Chandra X-ray images of the central part of M82 integrated over the 13 observations. Top-left: 2-4 keV image; Top-right: 4-8 keV band; The colour scale of these two images is in logarithmic with the brightest point sources are saturated. Bottom-left: The 4-6 keV image of the ellipse where the data of the diffuse emission spectrum were taken. The detected discrete sources were masked. The region where the brightest ULXs along with a few others are clustered is masked in a block. This image is in a linear scale as the dynamic range of the brightness is narrow: 1-19, as a result of the removal of discrete sources. Bottom-right: The same image as Top right overlaid by the ellipse and the 38 region segments for spatially resolved spectral study.

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... 4-8 keV Chandra image integrated over all the 13 observations is shown in Fig. 1. The diffuse emission, as well as a number of discrete sources, is visible. It lies along the starburst disc where the discrete sources are concentrated. Its extension along the major axis (PA 70 • ) is ∼ 700 pc, coinciding with the disc size. The height of the extension above and below the disc is less than 200 pc and the overall ...

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... so that a sufficiently large area around them is masked to avoid their emission in the PSF wings as well as the bright core emission. This block contains Xp12 through Xp17. These masks for the discrete sources take up 23% of the entire ellipse area of 591 arcsec 2 . The 4-6 keV continuum image from the ellipse with the masking is also shown in Fig. 1. We note that the 4-6 keV image of the diffuse emission alone has a relatively flat brightness distribution, in the absence of peaky discrete sources, and is shown in a linear ...

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... across the 4-8 keV diffuse emission of M82. We set 38 rectangular region-segments 2 We use energy index, α, for a spectral slope appropriate for a spectrum in flux density unit, as defined as f E ∝ E −α , where f E is flux density and E is energy. The conventional photon index, Γ, is related as Γ = 1 + α. covering the diffuse emission ( Fig. 1) and examined the individual spectra taken there. These region segments were placed by avoiding the discrete sources, except for the two transient sources (the reason for this exception is described below). We label them as dX0 through dX37. The segments dX0 through dX20 are on the northern side of the starburst disc while dX21 through ...

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... of source counts contained between the individual segments where allowed by the positions of the discrete sources. As a result, the 4-8 keV counts of those segments range between 150 and 400 counts, with the mean of 274 counts and the standard deviation of 72 counts. The broad-band spectra of these 38 region segments are shown in Appendix F (Fig. ...

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... continuum emission shows high surface brightness near the starburst disc which declines gradually towards larger radii, in agreement with the 4-6 keV count image shown in Fig. 1. With respect to the disc plane, the brightness distribution appears to be similar between the northern and southern sides, when discounting the lowest surface brightness regions on the north-east corner. On the contrary, the Fe xxv emission is markedly stronger on the northern side, particularly evident in its eastern part. Such a ...

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... Background-corrected 4-8 keV counts were collected from each rectangular region and the surface brightness profiles along each strip was constructed. On inspecting those projected profiles, the brightness peak can be located at around the reference point. The light distribution along the major axis is relatively flat and skewed towards the east (Fig. 10a). Along the minor axis, the light distribution is nearly symmetric at the brighter inner part, as already seen in the 4-6 keV map (Fig. 4), but shows an enhanced extension towards north at the fainter brightness level (Fig. 10b). Along each axis, the profile shapes of the two strips are found to be similar, as shown in the standardised ...

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... around the reference point. The light distribution along the major axis is relatively flat and skewed towards the east (Fig. 10a). Along the minor axis, the light distribution is nearly symmetric at the brighter inner part, as already seen in the 4-6 keV map (Fig. 4), but shows an enhanced extension towards north at the fainter brightness level (Fig. 10b). Along each axis, the profile shapes of the two strips are found to be similar, as shown in the standardised brightness plots (Fig. 10c), while their absolute fluxes ...

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... the minor axis, the light distribution is nearly symmetric at the brighter inner part, as already seen in the 4-6 keV map (Fig. 4), but shows an enhanced extension towards north at the fainter brightness level (Fig. 10b). Along each axis, the profile shapes of the two strips are found to be similar, as shown in the standardised brightness plots (Fig. 10c), while their absolute fluxes ...

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... from the other masked areas of discrete sources was computed by applying the same conversion factor and it is found to be 0.9 × 10 −13 erg s −1 cm −2 . Adding these estimated fluxes from the masked areas to the 7.2 × 10 −13 erg s −1 cm −2 of the diffuse emission spectrum (Sect. 3.2), the total 4-8 keV flux of diffuse emission in the ellipse in Fig. 1 is thus 1.1 × 10 −12 erg s −1 cm −2 , which corresponds to the 4-8 keV luminosity of 1.7 × 10 39 erg s −1 . This luminosity, however, contains some contribution of unresolved discrete sources, that is, X-ray binaries and SNRs. We estimated their contributions using the cumulative luminosity distribution, N(> L), which can be ...

