------------------------------------------------------------------------ From: melia@spacetime.physics.Arizona.EDU (Fulvio Melia) To: gcnews@aoc.nrao.edu Subject: Another new paper %astro-ph/9806324 \documentstyle[aasms4,amstex,amsfonts,epsfig,rotating,float,12pt]{article} \def\ledd{L_{\rm Edd}} \def\taut{\tau_{\rm T}} \def\taup{\tau_{\rm p}} \def\msun{{\,M_\odot}} \def\lsun{{\,L_\odot}} % % Reference macros % % To generate reference to a paper in Ap.J. volume 300, p.123 % write \apj{Claus, S. 1990}{300}{123} % \def\refindent{\par\noindent\hangindent=3pc\hangafter=1 } \def\aa#1#2#3{\refindent#1, A\&A, #2, #3} \def\aasup#1#2#3{\refindent#1, A\&AS, #2, #3} \def\aj#1#2#3{\refindent#1, AJ, #2, #3} \def\apj#1#2#3{\refindent#1, {\it ApJ}, {\bf#2}, #3.} \def\apjlett#1#2#3{\refindent#1, {\it ApJ (Letters)}, {\bf #2}, #3.} \def\apjsup#1#2#3{\refindent#1, ApJS, #2, #3} \def\araa#1#2#3{\refindent#1, ARA\&A, #2, #3} \def\baas#1#2#3{\refindent#1, BAAS, #2, #3} \def\icarus#1#2#3{\refindent#1, Icarus, #2, #3} \def\mnras#1#2#3{\refindent#1, {\it MNRAS}, {\bf#2}, #3.} \def\nature#1#2#3{\refindent#1, {\it Nature}, {\bf #2}, #3.} \def\pasj#1#2#3{\refindent#1, PASJ, #2, #3} \def\pasp#1#2#3{\refindent#1, PASP, #2, #3} \def\qjras#1#2#3{\refindent#1, QJRAS, #2, #3} \def\science#1#2#3{\refindent#1, Science, #2, #3} \def\sov#1#2#3{\refindent#1, Soviet Astr., #2, #3} \def\sovlett#1#2#3{\refindent#1, Soviet Astr.\ Lett., #2, #3} \def\refpaper#1#2#3#4{\refindent#1, #2, #3, #4} \def\refbook#1{\refindent#1} \def\degs{$^\circ$} \def\biggldb{\biggl[\!\!\biggl[} \def\biggrdb{\biggr]\!\!\biggr]} \def\um{{\,\mu\rm m}} \def\cm{{\rm\,cm}} \def\km{{\rm\,km}} \def\au{{\rm\,AU}} \def\pc{{\rm\,pc}} \def\kpc{{\rm\,kpc}} \def\mpc{{\rm\,Mpc}} \def\sec{{\rm\,s}} \def\yr{{\rm\,yr}} \def\gm{{\rm\,g}} \def\kms{{\rm\,km\,s^{-1}}} \def\kelvin{{\rm\,K}} \def\erg{{\rm\,erg}} \def\ev{{\rm\,eV}} \def\hz{{\rm\,Hz}} \def\>{$>$} \def\<{$<$} \def\bsl{$\backslash$} \def\simlt{\lower.5ex\hbox{$\; \buildrel < \over \sim \;$}} \def\simgt{\lower.5ex\hbox{$\; \buildrel > \over \sim \;$}} \def\sqr#1#2{{\vcenter{\hrule height.#2pt \hbox{\vrule width.#2pt height#1pt \kern#1pt \vrule width.#2pt} \hrule height.#2pt}}} \def\square{\mathchoice\sqr34\sqr34\sqr{2.1}3\sqr{1.5}3} %def\ledd{1} %\slugcomment{To be submitted to Astrophysical Journal} \begin{document} \centerline{Submitted to the Astrophysical Journal (Letters)} \centerline{Revised June 8, 1998} \bigskip \title{A Model of the EGRET Source at the Galactic Center:\\ Inverse Compton Scattering Within Sgr A East and its Halo} \author{Fulvio Melia\altaffilmark{1}$^{*\dag}$, Farhad Yusef-Zadeh$^{\ddag}$ and Marco Fatuzzo$^*$} \affil{$^*$Physics Department, The University of Arizona, Tucson, AZ 85721} \affil{$^{\ddag}$Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208} \affil{$^{\dag}$Steward Observatory, The University of Arizona, Tucson, AZ 85721} %\author{C. D. Biemesderfer\altaffilmark{4,5}} %\affil{National Optical Astronomy Observatories, Tucson, AZ 85719} %\and %\author{R. J. Hanisch\altaffilmark{5}} %\affil{Space Telescope Science Institute, Baltimore, MD 21218} % Notice that each of these authors has alternate affiliations, which % are identified by the \altaffilmark after each name. The actual alternate % affiliation information is typeset in footnotes at the bottom of the % first page, and the text itself is specified in \altaffiltext commands. % There is a separate \altaffiltext for each alternate affiliation % indicated above. \altaffiltext{1}{Presidential Young Investigator.