Diffuse X‐Ray Emission in a Deep Chandra Image of the Galactic Center

“The plasma model we used (apec7 in XSPEC; see also, e.g., Raymond & Smith 1977; Mewe, Lemen, & van den Oord 1986; Borkowski, Sarazin, & Blondin 1994) assumes that it is in collisional ionization equilibrium, and is able to self-consistently account for both the continuum emission and the lines from…”

“It is likely that these represent an outflow from the central parsec (Baganoff et al. 2003; Morris et al. 2003).”

“Although computing the energy loss rate rigorously would require a full solution of wind equations such as those in Chevalier & Clegg (1985), we can make a rough estimate of it by assuming Ẇexp = PdV/dt ≈ PV 2/3cs.”

“The inner 20 pc of the Galaxy have been observed on twelve occasions as of June 2002 (Table 1) using the Advanced CCD Imaging Spectrometer imaging array (ACISI) aboard the Chandra X-ray Observatory (Weisskopf et al. 2002).”

“These widths are lower than those reported from ASCA data by Koyama et al. (1996) (75 ± 10 eV).”

“…& Wilson 2003b; Pellegrini 2005; Rinn, Sambruna & Gliozzi 2005; Cappi et al. 2006; González-Mart́ın et al. 2006; Soria et al. 2006), including our own (Muno et al. 2004) and our close neighbor M31 (Garcia et al. 2005), but quantitative, statistical properties of the gas are now becoming available.”

“Diffuse X-ray Emission— Muno et al. (2004) studied the diffuse X-ray emission within ∼ 20 pc of the GC in detail using Chandra observations.”

“Further away at the 40 pc scale 28 ( 17 ′ ), the density has decreased to 0.1–0.5 cm − 3 and we can roughly describe the central parsecs with a density profile of the form n ( r ) ≃ 26 cm − 3 ( r/ 0 .”

“The deep Chandra survey of the galactic centre has revealed a large number of low-luminosity hard X-ray sources attributed to MCVs of the IP type (Muno et al. 2004a; Hong et al. 2012).”

“…remnant ejection in massive star formation complexes, X-ray images have the potential to help understand these important paths of energy input and chemical enrichment of the interstellar medium, providing a foundation for studies of starburst galaxies (e.g. Townsley et al. 2003; Muno et al. 2004).”

“Such a mechanical power input into the galactic center could play a significant role in the production of the TeVγ-rays recently observed by the HESS (High Energy Stereoscopic System) collaboration (Aharonian et al. 2004; Atoyan & Dermer 2004), as wellas in the heating of the hot (> 8 keV) diffused plasma detected by…”

“We also include sources previously identified in 630 ks of data on the inner 20 pc around Sgr A∗ (Muno et al. 2003a, 2004b); 30 12 ks exposures of the 2◦ × 0.”

“…physics of the flares (as well as to continue detailed study of diffuse emission and transients in the Galactic center; e.g., Baganoff et al. 2003; Muno et al. 2004, 2007, 2009; Park et al. 2004; Ponti et al. 2010; Capelli et al. 2012), Chandra undertook an unprecedented X-ray Visionary Project…”

“…and XMM-Newton in the Galactic plane and in the Galactic center region resolved only ∼10–30% of the ridge emission (above 2 keV) into point sources (Muno et al. 2004; Ebisawa et al. 2005; Hands et al. 2004), leaving the question open as to what fraction of the unresolved emission is truly…”

“The spectrum of the hard X-ray continuum near the GC can be fit by eitherkT ∼ 8 keV thermal models orΓ∼ 1.9 power law models (Muno et al. 2004).”

“Since Sgr A∗ region is rather dim in X-rays (Baganoff et al., 2003; Muno et al., 2003, 2004), we were able to place an upper limit of ∼ few thousand of low mass PMS stars populating the same region as the He-I stars of the better defined stellar ring (Levin & Beloborodov, 2003; Genzel et al., 2003;…”

“…region associated with the Galactic center and encompassing a diameter of about 20 arcminutes, the total X-ray luminosity in the range 2 to 40 KeV (Muno et al. 2004; Koyama et al. 2007; Yuasa et al. 2008; Dogiel et al. 2010) is 6×1036 erg s−1, yielding an X-ray flux density of 2.0×10−3 erg cm−2…”

“We simulated the point-source flux in the 4.7–8.0 keV band observed with XIS, putting the positions and fluxes (2.0–8.0 keV band) of the Chandra cataloged sources (Muno et al. 2003) in the same rectangles of figure 4.”

