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# North Polar Spur/Loop I: gigantic outskirt of the Northern Fermi bubble or
nearby hot gas cavity blown by supernovae?

Rosine Lallement  GEPI/Observatoire de Paris,PSL*, 5 pl. J. Janssen, 92195
Meudon, France  [

###### Abstract

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 the most
conspicuous region shining in soft X-rays, whose eastern base is a remarkably
bright, elongated structure seeming to emerge from the Galactic plane, dubbed
the North Polar Spur (NPS). After 40 years of debates, two very different,
contradictory views of Loop I/NPS are still defended: on the one hand, the NPS
is a gigantic 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 totally
independent of the northern FB, it is a nearby, ordinary cavity of hot gas
blown by supernovae, Loop I is its shock front and both are coincidentally
located in the direction of the FB. To an observer at the Sun, both 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
important consequences on the structure and history of our Galactic
neighborhood, on the age of the North and South FBs and the activity at the
Galactic center. Moreover, whatever are the actual shape and distance of Loop
I/NPS, accurate modeling of the polarized emission associated with Loop I is
important for CMB foreground removal. After a short review, I discuss recent
results which have a connection with Loop I/NPS. Some of them have been used
as arguments in the two opposite ways, while for others the connections with
LoopI/NPS have been overlooked. 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 updated 3D maps of dust and a recent massive star
catalog. I distinguish arguments based on geometric similarity or
dissimilarity from those derived from measurements and physical models.
Considering all past and recent 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.

###### keywords:

Galaxy; Galactic interstellar medium; X-rays; Gamma rays; Microwave emission;
Galactic Center; Galactic halo

##  1 Introduction

Figure 1: 408 MHz radio continuum (from [1], reprocessed by [2]) and Spektr-
RG/eROSITA soft X-rays (from [3]) all-sky maps revealing radio Loop 1 in the
northern hemisphere and its associated X-ray bright area, in particular the
eastern low latitude part of the latter, the NPS. The X-ray image is a
composite of three bands: red(0.3-0.6 keV), green(0.6-1.0 keV) and
blue(1.0-2.3 keV). Both maps are in Aitoff Galactic coordinates. North is at
top, East is left of center. The black signs in the 408 MHz map are also
reported in the ROSAT X-ray maps used in Section 5.

Loop I is a spectacular $\simeq 70$∘ x 50∘ loop-like feature shining above the
Galactic Center at radio continuum, microwave, and more weakly gamma-ray
wavelengths. Its contour delimitates a $\simeq 1$-2 keV X-ray bright area
which is characterized by a strong brightness enhancement of it eastern low
latitude part, the North Polar Spur (NPS). Among the long-lasting
controversies in astrophysics, the 40-year-old debate about the physical size,
the location and the origin of the NPS/Loop I association (hereafter LNPS) is
one of the most surprising. As a matter of fact, here the scene is not the
distant Universe, instead it is our Milky Way, i.e. a well-observed
astrophysical object. Moreover, the structure in debate is not a faint or
minor feature, instead it is a major, conspicuous object. Last but not least,
the two debated interpretations are so radically different that it is hard to
believe that they both co-existed for so long.

After an introduction to the observations and debate, a series of recent
results related directly or indirectly to the origin of the LPNS are
presented, along with their implications. Although all arguments influence the
entire LNPS structure and are often interconnected, they are separated into
three categories related to the overall geometry, a fourth section devoted to
inferences from X-ray data, and finally a section on stellar and interstellar
constraints on the nearby super-bubble hypothesis. The final Section 7
contains a summary and discussion of potential solutions and future
measurements that could definitely close the debate.

###  1.1 First observations

Observations directly or indirectly related to LNPS could fill long chapters,
and we restrict this introduction to early multi-wavelength observations and
to the main findings of the last decade. Several presentations of Loop I and
discussions about its origin have been published, see the very deep analysis
by [4] and references therein, and for a very recent discussion see [5]. Loop
I was first detected in radio as the most prominent Galactic loop, with the
NPS at its bottom([6, 7, 1, 8]). The two structures were later found to
bound the most prominent diffuse feature of the entire sky in soft X-rays
([9]), along with a wide region in the general direction of the Galactic
center, the X-ray bulge. To illustrate the similarity between radio and soft
X-rays, Figure 1 shows the 408 MHz continuum full-sky map (from [1],
reprocessed by [2]) and the very recent Specktr-RG/eROSITA composite soft
X-rays (0.3-2.0 keV) full-sky image ([3]). The contour drawn in the radio
map on top of Loop I is superimposed on the ROSAT maps of [9] used in Section
5 and available numerically. Unambiguously, radio and X-rays trace the same
object. Both NPS and Loop I were also mapped from ground in polarized radio
continuum ([10, 7, 11]) and later from space in total-intensity and
polarized microwave emission during missions devoted to the CMB. More
recently, a counterpart in gamma-rays was also found by [12], based on Fermi-
LAT data (see Figure 2).

After its discovery, it was hypothesized that LNPS is associated with a nearby
hot gas cavity blown by a recent supernova (SN) or a series of supernovae.
According to the classical scheme, soft X-rays are thermally emitted by
million degree hot gas blown by the massive precursor stellar winds and SN
ejecta to form a large remnant cavity, while the radio signal is due to
synchrotron emission by supra-thermal electrons accelerated in external,
shocked regions of the SN remnant (SNR). Indeed, spectra slopes of radio data
were found compatible with the synchrotron mechanism. The proximity of the
LNPS source was considered as inevitable. Due to its exceptionally large
angular size, maintaining a physical dimension of LNPS in a realistic range
implies a short distance, and, in addition, below Loop I are a series of hot
young O, B stars associations, in particular the Scorpius-Centaurus group at
$\simeq 130$ pc, ideally located above the Plane and a very likely source of
recent supernovae. A first potentially contradicting observation is the
absence of high velocity expanding shell at the periphery of LNPS, because
million degree gas implies shock speeds on the order of tens to hundreds of
km/s. This was explained by a two- or multiple-stage formation of the X-ray
emitting volume associating an initial old (few million years) SNR episode and
a recent re-heating of the blown cavity due to more recent explosions (see
Section 6). The Local Cavity surrounding the Sun (often called Local Bubble,
LB) and filled with hot, highly ionized gas is also generally believed to be a
relic of similar events. An additional argument in favor of a local origin of
LNPS was also very early proposed, namely the presence of high Galactic
latitude HI filaments seeming to envelop Loop I (see, e.g., [7]). All along
the following decades, owing to increasingly detailed and sensitive gas and
dust emission surveys, it was confirmed that there are nearby concentrations
of gas and dust and especially high latitude arches in the same longitude-
latitude range than the LPNS. Although no clear 3D view of their association
with the large SNR episode responsible of LNPS was established, it was
implicitly assumed that they are debris of a large shell formed around the hot
gas (but see below).

In parallel, a very different interpretation of the LNPS started to be
defended. In an attempt to explain the wide empty cavity (3 kpc radius) around
the Central Molecular Zone (CMZ) at the Galactic center (GC), and the
surrounding expanding ring of dense interstellar matter, it was proposed in
1977 that both were due to explosive events at the GC ([13]), and, as part
of the same model, that the propagation in the halo of the shock generated by
the latest event has created huge volumes of hot gas and accelerated particles
at the shock front. In the northern hemisphere the hot gas and the shock front
are seen from the Sun vantage point as the LNPS. A series of models were since
then regularly published by Sofue and co-workers, based on all new relevant
observations, including, from 2010, those of the Fermi Bubbles (FBs)
discovered at that time and unanimously recognized as generated at the GC (see
below). However, the local origin of LNPS and a total absence of link between
LNPS and the FBs remained the preferred scenario for most astrophysicists.