} % The abstract environment prints out the receipt and acceptance dates % if they are relevant for the journal style. For the aasms style, they % will print out as horizontal rules for the editorial staff to type % on, so long as the author does not include \received and \accepted % commands. This should not be done, since \received and \accepted dates % are not known to the author. \begin{abstract} Continuum low-frequency radio observations of the Galactic Center reveal the presence of two prominent radio sources, Sgr A East and its surrounding Halo, containing non-thermal particle distributions with power-law indices $\sim 2.5-3.3$ and $\sim 2.4$, respectively. The central $1-2$ pc region is also a source of intense (stellar) UV and (dust-reprocessed) far-IR radiation that bathes these extended synchrotron-emitting structures. A recent detection of $\gamma$-rays (2EGJ1746-2852) from within $\sim 1^o$ of the Galactic Center by EGRET onboard the Compton GRO shows that the emission from this environment extends to very high energies. We suggest that inverse Compton scatterings between the power-law electrons inferred from the radio properties of Sgr A East and its Halo, and the UV and IR photons from the nucleus, may account for the possibly diffuse $\gamma$-ray source as well. We show that both particle distributions may be contributing to the $\gamma$-ray emission, though their relevant strength depends on the actual physical properties (such as the magnetic field intensity) in each source. If this picture is correct, the high-energy source at the Galactic Center is extended over several arcminutes, which can be tested with the next generation of $\gamma$-ray and hard X-ray missions. \end{abstract} % The different journals have different requirements for keywords. The % keywords.apj file, found on aas.org in the pubs/aastex-misc directory, % contains a list of keywords used with the ApJ and Letters. These are % usually assigned by the editor, but authors may include them in their % manuscripts if they wish. \keywords{acceleration of particles---black hole physics---Galaxy: center---galaxies: nuclei---gamma rays: theory---radiation mechanisms: non-thermal} % That's it for the front matter. On to the main body of the paper. % We'll only put in tutorial remarks at the beginning of each section % so you can see entire sections together. % In the first two sections, you should notice the use of the LaTeX \cite % command to identify citations. The citations are tied to the % reference list via symbolic KEYs. We have chosen the first three % characters of the first author's name plus the last two numeral of the % year of publication. The corresponding reference has a \bibitem % command in the reference list below. % % Please see the AASTeX manual for a more complete discussion on how to make % \cite-\bibitem work for you. \section{Introduction} In 1992, EGRET on board the Compton GRO identified a central ($< 1^o$) $\sim 30$ MeV - $10$ GeV continuum source with luminosity $\approx 2\times 10^{37}$ ergs s$^{-1}$ (\cite{mayer98}). Its spectrum can be represented as a broken hard power-law with spectral indices $\alpha=-1.3\pm 0.03$ and $-3.1\pm 0.2$ ($S=S_0\,E^\alpha$), with a cutoff between $4-10$ GeV. This EGRET $\gamma$-ray source (2EGJ1746-2852) appears to be centered at $l\approx 0.2^o$, but a zero (or even a negative) longitude cannot be ruled out completely. The $\gamma$-ray flux does not appear to be variable down to the instrument sensitivity (roughly a $20\%$ amplitude), which has led some to postulate that the observed $\gamma$-rays are produced by diffuse sources, either within the so-called