North Polar Spur/Loop I: gigantic outskirt of the Northern Fermi bubble
  or nearby hot gas cavity blown by supernovae?
  Radio continuum, microwave and gamma-ray images of the Milky Way reveal a
conspicuous, loop-like structure that fills almost half of the northern
Galactic hemisphere, called Loop I. The interior of Loop I is shining in soft
X-rays, and its eastern base is a bright, elongated structure dubbed the North
Polar Spur (NPS). After 40 years of debates, two contradictory views of Loop
I/NPS are still defended: on the one hand, the NPS is a volume of expanding hot
gas that envelops and extends the northern Fermi Bubble (FB) known to be blown
by the Galactic center, and Loop I marks the shock front; on the other hand,
the NPS is a nearby cavity of hot gas blown by supernovae, Loop I is its shock
front and they are coincidentally located in the direction of the FB. To an
observer at the Sun, both structures can produce the same perspective view,
although the former has a size comparable to the Milky Way itself, and the
latter a diameter of a few hundreds parsecs. The energy involved varies by
3-4~orders of magnitude, and the solution has various important consequences. I
discuss recent results which have a connection with Loop I/NPS, overlooked or
used as arguments in the two opposite ways. They involve very different
spacecraft, from a 12 Kg Cubesat (HaloSat) to major space-borne observatories
(HST, Gaia, and Spektr-RG). I make use of 3D~maps of dust and a massive star
catalog. Considering all constraints, it is clear that there is no entirely
local or entirely distant scenario that is free from contradictions with some
of the data analyses or from improbable coincidences. I discuss a speculative
scenario, partially inspired by recent BF and Milky Way gas models, combining
near and far aspects and seeming to be able to meet the various constraints.
However, new data and models are needed to bring the controversy to a close and
we can still expect new episodes of this long story.