###  1.2 The Fermi _bubbles_ discovery

A turning point occurred at the time of the successive discoveries of (i) the
so-called WMAP _haze_ around the Galactic center, (ii) its Fermi gamma-ray
counterpart and, finally, (iii) the “Fermi bubbles”. Shortly, after the WMAP
mission devoted to the CMB mapped the entire sky in several microwave
frequency bands, [14] noticed a $\simeq$ 70∘ wide brightness enhancement
around the GC area. The initially proposed origin of the “haze” was dark
matter annihilation, but later analyses favored instead synchrotron emission
by energetic electrons ([15]). Subsequently, [16] discovered the gamma-ray
counterpart of the haze and suggested that the same population of relativistic
electrons produces both microwave synchrotron emission and gamma rays through
inverse Compton with the interstellar radiation field. Soon after, a
reanalysis of the Fermi data by [17] revealed two large gamma-ray bubbles,
extending 50∘ above and below the Galactic center, with a width of about 35∘
in longitude, the spectacular Fermi bubbles (FBs). Without any doubt, such
giant volumes of highly energized gas with sizes comparable to the one of the
Milky Way, are due to bursting activity at the Galactic center.

The existence and observed geometry of the northern FB, and the fact that it
appears encircled by the LNPS (see Section 2) gave more weight to Sofue’s
hypothesis, and several models were developed to reproduce these gigantic,
$\simeq 10$ kpc high structures, and study links between FBs and LNPS ([18,
19, 20]). The detailed mechanisms at the origin of the FBs are currently
lively debated. A series of X-ray, radio and IR data recently confirmed the
existence of powerful jet-type outflows from the central parsecs at the GC,
able to feed FB-type structures and suggesting that today’s FBs are the result
of a particularly strong activity several Myrs ago (see e.g. [21, 22]).

Figure 2: Top left: Gamma-ray sky (from [23]). The FBs correspond to the
double-lobed structure centered exactly on the GC and reaching up to b$\simeq
50$-55∘ above and below the plane. A fainter, wider loop similar in location
to Loop I is also visible in the north, surrounding the FB bubble and reaching
b$\simeq+80$∘ . Top right: Soft X-ray sky measured by e-Rosita (cyan), with
the Fermi image superimposed (in red), and showing the respective locations of
the X-ray bright areas and the FBs in both hemispheres (from [3]). The
southern X-ray structure is much fainter than its northern counterpart,
however, in both hemispheres the X-ray bright regions encircle the FBs in a
similar way. The northern, fainter $\gamma$ray loop is not visible in the
superimposed image, however it can be seen that it would encircle closely the
X-rays. The external regions of the X-ray extended area correspond quite
closely to the radio loop shown in Figure 1.
Main figure and illustration of the two interpretations: Schematic view of the
meridian plane perpendicular to the Galactic plane and containing the Sun and
the GC. The Milky Way thin disk of dust and gas is shown as a 800 pc wide dark
and light grey band extending about 15 kpc on both sides of the GC, except for
the region internal to the 3 kpc molecular ring around the GC white band, and
for the 6 kpc wide region around the Sun where we use the dust distribution
(from [24]). Assuming that the FBs are symmetric around an axis perpendicular
to the Plane, the northern FB would occupy the region within the dashed yellow
ellipse. If LNPS X-rays are associated to the FBs and surrounding them, the
X-ray emitting regions could be bounded by the dashed blue line B (see text).
This corresponds to the interpretation of NPS/Loop I as the envelop and
outskirts of FB north. A zoom showing the dust distribution within $\pm 3$ kpc
from the Sun is shown at left (dashed red-line rectangle, from [24]). Units
are parsecs, the Sun is at (0,0), the GC direction is to the right and the
north Galactic pole direction to the top. In the local interpretation of LNPS,
X-ray, radio and microwave emissions would come from a very close region seen
from low to high latitudes, whose periphery is schematized as arc A (blue
dashed line in the zoom).

###  1.3 Illustration of the two interpretations

Today, the debate about the size of the LNPS and its potential link with GC
events and FBs is still very lively, as shown by very recent publications in
favor of the two different interpretations (see below). Figure 2 schematizes
the two interpretations and illustrates the strong difference in size scales.
According to the $\simeq 50$∘ latitude of the northern FB top and the FB
shape, _its_ height and maximal width reached at $\simeq 2$ kpc altitude are
on the order of 8.5 and 6 kpc respectively, if one assumes that the FBs are
symmetric around the polar axes. In the GC scenario for the LNPS, the shocked
halo gas envelops the FBs and, in the north, the high latitude limit of the
X-rays corresponds to the highest latitude tangential direction to the X-ray
boundary (schematized as arc B). To illustrate the local origin of the LNPS,
an image of the dust distribution in the meridian plane within $\pm 3$ kpc
from the Sun along the Sun-GC axis and within $\pm 400$ pc from the Plane
towards the poles is inserted into the sketch, and a zoom on this image in the
vicinity of the Sun is added. The zoom is used to illustrate the location and
shape of a _hypothetical shell generated by nearby SNRs_ (arc A) and viewed
from the Sun within the same latitude range than _the arc B in the GC
scenario_.

###  1.4 On the link between NPS/Loop I and interstellar high-latitude dust
and gas

It has been often suggested that the high latitude HI and dust filamentary
features between $\rm l=250$∘ and $\rm l=60$∘ are connected with the LNPS,
based on morphological similitude between their location and orientation along
the periphery of the X-ray emitting region, and this has been used as an
argument in favor of the local hypothesis. Gas and dust filaments would mark
the outer boundary of a shell surrounding the volume of heated gas. If proven,
the existence of such an association would definitely close the controversy
and favor a local LNPS. As a matter of fact, these high latitude dust or gas
structures are all located at short distances, as demonstrated by various
works on gas absorption [25], stellar polarization (e.g., [5], and 3D
extinction maps ([24], also see the $\rm b\simeq+60$∘ dust cloud in Figure 2
or other high latitude nearby (100-200 pc) faint structures in Figure 7
(right)). However, the association between gas and dust on one side, and
X-ray, gamma-ray and synchrotron structures on the other side, has never been
proven. A careful look at the various maps shows that there is no close
correspondency (see, e.g., [4, Figure 21]) and, as mentioned above, HI 21 cm
spectra show that the gas is not expanding. None of the new sources of
information discussed below involves directly the high latitude filaments of
gas and dust, and we come back to them and their potential links with LNPS in
the last section only.

##  2 The North-South asymmetry

One of the arguments against a GC origin of the LNPS is the absence of its
southern counterpart. Because the FBs are nearly symmetric around the GC, one
would expect a conspicuous X-ray-bright wide area around the southern FB,
bordered by a South Polar Spur and a southern wide radio loop, similar to LNPS
in the north. In the local source hypothesis, one may instead expect a non-
symmetric prolongation to the south of X-ray and synchrotron features, if the
multi-SNR cavity extends on both sides of the Plane, or no corresponding
structure at all, in the case of a reheated SNR cavity limited to positive
latitudes. Clearly, X-ray data lack a remarkable brightX-ray area in the south
and a bright spur. ROSAT [9] and Suzaku/MAXI [26] images reveal weak soft
X-ray features, however they were judged too faint to be counterparts of the
LNPS and overlooked. In the synchrotron domain, detailed WMAP and Planck
measurements of diffuse polarized emission revealed a large series of arches
and filaments, and among those the WMAP filament XII (see [27]) was proposed
as a southern continuation of Loop I [4]. According to the authors, the closed
loop made from Loop I and filament XII could mark the tangential surface to
the wide cavity blown by nearby SNRs (see Figure 7 and Section 3 for
discussion of such a structure).

The situation has evolved very recently, with the first soft X-ray images
recorded by thee-Rosita instrument on-board the Spektr-RG observatory [3]. The
e-Rosita images in the 0.3-2.3(resp. 0.6-1.0) keV range shown in Figure 1
(resp. Figure 2) clearly show a brightness enhancement in the south below the
GC, and, in particular, an enhanced spur on the western side, quasi symmetric
of the eastern NPS with respect to the GC (see next section). Brightnesses are
significantly weaker than the north, though, especially at high negative
latitudes, but the overall shape of the X-ray bright area corresponds well to
an envelope of the southern FB, as shown in the top-right insert of Figure 2,
where $\leq 10$ GeV gamma-ray are superimposed on the e-Rosita image (Figure
taken from [3]. The very weak southern boundary at $\rm b\simeq-80$∘ can be
seen in pale blue if one looks carefully at this composite image.

