------------------------------------------------------------------------ From: Lara Sidoli sidoli@ifctr.mi.cnr.it X-Sender: sidoli@gaia To: Galactic Center Newsletter Subject: submit sidoli.tex to appear in A&A Letters, 1999 MIME-Version: 1.0 % astro-ph/9908164 %\documentstyle[referee]{l-aa} \documentstyle[psfig]{l-aa} \newcommand{\be}{\begin{equation}} \newcommand{\en}{\end{equation}} \begin{document} \newfont{\mc}{cmcsc10 scaled\magstep2} \newfont{\cmc}{cmcsc10 scaled\magstep1} \newcommand{\bc}{\begin{center}} \newcommand{\ec}{\end{center}} %---------------------------------- \title{The X--ray Diffuse Emission from the Galactic Center} \author{L.~Sidoli(1,2) \& S.~Mereghetti(1)} \institute{Istituto di Fisica Cosmica ``G. Occhialini" -- C.N.R., Via Bassini 15, I-20133 Milano, Italy; \\ e-mail: (sidoli, sandro)@ifctr.mi.cnr.it \and Dipartimento di Fisica, Universit\`a di Milano, Sez. Astrofisica, Via Celoria 16, I-20133 Milano, Italy } \date{Received 13 July 1999 / Accepted 2 August 1999} \maketitle \label{sampout} \begin{abstract} A long BeppoSAX observation of the Galactic Center region shows that the spectrum of the diffuse X--ray emission from the SgrA Complex can be described with the sum of two thermal plasma models with temperatures of $\sim$0.6 keV and $\sim$8-9 keV. The spatial distribution of the diffuse emission is energy dependent. While the hard X--ray emission has a nearly symmetric distribution elongated along the Galactic Plane, the soft X--rays (E $<$ 5 keV) are remarkably well correlated with the triangular radio halo of SgrA East. The parameters derived for the soft component support the interpretation of the SgrA East shell in terms of an evolved supernova remnant. \end{abstract} \keywords{Galaxy: center - Individual: SgrA East halo - X--rays: supernova remnants} \section{Introduction} The diffuse X--ray emission from the Galactic Center (GC) region was imaged for the first time with the Einstein Observatory in the 0.5--4 keV band (Watson et al. 1981) and later studied in more detail with several satellites (Kawai et al. 1988, Skinner et al. 1987, Koyama et al. 1989, 1996). The real nature of this emission is still an open issue. Though at least part of it can be due to the integrated emission from weak unresolved sources (Watson et al. 1981; Zane, Turolla \& Treves 1996), there is evidence that a hot plasma, responsible for the observed emission lines, permeates the GC region. This was indicated by the Ginga discovery of a strong iron line at 6.7 keV with an equivalent width of $EW=600\pm$70 eV (Koyama et al. 1989). More recently, the ASCA satellite confirmed the presence of a hot plasma (kT$\sim10$~keV) and discovered another diffuse component in the 6.4 keV Fe-line with an asymmetric spatial distribution with respect to the GC and well correlated with the distribution of the giant molecular clouds (Koyama et al. 1996). The data on the diffuse X--ray emission reported here were obtained during a survey of the GC performed with the BeppoSAX Narrow Field Instruments (Sidoli et al. 1998). The results on the point sources have been reported elsewhere (Sidoli et al. 1999a, 1999b). \section{The Galactic Center environment} Before presenting our results, we briefly describe the most important radio emitting sources present in the region within $\sim8'$ from SgrA* covered by our observation (see Morris \& Serabyn 1996 for a recent review). These extended sources are collectively known as the Sgr~A~Complex (Fig.