------------------------------------------------------------------------ SgrA* paper to appear in the ApJ Letter Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Content-MD5: AmzIw6QQqWiYRdZxLvTe4g== Content-Length: 31596 %astro-ph/0302062 \documentclass[12pt,preprint]{aastex} %% %% Beginning of file 'sample.tex' %% %% Modified 03 Nov 99 %% %% This is a sample manuscript marked up using the %% AASTeX v5.0 LaTeX 2e macros. %% The first piece of markup in an AASTeX v5.0 document %% is the \documentclass command. LaTeX will ignore %% any data that comes before this command. %% The command below calls the default manuscript style, %% which will produce a double-spaced document on one column. %% Examples of commands for other substyles follow. Use %% whichever is most appropriate for your purposes. %\documentclass{aastex} %% preprint produces a one-column, single-spaced document: %% preprint2 produces a double-column, single-spaced document: %\documentclass[preprint2]{aastex} %% If you want to create your own macros, you can do so %% using \newcommand. Your macros should appear before %% the \begin{document} command. %% %% If you are submitting to a journal that translates manuscripts %% into SGML, you need to follow certain guidelines when preparing %% your macros. See the AASTeX v5.0 Author Guide %% for information. \newcommand{\vdag}{(v)^\dagger} \newcommand{\myemail}{skywalker@galaxy.far.far.away} %% You can insert a short comment on the title page using the command below. %\slugcomment{Submitted to ApJ Letter} \def\etal{{\it et al.~}} \def\ds{\displaystyle} %% If you wish, you may supply running head information, although %% this information may be modified by the editorial offices. %% The left head contains a list of authors, %% usually a maximum of three (otherwise use et al.). The right %% head is a modified title of up to roughly 44 characters. Running heads %% will not print in the manuscript style. \shorttitle{Variability of Sagittarius A*} \shortauthors{Zhao \etal} %% This is the end of the preamble. Indicate the beginning of the %% paper itself with \begin{document}. %\received{2002 October 2} \begin{document} %% LaTeX will automatically break titles if they run longer than %% one line. However, you may use \\ to force a line break if %% you desire. \title{Variability of Sagittarius A* - Flares at 1 Millimeter} %% Use \author, \affil, and the \and command to format %% author and affiliation information. %% Note that \email has replaced the old \authoremail command %% from AASTeX v4.0. You can use \email to mark an email address %% anywhere in the paper, not just in the front matter. %% As in the title, you can use \\ to force line breaks. \author{Jun-Hui Zhao 1, K. H. Young 1, R. M. Herrnstein 1, P. T. P. Ho 1, T. Tsutsumi 1, K. Y. Lo 2, W. M. Goss 3, and G. C. Bower 4} \altaffiltext 1 {Harvard-Smithsonian CfA, 60 Garden St, MS 78, Cambridge, MA 02138; jzhao@cfa.harvard.edu, kyoung@cfa.harvard.edu, rmcgary@cfa.harvard.edu, pho@cfa.harvard.edu, ttsutsumi@cfa.harvard.edu} \altaffiltext 2 {Academia Sinica Institute of Astronomy \& Astrophysics, Taipei, Taiwan;flo@nrao.edu} \altaffiltext 3 {NRAO-AOC, P. O. Box 0, Socorro, NM 87801; mgoss@aoc.nrao.edu} \altaffiltext 4 {University of California, Berkeley, CA94720; gbower@astron.berkeley.edu} %% Notice that each of these authors has alternate affiliations, which %% are identified by the \altaffilmark after each name. Specify alternate %% affiliation information with \altaffiltext, with one command per each %% affiliation. %% Mark off your abstract in the ``abstract'' environment. In the manuscript %% style, abstract will output a Received/Accepted line after the %% title and affiliation information. No date will appear since the author %% does not have this information. The dates will be filled in by the %% editorial office after submission. \begin{abstract} We report the results from recent observations of Sgr~A* at short-/sub-millimeter wavelengths made with the partially finished Sub-Millimeter Array (SMA) on Mauna Kea. A total of 25 epochs of observations were carried out over the past 15 months in 2001 March to 2002 May. Noticeable variations in flux density at 1.3 mm were observed showing three ``flares''. The SMA observations suggest that Sgr~A* highly increases towards