Arc of non-thermal filaments (\cite{po97}), or as a result of the explosive event forming the large supernova-like remnant Sgr A East (\cite{yz97}). (A schematic diagram of the morphology of the central parsecs is shown in Fig. 1 below.) Markoff, Melia \& Sarcevic (1997) also considered in detail a possible black hole origin for the $\gamma$-rays under the assumption that the ultimate source of power for this high-energy emission may be accretion onto the central engine. They concluded that it is not yet possible to rule out Sgr A* (which appears to be coincident with the central dark mass concentration) as one of the possible sources for this radiation, and that the expected spectrum is a good match to the observations. The lack of variability larger than $\sim 20\%$ in the high-energy flux would then be consistent with the maximum amplitude of the turbulent cell fluctuations seen in three-dimensional hydrodynamical simulations of accretion onto Sgr A* (\cite{ruf94}; \cite{cm97}). It appears that a true test of Sgr A* as the source for the EGRET emission would be the detection (or non-detection) of variability with an amplitude significantly smaller than this. To answer the question of whether or not 2EGJ1746-2852 is coincident with Sgr A*, it is essential to fully understand the alternative contributions to the high-energy flux from the Galactic Center. The unique environment in this region facilitates the co-existence of thermal and non-thermal particles, which can lead to interactions that produce a substantial diffuse Compton upscattering emissivity. There is now considerable evidence that the radio spectrum of Sgr A East and the Halo is likely synchrotron radiation by nonthermal particles at high energy (Pedlar, et al. 1989). However, this region is also bathed with an intense ultraviolet (UV) and infrared (IR) photon field from the central $1-2$ parsecs and these particles must therefore be subjected to numerous Compton scattering events. Our focus in this {\it Letter} is to see whether the properties of this relativistic electron distribution, inferred from their observed radio characteristics, also make them a viable source for the $\gamma$-rays detected by EGRET. This is particularly important in view of the fact that it may be possible to distinguish between Sgr A* and an extended $\gamma$-ray source with high-resolution $\gamma$-ray (or hard X-ray) imaging. For example, the proposed balloon flight instrument UNEX (Rothschild 1998) may have sufficient sensitivity to image the hard X-ray counterpart to 2EGJ1746-2852. \section{Sgr A East, the Halo and the Galactic Center Radiation Field} Radio continuum observations of the Galactic center show a prominent nonthermal radio continuum shell-like structure, Sgr A East, as well as thermal ionized gas, known as Sgr A West, orbiting Sgr A$^*$ (Ekers, et al. 1983; Pedlar, et al. 1989; Serabyn et al. 1991). The latter two are themselves surrounded by a torus of dust and molecular gas known as the Circumnuclear Disk (CND). Figure 1 shows a schematic diagram of Sgr A East, its Halo, and their location relative to the black hole candidate Sgr A*, centered within the CND. Low-frequency continuum observations show a deep depression in the brightness of the Sgr A East shell at the position of Sgr A West, which results from free-free absorption of the radiation from Sgr A East by the thermal gas in Sgr A West. Sgr A East must therefore lie behind Sgr A West (Yusef-Zadeh \& Morris 1987; Pedlar et al. 1989). The exact distance of Sgr A East behind Sgr A West is not known, but a number of