) with no rise in gamma rays. One-zone lepto-hadronic models cannot give that neutrino luminosity without overproducing X rays and gamma rays from cascades. Determine the emission geometry (multiple zones, a compact core dense in target photons, jet interaction with clouds or stars) that fits both episodes, and whether it allows gamma-ray blazars to supply more than $\sim 10\ \text{percent}$ of the diffuse flux. Answer: a model fitting both episodes and the multiwavelength data.
What would settle it
Further neutrino flares from blazars with simultaneous X-ray and MeV coverage that fix the cascade budget.