submillimeter wavelengths during a flare suggesting the presence of a break wavelength in spectral index around 3 mm. A cross-correlation of the SMA data at 1 mm with the VLA data at 1 cm show a global delay of ${\rm t_{delay}> 3 d}$, suggesting that sub-millimeter wavelengths tend to peak first. Only marginal day-to-day variations in flux density (2-3$\sigma$) have been detected at 1.3 mm. No significant flares on a short time scale ($\sim1$ hr) have been observed at 1.3 mm. We also failed to detect significant periodic signals at a level of 5$\%$ (3$\sigma$) from Sgr A* in a periodic searching window ranging from 10 min to 2.5 hr. The flares observed at the wavelengths between short-centimeter and sub-millimeter might be a result of collective mass ejections associated with X-ray flares that originate from the inner region of the accretion disk near the supermassive black hole. \end{abstract} %% Keywords should appear after the \end{abstract} command. The uncommented %% example has been keyed in ApJ style. See the instructions to authors %% for the journal to which you are submitting your paper to determine %% what keyword punctuation is appropriate. \keywords{Galaxy:center --- accretion, accretion disks --- galaxies:active --- radio continuum: galaxies --- black hole physics} %% From the front matter, we move on to the body of the paper. %% In the first two sections, notice the use of the natbib \citep %% and \citet commands to identify citations. The citations are %% tied to the reference list via symbolic KEYs. The KEY corresponds %% to the KEY in the \bibitem in the reference list below. We have %% chosen the first three characters of the first author's name plus %% the last two numeral of the year of publication as our KEY for %% each reference. \section{Introduction} Sgr~A*, a compact radio source, is believed to be associated with the supermassive black hole at the Galactic center (Eckart \etal, 2002; Ghez \etal, 2000). The inferred bolometric luminosity (L$\sim10^{-8.5}$ L$_{\rm Edd}$) is far below the Eddington luminosity for the black hole mass of $\sim$2.6$\times10^6$ M$_\odot$. Sgr~A* represents an extremely dim galactic nucleus. The low luminosity of Sgr A* is explained by the low efficiency radiative advection dominated accretion flow (ADAF) model (Narayan \etal~1998). However, the ADAF alone can not explain the detailed activities, such as the radio outbursts and the X-ray flares. The recent work by Yuan \etal~(2002a) provides an alternative model to link a jet outflow (Falcke \etal~1993) with the ADAF. This new model appears to be able to fit reasonably well the overall spectrum of Sgr~A* from radio, sub-millimeter, IR to the X-ray. The apparent structure at radio wavelengths longer than 3 mm appears to be mainly dominated by the scattering effects due to the ISM. At 3 mm, the scattering effects are finally small enough that the intrinsic source size is estimated to be less than 0.27 mas (Doeleman \etal~2001), or about 40 times the Schwarzchild radius (${\rm R_{sc}}$ hereafter). A promising way to explore this extremely compact source is to monitor the variations of the emitted flux density from radio to X-ray. The variations in radio flux density of Sgr~A* have been known for two decades (Brown \& Lo 1982). The nature of the radio variability has not been well understood. At the long wavelengths, the flux density of Sgr~A* might be modulated by the scintillation due to the turbulence in the ISM (Zhao \etal~1989). The radio light curves observed with the VLA at wavelengths from 20 to 1.3 cm during the period of 1990-1993 suggest that the fractional amplitude variations increased towards short wavelengths and that the rate of radio flares appeared to be about three per year (Zhao {\it et al.