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).”

“, it is located between young, massive star clusters at the Galactic center (Krabbe et al. 1995; Paumard et al. 2001) and the Radio arches region (Figer et al. 1999), and it is the site of the strongest low-ionization Fe emission (Park et al. 2004).”

“…for 1023 cm−2 of absorption through the Galaxy, and the fact that sources brighter than 6 × 10−15 erg cm−2 s−1 (2–8 keV; de-absorbed to match the deep-field surveys) should have been detected, less than 1% of the observed diffuse emission is from extragalactic background (e.g., Rosati et al. 2002).”

“, it is located between young, massive star clusters at the Galactic center (Krabbe et al. 1995; Paumard et al. 2001) and the Radio arches region (Figer et al. 1999), and it is the site of the strongest low-ionization Fe emission (Park et al. 2004).”

“We reduced the data from each observation using the methods described in detail in Muno et al. (2003a) and Townsley et al. (2003).”

“Second, we have converted the photon fluxes in Muno et al. (2003a) to energy fluxes by assuming 1 photons cm−2 s−1 = 8 × 10−9 erg cm−2 s−1 (2.0–8 keV), which is appropriate for a Γ = 0.5 powerlaw spectrum absorbed by a 6 × 1022 cm−2 column of gas and dust.”

“Individual magnetic flux tubes are observed as synchrotron-emitting radio filaments oriented perpendicular to the Galactic plane (Yusef-Zadeh & Morris 1987; LaRosa et al. 2000a).”

“As noted by Wang et al. (2002), the shapes and relative intensities of the He-like and H-like iron lines appear qualitatively similar in the point sources and diffuse emission.”

“For instance, low-energy (< 1 MeV) cosmic rays could interact with the neutral ISM to produce continuum emission through bremsstrahlung radiation and line emission through charge-exchange interactions (e.g., Valinia & Marshall 1998; Tanaka et al. 1999; Valinia et al. 2000).”

“The remaining long-term variations in the particle background should be no larger than 5% (see Markevitch et al. 2003).”

“The differences in the soft emission is the most dramatic: it is lowest in the Northwest, Southeast, and Southwest, 50% higher in the region close to the Galactic center, a factor of 3 higher in the East, and a factor of 9 brighter in the Northeast.”

“Individual magnetic flux tubes are observed as synchrotron-emitting radio filaments oriented perpendicular to the Galactic plane (Yusef-Zadeh & Morris 1987; LaRosa et al. 2000a).”

“The line energies and ratios are consistent with those expected from plasma in thermal equilibrium (compare Kaneda et al. 1997; Tanaka 2002).”

“However, it has been previously noted that an 8 keV plasma is too hot to be bound to the Galactic plane (e.g., Yamauchi et al. 1990; Kaneda et al. 1997; Yamasaki et al. 1997), so it is important to consider the amount of energy that could be lost as the plasma expands.”

“We have computed the luminosity from the de-absorbed 2–8 keV fluxes in Table 3 by applying a bolometric correction that we determined by integrating the total flux from the vapec model in XSPEC; this is equivalent to using the cooling function in, for example, Raymond, Cox, & Smith (1976).”

“…of the Galactic center, the most similar region may well be the Carina Nebula, which exhibits ≈ 2 × 1035 erg s−1 of X-rays from the outflow around η Car, the WR and O stars in the cluster Trumpler 14, and diffuse emission that may have resulted from recent supernovae (Seward & Chlebowski 1982).”

“, it is located between young, massive star clusters at the Galactic center (Krabbe et al. 1995; Paumard et al. 2001) and the Radio arches region (Figer et al. 1999), and it is the site of the strongest low-ionization Fe emission (Park et al. 2004).”

“…fluorescence that are ascribed to molecular clouds being illuminated by X-rays from a bright point source (Sunyaev, Markevitch, & Pavlinsky 1993; Murakami et al. 2000) or bombarded by low-energy cosmic-ray electrons (Yusef-Zadeh, Law, & Wardle 2002); arcminute-scale features with hard spectra…”

“It has also been proposed that there is a non-thermal component to the spectrum between 2–8 keV, which represents a lowenergy extension of the power-law emission seen above 10 keV (e.g., Valinia et al. 2000, but see Lebrun et al. 2004).”

“On the other hand, Chandra observations clearly demonstrate that there are not enough discrete sources with LX & 10 31 erg s−1 to account for more than 10% the Galactic ridge X-ray emission (Ebisawa et al. 2001).”

“Moreover, Sgr A East (Maeda et al. 2002) and the radio wisp ’E’ (Ho et al. 1985) are already thought to be remnants of recent supernova, so it seems likely that the inner 20 pc of the Galaxy has experienced a supernova rate at least this high.”

“…from the interstellar medium could make up ∼ 1% of the mass of the Galactic center (Zane, Turolla, & Treves 1996), they must be less luminous than 1029 erg s−1, or else many thousands of similar systems would have been detected in the local Galaxy in the ROSAT AllSky survey (Perna et al. 2003).”