\ref{sgra}). The most peculiar object is the compact radio source SgrA* (Balick \& Brown 1974). It has a radio luminosity of $\sim2\times10^{34}$~ergs~s$^{-1}$ and a flux density rising as S$_{\nu}\propto\nu^{1/3}$ between a low frequency turnover at $\nu\sim$800~MHz and a cut--off at 2000--4000 GHz (Mezger, Duschl \& Zylka 1996). Lo et al. (1998) measured an intrinsic size of less than 5.4$\times10^{13}$~cm with multi--wavelength VLBA observations. From the proper motion of the nearby IR stars a dark mass of 2.6$\times10^{6}$~M$_{\odot}$ has been deduced to reside within a central volume of 10$^{-6}$~pc$^{3}$ (Ghez et al. 1998). Sgr~A* is generally considered to be a super--massive black hole due to its unique radio spectrum (see e.g. Krichbaum et al. 1998), its compactness, its location at the GC (Yusef-Zadeh, Choate \& Cotton 1999) and its low proper motion ($\leq$40 km~s$^{-1}$, Reid et al. 1999). The high energy emission of SgrA* is surprisingly low, corresponding to a 2--10 keV luminosity of only $\sim$10$^{35}$~ergs~s$^{-1}$ (Koyama et al. 1996, Sidoli et al. 1999b), well below the Eddington limit for a 2 million solar masses black hole. SgrA* is embedded in the ionized gas of SgrA~West, an HII region with a characteristic ``mini-spiral" shape of thermal radio emission. With an angular extent of $\sim1'$, SgrA~West is the innermost and ionized part of the ``Circumnuclear Disk", a structure of molecular gas extending from 1.7 pc up to about 7 pc from the GC and rotating around it (Marshall, Lasenby \& Harris 1995). Proceeding outward, we find the $2'-3'$ shell of SgrA~East (Eckers et al. 1983), which is characterized by a non--thermal radio emission with spectral index $\alpha=-0.64$. It is centered about $50''$ offset of SgrA* and is probably located behind SgrA~West (Davies et al 1976, Pedlar et al. 1989). Its synchrotron radio emission and its shell morphology were explained in terms of a supernova remnant. Mezger et al. (1989) discovered a dust ring compressed by the SgrA~East shell, possibly indicating that this shell originated from an extremely energetic explosion (4$\times10^{52}$~ergs), maybe associated with SgrA*, occurring inside a dense molecular environment (10$^{4}$~cm$^{-3}$). Other alternative explanations are the simultaneous explosion of about 40 supernovae, or a single SN explosion inside a medium with a much lower density. In this case the dust shell could be due to the stellar wind from the normal supernova progenitor (Mezger et al. 1989). Another possibility is the tidal disruption of a star by the central supermassive black hole (Khokhlov \& Melia 1996). A triangular shaped non--thermal halo ($\sim$20 pc in diameter) surrounds in projection the SgrA~East shell (Yusef-Zadeh \& Morris 1987), but it is probably not physically related with it (see Pedlar et al. 1989 for a detailed discussion). Its origin is unknown. It could be centered at the SgrA* position and related with the high energy activity at the GC (Yusef-Zadeh et al. 1997, Melia et al. 1998). The total energy in relativistic particles estimated from radio observations is about 5$\times10^{50}$~ergs~s$^{-1}$. This value, together with its non--thermal radio spectrum and its size, suggest the possibility that it is an evolved supernova remnant (Pedlar et al. 1989). \section{Observations and Data Analysis} Our results were obtained with the Medium Energy Concentrator Spectrometer (MECS, Boella et al. 1997) that provides images in the $\sim$ 1.3--10 keV energy range over a nearly circular field of view with $\sim28'$ radius. The MECS has a good angular resolution (50\% power radius of $\sim75''$ at 6 keV, on-axis) and a moderate energy resolution (FWHM $\sim$8.5$\sqrt{6/{\rm E_{keV}}}$\%). In order to exploit the best spatial and spectral resolution of the detector, and to avoid the artifacts due to the absorbing strongbacks of the MECS entrance window, we only considered the inner region of the field of view (radius $\sim8'$). For the spectral analysis we have used the MECS effective area values appropriate for extended sources. These were properly derived by convolving a flat surface brightness distribution with the energy and position dependent vignetting and