Origin of Galactic Spurs: New Insight from Radio/X-ray All-sky Maps
Jun Kataoka, Marino Yamamoto, Yuki Nakamura, Soichiro Ito, Yoshiaki Sofue, Yoshiyuki Inoue, Takeshi Nakamori, Tomonori Totani
DDraft version January 12, 2021
Typeset using L A TEX twocolumn style in AASTeX62
Origin of Galactic Spurs: New Insight from Radio/X-ray All-sky Maps
Jun Kataoka, Marino Yamamoto, Yuki Nakamura, Soichiro Ito, Yoshiaki Sofue, Yoshiyuki Inoue,
3, 4, 5
Takeshi Nakamori, and Tomonori Totani Faculty of Science and Engineering, Waseda University, 3-4-1, Okubo, Shinjuku, Tokyo 169-8555, Japan Institute of Astronomy, The University of Tokyo, 2-21-2, Osawa, Mitaka-shi, Tokyo 181-0015, Japan Department of Earth and Space Science, Osaka University, 1-1 Machikaneyamacho, Toyonaka, Osaka, 560-0043, Japan Interdisciplinary Theoretical & Mathematical Science Program (iTHEMS), RIKEN, 2-1 Hirosawa, Saitama 351-0198, Japan Kavli Institute for the Physics and Mathematics of the Universe (WPI), UTIAS, The University of Tokyo, Kashiwa, Chiba 277-8583,Japan Department of Physics, Faculty of Science, Yamagata University, 990-8560, Japan Department of Astronomy, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan (Received December 12, 2020; Accepted January 12, 2021)
Submitted to ApJABSTRACTIn this study, we analyze giant Galactic spurs seen in both radio and X-ray all-sky maps to revealtheir origins. We discuss two types of giant spurs: one is the brightest diffuse emission near themap’s center, which is likely to be related to Fermi bubbles (NPSs/SPSs, north/south polar spurs,respectively), and the other is weaker spurs that coincide positionally with local spiral arms in ourGalaxy (LAS, local arm spur). Our analysis finds that the X-ray emissions, not only from the NPSbut from the SPS are closer to the Galactic center by ∼ ◦ compared with the corresponding radioemission. Furthermore, larger offsets of 10 − ◦ are observed in the LASs; however, they are attributedto different physical origins. Moreover, the temperature of the X-ray emission is kT (cid:39) kT (cid:39) Keywords:
X-ray astronomy (1810); Radio astronomy (1338); Milky Way stellar halo (1060); Interstel-lar medium (847); Superbubbles (1656); Spiral arms (1559) INTRODUCTIONThe all-sky survey is a unique albeit the only approachfor revealing giant structures in the sky that are rarelyseen in pointing observations with a limited field of view(FOV). Haslam et al. (1982) conducted the first com-plete radio survey, which was measured at 408 Hz; thesurvey confirmed various giant spurs and loop structuresextending over the entire sky. Of particular note wasLoop I, a continuum loop spanning across 100 ◦ in the Corresponding author: Jun [email protected] sky, and its brightest arm, known as the north polarspur (NPS). Initially, it was argued that NPS/Loop Iwas an old supernova remnant that was extremely closeto the Sun (Berkhuijsen 1971); however, an alternativeidea was proposed, suggesting that it was the remnantsof starburst or nuclear outbursts in the Galactic cen-ter (GC) that occurred over 10 million years ago (Sofue1977). Although the latter idea successfully explainedsimilar structures observed in the south (SPS, south po-lar spur: Sofue et al. 2000), it was almost neglected;however, it received renewed attention after the discov-ery of Fermi bubbles (Su et al. 2010). a r X i v : . [ a s t r o - ph . H E ] J a n Kataoka et al.