} 1992; and Zhao \& Goss 1993). The typical time scale of these radio flares is about a month. The observed large amplitude variations in flux densities at 3 mm (Wright \& Backer 1993; Tsuboi \etal~1999) are consistent with the wavelength-dependence of the variability as observed at centimeter wavelengths. The presence of a 106 day cycle in the radio variability of Sgr A$^*$ was suggested from an analysis of data observed with the VLA in the period of 1977-1999 (Zhao \etal~2001). The periodic oscillation at a period around 100 days appears to persist in the densely sampled light curves obtained with the VLA at 2, 1.3 and 0.7 cm over the past two years (Zhao \etal~2002). The period of the fluctuation cycle appears to increase to $\sim$ 130 days (Bower \etal~2002a; Zhao \etal~2002). In addition, a longer periodic fluctuation feature with a period 2.4-2.5 times the short one has also been seen (Zhao \etal~2002). Similar periodic fluctuation in flux density was also observed in the GBI monitoring data (Falcke, 1999). Observations of Sgr A* at sub-millimeter can penetrate into the deep region of this intriguing source. In this letter, we report results obtained from a monitoring program at 1 mm, with the partially finished Sub-Millimeter Array (hereafter SMA; Moran 1998). \section{Observations \& Data Reduction} Observations of Sgr~A* at 1.3 mm and 0.87 mm were made using the partially completed SMA with three or four antennas and baselines ranging from 7 to 55 kilo wavelengths at 1.3 mm. A total of 24 epochs of observations at 1.3 mm were carried out in reasonably good weather conditions (the sky opacity $\tau_{\rm zenith}<$ 0.3 at 1.3 mm). One observation was made at 0.87 mm on 2002 March 22 with $\tau_{\rm zenith}\sim$0.5 at 0.87 mm. The observations were carried out with a total bandwidth of 328 MHz for each sideband. A typical system temperature is 200-300 K at 1.3 mm. A typical r.m.s. noise of $\sim$ 20 mJy was achieved from an observation with four antennas for a typical on-source integration time of 2 hrs. In each epoch of observation, we interleaved Sgr A* with Sgr B2(N), a compact ($<5$\arcsec~in size) nearby (${\rm \sim1^o}$) HII region, and two nearby QSOs, OV236 (${\rm \sim22^o}$) and NRAO 530 (${\rm \sim16^o}$). The flux density scale in each observation was determined by observing a compact planet (Neptune ($<2.5$\arcsec~in diameter) and Uranus ($<4$\arcsec)). Sgr~B2(N) ($\sim$50 Jy at 1.3 mm) was used to monitor the stability of the telescope during the observations, for example the effect due to the possible telescope pointing drift. Further calibration was done by observing two QSOs, OV236 and NRAO 530. We fitted the secular variations of each calibrator with polynomials. Any correlated offsets in flux density from the secular variations of the two QSOs are considered as systematic offsets, such as telescope pointing error. The residual gain correction determined from the offsets are applied to the Sgr A* data in order to minimize the the systematic errors. The final uncertainty of $\sim10\%$ in the flux density calibration is assessed by calculating the standard deviations of the residual offsets from the calibrators. However, the variation of a possible linear polarization as a function of parallactic angle was not corrected. A fractional linear polarization of $\sim$7\% from Sgr A* has been detected (Bower {\it et al.}, 2002b). The error due to this effect is less significant and is embedded in the final uncertainty. In addition, Sgr~A* is embedded in the complex, extended source Sgr A West. Fig. 1 shows the image of Sgr A* and its vicinity observed with the SMA at 1.3 mm with a beam of 7.4\arcsec$\times$2.3\arcsec (P.A. =7\arcdeg). At this wavelength and this angular resolution, Sgr A* is about 10 times brighter than the surrounding components. By examining the visibilities as a function of baseline lengths, we find that, for baselines $\sim$ 20k$\lambda$ or longer, Sgr~A* is the dominant source and the confusing flux density at 1 mm from the surrounding free-free and dust emission is less than 0.3 Jy. The flux density measurements were made in both the visibility and imaging domains. In the visibility domain, we measured the amplitude of the baselines of $\sim$20 k$\lambda$ and longer. The flux density was double checked by constructing images with the self-calibrated (phase-only) visibility data. The measurements of the point source flux density were done by deconvolving the telescope beam and taking out the contribution from surrounding components using IMFIT in AIPS. The flux densities derived from the two domains are in an agreement within 10\% of the mean value. This additional 10\% level of the uncertainty in the measurements is mainly due to the confusion from the surrounding emission and is added to the final error assessment. The error bars of