The southern X-ray envelope and spur, although rather convincing, are
remarkably fainter, and the origin of the north-south asymmetry must be
explained. Quantitative arguments were brought by [20] who performed
hydrodynamical simulations of the FBs and their interactions with the halo,
and addressed the question of north-south X-ray brightness difference by
varying halo characteristics. The author showed that the X-ray brightness
difference may be explained by 20% deficiency of halo gas density in the
southern hemisphere in comparison with the northern one. According to the
author, it would additionally explain why the southern FB is 5% higher above
the Plane than the northern FB: this would simply be due to the easier
expansion in the less dense gas. Note that Sarkar’s model addresses the case
of FB generation due to quasi-continuous starburst at the GC and series of
SNRs, while, today, the favored mechanism is accretion-fed explosive ejection.
However, in both cases the expansion in the halo is strongly dependent on the
halo density, and one may assume that his argument is also valid in the jet-
like case. The result of [20], presented by the author as confirmation of a
distant origin of the LPNS, was also used in an opposite way: arguing that
there is no established reason for such a north-south difference of halo gas
density, [5] considered the result as evidence against the distant LNPS.

We performed a search in the recent literature for measurements related to the
halo density and found some interesting results:

  1. 1.

[28] compiled and analyzed all past measurements of interstellar HI Lyman-
series absorption with the Far Ultraviolet Spectroscopic Explorer (FUSE) and
all HI Ly$\alpha$absorption data towards extragalactic sources at latitudes
$|b|\geq 20$∘ with the HST. The authors derived a larger fraction of
intermediate velocity (IVC) gas in the northern halo. More quantitatively,
they computed the sky covering fraction of IVCs as a function of their
hydrogen column density and find that 70% of sky covering fraction is reached
for Log(N(HI)= 18.7 cm-2 in the north and only Log(N(HI)= 16.7 cm-2 in the
south. For the high velocity gas (HVC) the sky coverage is about the same, but
there are more high column-density sightlines in the north.

  2. 2.

[29] compiled and analyzed absorption by the SiIV ion towards AGNs using
HST/COS data and developed a model of the halo warm gas associated with the
ion. The authors found that the hot gas vertical scale height r0 is larger in
the north than in the south with r${}^{N}_{0}=6.3(+1.6,-1.5)$ kpc and
r${}^{S}_{0}=3.6(+1.0,-0.9)$ kpc. If we use their results, this implies that
between 1 and 10 kpc altitude, i.e. where the FBs and the heated X-ray gas
volume are formed, the hot ionized halo gas density is significantly higher in
the north (e.g., 25% higher at 2kpc, and 80% higher at 5 kpc).

  3. 3.

[30] analyzed HST-COS UV absorptions in the spectra of 5 quasars whose sight-
lines cross the FBs with $30\geq|b|\geq 20$∘ . They found accelerating HI
clouds in both hemispheres, but, interestingly, they also noticed anomalous
blue-shifted clouds at b$\simeq 30$∘ and interpret these signatures as due to
remnants of past outflows in front of the FBs.

  4. 4.

It is finally possible to infer some asymmetry by taking a closer look at
ROSAT ultra-soft X-rays images (0.25 keV). This easily absorbed emission is
detected only towards the halo, and in the direction of the _holes_ in the
local ISM. The emission reaches significantly stronger intensities in the
north, even after exclusion of the LNPS area. In the past, this was
interpreted as a sign for weaker absorption by cool gas at lower altitude,
however, today measurements show that gas and dust columns in the northern
hemisphere are similar or even higher than in the south, and this explanation
in term of absorption does not hold. Instead, a stronger emission, must be
preferred, i.e., the excess ultra-soft X-ray emission is potentially due to a
higher hot gas density in the northern halo.

This series of measurements and analyses all suggest the existence of non
negligible differences between halo characteristics in the two hemispheres,
and point to a larger density in the north, bringing support to Sarkar’s
explanation of the weak southern X-ray emission. Along with the X-ray
brightness decrease, the model shows that radio features are also weaker in
the south and have very different shapes. As a matter of fact, expansion in a
low density medium implies lower energy transported by the shock wave and
shock accelerated particles of less energy. If the southern halo is less
dense, it may potentially explain the weakness of both southern X-ray and
synchrotron emission. It remains that the rare, weak radio/micro-wave southern
arcs are not well explained yet by a model. On the other hand, and
interestingly, in an asymmetric configuration such like the one modeled by
[20], the outer shock has reached a distance from the GC larger than in the
north. In one of the model results, the distance reached in the south locates
it slightlybeyond the Sun circle. In other words, the outer shock may have
just crossed us in the south while the northern shock is still in front of us.
This would mean that, at variance with the northern Loop I seen in the
Galactic Center hemisphere at very high latitude, the synchrotron emission
generated by the southern loop may appear differently. We will come back to
this point in the discussion section.

Figure 3: Left: Fraction of the combined Planck/WMAP polarization intensity
map (from [4]). Superimposed as yellow arrows are the approximate bases (at
latitude 0∘ ) of the sharpest spurs/arches in the northern hemisphere.
Right: High resolution hydrodynamical simulation of the Galactic gas adjusted
to HI 21 cm data and the most recent Gaia and survey data (image from [31]).
The Galactic plane is seen from the northern pole. Yellow dashed lines issued
from the Sun have been superimposed on the image to indicate the longitudes of
the low latitude ends of the spurs as defined in the left figure. One argument
in favor of the GC origin of the LNPS is the coincidence between the direction
of the low-latitude extremity of the LNPS ($\rm l\simeq 25$∘ ) and the
direction of the high gas density area of the eastern 3-kpc arm (second
direction from left). But there are also other interesting associations
between spur and high gas concentration longitudes according to the new model
of the entire Milky Way.

##  3 The East-West asymmetry

Another argument against a GC origin of the LNPS is the absence of east-west
symmetry. Since the FBs are nearly symmetric around the GC, for homogeneous
Milky Way disk and halo one would expect the X-ray and radio associated
features to be nearly symmetric with respect to the meridian plane (zero
longitude plane), as viewed from the Sun, which is not observed. In the north,
X-ray data are characterized by a much stronger brightness in the east, with a
fuzzier, weaker west counterpart. In the south, it is the opposite. Radio data
do not show any high latitude “spur” in the west, and the low latitude arcs
seen in polarized intensity (see Figure 3) are more numerous in the east and
are all oriented in the same way.

Several recent works have shown strong evidence for an accretion-fed jet-type
(collimated) event at the origin of the FBs, similar to jets in AGNs despite
orders of magnitude of difference in power (e.g. [21, 22]). The jets are
collimated by the dense gas from the Central Molecular Zone (CMZ), which
explains the FBs orientation at right angle from the Plane. It follows that,
in case the LNPS is linked to the FBs, the observed east-west asymmetry seen
in X-ray and radio must be related to posterior, differential evolution of the
halo gas at large distances from the GC. Pursuing (in a different way) their
early idea that the wide, 3 kpc radius very low density region of the inner
disk is a remnant of GC past explosive events, [32] recently suggested that
the giant X-ray bubbles of shocked, expanding halo gas brighten where the flow
encounters the 3 kpc molecular ring,to form the X-ray then radio features. As
a main argument, the authors use the fact that at the lowest latitude where
the NPS can be detected, the longitude is $\rm l=22$∘ , i.e. very close to
the direction of the tangent to the 3kpc ring.