arguments suggest that it is located very close to the Galactic Center (e.g., G\"usten \& Downes 1980; Goss et al. 1989). On a larger scale, there is a diffuse $7^{\prime}-10^{\prime}$ Halo of nonthermal continuum emission surrounding the oval-shaped radio structure Sgr A East. Assuming a power-law distribution of relativistic particles, the energy spectrum of the relativistic electrons within the shell and the Halo are estimated to be $\sim 2.5-3.3$ and $\sim 2.4$, respectively (Pedlar et al. 1989). The Halo may be a secondary manifestation of the explosion that produced Sgr A East. However, the fact that the particle spectral index is steeper in the latter suggests that significant cooling of its relativistic particles may already have taken place which may not be consistent with a model in which the cosmic-ray electrons leak through the shell and produce the extended Halo radio emission. Thus, the Halo may be unrelated to the creation of Sgr A East, as Pedlar et al. (1989) have suggested. It may instead be associated with continued activity at the Galactic Center, possibly from the expansion of relativistic particles that are not confined by Sgr A*. This may also be taken as indirect evidence that the Halo, unlike Sgr A East, may be centered on Sgr A* (see Fig. 1). In either case, what is of interest to us here is the indication from radio observations of the presence of these power-law particle distributions in the extended region surrounding Sgr A*. The Compton spectrum from a lepton distribution with index $p \equiv 2.4-3.3$ is expected to have a spectral index $\alpha\sim (1+p)/2\approx 1.7-2.2$, close to that of 2EGJ1746-2852. The optical depth toward Sgr A East and the Halo at low frequencies led Pedlar et al. (1989) to consider a mixture of both thermal and nonthermal gas, though displaced to the front side of Sgr A East. Pedlar et al. (1989) also showed evidence that the nonthermal emission from the Halo is located in front of the thermal gas in Sgr A West. The schematic diagram in Figure 1 depicts a geometry in which the Sgr A East shell lies close to, but behind, the Galactic Center whereas the diffuse Sgr A East Halo surrounds the Galactic Center and the shell. \begin{figure}[H] % fig 1 \centerline{\begin{turn}{0}\epsfig{file=fig1.eps,width=4.0in}\end{turn}} \vspace{10pt} \caption{Schematic diagram showing the relative positions and sizes of the Halo and Sgr A East relative to Sgr A*, which is shown here as a point centered within the CND. The thermal 3-arm spiral radio source Sgr A West is also contained within the CND.} \label{fig1} \end{figure} At $\lambda 20$ cm, Sgr A East and the Halo are among the brightest radio sources in the sky, with integrated flux densities of $222$ and $247$ Jy, respectively (Pedlar, et al. 1989). Their average brightness is about 900 and 350 mJy per 12" beam, respectively, and in order to fit the radio spectrum, the maximum particle Lorentz factor in these sources should be $\gamma_{max}\sim 2\times 10^5$. Thus, with a minimum Lorentz factor $\gamma_{min}\sim 6,000$ (see discussion in the following section), the total number of radiating electrons is \begin{equation}\label{Nhalo} N_0(\hbox{Halo})\approx 1.0\times 10^{52}\;\left({10^{-5}\;\hbox{G}\over B\sin\theta}\right)^{1.7}\;, \end{equation} and \begin{equation}\label{Nsgrae} N_0(\hbox{Sgr A East Shell})\approx 4.2\times 10^{52}\;\left({10^{-5}\;\hbox{G}\over B\sin\theta}\right)^{2.15}\;, \end{equation} where $B$ is the magnetic field and $\theta$ is the pitch angle. The Halo particles are assumed to be distributed uniformly throughout