1 Introduction

The energetic radio source SgrA^* located at the center of our Galaxy is now widely believed to be the signature of a massive black hole with mass $M=2.6 \times 10^6~M_{\odot}$ (Melia & Falcke 2001; Haller et al. 1996; Eckart & Genzel 1996; Ghez et al. 1998; Reid et al. 1999; Backer & Sramek 1999). Its radio spectrum seems to consist of two components, with a break around $\sim$50 GHz. The spectral dependence is $F_{\nu} \propto \nu^{0.2}$ for $\nu < 50$ GHz, while above this break there is a submm bump which is described by $F_{\nu} \propto \nu^{0.8}$ up to $\sim$10³ GHz followed by a steep cut-off towards the infrared (IR) (Zylka et al. 1992; Serabyn et al. 1997; Falcke et al. 1998). The upper limits from IR (Menten et al. 1997) and ROSAT X-ray observations (Predehl & Trümper 1994) indicate that this source is quite dim.

On the theoretical side, a number of models have been proposed in the past years for SgrA^*. Most models are based on accretion onto the central massive black hole. Possible sources of accretion material include the stellar winds emitted by the nearby massive stars and the hot interstellar medium. Since in either case the angular momentum of the accretion flow should be small, Melia (1992, 1994) proposed a spherical accretion model. In this model the accretion flow is assumed to free-fall until a Keplerian disk is formed within a small "circularization'' radius. The main contributors to the radio and X-ray spectra are synchrotron radiation and bremsstrahlung, respectively, from the roughly free-fall flow beyond the small disk. However, spherical accretion is likely to be an over-simplification, since the accretion flow still possesses some angular momentum. An advection-dominated accretion flow (ADAF) model therefore is more dynamically exact in this sense (Narayan et al. 1995; Manmoto et al. 1997; Narayan et al. 1998). The most attractive feature of the ADAF model is its ability to explain the unusual low-luminosity of SgrA^* given the relatively abundant accretion material. This is because most of the viscously dissipated energy is stored in the flow and advected beyond the event horizon rather than radiated away (Ichimaru 1977; Rees et al. 1982; Narayan & Yi 1994, 1995; Abramowicz et al. 1995; Chen et al. 1995; Narayan et al. 1997; Chen et al. 1997). In the application to SgrA^*, the radio spectrum is produced by the synchrotron process in the innermost region of the disk while the X-rays are due to bremsstrahlung radiation of the thermal electrons in a large range of radii $\sim$ $10^3{-}10^4~R_{\rm s}$, where $R_{\rm s} =2 GM/c^2$ is the Schwarzschild radius. However, the ADAF under-predicts the low-frequency radio emission of SgrA^* by over an order of magnitude and additional assumptions must be imposed in order to match the spectrum (Mahadevan 1998; Özel et al. 2000).

Following the initial paper by Reynolds & McKee (1980) (see also Blandford & Königl 1979), Falcke et al. (1993) proposed that it is the jet stemming from the disk rather than the disk itself which is responsible for the radio spectrum of SgrA^*. In this model, the submm bump is produced by the acceleration zone of the jet, called nozzle, while the low-frequency radio spectrum comes from the part of the jet beyond the nozzle (Falcke 1996b; Falcke & Biermann 1999). The nozzle is of order 10 $R_{\rm s}$ and forms from the disk at a radius of $\sim$ $2~R_{\rm s}$. This model gives an excellent fit to the radio spectrum of SgrA^*, including the low-frequency spectrum below the break and the submm bump, but the expected X-ray emission was not calculated explicitly.

The latest observational constraints for SgrA^* come from the high spatial resolution ($\approx$ $1\hbox{$^{\prime\prime}$ }$) Chandra X-ray Observatory (Baganoff et al. 2001a, 2001b). Baganoff et al. observed SgrA^*twice and they found that SgrA^* comes in two states: quiescent and flares. In the present paper we concentrate on the quiescent state, whereas the flare state is considered in Markoff et al. (2001b). The main observational results for the quiescent state are summarized as follows:

• The absorption-corrected 2-10 keV luminosity is (2.2 ^+0.4_-0.2) $\times10^{33}~{\rm erg ~s}^{-1}$;
• The spectrum is well fitted by an absorbed power-law model with photon index $\Gamma=2.2^{+0.5}_{-0.7}$;
• The inner region of the source is rapidly variable on short timescale of $\simeq$ $1~{\rm hr}$. A rapid drop of flux on a timescale of 10 min is detected in the flare state. On the other hand, the comparison between the two observations with an interval of about one year indicates that the steady X-ray flux remains almost constant;
• Some fraction of the X-ray flux may come from a partly extended region with diameter $\approx$ $1\hbox{$^{\prime\prime}$ }$;
• There is tentative evidence for a FeK$\alpha$ line at 6.7 keV.