Recent high resolution hydrodynamical simulations of the Galactic gas
reproduce the 3 kpc “ring” in the gas distribution, as a consequence of the
dynamical influence of the Galactic bar and the spiral arms. When based on a
realistic barred Milky Way potential constrained by recent observations from
Gaia and massive stellar surveys, the latest models from [31] reproduce most
features in the observed HI 21 cm longitude-velocity diagram, including the
Central Molecular Zone, the Near and Far 3-kpc arms, the Molecular Ring, and
the spiral arm tangents. The gas distribution that comes out from these state-
of-the-art models is strongly influenced by the bar and is roughly symmetric
with respect to an axis oriented by about 25 degrees from the Sun-GC axis,
i.e., it introduces a strong east-west asymmetry. One of the models is shown
in Figure 3, and it can be seen that the gas over-densities, as viewed from
the Sun vantage point, are seen at very different distances in the east and in
the west. If, as suggested by Sofue and co-workers, the sources of the main
soft X-rays brightness enhancements are associated with the gas flow encounter
with 3 kpc structures, then it is clear that the “east source” is much closer
(at $\simeq 5$ kpc from the Sun) than the “west source” at $\simeq 11$ kpc.

In an attempt to go one step further beyond the association between the LNPS
base and tangential direction to the eastern ring, we have the same model of
[31] and searched for potential associations between the longitudes of the
polarized arches/spurs where they cross the Plane and the longitudes of the
densest structures in the model (except the CMZ). In Figure 3 are shown the
approximate directions of the low latitude ends of the radio arcs/spurs seen
in polarized emission and taken from [4, Figure 20]. We have reported this set
of directions in the model map of [31]. There are several interesting
associations that may mark the interaction of the shocked gas with disk gas
concentrations, calling for further work in this direction. In any case, such
an east-west asymmetry has important potential implications in the frame of
the LPNS distant hypothesis: it will generate different characteristics of the
propagation of the outer shock in the low altitude halo and modify the
observed pattern as seen from the Sun vantage point, both in X-ray and radio,
and create asymmetries.

##  4 Microwave-radio and stellar polarization: the dual top-bottom structure
hypothesis

The LNPS synchrotron emission is highly polarized ($\simeq 40$-50%), and
spectacular images of the radio/microwave polarized fraction have been
produced (see [4, Figure 20], Figures 3 and 7). Information on the source
distance based on the synchrotron signal has been searched for in several
ways.

  * •

A first estimate of the LPNS source distance is based on the amount of
depolarization due to Faraday rotation in the foreground and the quantity of
interstellar matter required to explain the depolarization (see [33, 4, 34]).
This technique has led to some constraints on the source location at low
latitude (b $\leq 30\hbox{${}^{\circ}$\leavevmode\nobreak\ }$), where the
radio signal is strongly depolarized, especially at low frequencies. A
distance of several hundred pc is suggested by the most recent works (see
[34]. At higher latitudes there is no sign for depolarization, instead [35]
showed that the rotation measure (RM) of the NPS emission is negligible (see
below other results from the same study). These studies suggest that there may
be a dichotomy of the LNPS, with two different sources located at short
distance (resp. higher distance) and viewed at high latitudes (resp. low
latitude).

  * •

A second technique is the comparison between the polarization angle of the
LNPS synchrotron emission and the direction of starlight polarization. A
strong similarity between the starlight and LNPS polarization angles was well
recognized for several decades in the case of nearby stars, but more recently
the strong improvements in quality and sensitivity of stellar polarization
measurements brought better constraints. [5] used the latest optical data on
polarization of stars closer than 1 kpc along with Planck and WMAP polarized
synchrotron emission, also including the polarization angle of the dust
thermal emission measured by Planck. The authors found that at $\rm b\leq 30$∘
(their field IA), there is no clear similarity in the directions of
polarization, due to the presence of multiple structures. Above 30∘ , there
is alignment between synchrotron and stellar polarization for stars located
between 100 and 200 pc, and in two from three regions (fields IB and IC) the
alignment holds at larger distances. In addition, where the dust is restricted
to the first hundred parsecs, the polarization of the dust emission is aligned
with the synchrotron. This is a very interesting result, and it strongly
favors a very nearby source for the high latitude LNPS. According to their
results, the authors suggest that the low and high latitude NPS emissions come
from different sources. As a caveat, it must be said that the alignment is a
necessary but not sufficient condition for a co-spatiality of the dust and the
energetic electrons that emit synchrotron. If the magnetic field keeps the
same orientation along large distances, both in the dusty region and beyond
the dust, the synchrotron emission may originate from beyond the dust, while
the angle between the polarization directions of the synchrotron emission and
the stellar light remains the same. This is not precluded in the two regions
mentioned above (fields IB and IC), since the good alignment holds until the
limit distance for the alignment study, about 600 pc. The conclusion drawn,
namely that above 30∘ latitude the synchrotron emission is generated as close
as $\simeq 100$ pc, may not be valid in those fields. It remains valid in the
highest latitude field ID.

  * •

A third and very promising technique is Faraday tomography. [35] used the
first data from the GMIMS Survey using 2048 frequency channels from 1280 to
1750 MHz to obtain a Faraday Rotation Measure (RM) of the NPS and in adjacent
regions. They combined the RMs with Faraday depth maps of the entire Galaxy
based on extra-galactic sources assembled by [36]. From the comparison between
the two measurements and using the difference between data directed towards
the NPS and data in adjacent regions, they estimated the Faraday depth in
front and beyond the NPS and used those estimates to draw several conclusions:
-below $\rm{b}=44$∘ the emission is very likely distant, or the large scale
magnetic field must have a reversal;-above this latitude, the Faraday depth is
close to zero and the LNPS emission must be local and originate within several
hundreds parsecs. Such a pioneering study is quite interesting; however, at
high latitudes the measurements and differences are close to the uncertainty
level, and there are several simplifying assumptions, such like the assumed
similarity between all physical quantities in adjacent regions outside and
inside the NPS, an assumption that may not be valid if the NPS corresponds to
the periphery of shocked expanding hot gas, and the additional assumption of
perfect mixing between thermal and non thermal emitting gas within the NPS.

As a conclusion, interestingly all three recent studies based on polarization
point to a double source, a low-latitude and distant source and a high-
latitude and nearby source. Again, we must note that at high latitudes the
total column of interstellar dust is reached at very short distance (see the
dust map section below), making the argument of negligible depolarization or
Faraday rotation as a sign of proximity of the emission disputable. We must
also note that such a dual source is in contradiction with the assumption that
there is a unique giant reheated cavity bounded by Loop I and the continuation
of Loop I under the form of Loop XII (from [27]) (see Section 6 and Figure 7).

A dual structure is contradicted by X-ray data. At variance with polarization
measurements, X-ray data favor the continuity of the LNPS source properties
from low to high latitudes, because there is no discontinuity nor sign of
inhomogeneity of the recorded spectra. [37] analyzed the entirety of Suzaku
spectra in the direction of the LNPS and found that all spectra can be fitted
with a $\simeq 0.3$ keV gas, with no need for any discontinuity, nor even
variation except for the emission measure. Such homogeneity is in agreement
with the previous analyses based on Suzaku, XMM-Newton and Swift observations
in various directions within both the LNPS and the northern FB contour ([38,
39, 26]). More recently, and totally independently, the analysis of the
spectra recorded with the nano-satellite HaloSat led to the same conclusion.
HaloSat ([40]) was dedicated to the diffuse emission from the hot gas in the
halo, but in parallel did a series of observations along the bright LNPS from
galactic latitudes $+15$ to $+74$∘ and found a total continuity in the
spectral shape ([41]).

Figure 4: Distances at which measured X-ray absorbing columns are reached
towards the NPS. The figure is the image of the dust extinction density
distribution in a vertical plane containing the Sun and the direction of the
northern Galactic pole, and oriented along the Galactic longitude $\rm l=29$∘
(the plane colored in orange in the small inserted scheme). The blue circles
are associated with a series of 19 XMM-Newton spectra recorded in this plane
at galactic latitudes comprised between $+5.6$ and $+11.2$∘  [42]. The
spectral analysis provided for each direction the value of the column of gas
absorbing the NPS signal. After conversion to extinction, the map is used to
calculate the distance along the corresponding line-of-sight at which this
value is reached, i.e. the minimum distance to the region responsible for the
NPS emission. For each pointing the corresponding location is indicated by an
arrow whose location marks this limit and the region beyond which the emission
starts to be produced. Note that in the case of the four circles at right in
the figure the computed distance would locate them out of the figure. The four
yellow triangles correspond to other additional measurements towards the NPS,
however the longitudes of the probed directions are not $+29$∘ (see text).