its volume. On the other hand, the Synchrotron emission from Sgr A East is concentrated within a shell with thickness $d\sim 1$ pc (e.g., Pedlar, et al. 1989). However, the gyration radius $a_{gyr}$ within this region with $B\sim 10^{-5}$ G is $\sim 3\times 10^{13}$ cm for the most energetic particles ($\gamma_{max}= 2\times 10^5$), and so the diffusion time out of the shell is expected to be $\sim (d/c)(d/a_{gyr})\approx 3\times 10^5$ years, much longer than the $\tau_{age}\sim 10,000$ year lifetime of the remnant. So most of the Compton scatterings associated with Sgr A East are expected to occur within its shell. These relativistic particles are immersed in an intense source of UV and IR radiation from the central $1-2$ parsecs. The ring of molecular gas (also known as the Circumnuclear Disk, or CND) is rotating, and is heated by a centrally concentrated source of UV radiation (predominantly the IRS 16 cluster of bright, blue stars). The CND encloses a central concentration of dark matter, which is believed to be a $\sim$ 2.6$\times$10$^{6}$ solar mass black hole (e.g., Haller et al. 1996; Genzel et al. 1996). The CND is a powerful source ($\approx 10^7\:L_\odot$) of mid to far-infrared continuum emission with a dust temperature of $\approx 100$ K (e.g., Telesco et al. 1996; Davidson et al. 1992). This radiation is due to reprocessing by warm dust that has absorbed the same power in the UV (Becklin, Gatley and Werner 1982; Davidson et al. 1992). Models of the photodissociation regions in the CND require an incident dissociating flux ($6$ eV $< h\nu < 13.6$ eV) of $10^2$--$10^3$ erg cm$^{-2}$ s$^{-1}$ (Wolfire, Tielens \& Hollenbach, 1990), implying a total UV luminosity of about $2\times 10^7\;L_\odot$, consistent with the radio continuum emission from Sgr A West (Genzel et al. 1985). This intensity is also suggested by the detection of radio continuum emission from the outer envelope of IRS 7, a cool supergiant being photoionized by the UV radiation field (e.g., Serabyn et al. 1991; Yusef-Zadeh and Melia 1992), and is consistent with the inferred ionizing flux in Sgr A West, corresponding to a centrally concentrated source of $2\times 10^{50}$ ionizing photons per second (Lacy, et al. 1982; Ekers, et al. 1983; Mezger and Wink 1986). \section{Inverse Compton Scattering within Sgr A East and the Halo} The dominant cooling mechanism for the relativistic electrons as they diffuse throughout the Sgr A East and Halo regions is inverse Compton scatterings with the Galactic Center stellar UV photons and the reprocessed IR photons from the CND. This radiation field has a specific photon number density per solid angle $n_{ph}^{tot}(\varepsilon)\equiv n_{ph}^{UV}(\varepsilon)+n_{ph}^{IR}(\varepsilon)$, where $n_{ph}^{UV}(\varepsilon) = N_0^{UV}(2\varepsilon^2/h^3 c^3) (\exp\{\varepsilon/kT^{UV}\}-1)^{-1}$, and $n_{ph}^{IR}(\varepsilon) = N_0^{IR}(2\varepsilon^3/h^3 c^3)(\exp\{\varepsilon/kT^{IR}\}-1)^{-1}$. Here, $\varepsilon$ is the lab-frame photon energy and $T^{UV}$ and $T^{IR}$ are, respectively, the temperature (assumed to be $30,000$ K) of the stellar UV component and of the reprocessed CND radiation, which is assumed to peak at $50\mu$m, corresponding to a characteristic temperature $T^{IR}\approx 100$ K. Note that these expressions take into account the energy dependence of the efficiency factor for dust emission, which leads to a modified blackbody form for the dust spectrum. The normalization constants $N_0^{UV}$ and $N_0^{IR}$ incorporate the dilution in photon number density as the radiation propagates outwards from the central