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... declines rapidly, within the diffuse emission region (excluding the far more luminous ULXs) also show γ ∼ 0.6. Using this slope, we estimated the 4-8 keV luminosity of unresolved X-ray binaries to be 2.0 × 10 38 erg s −1 . The N(< L) slope for X-ray SNRs appears to have a similar value γ =0.6-0.7, based on the seven X-ray SNRs (Iwasawa 2021 Fig. 1, the mean electron density would be n e ∼ 0.6 cm −3 , as the volume of the spheroid is ∼ 7.9 × 10 7 pc 3 ...

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... used the FIR image ( Fig. 11) taken by the Herschel PACS ( Bendo et al. 2012, data were obtained through NED) for a comparison. It provides a 70 µm image at the angular resolution of ∼ 5 . 5 (PACS Handbook 2019, version 4.01 ESA). A similar image at 59 µm (the beam FWHM of 5 . 5) has been also taken by SOFIA ( Jones et al. 2019). We used a radio continuum image ...

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... ( Fig. 11) taken by the Herschel PACS ( Bendo et al. 2012, data were obtained through NED) for a comparison. It provides a 70 µm image at the angular resolution of ∼ 5 . 5 (PACS Handbook 2019, version 4.01 ESA). A similar image at 59 µm (the beam FWHM of 5 . 5) has been also taken by SOFIA ( Jones et al. 2019). We used a radio continuum image (Fig. 11) taken by the VLA at 3 cm ( Adebahr et al. 2013) with D-configuration with a matching beam size (7 . 6 × 7 . 3) to the FIR image. The radio continuum at 3-cm represents the highfrequency part of the synchrotron emission with a power-law spectrum ν −0.7 in the starburst region while the electron population responsible for the Compton ...

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... FIR and radio images are, unsurprisingly, very similar (Fig. 11 Right). Their image morphology agrees well with that of the 4-8 keV diffuse emission (Fig. 11 Left; see also Fig. 10d). The matching morphology between the three band images is encouraging for the inverse Compton scattering origin for nonthermal emission. A likely contribution of this non-thermal component to the 4-8 keV continuum luminosity ...

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... FIR and radio images are, unsurprisingly, very similar (Fig. 11 Right). Their image morphology agrees well with that of the 4-8 keV diffuse emission (Fig. 11 Left; see also Fig. 10d). The matching morphology between the three band images is encouraging for the inverse Compton scattering origin for nonthermal emission. A likely contribution of this non-thermal component to the 4-8 keV continuum luminosity is further discussed in Sect. ...

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... at 8.3 GHz and 43 GHz and detected with 4 mJy in both frequencies. It is tentatively classified as a H ii region because of the flat spectrum. These positions, including the previously measured 2.2 µm peak positions ( Rieke et al. 1980;Lester et al. 1990) and the WISE position (Jarrett et al. 2019, WXSC), are plotted over the 2MASS K s band image (Fig. 12, retrieved from the 2MASS Image Service of IRSA). The positions of Xp9 and the radio source agree with the kinematic centre measurements and the WISE position, and is < ∼ 1 off to the west from the 2MASS peak. This positional coincidence with the kinematic centre, albeit being tentative, offers a possibility that Xp9 might be a ...

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... hypothesis is that the diffuse emission is inverse Compton emission and soft X-ray wind emission, added to the hot gas of kT ∼ 5 keV which occupies specific regions where the Fe xxv Fig. 12: K s band image of the M82 nuclear peak emission obtained from 2MASS in grey scale (fainter emission is cut out), overplotted by positions of the X-ray source, Xp9 (white), radio source 43.21+61.3 (red, Rodriguez- Rico et al. 2004), H i kinematic centre (green Weliachew et al. 1984), Ne ii kinematic centre (blue, Achtermann & Lacy ...

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... the three EW-sliced spectra and dX37 spectrum in the 4-7.2 keV band jointly, leaving the This fit gives a good description of the data with C-stat = 785.3 with 774 bins (754 dof). The temperature was found to be kT = 5.0 ± 0.8 keV. The best-fit Fe metallicity was driven to the highest bound of 5 Z but the likelihood remains similar above 2.5 Z (Fig. 13). This can be compared with the Fe metallicity of ∼ 2 Z , which would be expected from the best-estimate of the mass-loading factor of 2.7 by Strickland & Heckman ...