Point Spread Function (PSF). All the spectra have been corrected for the the instrumental background by subtracting a spectrum obtained from MECS observations of the dark Earth. This procedure does not account for the subtraction of the cosmic X--ray background (CXB). Indeed, the subtraction of the MECS standard background obtained from high galactic latitude pointings would overestimate the CXB contribution, which is highly absorbed in the GC region. The CXB contribution has been included as a known additive component in the spectral fits with an absorbing column density of 8$\times10^{22}$~cm$^{-2}$ and spectral parameters fixed at the values determined with the MECS (photon index = 1.44, F(1 keV)=12.9~phot~cm$^{-2}$~s$^{-1}$~sr$^{-1}$~keV$^{-1}$; S. Molendi, private communication). \section{X--rays from the Sgr~A~Complex} The location of SgrA* was imaged on August 24-26, 1997 for a net exposure time of 99.5 ks. In order to study the surface brightness and temperature profile of the diffuse emission, we extracted the MECS counts from four annular regions with different inner and outer radii ($0'-2'$,$2'-4'$,$4'-6'$,$6'-8'$) centered on SgrA*. All these spectra contain several emission lines, with the K-lines from iron (E$\sim6.7$~keV) and sulfur (E$\sim2.4$~keV) particularly bright. We fitted them with a single temperature plasma model (MEKAL in XSPEC v.10.00), deriving the surface brigthness and temperature profiles shown in Fig.\ref{fsb}. A radial spectral variation is evident: while the $\sim7-8$ keV temperature in the three external regions is almost constant, the emission from the inner $2'$ is significantly softer. This is probably due to the presence, in addition to the diffuse emission, of further contributions from the bright point sources present near the GC (Maeda et al. 1996, Predehl \& Tr\"umper 1994, Sidoli et al. 1999b). The fit with a single temperature thermal model does not account for all the emission lines at low energies (in particular the intense emission from sulfur at E$\sim$2.4 keV), nor for an excess around 6.4 keV, due to the presence of emission of fluorescent origin. A gaussian line added at 6.4 keV accounts for these residuals. While this line is present in all the spectra extracted from $2'$ to $8'$, it seems to be absent within $2'$ (EW$\leq$12eV). This is consistent with the 6.4 keV map produced with ASCA (Maeda \& Koyama 1996) where a strong peak is visible in the region of the Radio Arc, NE to the GC. Indeed the EW of this line increases towards the outer regions ($EW_{2'-4'}=80-100$ eV,$EW_{4'-6'}=120-130$ eV, $EW_{6'-8'}=140-150$ eV). The MECS spectra extracted from two semi--annular regions at NE and SW of the GC, confirm this interpretation: the EWs of the 6.4 keV line are $\sim100\pm{30}$ eV (NE sector; errors are at 90\% confidence level) and $\sim40\pm{20}$ eV (SW sector). Since the temperature profile does not show evidence of strong spectral variations in the region from $2'$ to $8'$, we analysed the overall spectrum by extracting all the counts from this larger circular corona. We started by fitting a thermal bremsstrahlung plus three gaussian lines at $\sim$1.8, 2.4 and 6.7 keV (Si, S and Fe respectively). Their estimated equivalent widths are about 120, 190 and 1000 eV. The best fit value for the bremsstrahlung temperature is $\sim$13 keV. This temperature is too high to be consistent with the presence of the low energy emission lines (Kaneda et al. 1997, Fig. 2b). A possible explanation is a multi-temperature plasma. Thus we fitted the spectrum with two thermal emission plasma models (two ``MEKAL" in XSPEC). Our best fit parameters ($\chi^2$=1.29, 368 d.o.f) are N$_{H}$=$7.93^{+0.09}_{-0.36}\times10^{22}$~cm$^{-2}$, T$_{1}=0.57^{+0.02}_{-0.05}$~keV and T$_{2}=8.69^{+0.26}_{-0.41}$~keV. A gaussian added at 6.4 keV gives an EW$\sim$120~eV. The total unabsorbed flux (2--10 keV) is $1.71^{+0.05}_{-0.08}\times10^{-10}$ ergs~\cmdue~s$^{-1}$, about one third of which is contributed by the soft component. To study the spatial distribution of the diffuse emission, we extracted images in different energy bands (Fig.