Fermi bubbles are giant structures extending approx-imately 50 ◦ (or 8.5 kpc) above and below the GC, witha longitudinal width of 40 ◦ . Notably, the NPS/Loop Iexhibited close contact with Fermi bubbles. Moreover,Fermi bubbles are spatially correlate with the WMAPhaze (Dobler & Finkbeiner 2008) measured between20 −
50 GHz, which was confirmed later via Planck ob-servations (Planck collaboration 2013). The connectionbetween the NPS/Loop I and Fermi bubbles was widelydiscussed based on
ROSAT all-sky X-ray maps (Snow-den et al. 1995), although a positional offset betweenradio and X-ray spurs was suggested (Sofue et al 2015;Kataoka et al. 2018). Using multiple observations with
Suzaku , the X-ray emission from the NPS/Loop I waswell represented by kT (cid:39) kT (cid:39) eROSIT A onthe Spectrum–Roentgen–Gamma mission (Predehl et al.2020a) launched in June 2019 provided a new, sharpall-sky map, in which a clear X-ray envelope surround-ing Fermi bubbles was observed in both the northernand southern skies (Predehl et al. 2020b). Hence, theNPS/Loop I and SPS can now be regarded as the rem-nant of a Galactic explosion over 10 million years ago.In addition to the abovementioned giant structures,which are likely located in the GC, weaker but evidentspurs were observed in all-sky radio maps. In fact, non-thermal spiral-arm emission along the Galactic plane isclosely associated with the discrete spiral pattern of theGalaxy (Mills 1959; Sofue 1976; see also Nakanishi & So-fue (2006) for H I and H patterns). Moreover, diffuseradio emissions emanating vertically from the Galacticplane are noteworthy; they are the most conspicuous atlongitudes of l ∼ ◦ and ∼ ◦ , i.e., exactly at the posi-tion of the Orion–Cygnus arms (or local arm), where ourSun is located. It has been argued that radio emissionsassociated with local arm spurs (LASs) are enhanced innonthermal banks, which extend vertically up to z ∼ ROSAT and eROSIT A all-sky maps; however, their characteristics are yet tobe elucidated. In this context, diffuse X-ray emissionalong spiral arms, whose morphology matched well with those in the mid-infrared or H α region, was discovered innearby spiral galaxies by recent Chandra observations(Tyler et al. 2004; Long et al. 2014).Herein, we provide systematic comparisons of giantGalactic spurs observed in radio and X-ray all-sky mapsto reveal their origins. The remainder of this paper is or-ganized as follows. In §
2, we present the method and re-sult of correlation analysis to quantitatively discuss thepositional offset between the radio and X-ray maps forall Galactic spurs. Next, we analyze the archival
Suzaku data to reveal the origin of thermal X-ray emission asso-ciated with SPS and LAS regions; subsequently, we com-pare the results with those reported in the NPS/Loop I(Kataoka et al. 2013; 2018). In §
3, we discuss the differ-ent characteristics of thermal emission in the NPS/SPSand LAS. We first discuss the origin of the comparablethickness and offset between the NPS and SPS; subse-quently, we consider the origin of radio and X-ray emis-sions associated with LASs. Finally, a brief summaryand future prospects are presented in § ANALYSIS AND RESULTS2.1.
Correlation analysis of radio and X-ray all-skymaps
We first compare the radio sky map obtained at408 MHz (Haslam et al. 1982: contour) and the X-ray map measured at 0.75 keV based on the
ROSAT all-sky survey (Snowden et al. 1995: color), as shown inFigure 1. The Galactic spurs analyzed herein are tagged yellow , with the projection example for each rectangularregion shown separately in the top panels. A significantoffset was observed between the radio and X-ray pro-files, at least for a certain fixed latitude b , as shown inFigure 1. Subsequently, we created a complete list ofthe radio/X-ray pixel values at the same Galactic posi-tion, ( l , b ), with a 1 ◦ resolution in both the longitudeand latitude directions. Next, we defined the source ex-traction region corresponding to the NPS, SPS, LAS1,and LAS2, as summarized in Table1. Figure 2 shows theclose-up view of the radio/X-ray maps for each region.To be more precise, we first calculated the correlationcoefficients ρ between all radio and X-ray pixel values,i.e., R ( l, b ) and X ( l, b ), for each spur region. We defined ρ [0] for raw radio and X-ray data as follows: ρ [0] = cov ( R, X ) σ R σ X , (1)where cov ( R, X ) is the covariance; σ R and σ X are thestandard deviations of variables R and X , respectively.Next, we calculated ρ [ θ ], which is the correlation coef-ficient between R ( l + θ , b ) and X ( l , b ), wherein the radiodata were shifted by θ [ ◦ ] in the longitude direction. In rigin of Galactic Spurs Table 1.