“Such a population explains the 20–40 keV X-ray emission seen by NuSTAR (Perez et al. 2015) and implies that many of the unidentified Chandra point sources may be MSPs (Muno et al. 2004; Perez et al. 2015).”

“From Figure 5 in Muno et al. (2004), the nonthermal X-ray flux is estimated to be about one-third of the continuum flux at the FeI Kα line band.”

“…cluster wind is required. c© 2005 RAS, MNRAS 000, 1–29 Amongst the dense stellar clusters known in our Galaxy, the central cluster around Sgr A* is unusual in many respects, including the recent detection of apparently thermal gas with a temperature kT ≈ 8 keV (Muno et al. 2004), and TeV emission.”

“CMZ molecular clouds have unusually high mean H2 densities ∼104 cm−3 and column densities ∼1023 cm−2 (e.g., Lis & Carlstrom 1994), and they are subject to an average gas pressure from hot gas of order 106 to 7 K cm−3 (Yamauchi et al. 1990; Spergel & Blitz 1992; Muno et al. 2004).”

“If the absorption column to the source is 6 × 1022 cm−2, then we can use Figure 13 in Muno et al. (2004b) to estimate that the spectrum is consistent with a Γ=1 power law with a luminosity is 1 × 1033 erg s−1.”

“This line emission, as well as the underlying continuum and the intra-line emission, are typically well fitted by a thermal model (e.g. APEC in XSPEC) with temperatures in the range 0.6−1.5 keV (Kaneda et al. 1997; Tanaka et al. 2000; Muno et al. 2004; Nobukawa et al. 2010; Heard & Warwick 2013b).”

“The very-hot plasma presents a severe energetics problem, however: were it a hydrogen plasma its sound speed would be ∼1500 km/s – considerably in excess of the local escape velocity of ∼ 900 km/s (Muno et al. 2004) – and it would escape (Yamauchi et al. 1990) on a short timescale.”

“Muno et al. (2004) report the presence of diffuse hard ( keV) and soft ( keV) X-ray emission from thekT ∼ 8 kT ∼ 0.8 inner 20 pc.”

“…emerges as r V n n v t v n n 2 70 500 km s 0.004 cm 0.1 cm pc, (2.6) 0 FB tot FB tot GC wind cool wind 1 1 2 tot FB 3 1 2 tot GC 3 1 2 p æ è ççç ö ø ÷÷÷ ´ æ è çççç ö ø ÷÷÷÷ æ è çççç ö ø ÷÷÷÷ - - - - - where we normalize ntot GC to a value informed by GC X-ray observations (Muno et al. 2004).”

“The X-ray luminosity of the soft component of the plasma in the inner 20 pc is LX ≈ 3× 10 36 erg s−1 (Muno et al. 2004).”

“From the X-ray observations Muno et al. (2004) estimated that the VH cooling time, tcool ∼ 108 yr.”

“A true origin would be a mixture of diffuse plasma and point sources, but the mixing ratios are unclear for both GRXE and GCXE (see Muno et al. 2004; Ebisawa et al. 2005; Koyama et al. 2009; Revnivtsev et al. 2007, 2009).”

“…near Sgr A* (Baganoff et al. 2003) are attributed to the atmosphere close to the SMBH, which can be explained as gas being shock-heated by the interaction of strong stellar winds of the surrounding massive stars (e. g. Krabbe et al. 1991; Melia 1992; Baganoff et al. 2003; Muno et al. 2004).”

“More recently, Muno et al. (2004) observed a smaller, 17′ × 17′ area around the GC, with better signal-to-noise ratio and finer angular resolution.”