core. For the UV radiation, this is calculated assuming that the radiation emanates from a sphere of radius $\approx 1$ pc, whereas for the IR radiation, we assume a total luminosity of $10^7\,L_\odot$ from a disk with radius $\approx 2$ pc. In the following expressions, primed quantities denote values in the electron rest frame, whereas unprimed parameters pertain to the (stationary) lab frame. An electron moving with Lorentz factor $\gamma$ through this field scatters $dN$ photons to energies between $\varepsilon_s$ and $\varepsilon_s + d\varepsilon_s$ and solid angles between $\mu_s \phi_s$ and $[\mu_s + d\mu_s][\phi_s + d\phi_s]$ at a rate (per energy per solid angle) \begin{equation}\label{dN} {dN\over dt d\varepsilon_s d\mu_s d\phi_s} = \int_{ph} d\varepsilon \;\int\;d\mu\,d\phi\; n_{ph}^{tot}(\varepsilon,\Omega)\; \left({d\sigma_{KN}\over d\mu_s' d\phi_s' d\varepsilon_s'}\right) \;{c (1-\beta\mu)\over \gamma(1-\beta\mu_s)}\;, \end{equation} where $\beta = (1-\gamma^{-2})^{-1/2}$, $\mu$ is the cosine of $\theta$ relative to the electron's direction of motion, and $\phi$ is the azimuthal angle that completes the integration over all solid angles. The differential Klein-Nishina cross-section ${d\sigma_{KN}/ d\mu_s' d\phi_s' d\varepsilon_s'}$ is evaluated in the electron rest frame. Using the general expressions relating the lab and rest frame energies ($\varepsilon' = \varepsilon\gamma[1-\beta\mu]$) and angles ($\mu' = [\mu-\beta]/[1-\beta\mu]; \phi'=\phi$), one finds the relation $d\varepsilon_s d\mu_s d\phi_s / d\varepsilon_s' d\mu_s' d\phi_s' = \gamma (1-\beta\mu_s)$, thereby allowing Equation (3) to be easily integrated over all scattered photon energies and solid angles to yield the single electron scattering rate. The inverse Compton (X-ray and $\gamma$-ray) emissivity can be determined by integrating Equation (3) over the entire scattering electron population. For simplicity, we assume that the electron distribution is locally isotropic, which then also implies that the upscattered radiation field is emitted isotropically from within a volume $V\sim 250$ pc$^3$ in the case of the Sgr A East shell and $\sim 2,500$ pc$^3$ for the Halo, and corresponding surface area $4\pi R^2$, where $R\approx 5$ pc for the former and $\approx 8.5$ pc for the latter (see Fig. 1). Thus, the rate at which photons are detected by an observer at a distance $D$ is given by the expression \begin{equation}\label{dNtot} {dN_{obs}\over dt d\varepsilon_s dA} = {1\over 2D^2}\; \int_{V}d^3x\;\int_{\gamma_{min}}^{\gamma_{max}}\; d\gamma \;\int_{-1}^1 d\mu_s\; n_e(\gamma)\; {dN\over dt d\varepsilon_s d\mu_s d\phi_s}\;, \end{equation} where azimuthal symmetry has been invoked to eliminate the need to average over $\phi_s$. Note that the integral over $\phi$ is still within ${dN/ dt d\varepsilon_s d\mu_s d\phi_s}$. This expression also uses the relationship between the scattered photon solid angle $d\mu_s \;d\phi_s$ and the detector area $dA$ ($dA = D^2 d\mu_s d\phi_s$). \section{Results and Discussion} The main results of our calculations are summarized in Figures 2 and 3, which show, respectively, the cumulative spectra from Sgr A East and the Halo. Both sources appear to account well for the high-energy spectrum of the Galactic center, though their actual $\gamma$-ray fluxes depend on the magnetic field intensity within each of the structures. The value of $B$ assumed for both of these figures is $1.8\times 10^{-5}$ G, which then fixes the relativistic particle numbers quoted there (see also eqs. 1 and 2). \begin{figure}[H] % fig 2 \centerline{\begin{turn}{0}\epsfig{file=fig2.ps,width=3.3in}\end{turn}} \vspace{10pt} \caption{Combined spectrum from inverse Compton scattering within Sgr A East. The components shown here are: the upscattered IR, and the upscattered UV. The cumulative spectrum is shown as a thin solid curve. The EGRET data are from Mayer-Hasselwander, et al. (1998).