##  5 Distances from X-ray data

###  5.1 Using global spectral shapes

More quantitative estimates of the distances to the LNPS source regions can be
obtained from spectral information on the absorbing foreground. Several X-ray
spectra have been recorded towards NPS regions, and various conclusions have
been drawn depending on the probed region and the spectral analysis. XMM-
Newton then later on Suzaku were used to record spectra towards the NPS above
$\rm b=+20$∘ ([43, 44]). The data analysis suggested a significant absorbing
column on the order of the estimated full Galactic column. In the frame of a
wider study of the whole X-ray emission supposedly associated with the Fermi
bubbles, [38] recorded and analyzed the Suzaku spectra for 8 sightlines
towards the high latitude NPS ($43\hbox{${}^{\circ}$\leavevmode\nobreak\
}\\!\\!\leq{\rm b}\leq 48\hbox{${}^{\circ}$\leavevmode\nobreak\ }$). Using a
single component for the NPS and Galactic halo (in addition to the Local
Bubble) the authors found that the column of the cold absorbing foreground is
on the order of the full Galactic column. This was confirmed by later analyses
of the same data augmented by several additional Suzaku and SWIFT spectra
([39, 26]).

Figure 5: Top: Isocontours of dust extinction in the visible, integrated
between 0 and 100 pc ([24]), superimposed on the ROSAT 0.75 keV map
([45]). Contours are drawn for A${}_{V}=0.02,0.03,0.04,0.06,0.08$ and 0.1
mag. The LNPS eastern limit is shown in red. Bottom: same as top, but the
extinction is integrated between 0 and 800 pc. and contours for
A${}_{V}=0.04,0.06,0.1,0.2,0.4,0.8$ (blue) and 1.5 (red) mag. White lines mark
the latitudes above which the integration is limited by the boundaries of the
3D map and does not reach 800 pc (the distance reached is between 400 and 800
pc). At those latitudes, however, beyond 400 pc the extinction is negligible.
Note the detailed anti-correlation with the X-ray brightness for most contours
in the bottom map, at variance with the 100 pc integration at top where only a
very weak anti-correlation is seen for the Aquila Rift at low latitude.

Figure 6: Top: Isocontours of dust extinction in the visible, integrated
between 0 and 100 pc ([24]), superimposed on the ROSAT 0.25 keV map
([45]). Note the strong change of the color scale. The recorded signal is
restricted to high latitudes directions for which only weak absorption occurs,
i.e. from the halo and the top part of the LNPS. The LNPS eastern limit is
shown in red. It allows to distinguish the contribution to the signal of the
LNPS from the one of the halo. Contours are drawn for A${}_{V}=0.02,$ 0.025,
0.03, 0.035, 0.04, 0.06 mag.Bottom: same as top, but the extinction is
integrated between 0 and 200 pc and contours at $\rm A_{V}=0.035,$ 0.04, 0.09,
0.09, 0.2 mag. Note the appearance of anti-correlation for several contours in
the bottom map. Also note the absorption due to the elongated cloud
overlapping the LNPS ridge (e.g. at locations indicated by red arrows).

An interesting region is the low latitude part of the X-ray NPS around $\rm
b=+10$∘ , a region characterized by an abrupt disappearance of the signal
below $\simeq 6$∘ latitude (see Figure 1. A series of XMM-Newton spectra were
recorded with the goal of determining whether this disappearance is due to
increased interstellar matter absorbing column (absorption-bounded case) or to
the true lack of emission (emission-bounded case) below this latitude (see
[42]). The analysis of the spectral features and intensities showed that the
absorption-bounded case is the actual situation, and a series of absorbing
columns were measured between $+5.6$ and $+11.2$∘ , all around the $l=+29$∘
longitude. The model used for this low-latitude part of the NPS was a
composite model of the NPS itself, the higher energy emission of the _bulge_
and the pre-determined small contribution of the Local Bubble at the softer
energies, and the absorption was assumed to be entirely due to cold gas.
Absorbing columns on the order of 5 to 50 1020 cm-2 were obtained from the
spectral fits. After conversion of gas columns to dust extinctions and
comparisons with existing 3D dust maps, estimates of the distance from the Sun
at which the measured absorbing column is reached could be obtained for each
sightline and a first estimate of the minimum distance to the X-ray source was
300 pc; however the good correlation of the absorbing columns with the optical
thickness of the intervening dust deduced from Planck, generated from the Sun
to infinity, was suggesting a larger distance. More recently, [46] used the
same measured absorbing columns and updated 3D extinction radial profiles
based on PanSTARRS and 2MASS to perform a new conversion from gas to dust and
estimate distances at which the absorption takes place. In order to visualize
the absorbing clouds, we interpolated in their table 1 to compute the
distances at which 100% of the converted gas column is reached. In Figure 4
are shown the locations of such limits, superimposed on the image of the
extinction density distribution in a vertical plane containing the Sun and
oriented towards $\rm l=29$∘ . The extinction distribution, here shown up to
1050 pc, is deduced from a recent inversion of individual measurements of
individual extinctions from GAIA eDR3 and 2MASS photometric data analyses and
from ground-based spectroscopic surveys [24]. Since the 19 XMM-Newton lines of
sight are within $0.7$∘ from the represented plane, the image gives a close
representation of the dust along all of them. Four limit distances are higher
than 1 kpc and fall out of the map and are shown at the map boundary. 4
additional points at high latitude are also shown, from two different works
and are discussed below.

Two comments can be made from such the X-ray and dust data: first, the limits
are either close to locations beyond which the quantity of dust is very low or
beyond all the dust. Given the large uncertainties on the gas to dust
conversion factor and on the absorbing columns (see below), this pattern is
compatible with a source of the emission from a volume beyond or above the
mapped dust clouds (this would happen in case the absorption is slightly
underestimated); second, and more important, even if the absorbing columns
(and minimum distance) were slightly overestimated, the minimum distance to
the emission source seems to increase with latitude among the 19 low-latitude
sightlines, a pattern certainly not suggestive of a source region located
close to the Sun, and certainly not at distances on the order of 150 pc above
the Sco-Cen associations. Instead, the pattern suggests a source located above
the dust layers or tangential to the long series of dust clouds from the Sun
to about 1000 pc. In Figure 4 we have added four limit distances based on
measurements of X-ray absorbing columns from [43, 44],and a conversion factor
of 5 1021 cm-2 per E(B-V) mag. The longitudes for these measurements are
significantly different from $\rm l=29$∘ , however dust images in the
corresponding planes show a close pattern. The location of the limits gives an
idea of the constraints, or more precisely, of the absence of constraints on
the distance to the emission: as a matter of fact the absorbing column places
the limits beyond the reconstructed dust, implying that the source may be
everywhere at larger distances.