“…et al. 1972; Scoville 1972); 6) North Polar Spur (NPS, Sofue 2000; Bland-Hawthorn & Cohen 2003); 7) the 8 keV diffuse X-ray emission in the center (Muno et al. 2004); 8) the excess of Hα emission of Magellanic Stream (BlandHawthorn et al. 2013); 9) the Suzaku observations to the NPS (Kataoka et…”

“The typical cooling time of the hot gas is ∼ 108 yr (Muno et al. 2004), which is much longer than tesc, and hence the expansion is adiabatic as argued by Sofue (2000) and Bland-Hawthorn & Cohen (2003).”

“…& Sazonov 2007; Revnivtsev et al. 2009) and the Galactic Center X-ray emission (Koyama et al. 1989; Yamauchi et al. 1990; Koyama et al. 1996; Muno et al. 2004; Koyama et al. 2007a; Revnivtsev et al. 2007) mostly consist of numerous dim point sources (Revnivtsev et al. 2009, and references…”

“However, interpretation of the X-ray emission remains controversial (e.g., Muno et al. 2004; Morris & Nayakshin 2006; Koyama et al. 2007).”

“As the low mass star with a stellar wind descends towards SgrA∗, the outer envelope will interact with the ambient hot X-ray emitting gas (Baganoff et al. 2003; Muno et al. 2004; Shcherbakov & Baganoff 2010).”

“Previous work on the question of the existence of the putative very hot plasma (e.g. Muno et al. 2004) has focused on the energetic difficulties apparently encountered were the plasma real and escaping at approximately the sound speed because of the inadequacy of known power sources to sustain the…”

“Unlike dark mat- ter, the existence of a Galactic Centre population of neutron stars has also been firmly established based on discovery of a handful of pulsars within 15′ of Sgr A∗ (Muno et al. 2004a; Deneva, Cordes & Lazio 2009).”

“(Muno et al. 2004b; Porquet et al. 2005a).”

“This is equivalent to the entirety of supernova kinetic energy output occurring every 3000 years and is unreasonably high (Muno et al. 2004).”

“The pervasive nature of these variations is not surprising, becuase then entire field contains molecular gas (Tsuboi et al. 1999) and exhibits diffuse iron Kα emission (Muno et al. 2004).”

“The requisite population of several thousand millisecond pulsars would conflict with the dominant IP interpretation of the Chandra point source population in the same region (Muno et al. 2004b; Pretorius & Mukai 2014).”

“The diffuse hard Xray emission in the Galactic centre region was used by Muno et al. (2004) to estimate an energy input of ∼ 1040 erg s−1.”

“In fact, it is expected that the interstellar medium (ISM) can be in a multi-temperature phase7 (e.g., McKee & Ostriker 1977; 7 It is also known that hot gas in other systems such as galaxy groups or clusters can be multi-temperature (e.g., Buote et al. Muno et al. 2004; Randall et al. 2006).”

“The spectrum of the emission from these diffuse features is sightly steeper than that of the faint point sources (with Γ = 0.9), and the Helike Fe emission is weaker (400±60 eV for point sources Muno et al. 2004b).”

“XMMNewton EPIC-PN spectra and response files from individual observations were similarly combined, after ensuring that individual spectra were consistent with each other.”

“The plasma temperature is around T ≃ 6.5 keV (see Koyama et al. 1996; Muno et al. 2004; Koyama et al. 2007b).”

“3), are subject to a high confining pressure of 106 to 7 K cm−3 (Yamauchi et al. 1990; Spergel & Blitz 1992; Muno et al. 2004), and are penetrated by strong magnetic fields with strengths ∼5 mG (Pillai et al. 2015).”

“This seems an attractive mechanism for explanation of the diffuse X-ray emission of the Galactic Center, given the serious problems of interpretation of this radiation within the " standard " (thermal and nonthermal) models (Muno et al., 2004).”

“Muno et al. (2004) showed that to account for the diffuse emissionwithin 20pc of the Galactic centre, approximately0.”

“In fact the implied source number density substantially exceeds the estimated 104 CVs in the region and, together with the additional luminosity and spectral constraints, matches no known population of sources (Muno et al. 2004a).”