Regions for correlation analysis.
Source Region Galactic longitude Galactic latitudeNPS 10 ◦ ≤ l ≤ ◦ ◦ ≤ b ≤ ◦ SPS 295 ◦ ≤ l ≤ ◦ − ◦ ≤ b ≤ − ◦ LAS1 60 ◦ ≤ l ≤ ◦ ◦ ≤ b ≤ ◦ LAS2 240 ◦ ≤ l ≤ ◦ ◦ ≤ b ≤ ◦ Table 2.
Suzaku observations and analysis results
SrcID ObsID a RA b DEC b l b b b Exposure c N H /N H , Gal d kT e EM f χ / d.o.f g [ ◦ ] [ ◦ ] [ ◦ ] [ ◦ ] [ksec] [keV] [ × − cm − pc]South Polar SpurS1 705013010 265.961 − − < +0 . − . +0 . − . − − − +0 . − . ± − − − +0 . − . +0 . − . − − − +0 . − . ± − +0 . − . +0 . − . < +0 . − . +0 . − . − ± +0 . − . < +0 . − . ± < +0 . − . +0 . − . − < +0 . − . ± − − +0 . − . ± Note — a : Suzaku observation ID. b : Right Ascension, Declination, Galactic Longitude, Galactic Latitude of Suzaku observations. c : Suzaku
XIS exposure in ksec. d : The ratio of absorbing column density to the full Galactic column along the line of sight when N H was left free in the spectral fitting. e : Temperature of the Galactic halo gas fitted with the apec model for the fixed abundance Z = 0.2 Z (cid:12) . f : Emission measure of the Galactic halo gas fitted with the apec model for the fixed abundance Z = 0.2 Z (cid:12) . g : χ of the spectral fitting to the model apec1+wabs*(apec2+pl) , where we fixed kT at 0.1 keV, and assumed a Solar abundance, Z (cid:12) , for apec1 .A photon indexof pl representing the Cosmic X-ray background (CXB) is 1.41 (Kushino et al. 2002). Since wabs is not well constrained due tolow photon statics, we fixed them to zero for the values listed here. this context, a “positive” offset is defined as the direc-tion to which the data are shifted toward the center ofthe Galactic east region (0 ◦ < l < ◦ ), and toward theanti-GC for the west region (180 ◦ < l < ◦ ).Figure 3 ( lef t ) shows ρ [ θ ] as a function of θ for theNPS, SPS, LAS1, and LAS2, separately. Is it notewor-thy that correlations were stronger for the NPS and SPSthan for LAS1 and LAS2. Although we did not excludepoint sources from all-sky X-ray maps, no such strongsources existed that affected the results of the correla-tion analysis presented herein. The amount of peak off-set is summarized in Figure 3 ( right ) against Galacticlongitude l . The errors in θ were estimated by approxi-mating ρ [ θ ] using a single Gaussian function around thepeak; hence, the uncertainties of peak positions were estimated in Figure 3 ( right ) . The positional offsetsbetween the radio and X-rays were θ = 6.31 ± ◦ forthe NPS and θ = − ± ◦ for the SPS. Meanwhile,the offsets of the LASs were significantly larger, i.e., θ =19.35 ± ◦ for LAS1 and θ = − ± ◦ for LAS2,respectively. We conclude that in all spurs, the X-rayemission is shifted toward the GC direction comparedwith the corresponding radio emission by 5 − ◦ .It is noteworthy that the observed X-ray all-sky mapis generally modified by absorption owing to the inter-stellar medium (mostly H I gas), particularly in the lowlatitude regions within or near the Galactic disk andbulge. The amount of absorption depended on the dis-tance to the structure of interest, which is however un-known for the NPS, SPS, and LASs analyzed in this Kataoka et al.
Figure 1.