These results provide new and strict constraints to the theoretical models for SgrA^*. In both the ADAF and spherical accretion models mentioned above, the X-ray radiation is produced by bremsstrahlung originating from 10³- $10^4~R_{\rm s}$. Hence the spectrum is very hard with photon index $\Gamma \sim 1.4$and the predicted variability timescale is thousands of hours, much longer than the observed $\sim$1 hour.

Therefore it is necessary to reexamine the theoretical models for SgrA^*. Melia et al. (2001) proposed that the electrons in the small Keplerian disk can attain a very high temperature through some magnetic processes, and the resulting synchrotron and self-Compton emission are responsible for the radio and X-ray spectrum. However, the formation of the small disk may not be a necessary result of such low angular momentum accretion. An accretion flow with very low angular momentum can still be described by an ADAF, although such accretion may belong to the Bondi-like type rather than disk-like type, as shown by Yuan (1999) (see also Abramowicz & Zurek 1981; Abramowicz 1998). Thus the dynamical scenario of this model needs to be studied carefully.

For the jet model, Falcke & Markoff (2000) take into account the contribution from synchrotron self-Compton emission (SSC) in the nozzle and find that the parameters required to interpret the submm bump give a very good fit to the Chandra spectrum without changing the basic parameters of the jet model. But the remaining important problem in the model is why the parameters of the jet possess the required values, particularly in reference to the inferred underlying accretion disk. Previous ideas of a standard optically thick accretion disk in SgrA^* (e.g., Falcke & Heinrich 1994) do not seem to work because the predicted IR flux from a standard thin disk with a reasonable accretion rate would be several orders of magnitude higher than the observed IR upper limit (Falcke & Melia 1997). Therefore, it is crucial to consider the jet and accretion flow as a coupled system in SgrA^*, and to consider what are their respective roles if both are truly present in SgrA^*. Yuan (2000) presented the first effort, by considering a combination of jet and ADAF models. However, the complete Chandra data was not available at that time and the detailed coupling mechanism was lacking in Yuan (2000) so it is necessary to revisit the model again.

The development of the theory provides a new chance to model SgrA^*. Since the Bernoulli parameter of the ADAF is positive, which means the gas can escape to infinity with positive energy, Blandford & Begelman (1999) propose an advection-dominated inflow-outflow solution (ADIOS) in which most of the gas is lost through winds rather than accreted past the horizon of the black hole. The concept of strong winds from accretion flow was also proposed and studied by Xu & Chen (1997) and Das & Chakrabarti (1999). The latter described pressure-driven winds from centrifugally supported boundary layers and shocks in the inner regions of disks, and the former proposed an advection-dominated flow where the central black hole redirects the inward flow at low latitudes into an outflow at high latitudes. We are not explicitly making use of the latter two models. The most appealing point of the ADIOS model as applied to SgrA^* is that the predicted X-ray spectrum is possibly much softer than that of the ADAF (Quataert & Narayan 1999), and therefore could possibly give a better fit to the Chandra data. This is because the density profile of the accretion flow becomes flatter due to the wind, while X-ray emission at higher frequencies is produced in the inner region of the accretion flow. If we assume that the mass accretion rate in the ADIOS is described by a power-law of radius, $\dot{M}\propto R^{p}$, the predicted photon index in Chandra band is approximately $\Gamma \approx 3/2+2p$. Thus it is necessary to investigate this model for the possibility of interpreting the Chandra results.

In this paper we explore several of the above-mentioned models for SgrA^*. By probing a larger parameter space than before, we find that ADAFs can give a marginal interpretation to the new Chandra results, although the fit is not very good in some points (Sect. 2), while the ADIOS model can't (Sect. 3). In Sect. 4 we propose that the combination of an ADAF and a jet could provide an excellent fit to the observations to SgrA^*, and present our model results. The last section is a summary and discussion.