“…to explain some and perhaps most of the line emission from more highly ionized species such as He-like and H-like Si, S, Ar, Ca, and Fe observed in diffuse emission from the Galactic Ridge (GR) and Galactic Center (GC) (Koyama et al. 1996; Kaneda et al. 1997; Ebisawa et al. 2001; Muno et al. 2004).”

“However, the medium in GC is quite complicated, there exist many giant molecular clouds (LaRosa et al. 2000), and hot gas discovered in Xrays (Muno et al. 2004).”

“The absorption column density NH of about 10 24 cm−2 is significantly larger than the value of 1023 cm−2 for typical sources in the GC (Muno et al. 2004), and hence, the major fraction would be owing to the absorption in the MCs.”

“Studies based on X-ray emission from the GC (Muno et al. 2004) suggest that, even on the smaller spatial scales of interest to us, the gas pressure may not be very different.”

“Muno et al.2004, Raassen et al. 2003).”

“The hot phase of the Galactic center and the hard one with a kT ∼ 8 keV (e.g., Muno et al. 2004; Koyama et al. 1996).”

“The absorption column density toward the GC region (Abs1) is ∼ 6.7× 1022 H cm−2, a typical value to the GC region (e.g., Muno et al. 2004).”

“…that this is integrated point sources (component-3), because Muno et al. (2004) reported that similar fractions of the GCDX come from point sources of ≥ 3×10−15 ergs cm−2 s−1 (2–9 keV), and the spectrum has a strong 6.7 keV line and a rather weak 6.4 keV line (see figure 7 of Muno et al. 2004).”

“Finally, the astrophysical relevance of Fe XXV and Fe XXVI emission in the Galactic center and ridge (Tanaka et al. 1999; Muno et al. 2004) makes these calculations useful for astrophysical modelers of these regions as charge exchange may be a dominant process.”

“Although they suggested electron origin, the discovery of time variability by Muno et al. (2007) strongly supports an X-ray irradiation scenario.”

“Recent observations with Chandra reveal diffuse hard X-ray emission from the central R ≈ 60 pc, with a luminosity of ∼ 2×1036 ergs s−1 (Muno et al. 2004).”

“The model proposed for the DHN is that of a Alfvén wave propagating from the molecular gas in the central parsec of the Galaxy and is unrelated to the GC lobe (Morris et al. 2006).”

“Indeed, using Chandra X-ray spectra for the GC fields, Muno et al. (2004a) and Muno et al. (2004b) measure the absorption column densities for various diffuse emission regions and for various point sources, respectively.”

“The observed diffuse X-ray luminosity in the 0.5–2 keV band of our Galactic bulge and M31 bulge is about 1038 ergs s−1 (Shirey et al. 2001; Takahashi et al. 2004; Muno et al. 2004; Li & Wang 2007).”

“These high-energy electrons and positrons would emit synchrotron radiations when they are subjected in a magnetic field strength B and surrounded by a background plasma with plasma frequency νp = 8890[n(r)/1 cm −3]1/2 Hz, where the electron density is n(r) ∼ 1 cm−3 (Muno et al. 2004).”

“This excess would be due to larger populations of high-mass stars in the east than the west (Park et al. 2004; Muno et al. 2004; Koyama et al. 2007d).”

“1033 erg s−1) X-ray source population in the galactic ridge (GRXE) and Galactic centre is a greatly debated topic for a decade (Muno et al. 2004; Sazonov et al. 2006; Revnivtsev et al. 2009, 2011; Warwick, Byckling & Pérez-Ramírez 2014; Perez et al. 2015; Hailey et al. 2016).”

“One of the remarkable discoveries from X-ray observations of the Galactic center (GC) region is the Galactic center diffuse X-ray emission (GCDX) extending over ∼300 pc (|l| < 1◦), and that the GCDX has a prominent highly ionized iron line at 6.7 keV (Koyama et al. 1989; Yamauchi et al. 1990).”

“Therefore, the pressure and the total energy are ∼ 2 × 10 7 K cm − 3 and E 15pc ∼ 6 × 10 50 erg, respectively (Muno et al. 2004; Ponti et al. 2019).”