###  5.2 Using anti-correlation between soft X-rays and dust opacities

The inter-comparison of diffuse soft X-ray images in various energy bands
provides primarily information on the hot gas temperature and density, but may
additionally provide constraints on the distance to the source, provided one
has some preliminary information on how the absorbing gas is distributed along
the line of sight. This is due to the enormous variation of the absorption
cross-section per H atom $\sigma_{X}$ over the soft X-ray energy range. In the
case of the 0.1 to 2 keV ROSAT maps ([45]), $\sigma_{X}$ decreases by a
factor above 500 with increasing photon energy. This is why 0.25 keV data
reveal solely the _unabsorbed_ or very weakly absorbed emission from hot gas
in the local bubble and from the high latitude halo (NH columns on the order
of1019-20 cm-2), while 0.75 keV can be used to detect hot gas beyond more
opaque regions (columns on the order of up to 1022 cm-2). The observed LNPS
soft X-ray spectrum becomes harder at low latitude (until signal disappearance
below $\simeq$ 6∘ ), while at high latitude the Galactic ISM columns become
small enough to allow detection in the softer range. This explains the color
variation in the e-Rosita map of Figure 1. Thanks to Gaia and ground-based
surveys, the increased availability of 3D information on the Galactic ISM, and
in particular on dust extinction allows to study in more detail the distance
to the emitting gas. Assuming a uniform dust to gas conversion factor, it is
possible to build maps of the columns of absorbing gas integrated from the Sun
to a given distance, and to compare the resulting images with diffuse X-ray
background images. Against a homogeneous background source, variations of the
absorbing columns should be anti-correlated with the observed, absorbed
signal. We have taken advantage of the new 3D extinction maps based on Gaia
distances and photometry to do this exercise ([24]), and Figures 5 and 6
illustrate some of the results. An entire analysis is beyond the scope of this
article.

Figure 5 shows two examples of comparisons between the 0.75 keV diffuse
emission and dust extinction in the visible AV. The extinction between the Sun
and 100 pc (resp. 800 pc) is shown under the form of iso-extinction contours
that can be compared with the X-ray intensity pattern. While no or very little
anti-correlation can be seen for the short distance extinction, the 800 pc
extinction map is clearly revealing detailed anti-correlation patterns. This
confirms the previous analysis of XMM data and extends to all longitudes the
conclusion reached at $\rm l=+29$∘ , namely that the emission is generated
well beyond the Aquila Rift and very likely beyond 800 pc. Figure 6 is of
similar nature for the ultra-soft energy map (0.25 keV). In this range, X-ray
emission is detected only at high latitude where absorption is very weak, and
the LNPS ridge is no longer a sharp limit as at higher energy. We show the
extinction iso-contours for 100 and 200 pc only. Again, no anti-correlation is
visible at 100 pc, showing that the absorbing medium is located at further
distance. Conversely, between 100 and 200 pc anti-correlations appear, in
particular the absorption effect of an elongated cloud covering the LNPS ridge
is visible. This cloud, whose HI counterpart has been already discussed by
[35], is also visible as a weak dark feature in Figure 4 at $(\rm
l,b)=(+29\hbox{${}^{\circ}$\leavevmode\nobreak\
}\\!\\!,+60\hbox{${}^{\circ}$\leavevmode\nobreak\ }\\!\\!)$. Isocontours
computed for longer distances are similar to those for 200 pc, because there
is no more dust beyond 200 pc at such high latitudes. This pattern suggests
that at high latitude the X-ray signal is generated beyond the totality of the
dust.

###  5.3 Using line ratios

After the published analysis of the early LNPS soft X-ray spectra recorded
with XMM-Newton and Suzaku above $\rm b=+20$∘ ([43, 44]), some peculiar
features in the spectra attracted the attention. [47] noticed a Doppler shift
of the OVII and NeIX He-like triplets (made from unresolved resonance,
intermediate and forbidden lines) and interpreted it as potential forbidden
line enhancement due to charge-exchange (CX) cascade emission, a mechanism
potentially at work at the interface between the NPS hot gas and the cold
matter of the disk. Later on, and almost at the same time as the XMM-Newton
study quoted above, [48] re-reduced and analyzed in detail all previous XMM
and Suzaku data above $\rm b=+20$∘ , introducing in the spectral analysis a
potential absorption by ionized gas, in addition to the absorption by dense,
neutral matter. Compared to the spectral modeling of [42], the analysis was
made differently. [48] used data recorded at the same latitude but out of the
NPS to perform a preliminary determination of the emission from the LB and the
one from the Galactic halo (at such latitudes the halo emission is a non-
negligible contributor, which not was not the case for the NPS between 6 and
11 degrees latitude where the emission from the bulge is the main additional
contributor). The authors built model spectra for various combinations of
thermal+CX emission and absorbing columns of both neutral and ionized
interstellar gas and found that a high absorbing column of hot (0.17-0.20 keV)
ionized gas is necessary to reproduce simultaneously the enhanced OVII and Ne
IX forbidden-to-resonance ratios and high OVIII Ly$\beta$ line relative to
other Lyman series. They found a minor CX contribution to the emission on the
order of 10% only. The similarity between fitted model spectra and data
obtained by the authors in this case is very strong. They concluded that the
high temperature and the high columns of this absorbing ionized gas (on the
order of 3-5 1019 cm-2) play in favor of halo gas absorption over large
distances (5 to 8 kpc). If the hot gas column were distributed over distances
$\leq 1$ kpc, the size of the proposed nearby cavities (see also below), its
average density would be higher than 0.01 cm-3. This is above average hot gas
densities in hot super-bubbles according to hydrodynamical models (see e.g.
[49]), instead long (above 5-6 kpc) paths through hot halo gas are more
likely. The conclusion of this overlooked study is in agreement with the
conclusion deduced from the NPS low latitude edge, however, in the case of hot
ionized gas absorption, the favored distances reach Galactic scale sizes.

##  6 A re-heated superbubble within the Local Arm?

Figure 7 shows the diffuse polarized emission from combined Planck and WMAP
data (taken from [4]). There are several loops and arches whose distances have
been discussed by the authors, who also proposed a prolongation of Loop I in
the south and in the west to form the closed loop indicated by a white contour
and marking the synchrotron-emitting tangential surface to a giant cavity of
recently reheated hot gas, centered at several hundred parsec. As a matter of
fact, the existence of strong depolarization below $\rm b=30$∘ ([33, 4,
34]), the clear imprints of Aquila rift absorption in ROSAT maps and first
evidences for the absence of a very nearby huge cavity in 3D maps of the ISM
([50]) led [4] to propose a two-steps scenario to explain both diffuse
X-rays and synchrotron radiation, namely (i) the creation more than 10 Myrs
ago of a huge hot gas cavity blown by a large series of SNRs, and centered at
several hundred parsecs, and (ii) the more recent (less than 2 Myrs ago)
reheating of this cavity by a second SNR episode. The giant shell boundary
crosses the Plane at $\rm l\simeq+25$∘ . In such a configuration the blown
and reheated hot gas cavity encircled by the shell is clearly asymmetric. We
have superimposed on the figure is a second close loop proposed and modeled by
[51]. According to this work, this shell marks the interaction between Loop I
and the Local Bubble. As part of the figure are drawn images of the dust
distribution in vertical (meridian) planes along the longitudes of the shell
centers (marked by I and III respectively). The directions of the high and low
latitudes tangential directions to the respective shells are also indicated.
The proposed cavities should be bordered by these two directions and centered
at several hundred parsec in the first case, and much closer in the second
case. According to the previous section, the strongest X-ray-based argument
against such a double episode of large cavity creation and reheating, as
proposed by [4], is the requirement for a very large column of hot ionized gas
in front of the LNPS, deduced from specific line intensities by [48]. As
discussed above, this is hardly compatible with a cavity size of about 1 kpc
or less. Assuming that such an inference is not granted, because it is is
based on a single type of spectral analysis, we consider the re-heating
scenario in the light of the above more numerous constraints on the distance
to the X-ray source.

First of all, we note that this is not an unusual configuration, as series of
SNRs occur episodically and super-bubbles are observed here and there.
Moreover, the recent detection at Earth of the 60Fe isotope (whose lifetime is
on the order of 2 Myrs) is reinforcing the existence of such a scenario
because it is naturally explained by one or more very recent SNR explosions at
very short distance from the Sun, as argued by [52]. The authors have traced
back the Sco-Cen stellar association and modeled the evolution and impact of
its members. Interestingly, they have shown that two of the most recent
supernovae from the group have very likely exploded 1.5-2 Myrs ago within 150
pc and may be at the origin of 60Fe, while the older members may have produced
14-20 supernovae and have gradually blown the Local Bubble. This is similar to
the re-heating scenario of [4], replacing the Local Bubble by a more distant
cavity centered at longitudes between $\rm l=330$∘ and $\rm l=0$∘ . In the
case of the LNPS, a super-bubble centered at several hundred parsecs (very
likely more than 700 pc as we discussed in Section 5), and culminating above
the Sun at b=+80∘ has a minimum radius on the order of 600 pc and a volume at
least one order of magnitude above the one of the LB. Models of super-bubbles
blown by stellar winds and supernovae predict that maintaining such gigantic
hot gas cavities requires a frequency of massive star deaths on the order of
five per Myr during more than 5 Myrs (see, e.g. [49]). In this case, OB
associations having given rise to the bubble must have left groups of early
and late B stars, and there should be a huge cavity in the ISM (see, e.g.
evolutive MHD models of [53], well suited for the Sun vicinity). Both types of
signatures are difficult to identify, however thanks to Gaia there has been
some progress in the two cases.