Sgr A* are derived from the quadrature addition of the uncertainty in flux density calibration and the uncertainty due to the confusion. The typical uncertainty at 1 mm is in a range of 10 to 20\%. \section{Results} \subsection{Light Curve and Flares} Fig. 2 shows the SMA light curve at 1.3 mm suggesting that Sgr~A* varies significantly. A few ``flares'' were observed from Sgr A* while the calibrators show secular variations with opposite drifts in flux density over the past year ( Fig. 2a). Three ``flares'' were observed over a 1-year period. Both the 2001-March and 2002-February flares (Flare 1 and Flare 3 as marked in Fig. 2b) were partially observed in their decreasing phase. The 2001-July flare (Flare 2) was observed covering an entire cycle from its inception to a slow decrease back to its steady state value. Flare 1, which started from 4.1$\pm$0.5 Jy after an unobserved peak and decreased to 1.1 $\pm$ 0.15 Jy within less than three months, appeared to be relatively stronger than others. The rising time for Flare 2 was about 2-3 weeks, reaching a peak of $3.2\pm0.3$ Jy on 2001 July 10. Then, a slow decrease lasted about 40-50d. We were not able to observe Sgr~A* for the next three months due to the proximity to the Sun. The monitoring program was resumed in 2002 February. A tail of a possible flare (Flare 3) was observed in early 2002. \subsection{Day-to-Day Variability and Non-detection Limit on Intra-day Variability} Based on the sparse data, marginal day-to-day variations at a level of 2-3$\sigma$ (or 20-30\%) were observed during Flare 1 and Flare 2 as well as in later May 2002. Intra-day variations on short time scales were searched based on the 24 epochs of observations at 1.3 mm. We averaged the data in a 5min bin and checked the visibility plots for each baseline. We also averaged all baseline together and checked intra-day light curves. No evidence for significant variations on a time scale of $\sim$1 hr has been found, {\it i.e.} a variability quantity ${{\left[S_{max}-S_{min}\right] \over \left[S_{max}+S_{min} \right]} < 20\%}$, where ${\rm S_{max}}$ and ${\rm S_{min}}$ are the maximum and minimum flux densities at 1.3 mm, respectively, in a single observing track of 6 hrs or less. We also searched for periodic signals in a period ranging between 10 min to 2.5 hrs based on the observations of 5.5 hr on May 29, 2002. There is a possible oscillation signal with a frequency of 1.1$\times10^{-4}$ Hz (or 2.5 hr in period) on an E-W baseline. This oscillation signal can be well modeled as a structure due to interference between Sgr A* and surrounding components. Combining all baselines, a $3\sigma$ non-detection limit of a periodic signal from SgrA* at a level of$\sim$5\% can be inferred. \subsection{Spectrum during A ``Flare''} We also observed Sgr~A* at 0.87 mm with the SMA on 2001 March 22. We observed Sgr~A* (S$_{\rm 0.87mm}=6.7\pm1.5$ Jy) at the sub-millimeter band using the three-element array of the partially completed SMA. Fig. 3a shows a spectrum derived from the mean flux density determined from the multiple observations within two weeks of the peak of Flare 1. The error bars were derived from the quadrature addition of the standard deviation and the maximum error in individual measurements. The spectral index $\alpha$ (S$_\nu\propto\nu^\alpha$) appears to be 0.1$\pm$0.1 at 100 GHz and below, and 1.5$^{+1.0}_{-1.1}$ between 232 and 345 GHz, suggesting a break frequency in spectral index of $\sim$100 GHz or higher. A flux density excess towards sub-millimeter wavelengths has been observed (Zylka, Mezger, \& Lesch 1992; Serabyn \etal~1997; Falcke \etal~1998). The overall spectrum can fit two power-law components, {\it i.e.} S$_\nu$ = S$_1(\nu/\nu_1)^{\alpha_1}$ + S$_2(\nu/\nu_2)^{\alpha_2}$. Three sets of combination of ${\alpha_1}$ and ${\alpha_2}$ are used in the fitting. First, for ${\alpha_1}=0.0$ and ${\alpha_2}=2$ (dashed lines in Fig. 3), the sub-millimeter component corresponds to either the thermal bremsstrahlung emission arising from the inner region of the accretion disk ({\it e.g.