} \label{fig2} \end{figure} The spectral turnover below $\sim 1$ GeV is difficult to produce with Compton scatterings without a low-energy cutoff in the particle distribution. Unlike the situation where the $\gamma$-rays result from pion decays, in which this turnover is associated with the pion rest mass (Markoff, Melia \& Sarcevic 1997), there is no natural characteristic cutoff energy here. To match the data, we have adopted a minimum Lorentz factor $\gamma_{min}\approx 6,000$, but we do not yet have a compelling argument for this value, though we can offer the following suggestion. If the protons and electrons continue to interact after they leave the shock acceleration region, either Coulomb scatterings or a charge-separation electric field can gradually shift the overall electron distribution to a higher Lorentz factor due to the mass differential between the two sets of particles. If the electrons and protons are energized more or less equally, then in a neutral plasma the lepton $\gamma_{min}$ must be much greater than $1$. For a proton index $\alpha_p$ and an electron index $\alpha_e$, it is then evident that $\gamma_{min}\approx [(\alpha_p-1)/(\alpha_p-2)] \times [(\alpha_e-2)/(\alpha_e-1)]\times (m_p/m_e)$. In this context, a $\gamma_{min}\approx 6,000$ for the electrons may therefore reflect the difference in particle mass and the relativistic distribution indices. For this to work, the energy equilibration would have to occur {\it in situ}, perhaps due to a uniform acceleration of the relativistic electrons by a charge separation-induced electric field, as mentioned previously. Clearly, if either Sgr A East and/or the Halo turn out to be the source of $\gamma$-rays, this explanation (or a viable alternative) must be developed more fully. \begin{figure}[H] % fig 3 \centerline{\begin{turn}{0}\epsfig{file=fig3.ps,width=3.3in}\end{turn}} \vspace{10pt} \caption{Same as Fig. 2, except here for the Halo. This fit assumes the same value of $B$ ($\sim 1.8\times 10^{-5}$ G) required to fit the spectrum with Sgr A East's emissivity, which then gives a Halo relativistic particle number $N_e=4\times 10^{51}$ as indicated.} \label{fig3} \end{figure} If the $\gamma$-rays detected by EGRET are indeed the upscattered UV photons from the Galactic center, it seems inevitable to us that the corresponding IR photons from the CND should result in a significant upscattered intensity at intermediate (i.e., $\sim 10-100$ keV) energies. This flux density ($\sim 10^{-5}$ photons cm$^{-2}$ s$^{-1}$ MeV$^{-1}$, or possibly higher if $\gamma_{min}<6,000$) may be above the sensitivity limit (which is $\sim 10^{-7}$ photons cm$^{-2}$ s$^{-1}$ MeV$^{-1}$ for a point source) of UNEX, a proposed balloon flight instrument (Rothschild 1998). With its expected spatial resolution of several arcmin or less, this experiment should therefore have little trouble imaging the hard X-ray counterpart to 2EGJ1746-2852, if this source is extended and is associated with either Sgr A East and/or the Halo. \section{Acknowledgments} This work was supported by NASA grant NAGW-2518. We are grateful to the anonymous referee, whose comments have led to a significant improvement of our paper. % That's the end of the main body of the paper. Now we will