Figure 7: Top left: combined Planck/WMAP polarization intensity, showing the
LNPS and all other arches/spurs ([4, Figure 21(c) from]). The contours of
the local shell proposed by [4] are shown by a thin dashed black line. We have
superimposed as a thick dashed black contour the local shell due to Loop I
interaction with the Local cavity proposed by [51]. We have selected three
meridian lines, marked by I to III and black arrows. I corresponds to the
center of the [4] shell, III to the center of the shell proposed by [51], and
II is the intermediate longitude. Right: dust extinction density images in the
three vertical meridian planes containing the Sun, the north pole direction
and oriented towards Galactic longitudes 330∘ (I), 342.5∘ (II) and 355∘ (III),
i.e. those containing the directions marked by black arrows in the left
figure. Massive B stars from the ALMA catalog ([54]) and located within 10
degrees from each plane are shown in projection onto the plane (note that some
targets appear on two figures). Colors refer to the stellar type, as indicated
on the color scale at bottom right. Blue dot-dashed lines indicate the
tangential directions to the boundaries of the shells I and III. A potential
solution for the near side boundary of a giant reheated cavity of hot gas is
indicated by dashed contours in III (see text).

First, an extensive new catalog of massive stars within 3kpc has been
established and very recently presented by [54]. Importantly, for most objects
from the catalog located within $\simeq 1$ kpc rather accurate Gaia parallaxes
are available. We have updated the Gaia DR2 distances given in the catalog to
their eDR3 re-evaluations and extracted all massive O, B stars from the
catalog with longitudes between $\rm l=320$∘ and $+5$∘ . The objects are
superimposed on the dust images of Figure 7. There are two groups of
associations, the nearby, well-known Sco-Cen association and a second group of
older stars at about 450 pc close to the Plane. Most stars from the former
group are still embedded within their massive parent molecular clouds. There
are relatively small(20-60 pc wide) cavities associated with the youngest and
most massive objects, in particular the cavity associated with $\rho$Oph
([55]). The 3D dust map resolution is too poor to reveal the details of the
smaller cavities, only a few are visible. These young objects may have been a
potential source of reheating of a more ancient super-bubble. The second group
of stars at about 450 pc contains about ten B stars which could belong to the
group having first produced the giant cavity. The number of objects, however,
appears rather low in comparison of expectations for a huge cavity. From the
geometrical point of view, the huge reheated cavity could be bounded in the
way schematized in Figure 7 for the $\rm
l=355\hbox{${}^{\circ}$\leavevmode\nobreak\ }$ vertical plane. In this plane
one can see that this cavity could extend in the southern hemisphere at large
distance (see the question marks in Figure 7), following the polarization full
loop proposed by [4]. However, cloud distributions at other longitudes, e.g.
at $\rm l=342.5$∘ in this figure, or at $\rm l\simeq+29$∘ in Figure 4, i.e.
close to the longitude at which the outer boundary of the cavity crosses the
Plane, are more difficult to reconcile with a southern extension to form a
giant north-south cavity.

On the second aspect of ISM distribution, we show in Figure 8 the distribution
of the dust within 3 kpc from the Sun as deduced from the very recent mapping
we already mentioned ([24]). The _old_ B star cluster is located close to
the boundary of the giant dust-free region separating the Local Arm (more
precisely its inner part, the conspicuous _split_ ([56]) from the $\geq 1$
kpc distant Sagittarius–Carina outer part. There is no indication of any large
scale cavity or even imprinted large scale curvature in the dust distributions
around the star group, instead the chains of dust clouds follow the same
orientation everywhere. As a conclusion, within the limits of current
knowledge, there are no hints of any kind of a super-bubble creation within
the Local Arm close to the Galactic Center direction, although all arguments
are not very strong. We do not discuss further a super-bubble generated within
Sagittarius (at $\simeq 1.2$ kpc), whose radius would be above 1.1 kpc.
Although this can not be excluded, we now reach a probability on the same
order as the interaction of a shock launched by FB-type events with the disk
material at and beyond the 3 kpc ring (potentially at 3-4 kpc from the Sun,
see Figure 3).

Figure 8: Dust distribution along the midplane within 3 kpc from the Sun. The
Galactic center direction is to the right, units are pc. The location of the
group of massive B stars from the ALMA catalog ([54]) considered as a
potential source of a nearby superbubble is indicated by a red square.

##  7 Summary and discussion

Figure 9: Simplistic scheme of the speculative solution discussed in this
draft. The gas distribution in the disk (as viewed from above the Plane) is
from the model of [31] already shown in Figure 3. The hypothetical boundary of
the shocked halo gas generated by the FBs and having flown at short distance
from the Plane is shown in projection onto the Plane (red circles). Due to the
asymmetry of the gas distribution, the shock front surface has been evolving
according to the obstacles that force its compression and expansion into the
halo. Its expansion is such that it has now reached the Sun circle with a
fully distorted boundary. As viewed from the Sun, X-rays emitted by the volume
of hot gas between the FB contours and the shock front (red hashes) start
abruptly from $\rm l\simeq 25$∘ , keep a strong brightness in the east and
towards the GC where the path length is the longer, while in the west their
brightness is gradually fading away down to $\rm l\simeq 270$∘ . The
intensity of the polarized synchrotron emission is maximal in directions
tangential to the shock (orange dashed arrows), i.e. mainly in the east and in
a much smaller extent in the west towards $\rm l\simeq 255$∘ . The latter
region could correspond to the western Loop I continuation proposed by [4]. At
high southern latitudes X-rays and synchrotron are significantly weaker than
in the north due to halo weaker density.

In this article we have reviewed recent measurements and models having a
direct or indirect connection with the LNPS structure and developed further
comparisons, with particular focus on the use of 3D maps of interstellar gas
and dust and on X-ray data. In some cases, the potential connection with the
LNPS has been overlooked. Our goal is to shed additional light on the
longstanding uncertainty on North Polar Spur-Loop I distance (LNPS). Several
conclusions can be drawn and we summarize them below.

###

The whole set of analyses of X-ray data, from the most recent to updated
analyses of the earliest spectra (XMM-Newton, Suzaku, Swift, HaloSat) gives
consistent results and favors a distant origin of the X-ray LNPS as well as an
homogeneity of the spectral shapes over the whole structure. There is no
firmly established upper limit on the source distance if one takes
uncertainties into account, whatever the latitude range.

  * –

Fitted neutral gas absorbing columns are on the order of full Galactic columns
at all latitudes from the lowest $\rm b=+5.6$∘ ([42]) to the highest $\rm
b=+55$∘ ([39]). Note that, due to the dust scale height, these columns are
reached at very different distances, from $\sim 100$ pc at high latitude to
$\sim 1$ kpc at the lowest latitudes of the measurements, see Figure 4.

  * –

At high latitude, the best fitted model spectra require a large absorbing
column of hot (million K) ionized gas in addition to the dense phase ([48]),
an evidence for a Galactic size source.

  * –

The geometrical pattern implied by constraints on dense interstellar matter
absorbing columns is hardly consistent with a nearby super bubble (Figure 4).