} Liu \& Melia 2002) or the thermalized synchrotron emission produced from a jet-nozzle (Falcke \& Markoff 2000). Second, for ${\alpha_1}=0.1$ and ${\alpha_2}=2.5$ (solid lines in Fig. 3), the spectral index of 2.5 suggests that a homogeneous opaque, non-thermal synchrotron source might be present in the inner region of the accretion flow. Such a model appears to be plausible if one considers the non-thermal synchrotron particles to be accelerated inside the compact source, perhaps within a jet nozzle as has been proposed for the case of NGC 4258 (Yuan {\it et. al.} 2002b). Finally, if the low frequency component has an exponential cut-off, {\it i.e.} ${\rm \sim S_1(\nu/\nu_1)^{0.25} exp(-\nu/\nu_0)}$, at $\nu_0 \sim 75$ GHz, a smaller value of $\alpha_2\sim1.5$ (dash-dotted lines in Fig. 3) for the sub-millimeter component is also consistent with a spectrum produced from the ADAF in which a gradient of ${\rm T_e}$ shallows the rising part of the spectrum (Narayan {\it et. al.}, 1998). The observed spectrum suggests an opaque nature of the sub-millimeter component at 1.3 and perhaps 0.87 mm. Observations at the shorter sub-millimeter wavelengths appear to be critical to differentiate between the models. The spectrum in a minimum state is also shown (Fig. 3b). The excess at 1.3 mm appears to be less significant. \subsection{Correlation with The VLA Data} The SMA data at 1 mm appears to show a correlation with the light curves observed with the VLA. The SMA light curve shows three ``flares'' from Sgr~A* in the past year from 2001 March to 2002 May. The variation in flux density that we observed in the SMA light curve suggests that flares are constantly occurring in this source. A quantitative analysis of cross-correlation properties between the light curves at 1.3 mm and 1.3 cm has been carried out. Due to the sparse data sampling, large uncertainty remains in the cross-correlation analysis for individual flares (Zhao 2002). However, a global delay between the 1.3 mm and 1.3 cm light curves can be searched for using the z-transformed discrete correlation function (ZDCF) without interpolating in the temporal domain (Alexander 1997). With no prior models assumed, the ZDCF is a reliable and efficient method to search for a delay. Fig.~4 shows the ZDCF between the SMA and VLA light curves at 1.3 mm and 1.3 cm. The peak in the correlation function corresponds to a delay of 4$^{+2}_{-1}$ d. A noticeable asymmetric shape of the ZDCF near the zero lag shows a significant excess in cross-correlated power towards the positive lags, suggesting that the true global delay is ${\rm t_{delay}>3 d}$. The ZDCF indicates that flares at 1.3 mm starts first. In addition, the strong X-ray flare with a time scale of $\sim$1 hr, observed by Baganoff \etal~(2001), occurred about 10 days earlier than a radio peak observed in all three VLA monitoring bands. During Flare 2 (2001 July), Chandra observed Sgr~A* on 2001 July 14, a few days past the 1 mm peak but no X-ray flares were observed. The X-ray flux level was consistent with that of a quiescent state (Baganoff 2001, private communication). \section{Discussion \& Summary} The SMA observations have shown that Sgr~A* varies significantly at 1 mm during the course of SMA monitoring in 2001 March to 2002 May. The derived lags from a cross-correlation analysis appear to be good evidence that the flaring occurs from the inside out starting from short wavelengths and then continuing to longer wavelengths. In the Jet-ADAF model connecting a jet outflow with the ADAF (Yuan \etal~2002a), the sub-millimeter excess is thought to arise from a sum of the emission from both the ADAF and the jet nozzle. On the other hand, from the observations with the SMA and the VLA, further constraints on the models can be derived. Taking a global delay time of ${\rm t_{delay} >3 d}$ and the source size of 40 R$_{\rm sc}$, an expansion velocity, v$_{\rm exp}\sim$1200 km s$^{-1}$ or $<$ 0.004 c, is inferred. The expansion velocity appears to be far below the escape velocity of 0.1 c at r $\sim$ 40 R$_{\rm sc}$. The bulk kinetic energy associated with the flares appeared to be too small to power a noticeable collimated jet in Sgr~A*. In addition, the break in the spectral index at $\sim$3 mm also indicates that a large fraction of the flaring plasma might well be confined within the characteristic radius at 3 mm. However, the possible outflow tries to expand to a larger scale. The data presented here does not exclude the possibility of a strong flare which can lead to an observable jet-like structure. The inferred small expansion velocity may imply that other processes contribute to the transport of high energy particles, {\it e.g.