Galactic spurs analyzed in this paper. NPS: North polar spur, SPS: South polar spur, LAS1: local arm spur at l ∼ ◦ , LAS2: local arm spur at l ∼ ◦ . X-ray: 0.75 keV ROSAT all-sky map is shown in logarithmic color in the unit of 10 − ctss − arcmin − . Radio : 408 MHz all-sky map is shown as 6-level contours ( magenta ) from 30 [K] to 600 [K] in logarithmic scale.Projections of radio/X-ray images along the Galactic longitude for each rectangular regions ( yellow : approximately 50 ◦ × ◦ inlongitude and latitude directions) are shown in the upper panels, where intensities are normalized with each mean values. study. Hence, we did not consider such modificationsfor the correlation analysis in this section. For this rea-son, we constrained the analysis regions to a relativelyhigh Galactic latitude of | b | > ◦ (see Table 1), wherethe absorption was almost negligible.2.2. Thermal emission from SPS and LAS
The X-ray emission properties of the NPS/Loop Ihave been summarized and discussed in our previouspapers (Kataoka et al. 2018 and reference therein).Similarly, in this study, we first investigated all thearchival
Suzaku data whose pointings were situated inthe SPS, LAS1, and LAS2 regions, as listed in Table 2.We excluded pointings that contained either bright X- ray sources or extended sources such as Galaxy clus-ters, whose tailed emission might affect the analysis inthe same FOV. Consequently, we discovered that four,five, and two pointings were used in the SPS, LAS1,and LAS2 regions, respectively. The pointing centersof the
Suzaku observations and exposure in kilosecondsare listed in Table 2. The analysis procedure used wasthe same as those provided in the literature; hence, theresults of
Suzaku observations in the NPS regions (sixpoints in the region defined in Table 1) were reproducedfrom the data by Kataoka et al. (2013).In summary, we extracted the XIS data from XIS 0, 1,and 3, where XIS 0 and 3 were front-illuminated CCDs(FI-CCDs), and XIS 1 was a back-illuminated CCD (BI- rigin of Galactic Spurs Figure 2.
Close-up of Galactic spurs as observed from
ROSAT (0 . keV ) and HASLAM (408 MHz) all-sky maps. (a) NPS:radio, (b) NPS: X-ray, (c) SPS: radio, (d) SPS: X-ray, (e) LAS1: radio, (f) LAS1: X-ray, (g) LAS2: radio, and (h) LAS2: X-ray.X-ray map ( ROSAT − cts s − arcmin − . Radio map (Haslam 408 MHz) is shown in unitsof Kelvin. Cross (X) indicates pointing center of Suzaku observations analyzed herein (see Table.1). In (b),
Suzaku pointingcenter for NPS (N1–6 of Kataoka et al. 2013) is shown as “+.”
CCD) that possessed better sensitivity than FI-CCDsbelow 1 keV but less sensitive above 5 keV. We ana-lyzed the
Suzaku data using headas software version6.22, xspec version 12.9 and a calibration database re-leased in April 2016. We applied sisclean to removehot and/or flickering pixels. Only data with a cutoffrigidity larger than 6 GV as well as day and night Earthdata with an elevation angle larger than 20 ◦ were usedfor the analysis. We extracted the spectrum after re-moving possible point sources within the same FOVand then generated redistribution matrix files and auxil-iary response files using xisrmfgen and xissimarfgen (Ishisaki et al. 2007). The non-X-ray background spec-tra from the night Earth observations were generatedwith xisnxbgen .For the spectral analysis, we used the 0.5 − − xspec with a model com-prising three plasma components, similar to previousstudies: apec1 + wabs * (apec2 + pl) . The apec model assumes an emission from collisionally ionized dif-fuse gas. Here, (1) apec1 is an unabsorbed thermalplasma with kT = 0.1 keV that mimics the local hotbubble (LHB), (2) wabs*apec2 is the absorbed thermalplasma emitted from the SPS or LAS, and (3) wabs*pl is an absorbed power-law component representing thecosmic X-ray background (CXB). We assumed metalabundance Z = Z (cid:12) for the LHB, whereas Z = 0.2 Z (cid:12) was assumed for the SPS and LAS, respectively (seeKataoka et al. 2013). For the CXB, we fixed the pho-ton index Γ = 1.41, as determined by Kushino et al.(2002).Table 2 lists the spectral fitting parameters for eachanalysis region, focusing particularly on the temperature kT and emission measure (EM) of plasma component(2). We first fitted the data with the absorption col-umn density ( N H ) as a free parameter, but the resultswere not constrained well owing to low photon statis-tics. Hence, we investigated the following two extremecases: (1) N H was fixed at the Galactic value providedby Dickey & Lockman et al. (1990); (2) N H was fixedat zero. In each case, the resulting kT value coincidedwithin uncertainties. The reduced χ values for the fit-ting model were listed with the number of degrees offreedom. Figure 4 ( lef t ) shows the variations in kT in apec2 along the Galactic longitude, whereas Figure 4( right ) shows the scatter plots of kT and EM for eachGalactic spur. The analysis results in the NPS region( open magenta ) were from N1 to N6 of Kataoka et al.(2013). It was clear that the kT of the SPS regionswas generally (cid:39) (cid:39) DISCUSSION3.1.