\ref{softhard}). The images corresponding to the 2--5 keV and 7--10 keV ranges show significantly different distributions of the diffuse emission. Both the soft and hard emissions are peaked at the GC position, but they have different spatial extents. In particular, the softer emission has a nearly triangular shape, with a rather sharp decrease in the south-western side, which is absent in the hard X--ray map. To study the spatial distribution of the iron line we have also extracted an image in the 5.5-7.5 keV band and subtracted from this map the continuum emission interpolated from the contiguous energies. The resulting iron line image shows a spatial distribution elongated along the galactic plane, very similar to that of the hard X--ray map. \section{Discussion} Our observation of the SgrA complex confirms the presence of a hot plasma with multiple temperatures and/or in non-equilibrium ionization, as already found with the ASCA instruments (Koyama et al. 1996). The increase in the 6.4 keV line equivalent width that we find in the North-East sector, is in agreement with the more detailed maps of this line obtained with ASCA that show a correlation with the molecular clouds. The limited spectral resolution of the MECS, compared with that of the ASCA solid state detectors, does not allow to study in more detail the energy profile and spatial distribution of the individual lines. On the other hand, the regular point spread function provided by the BeppoSAX mirrors has allowed us to produce unbiased maps in wide energy bands that demonstrate a clear difference in the spatial distribution of the softer and harder X--ray emission. Since also our spectral data could be well described by the sum of two thermal models with kT$\sim$0.6 and 8 keV, it is tempting to give an interpretation in terms of two plasma components at different temperatures and with different spatial distributions (although in reality the situation is certainly much more complex, with, e.g., a distribution of temperatures). In this interpretation, it is remarkable that the lower temperature plasma is well correlated with the SgrA~East triangular radio halo (Fig.\ref{radio}). The presence of unresolved point sources could affect the apparent distribution of the diffuse emission. We note however that previous observations, e.g. with Einstein and ROSAT (Watson et al. 1981, Predehl \& Tr\"umper 1994) do not show the presence of strong sources distributed in such a way to reproduce the triangular shape visible in our low energy map. In particular, the straight contours of the soft emission corresponding to the SW side of the triangular radio halo seem hardly explainable by a distribution of sources. Although we cannot exclude that the apparent shape of the softer X--rays is due to an absorption effect, we favour the interpretation of the lower temperature plasma as a component physically related to the halo of SgrA~East. In the following we will adopt this working hypothesis and assume a distance of 8.5 kpc. We derive for the soft component an emission measure $EM$=(1.2$\pm{0.4})\times10^{14}$~cm$^{-5}$, which assuming emission from a spherical region with radius 10 pc and an electron filling factor $f$, corresponds to n$_{e}\sim(3$~$\times$$f^{-1/2})$~cm$^{-3}$ and to a total mass $M_{g}\sim250~M_{\odot}$$f^{1/2}$. The average thermal pressure in the SgrA~East halo, $P\sim3\times10^{-9}$~ergs~cm$^{-3}$, is consistent with the pressure $P_{Sedov}=0.106\times$$(E_{SN}/R^{3})$~ergs~cm$^{-3}$ derived for a SNR in a Sedov phase, where $E_{SN}$ is the explosion