have some % back matter. % % Now comes the reference list. In this document, we used \cite to call % out citations, so we must use \bibitem in the reference list, which % means we use the LaTeX thebibliography environment. Please note that % \begin{thebibliography} is followed by a null argument. If you forget % this, mayhem ensues, and LaTeX will say "Perhaps a missing item?" when % you run it. Do not call us, do not send mail when this happens. Put % the silly {} after the \begin{thebibliography}. % % Each reference has a \bibitem command to define the citation format % to be placed in the text (in []) and the symbolic tag used for % cross referencing (in {}). % % See sample1.tex, or the AASTeX guide, for an alternative to the \cite- % \bibitem command. \begin{thebibliography}{} \bibitem[Becklin, Gatley \& Werner 1982]{beck82}\apj{Becklin, E.E., Gatley, I. \& Werner, M.W. 1982}{258}{134} \bibitem[Coker \& Melia 1997]{cm97}\apjlett{Coker, R.F. \& Melia, F. 1997}{488L}{149}. \bibitem[Davidson et al. 1992]{dav92}\apj{Davidson, J.A., Werner, M.W., Wu, X., Lester, D.F., Harvey, P.M., Joy, M. \& Morris, M. 1992}{387}{189} \bibitem[Ekers, et al. 1983]{ek83}\aa{Ekers, R.D., Van Gorkom, J.H., Schwarz, U.J. \& Goss, W.M. 1983}{122}{143} \bibitem[Genzel, et al. 1985]{gen85}\apj{Genzel, R., Crawford, M.K., Townes, C.H. \& Watson, D.M. 1985}{297}{766} \bibitem[Genzel, et al. 1996]{gen96}\apj{Genzel, R., et al. 1996}{472}{153} \bibitem[Goss, et al. 1989]{goss89}\refindent Goss, M. et al. 1989, The center of the Galaxy, IAU 136 ed. M. Morris, p345. \bibitem[G\"usten \& Downes 1980]{gd80}\aa{G\"usten, R. \& Downes, D. 1980}{87}{6} \bibitem[Haller, et al. 1996]{hall96}\apj{Haller, J.W., Rieke, M.J., Rieke, G.H., Tamblyn, P., Close, L. \& Melia, F. 1996}{456}{194} \bibitem[Lacy, Townes \& Hollenback 1982]{lac82}\apj{Lacy, J.H., Townes, C.H. \& Hollenbach, D.J. 1982}{262}{120} \bibitem[Markoff, Melia \& Sarcevic 1997]{mar97}\apjlett {Markoff, S., Melia, F. \& Sarcevic, I. 1997}{489L}{47} (Paper I) \bibitem[Mayer-Hasselwander, et al. 1998]{mayer98}\refindent Mayer-Hasselwander, H.A., et al. 1998, A\&A, in press. \bibitem[Mezger \& Wink]{mez86}\aa{Mezger, P.G. \& Wink, J.E. 1986}{157}{252} \bibitem[Pedlar, et al. 1989]{ped89}\apj{Pedlar, A., et al. 1989} {342}{769} \bibitem[Pohl 1997]{po97}\aa{Pohl, M. 1997}{317}{441}. \bibitem[Rothschild 1998]{roth98}\refindent Rothschild, R. 1998, private communication. \bibitem[Ruffert \& Melia 1994]{ruf94}\aa{Ruffert, M. \& Melia, F. 1994} {288}{L29}. \bibitem[Serabyn, Lacy \& Achtermann 1991]{sera91}\apj{Serabyn, E., Lacy, J.H. \& Achtermann, J.M. 1991}{378}{557} \bibitem[Telesco, et al. 1996]{tel96}\apj{Telesco, C.M., Davidson, J.A. \& Werner, M.W. 1996}{456}{541} \bibitem[Wolfire, Tielens \& Hollenback 1990]{wolf90}\apj{Wolfire, M.G., Tielens, A. \& Hollenbach, D. 1990}{358}{116} \bibitem[Yusef-Zadeh \& Melia 1992]{ym92}\apjlett{Yusef-Zadeh, F. \& Melia, F. 1992} {385}{41L} \bibitem[Yusef-Zadeh \& Morris 1987]{ym87}\apj{Yusef-Zadeh, F. \& Morris, M. 1987}{320}{545} \bibitem[Yusef-Zadeh et al. 1997]{yz97}\refindent Yusef-Zadeh, F., Purcell, W., \& Gotthelf, E. 1997, Proceedings of the Fourth Compton Symposium, (New York: AIP), 1027. \bigskip\bigskip \end{thebibliography}{} \end{document} Fulvio Melia The University of Arizona ------------------------------------------------ |Department of Physics |Department of Astronomy | |Rm 447, PAS #81 |N312 | |(520) 621-9651 |(520) 621-5698 | ------------------------------------------------ http://www.physics.arizona.edu/~melia ------------- End Forwarded Message -------------