Difference of kT between NPS/SPS and LAS Kataoka et al.
In the previous section, we showed that the diffuse X-ray emission associated with the SPS and LAS was gen-erally reproduced well by a thin plasma model assumingcollisional ionized gas, but the temperature in the SPSwas slightly higher than that in the LAS. Interestingly,the observed kT (cid:39) eROSIT A observations,which clearly exhibited X-ray bubbles with sharp edges,both in the north and south of the GC surrounding theFermi bubbles, where the SPS was situated at the lowestedge of the south bubble (Predehl et al. 2020b).By contrast, kT (cid:39) −
30 [km s − ] (Roberts 1969; Sofue 1973). It is note-worthy that the sound velocity of the interstellar gas isexpressed as follows: c s = (cid:113) γk B T /µm p (cid:39) (cid:18) T K (cid:19) / km s − , (2)where µ (cid:39) T ∼ [K] is the temperature of the interstellar gas. Hence,the falling gas causes a strong shock of M ∼ −
3, where M is the Mach number, resulting in a significant increasein the star formation rate because the compression ofthe gas density scales as n ∝ M . Moreover, if theGalactic shock is isothermal and the magnetic field B isparallel to the arm, B ∝ M , then the radio emissivityscales as ∝ nB ∝ M . Therefore, the local arms willbe sufficiently bright in the radio map, as first confirmedby Mills (1959).Meanwhile, the X-ray emitting gas had a sound ve-locity of c s ∼
200 km s − for kT (cid:39) ∝ n , only a 20 − n , is required to enhance the X-ray gas, whose temperature is kT (cid:39) Thickness and offset of NPS/SPS
We observed a significant offset between the radio andX-ray intensity distributions not only in the NPS but inthe SPS and LAS structures, although their origins arelikely to be different, as indicated in the previous section.For the NPS/SPS, the situation can be interpreted basedon the diffusive shock process. The relativistic electronsaccelerated via the forward-shock emit synchrotron radi-ation in the radio band, whereas swept-up, compressedgas emitted thin-thermal X-ray radiation in the reverse-shocked region. In this context, the observed offset be-tween radio and X-rays might be related to the thick-ness and positional offset of the forward/reverse shockregions. In fact, X-ray emitting shells are typically ob-served on the inner side of the radio shell in the case ofsupernova remnants (e.g., Gaetz et al. 2000 for the caseof E0102-72.3).From Kataoka et al. (2015) we adopt a simple modelin which two spherical bubbles that mimic the north andsouth are embedded in the center of a gaseous halo. Theradius of the outer shell corresponding to the NPS/SPSis R (cid:39) v sh (cid:39)
300 km s − ,as indicated by X-ray observations. First, the dynamicaltime scale t dyn in which the bubble expands to radius R is expressed as t dyn (cid:39) R/v sh . For example, t dyn is 16Myr in case of R = 5 kpc and v sh = 300 km s − .The thickness of forward shock (FS) region can beapproximated as follows: d F S (cid:39) ( k u + k u ) (cid:39) v s ξr g c , (3)where k and k are the diffusion coefficients; u and u are the plasma velocities in the upstream/downstreamof the FS, respectively; v s is the shock velocity; r g is thegyro radius; ξ ( >