“Similarly, we computed a mean re- sponse by averaging the response functions provided by Townsley et al. (2002b) from the range of detector rows covered by each region, also weighted by the number of counts in each observation.”

“The local space density of CVs is at most 3× 10−5 pc−3 (Warner 1995; Schwope et al. 2002), so the space density at the Galactic center should be less than 0.03 pc−3.”

“In observations of massive star clusters, about 10% of the wind kinetic energy is converted into X-rays (Stevens & Hartwell 2003; Townsley et al. 2003).”

“The spectrum of the diffuse emission from the Galactic center is dominated by line emission from He-like and H-like Si, S, Ar, Ca, and Fe (Figure 5; see also Yamasaki et al. 1997; Tanaka 2002).”

“We assume a distance of 8 kpc to the Galactic center throughout the rest of the paper (McNamara et al. 2000).”

“…by low-energy cosmic-ray electrons (Yusef-Zadeh, Law, & Wardle 2002); arcminute-scale features with hard spectra that resemble supernova shocks (Bamba et al. 2003); and X-ray counterparts to radio features that are thought to be magnetic filaments (Sakano et al. 2003, Lu, Wang, & Lang 2003).”

“Similarly, we computed a mean re- sponse by averaging the response functions provided by Townsley et al. (2002b) from the range of detector rows covered by each region, also weighted by the number of counts in each observation.”

“…possible that the hot diffuse X-ray emission is a low-energy extension of the emission with a power-law spectrum observed above 10 keV, which suggests that it may result from a non-thermal mechanism (see Skibo et al. 1997; Yamasaki et al. 1997; Valinia & Marshall 1998, but see Lebrun et al. 2004).”

“However, observations with INTEGRAL have resolved ≈ 85% of the emission above 20 keV into discrete point sources (Lebrun et al. 2004).”

“For instance, a Type II supernova in the Northeast could produce both the bright, soft diffuse emission and the neutral Fe emission (e.g., Maeda et al. 2002; Bykov 2002).”

“At larger radii from the center, the most prominent features are filaments dominated by low-ionization Fe emission at 6.4 keV in the Northeast (Park et al. 2004), and a hard, continuumdominated feature in the south (Sakano et al. 2003; Lu et al. 2003; Yusef-Zadeh 2003).”

“At larger radii from the center, the most prominent features are filaments dominated by low-ionization Fe emission at 6.4 keV in the Northeast (Park et al. 2004), and a hard, continuumdominated feature in the south (Sakano et al. 2003; Lu et al. 2003; Yusef-Zadeh 2003).”

“On the one hand, ASCA observations revealed degree-scale differences in the surface brightness of the diffuse emission that could only be produced by Poisson fluctuations in the numbers of undetected point sources if they have a luminosity of ∼ 1033 erg s−1 (Yamauchi et al. 1996).”

“…is not clear whether the filaments represent only a small fraction of vertical fields that pervade the Galactic center (Serabyn & Morris 1996; Chandran, Cowley, & Morris 2000), or whether they represent purely local magnetic features (Chevalier 1992; LaRosa, Lazio, & Kassim 2001; Yusef-Zadeh 2003).”

“Taken together, these observations suggest that the magnetic fields are predominantly vertical at the Galactic center, with a toroidal component produced by orbital shear in the molecular clouds (Uchida & Güsten 1995; Chandran et al. 2000; Novak et al. 2003).”

“…if the hard diffuse emission originates from non-thermal processes, such as the shocks that accelerate cosmic rays, the energies and ratios of the intensities of the line emission should deviate measurably from the values expected for a plasma in thermal equilibrium (e.g., Masai et al. 2002).”

“…a result, several authors have modeled the diffuse emission as originating from two plasma components (e.g., Koyama et al. 1996; Kaneda et al. 1997; Tanaka et al. 2000): one with kTcool ≈ 0.8 keV (which we refer to here as the “cool” or “soft” component), and a second with kThot ≈ 8 keV (which we…”

“5000 YSOs lie near the Galactic center (Feigelson 2004; Grosso et al. 2004), which is still far too few to produce the hard diffuse emission.”

“Second, we have converted the photon fluxes in Muno et al. (2003a) to energy fluxes by assuming 1 photons cm−2 s−1 = 8 × 10−9 erg cm−2 s−1 (2.0–8 keV), which is appropriate for a Γ = 0.5 powerlaw spectrum absorbed by a 6 × 1022 cm−2 column of gas and dust.”

“Moreover, neither non-thermal electrons nor protons are efficient at producing bremsstrahlung radiation, so the energy required to produce the observed diffuse emission is nearly as large as that required to replenish a continually-expanding thermal plasma (Dogiel et al. 2002).”

“Bright, diffuse X-ray and γ-ray emission has been observed all along the Galactic plane, but is particularly bright toward the Galactic center (e.g., Worrall et al. 1982; Koyama et al. 1986b; Yamauchi et al. 1996; Yamasaki et al. 1997; Skibo et al. 1997; Valinia & Marshall 1998).”

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

“…& 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 .”