energy of the SNR and R is the shell radius in cm. Indeed, if we assume $E_{SN}=10^{51}$~ergs~s$^{-1}$ and R=10~pc, we find $P_{Sedov}\sim4\times10^{-9}$~ergs~cm$^{-3}$. The X--ray luminosity (2--10 keV) of the soft component is $\sim4.5\times10^{35}$~ergs~s$^{-1}$. If we assume that this thermal emission is mostly produced by the SgrA~East halo, its X--ray luminosity, pressure, density, temperature of the emitting gas (0.6 keV) and size ($\sim$20 pc), match well with a supernova remnant origin in which thermal line emission is produced when the expanding supernova ejecta heats the ISM to X--ray temperatures. Another test of the SNR hypothesis can be done by comparing the X--ray and radio surface brightnesses: indeed a strong correlation $\Sigma_{R}\propto\Sigma_{X}^{0.69\pm0.08}$ between the radio $\Sigma_{R}$ (at 1 GHz) and the X--ray surface brightness $\Sigma_{X}$ (0.15--4.5 keV) was found by Berkhuijsen (1986). >From the radio spectrum reported by Pedlar et al. (1989) we derived $\Sigma_{R}\sim5\times10^{-19}$~W~m$^{-2}$~Hz$^{-1}$~sr$^{-1}$. The $\Sigma_{X}$ measured from our X--ray data and converted to the 0.15--4.5 energy band is $\Sigma_{X}\sim10^{34}$~ergs~s$^{-1}$~pc$^{-2}$. These values fall well inside the region defined by other typical SNRs. In conclusion, all the physical quantities derived from the analysis of the MECS are consistent with a SNR origin for the SgrA~East halo. \section{Conclusions} The analysis of the diffuse emission from the GC region reveals the presence of at least two distinct components : a soft one (T$_{mekal}\sim$0.6 keV), spatially correlated with the SgrA~East radio halo and a hard component (T$_{mekal}\sim$8--9 keV). While the hard component is elongated along the Galactic Plane and does not spatially correlate with physical structures observed at other wavelengths, the spatial distribution of the soft component is remarkably well correlated with the triangular radio halo of SgrA~East. The parameters derived for the softer component match extremely well with the idea that this thermal X--ray emission is due to a SNR origin for the SgrA~East halo. \begin{acknowledgements} We thank L.~Chiappetti, G.~Matt, S.~Molendi and A.~Treves for useful discussions and help with the data analysis. \end{acknowledgements} \begin{thebibliography}{} \bibitem{} Anantharamaiah K.R., Pedlar A., Ekers R.D. \& Goss W.M. 1991, MNRAS 249, 262 \bibitem{} Balick B. \& Brown R.L. 1974, ApJ 194, 265 \bibitem{} Berkhuijsen E.M., 1986, A\&A 166, 257 \bibitem{} Boella G. et al., 1997, A\&AS 122, 327 \bibitem{} Cremonesi D.I., Mereghetti S., Sidoli L. \& Israel G.L., 1999, A\&A 345, 826 \bibitem{} Davies R.D., Walsh D.\& Booth R.S. 1976, MNRAS 177, 319 \bibitem{} Eckers R.D, van Gorkom J.H, Schwarz U.J. and Goss W.M. 1983, A\&A 122, 143 \bibitem{} Ghez A.M., Klein B.L., Morris M. \& Becklin E.E. 1998, ApJ 509, 678 \bibitem{} Kaneda H. et al. 1997, ApJ 491, 638 \bibitem{} Kawai N. et al. 1988, ApJ 330, 130 \bibitem{} Khokhlov A. \& Melia F. 1996, ApJ 457, L61 \bibitem{} Koyama K. et al. 1989, Nature 339, 603 \bibitem{} Koyama K. et al. 1996, PASJ 48, 249 \bibitem{} Krichbaum T.P. et al. 1998, A\&A 335, L106 \bibitem{} Lo K.Y., Shen Z., Zhao J. \& Ho P.T.P 1998, ApJ 508, 61 \bibitem{} Marshall J., Lasenby A.N. \& Harris A.I. 1995, MNRAS 277, 594 \bibitem{} Maeda Y. et al. 1996, PASJ 48, 417 \bibitem{} Maeda Y. \& Koyama K. 1996, ASP Conf. 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Occhialini" -- C.N.R., Via BassiFIELD: 1 Address FIELD: 1 Address FIELD: 1 Address FIELD: 1 Address FIELD: 1 Address FIELD: 1 Address FIELD: 1 Address FIELD: 1 Address FIELD: 1 Address FIELD: 1 Address FIELD: 1 Address nawk: syntax error in regular expression \ at input record number 72 source line number 107 cat: write error: Broken pipe ------------- End Forwarded Message -------------