1) is a constant factor (Drury 1983).Furthermore, we assume that the diffusion coefficient k = k (cid:39) ξ r g c /3. In addition, v s = u = 4 u , r g = γm e c / eB , where γ is the Lorentz factor of the acceler-ated electrons. The acceleration time scale of electronswith energy γm e c is expressed as t acc = 20 ξr g c u = 4 d F S v sh . (4)For comparison, the radiative cooling time of electronsemitting 408 MHz of radio emission is expressed as t cool,R = 3 m e c σ T U B γ (cid:39) (cid:18) B µ G (cid:19) − / Myr , (5)where m e is the rest mass of electron, σ T = 6.65 × − cm − is the Thomson cross section, and U B is the mag- rigin of Galactic Spurs Figure 3. ( left ) Correlation coefficient calculated between radio (408 MHz) and X-ray (0.75 keV) spurs, as defined in Figure 2.Angle in horizontal axis becomes positive when X-ray structure is offset toward GC for spurs in Galactic east, whereas negativefor spurs in the west. ( right ) Positional offset of each Galactic spur plotted against Galactic longitude. In all cases, X-ray spursare located on inner side of corresponding radio spurs. netic field density. We assume that the shock compres-sion ratio of the magnetic field is (cid:39)
4. Hence, the cool-ing time is slightly longer but almost comparable to t dyn .Therefore, the thickness of the radio shell, d R , is approx-imated by equating t acc and t dyn (cid:39) t cool,R , resulting in d R (cid:39) d F S (cid:39) R . This is on the order of 1 kpc,which is consistent with the observed thickness of theradio emission region of the NPS/SPS.Next, we consider the dynamics of the X-ray shell.First, the radiative cooling time scale of the X-ray emit-ting gas can be expressed as t cool,X (cid:39) . × T . n cm − yr for 10 K < T < . K , (6)where n is the number density of the X-ray emitting gas, T is the temperature in the unit of 10 K (Eq. 34.4 ofDraine (2011)). We further modified the above equationassuming subsolar metallicity Z (cid:39) Z (cid:12) (Kataoka etal. 2013), which leads to t cool,X (cid:39) (cid:16) v sh
300 km s − (cid:17) . (cid:16) n .
01 cm (cid:17) − Myr , (7)(see also Inoue et al. (2017)). We assume a The X-ray-emitting gas does not cool during the expansion time, t dyn ; hence, the thickness of the X-ray shell is deter-mined by the amount of halo gas piled up in the reverseshock (RS) region to form the NPS. As the underlying halo gas density profile, we assumea hydrostatic isothermal model expressed as n ( r ) = n (cid:34) (cid:18) rr c (cid:19) (cid:35) − , (8)where n ( r ) is the gas density [cm − ] at radius r fromthe GC; n is the density at r = 0; r c is the core ra-dius, which we set as r c = 0.5 kpc from the observa-tion (Kataoka et al. 2015). The thickness of the X-rayshell, d X , can be determined from the conservation ofthe swept gas mass as follows: (cid:90) R πr n ( r ) dr = 4 πR [4 n ( R )] d X , (9)where 4 n ( R ) is the density of the shocked halo gas inthe downstream, assuming the strong shock of a specificheat ratio of 5/3. Using Eq. (8), we obtained d X ∼ R ∼ R = 5 kpc. This is consistent withthe observed thickness of the NPS/SPS, as shown in theX-ray map, and explains the approximate similarity be-tween the radio/X-ray thicknesses, d R ∼ d X . Hence, theobserved offset of ∼ ◦ , which corresponds to 0.7 kpcbetween the radio and X-ray NPS, may indicate that theFS is located outside the RS by 0.7 kpc on average; how-ever, approximately half of the FS/RS regions may havebeen overlapped. This is because the radio emission be-comes maximum around the forefront of the FS owingto compressed magnetic field via the synchrotron emis-sion, whereas the X-ray emission would have a broad Kataoka et al. maximum in the RS reflecting a density gradient of theswept up halo gas.3.3.