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... mean over the expected metallicity range of 1-5 Z . Their proportions of the total 4-8 keV flux in each spectrum are given in Table 4 (where the contribution of the 4.5 keV and 6.4 keV lines are left out). With the same set of spectral components, a spectral decomposition for the total diffuse emission spectrum (Fig. 2) is illustrated in Fig. 14. The relative contributions of the respective components are also given in Table ...

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... of hot gas. A northern asymmetry in outflow structures have also been observed in other wavelengths. In the mid-infrared (mid-IR) image taken by the Subaru-COMICS with the [Ne ii] filter at 12.8 µm, a few wisps are seen only on the northern side, extending from the starburst disc ( Gandhi et al. 2011). The same features are noted also in radio. Fig. 16 shows the deep JVLA image of the central part of M82 at 3-GHz, taken on 2015 August 16. The overlaid COMICS contours shows the coincidence of the northern wisps between mid-IR and radio. Wills et al. (1999) noted these radio counterparts in the earlier VLA images and identified four galactic chimneys, based on the emission void to the ...

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... energy source here is a sequence of SNe produced by a starburst and its formation has been formulated by earlier study (Tomisaka & Ikeuchi 1986;Mac Low & McCray 1988;Norman & Ikeuchi 1989). The mid-IR and radio emission voids, embraced by the wisps, correspond to chimneys that are expected to be filled by hot gas (Tomisaka & Ikeuchi 1988). Fig. 15 shows that the Fe xxv EW enhancement regions, as indicated by arrows I, II, and III. They appear to fill the mid-IR voids and to be traced back to the mid-IR knot 'E1' of Achter- mann & Lacy (1995). This can be understood that the metal-rich hot gas produced in the E1 knot was channelled into the chimneys and fills their ...

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... above process produces metal-rich hot fluid of a temperature of 10 8 K (Chevalier & Clegg 1985). In our Chandra observations, the region-segment dX34 coincides with the central part of E1 (Fig. 15). The X-ray spectrum of dX34 shows no Fe xxv line (Fig. 5), suggesting that the putative hot fluid, too, radiates little with its low emission measure. On the other hand, abundant amounts of dust are produced by SNe ( Gall & Hjorth 2018), providing a source of strong FIR emission. The region also acts as a cosmic ray reservoir, since a ...

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... a drop in temperature, as demostrated by simulations (e.g. Tomisaka & Ikeuchi 1988), and an increased density makes the fluid sufficiently radiative, resulting in sub-10 8 K gas with strong Fe xxv. This transition can be illustrated by the spectrum of dX35, which lies immediately outside E1 and at the base of the south-bound Fe xxv extension III (Fig. 15). The segments dX34 and dX35 are both located in the disc plane and separated by only ∼ 20 pc, yet the strong Fe xxv line in the dX35 spectrum contrasts sharply with that of dX34 with no Fe ...

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... brightest five radio SNRs have X-ray counterparts (Iwasawa 2021, see Fig. 16). An inspection of the deep JVLA image found a faint radio source which appears to coincide with Article number, page 12 of 19 2004), which coincides with the X-ray source, Xp9, at the position of the radio kinematic centre. The five brightest radio SNRs, all of which have X-ray counterparts, are encircled in white. The radio SNR ...

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... Here, we examine how plausible the faint X-ray source, Xp9, at the galactic centre might be an AGN. Xp9 has been assumed to be an X-ray binary but the presence of the radio counterpart makes it unique among the X-ray binaries detected in the central region. The deep JVLA image verifies the radio counterpart, as marked by a magenta diamond in Fig. 16, whilst none of the other X-ray binaries (including the ULXs) have a point-like radio counterpart in the JVLA image (Xp6 and Xp16 coincide with compact radio sources but they are radio SNRs, Fenech et al. (2010)). An X-ray binary could have a radio jet in the hard state. However, as Galactic black hole binaries typically have radio ...

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... radio-to-X-ray spectral energy distribution of Xp9 is shown in Fig. 17. The mid-IR points are upper limits from Gandhi et al. (2011). The 12-µm luminosity predicted by the ADAF models (Mahadevan 1997;Kino et al. 2000) and the mid-IR-X-ray relation of Gandhi et al. (2009) is around 10 38 -10 39 erg s −1 . This is translated to a flux density of 0.7-7 µJy, which may be barely detectable with the upcoming ...

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... is a possible line-like excess at 4.5 keV in the total diffuse emission spectrum. The Ca K band spectrum at ∼ 4 keV indicates the temperature kT ∼ 1.8 keV (Fig. B.1). No emission-line feature is expected at 4.5 keV in a thermal emission spectrum of a similar temperature. We examined the 4.1-5 keV data for its detection with a power-law model with or without a narrow Gaus- sian line. The lower energy bound was chosen to avoid the Ca K emission. To optimise a line detection, we rebinned the data with ...