  * –

Anti-correlations between extinctions deduced from recent 3D maps of
extinction density and ROSAT soft X-rays full-sky maps point to a distant
source, from beyond 700-800 pc at low latitude to beyond 150 pc at high
latitude.

  * –

All spectral analyses point to homogeneous or slowly varying spectral shapes
and subsequently continuously varying properties of the emitting hot gas.

  * –

The spectral homogeneity is consistent with the regularity of the NPS eastern
edge contour from low to high latitude, strongly suggestive of a unique
structure. The regularity is particularly visible in the new e-Rosita map,
because it is not affected by time-variable solar wind heliospheric and
magnetospheric charge-exchange emissions, at variance with ROSAT.

The results on the X-ray source distance and homogeneity have strong
implications. The homogeneity is not compatible with the dual top-bottom
structure proposed by [46, 5] and first suggested by [35]. The argumentation
of [46] is mostly based on a claimed contradiction between the large distance
to the X-ray source at low latitude and the short distance to the source at
high latitude. However, there is no actual determination of the high latitude
distance limit in this work, because there is no use of absorption data, only
a determination of the distance to the dust clouds, hypothesized to be
equivalent to the distance to the source. On the contrary, there are
contradictions between the X-ray homogeneity and the arguments presented by
[5] and [35]. In Section 4, we detailed their arguments in favor of a dual
structure based on depolarization and Faraday depth ([35]) or based on
stellar light polarization and Planck-WMAP polarization ([5]), and we come
back to this issue below.

The second consequence of a distant X-ray source, (and this holds even if it
is only several hundreds pc away), is an absence of link between the nearby
high latitude tenuous interstellar clouds and the LPNS, because high latitude
dust or gas structures filaments are unambiguously all very nearby at the
periphery of the Local Cavity, as confirmed by recent 3D dust maps. As already
mentioned in Section 1.4, such an absence of link is not so surprising if one
makes a close inspection of the X-ray and dust maps. Importantly, this absence
of link removes several difficulties associated with the hypothesis of a
nearby re-heated super-bubble:

  * (i)

the absence of accelerated gas at the periphery of the super-bubble, since
high velocity shocks are needed to explain gas heating and X-ray emission, and
one would expect at least some high velocity fragments of the shell,

  * (ii)

the absence of optical recombination lines, characteristics of SNR shell
remnants.

  * (iii)

the required coincidence between the distance reached by the expanding, most
recent shock and the distance to the nearby filaments surrounding the _old_
super bubble.

For those reasons it makes more realistic the scenario presented by [4]
according to which hot gas and synchrotron emissions are due to the recent re-
reheating of a previously blown super bubble (see e.g. [4]).

### Distance to the synchrotron source

The most convincing arguments in favor of a nearby LNPS source come for high
latitude synchrotron data, whose different aspects have been presented in
Section 4.

  * –

At $\rm b\geq 30$∘ there is a strong similarity between the polarization angle
of nearby starlight for stars beyond $\simeq$100 pc and LNPS synchrotron
emission. At $\rm b\simeq 75$∘ the similarity is restricted to star distances
between 100 pc and $\simeq 400$ pc, suggesting a source region within these
limits. Consistently, at these latitudes there is a strong similarity between
the polarization angle of nearby starlight and the one of the dust emission
measured by Planck/WMAP.

  * –

There is no measurable depolarization for $\rm b\geq
30\hbox{${}^{\circ}$\leavevmode\nobreak\ }$.

  * –

At $\rm b\geq 45$∘ the rotation measure due to the LNPS foreground is
consistent with zero.

The various arguments suffer from the weakness of the dust and gas columns at
high latitude and the resulting uncertainties, however the conclusions are all
consistent.

### The reheated super-bubble

A way to reconcile X-ray and synchrotron data analyses is a reheated super-
bubble centered several hundred parsecs away, as proposed by [4], and to
assume that its near side at high latitude is as close as $\simeq 100$-200 pc
(see Figure 7(III)). As we have seen, this is not favored byO, B stars
distributions, nor by 3D distribution of clouds, especially for a SB extending
on both sides of the Plane. One may argue that these arguments are not
quantitative, therefore not indisputable. What is indisputable is the
remarkable coincidence between the huge X-ray feature produced by the SB in
the north and the similarly huge X-ray feature in the south, both surrounding
the northern and southern FBs respectively.

### Geometrical arguments

We have seen that very recent data and models as well as overlooked results
bring some answers to geometrical difficulties with the hypothesis of a
distant FB-connected source.

  * –

There is an X-ray counterpart of Loop I in the south [3].

  * –

There are several evidences for a higher gas density in the northern halo
compared to the southern one, possibly at the origin of the weakness of the
southern X-ray loop, and in different distances and shapes of outer shock
fronts in the northern and southern halos.

  * –

State-of-the-art models of the Milky Way gas confirm very strong west-east
asymmetries. They may influence the hot gas expansion close to the Plane. Some
of gas strong over-densities other than the eastern 3 kpc ring may correspond
to the bases of arches/spurs seen by Planck/WMAP (Figure 3).

### Conclusion and speculated scenario

Although new observations and models in favor of the distant hypothesis are
more numerous than those in favor of the local source, it remains that the
full set of constraints derived from the various analyses, if entirely taken
for fully granted, can not be fulfilled by any purely nearby nor purely
distant model. The purely nearby configuration requires a super-bubble with a
geometry and physical properties that are not favored by stellar and
interstellar data and a remarkable, unlikely resemblance between the contours
of the super-bubble in the north and those of theX-ray bubble seen by Suzaku
and e-Rosita in the south, both enveloping in the same way the Fermi bubbles
in 2D images. The purely distant configuration is not compatible with the
estimated proximity of the synchrotron source at high latitude in the north
and lacks loops in the west and in the south that could be identified with the
expected huge synchrotron emitting shell.

On the other hand, several models we already mentioned are an inspiring source
for a speculative scenario potentially consistent with all data. Models of FBs
and associated halo heating from [20] predict that X-rays and synchrotron
shells may be different in the north and in the south due to halo density
asymmetry, and that the outer shock may have reached the Sun in the south and
not yet in the north. These models do not include the complex gaseous disk
structure and the gap between the CMZ and the 3 kpc ring (from, e.g., [31]).
[32] emphasizes the role of the ring as responsible for the low latitude spur
but uses an axisymmetric halo. Combining in the same model a realistic
distribution of the gas in the disk and a non-symmetric halo is a challenging
task. However one may speculate that in this case the volume of shocked halo
gas at low altitude above the Plane _may be_ very irregular, having expanded
quasi-freely outside of the CMZ, then encountered beyond 3 kpc very different
gas concentrations as a function of longitude. The volume of hot gas may be
also very different in the southern and northern hemispheres due to different
thick disk and halo gas density distribution. We have searched for a
configuration able to explain the extent of the X-rays and the
presence/absence of observed synchrotron shells. Figure 9 shows a _very_
speculative outer shock contour at short distance from the Plane, distorted
under the influence of the disk structures and having reached the Sun. This
type of contour may potentially explain east-west differences, namely a large
X-ray brightness in the east and a gradual decrease to the west, and the
absence in the west of a synchrotron counterpart of the bright LPNS (see
figure and caption). Evidently, such a configuration is not purely local nor
purely distant but intermediate in the sense that the origin of the LNPS is
distant but part of the source is nearby.

This is certainly not the end of the story and we emphasize that the suggested
speculative scenario lacks modeling support. New large-scale models of the FBs
and of their expansion are needed, using more complex gas distributions in the
disk, like the one shown in Figure 3, and in the halo, taking into account the
density asymmetry. This is a particularly difficult task, however the results
may show whether or not the observed radio/microwave spurs and Loop I can be
reproduced in 3D, and not only their approximate directions as in Figure 3.
More sensitive and numerous radio/submm data and especially Faraday tomography
studies (with LOFAR) will be also crucial to confirm or not the proximity of
the high latitude Loop I. Future Gaia data releases and resulting dust maps
with higher resolution and larger extent are also expected to help, and in the
far future Athena X-ray spectra should bring further constraints.

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