Origin of radio/X-ray offset in LAS
As reviewed in §
1, the radio emissions associated withthe LASs have been well known since the 1950s; how-ever, the origin of radio spurs in the tangential direc-tion of the local arm is yet to be elucidated. Sofue(1973;1976) suggested that spurs immediately above theGalactic shock may be generated by the inflation of mag-netic fields through the Parker instability triggered bythe Galactic shock wave. Such an inflation is promotedby the strong compression of the gas and magnetic fields;subsequently, the magnetic force lines above the shocklane will be stretched into the halo perpendicular to theGalactic plane. This is consistent with measurementsof vertically extended magnetic fields, as inferred frompolarized far-infrared dust emissions in the local arm di-rections (Mathewson & Ford 1970; Planck Collaborationet al. 2015).In such a scenario, the formation of radio spurs maybe substantially delayed after the Galactic shocks arebeing activated. First, shock-compressed interstellargas (mostly H I and H ), increases the magnetic fieldstrength along the arm, and increases the rate of starformation at the shock front. Thus, the regions of thenewly born stars (or H II regions) lies just outside theshock front. Typical time-scale for the evolution of mas-sive stars is t SF ∼ − years, thus the H II region lags behind the shock by (cid:39) t SF × ( v rot − v Ω ) (cid:39) v rot (cid:39)
220 km s − is the rotation speed of the Galaxy and v Ω ∼ − is the pattern speed of the spiral den-sity wave. Accordingly, offset by ∼ × sin( p ) ∼
100 pc is anticipated in the direction perpendicular tothe Galactic arm for a pitch angle of p ∼ ◦ . Whenthe stars end their lives, supernovae happen to accel-erate cosmic-ray electrons, which may take 10 − yearsfurther. Then accelerated electrons propagate into thenonthermal bank perpendicular to the disk along thearm within a timescale of h / v A (cid:39) ∼ yr, where h (cid:39)
100 pc is the height of non-thermal bank and v A (cid:39) −
100 km s − is the Alfven velocity.In this context, we can qualitatively interpret the off-set between radio and X-rays in the LAS as follows.The position of the radio spur may include an offsetcompared with the Galactic shock position because weexpect a ∼ − years of delay for their formation. Ra-dio spurs originate in synchrotron emission by cosmic-ray electrons, which are likely generated by supernovaremnants. Hence, time delays due to stellar evolution,particle acceleration, and diffusion should occur. By contrast, the LAS diffuse X-ray emission may be associ-ated with the Galactic shock region if the Galactic halogas is trapped by the local potential minimum of the spi-ral arm. Such an association is often observed in nearbygalaxies (Tyler et al. 2004; Long et al. 2014). In suchcases, the X-ray emission from the LAS is expected to lead the radio emission by ∼ a few 10 years. Assum-ing a typical distance of (cid:39) − ◦ offset corresponds to ∼
100 pc, which is approximatelyconsistent with the horizontal size ( ∼ ◦ from Figure 1)of the radio bank in the all-sky map, where we assumed ∼
100 pc.3.4.
Further comments on the NPS
Finally, we revisit some aspects related to the distanceand possible origin of the NPS. As noted in §
1, the NPSwas initially thought to be the remnants of an old su-pernova close to the Sun rather than a distant structureassociated with the GC activity. Although this idea isnot well supported by a number of recent X-ray andgamma-ray observations, it was recently claimed thatthe NPS distance between 70 and 135 pc depends onthe galactic latitudes, according to near-infrared andoptical photometry, and Gaia DR2 (Das et al. 2020).However, note that Das et al. (2020) observed the ex-tinction of stars toward the NPS; thus, they measuredthe distance to the cold dust (typical temperature T (cid:39) K) rather than the X-ray brightness of the NPS itself( T (cid:39) − K). In fact, such foreground dust absorptionin the Aquila Rift is generally used to provide a lowerlimit on the distance to the NPS (e.g., Sofue 1994, 2015;Lallement et al. 2016), but it is unlikely that cold dustand the hot X-ray plasma of the NPS coexist. Similarly,a wide range of the distance depending on the positionin the NPS, as proposed by Das et al. (2020), is alsounlikely if the NPS is a single continuous structure.However, let us consider the situation if the NPS isreally a structure close to the Sun, in the context of ob-served thickness and offset, as discussed in § R (cid:39)
100 pc, andthe width of the forward shock, d F S , is (cid:39) R , as an-ticipated from t dyn (cid:39) t acc (cid:39) B (cid:39) µ G for an old SNR (e.g., Loru et al. 2020), thecooling time of electrons, t cool,R , is much longer than t acc ; thus, the accelerated electrons remain uncooled,and the radio-bright NPS would be more spherical inshape rather than forming a shell. In contrast, the X-rayshell is formed as previously mentioned, i.e., by sweep-ing up the interstellar medium. We would expect quite rigin of Galactic Spurs Figure 4. ( left ) Temperature ( kT of apec2 in Table 2) variation in halo gas along Galactic plane. ( right ) Scatter plot oftemperature vs. emission measure of halo gas ( kT and EM of apec2 in Table 1). Analysis results in NPS region were from N1to N6 of Kataoka et al. (2013). different morphologies for the radio (sphere-like) and X-ray (shell), which are far from the observation. Thisis an indirect but additional reason supporting why wethink the NPS is a giant structure associated with theGC.Finally, note that the most recent observation byHaloSAT enabled coverage of the entire bright NPSthrough 14 observations of approximately 30 ks each(LaRocca et al. 2020). These observations provide thefirst complete survey of the NPS thanks to a wide fieldof view. While the observed X-ray spectrum is well fit-ted by two thermal components of kT cool (cid:39) kT hot (cid:39) kT hot (cid:39) kT hot (cid:39) kT = 0.30 ± Suzaku pointings were arranged in the inner arc of the NPS.Moreover, LaRocca et al. (2020) provided a new indica-tion that the cool component also belongs to the NPSrather than to local hot bubbles (LHB). Similarly, eventhe modeled hot component of kT (cid:39) kT (cid:39) HaloSAT and eROSIT A will provide a new insight to resolve such complexity inthe modeling of NPS spectra. CONCLUSIONHerein, we analyzed two types of Galactic spurs ob-served from both radio and X-rays, i.e., the NPS andSPS. These spurs are likely to be the remnants of Galac-tic explosions in the past and are hence located near theGC, whereas LASs are associated with local spiral arms.The important results of this study are as follows: • the X-ray emission from the NPS and SPS werewell represented by a thin thermal plasma of kT (cid:39) • the other X-ray emissions from the LAS were rep-resented by a thin thermal plasma of kT (cid:39) • the positional offset observed from the NPS andSPS in the radio and X-ray maps were (cid:39) ◦ , wherethe X-ray emissions were from the inner side ofthe radio shell. A similar width of radio/X-raystructures as well as positional offsets are naturallyinterpreted if the radio emission is primarily fromthe forward shock front, whereas the X-rays werefrom the downstream of the reverse shock. Thethickness of each shock was ∼ • the positional offset of 10 − ◦ was observed in theLAS; however, it might be attributed to differentorigins. Radio spurs might possibly be associated0 Kataoka et al. with nonthermal bank, which might have an off-set of ∼
100 pc compared with the Galactic shockposition. By contrast, the X-ray spurs might becloser to the Galactic shock if the halo gas wassimply trapped by the local arm potential or re-lated to the star formation activity in the spiralarm.We are cognizant that there exist more structures thatare possibly related to various Galactic spurs and evento Fermi bubbles’ edges, which are marginally visiblebut not conclusive based on the current HASLAM and
ROSAT datasets. Moreover, a correlation analysis forthe entire Loop-I structure is expected to be conducted,but it is still limited to the brightest eastern part known as the NPS. All-sky observations with unprecedentedsensitivity and resolution, for example,
P lanck and eROSIT A at different energies, will provide further op-portunities to confirm the physical origin and structureof Galactic spurs in the near future.We thank an anonymous referee for his/her construc-tive comments to improve this manuscript. This re-search made use of Astropy, a community-developedcore Python package for Astronomy (The Astropy col-laboration 2013; 2018). J.K. acknowledges the supportfrom JSPS KAKENHI Grant Numbers JP20K20923.Y.I. is supported by JSPS KAKENHI Grant NumbersJP18H05458 and JP19K14772. T.T. was supportedby JSPS/MEXT KAKENHI Grant Numbers 18K03692and 17H06362.REFERENCES
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