} diffusion and convection may also play a role in powering Sgr A* at lower radio frequencies. The time scale (weeks) of 1 mm flares differs from the time scale (1 hr) of the X-ray flare (Baganoff, 2001). The lack of strong flares on a short time scale at 1 mm places a critical constraint on the models of the inverse Compton scattering as has been proposed for the short duration X-ray flares (Falcke \& Markoff 2000; Markoff {\it et. al.}, 2001; Liu \& Melia, 2002 ). Considering the opaque nature of the sub-millimeter component at 1.3 mm, the X-ray flares could hide at 1.3 mm due to a self-absorption. Alternatively, the flares at sub-millimeter wavelengths might be a result of collective mass ejections associated with the X-ray flares that originate from the inner region of the accretion flows near the event horizon of the supermassive black hole at the Galactic center. \acknowledgments We would like to thank the SMA staff from both the SAO and the ASIAA for supporting this monitoring program. We are grateful to Jim Moran and Heino Falcke (the referee) for their valuable comments. JHZ thanks Irwin Shapiro for his initial suggestion of the SMA monitoring program. The Very Large Array (VLA) is operated by the National Radio Astronomy Observatory (NRAO). The NRAO is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. %% The reference list follows the main body and any appendices. %% Use LaTeX's thebibliography environment to mark up your reference list. %% Note \begin{thebibliography} is followed by an empty set of %% curly braces. If you forget this, LaTeX will generate the error %% "Perhaps a missing \item?". %% %% thebibliography produces citations in the text using \bibitem-\cite %% cross-referencing. Each reference is preceded by a %% \bibitem command that defines in curly braces the KEY that corresponds %% to the KEY in the \cite commands (see the first section above). %% Make sure that you provide a unique KEY for every \bibitem or else the %% paper will not LaTeX. The square brackets should contain %% the citation text that LaTeX will insert in %% place of the \cite commands. %% We have used macros to produce journal name abbreviations. %% AASTeX provides a number of these for the more frequently-cited journals. %% See the Author Guide for a list of them. %% Note that the style of the \bibitem labels (in []) is slightly %% different from previous examples. The natbib system solves a host %% of citation expression problems, but it is necessary to clearly %% delimit the year from the author name used in the citation. %% See the natbib documentation for more details and options. \begin{thebibliography}{} \bibitem[Alexander (1997)]{ale1997} Alexander, T., 1997, in Astronomical Time Series, eds. D. Maoz, A. Sternberg, E. Leibowitz (Dordrect: Kluwer), p. 163 \bibitem[Baganoff et al (2001)]{bag2001} Baganoff, F.~K., Bautz, M.~W., Brandt, W.~N. et al., 2001, \nat, 413, 45 \bibitem[Brown and Lo (1982)]{bro1982} Brown, R. L. and Lo, K. Y. 1982, \apj, 253, 108 \bibitem[Bower et al (2002a)]{bow2002_a} Bower, G. C., McGary, R. S., Zhao, J.-H., \& Goss, W. M., 2002a, AAS Meeting 199, \#85.08 \bibitem[Bower et al (2002b)]{bow2002_b} Bower, G. C. {\it et.al.}, 2002b, ApJ, submitted \bibitem[Doeleman et al (2001)]{Doe2001} Doeleman, S., Shen, Z., Rogers, A.E.E., \etal, 2001, AJ, 121, 2610 \bibitem[Eckart et al (2002)]{eck2002} Eckart, A., Genzel, R., Ott, T., \& Schodel, R., 2002 \mnras, 284, 576 \bibitem[Falcke etal (1993)]{fal1993} Falcke, H., \& Mannheim, K. \& Biermann, P. L. 1993, AA, 278, L1 \bibitem[Falcke (1998)]{fal1998} Falcke, H., {\it et.al.}, 1998, ApJ, 499, 731 \bibitem[Falcke (1999)]{fal1999} Falcke, H. 1999 in ASP Conf. Series 186, p113 \bibitem[Falcke (2000)]{fal2000} Falcke, H., and Markoff, S., 2000, AA, 362, 113 \bibitem[Ghez et al (1998)]{ghe1998} Ghez, A., Morris, M., Becklin, E., Tanner, A., \& Kremenek, T., 2000, Nature, 407, 349 %\bibitem[Herrnstein et al (2002)]{her2002}Herrnstein, R. M. \etal, 2002, in preparation \bibitem[Liu and Melia (2002)]{liu2002} Liu, S. \& Melia, F., 2002, ApJ, 566, L77 \bibitem[Markoff et al (2002)]{yua2001} Markoff, S., \& Falcke, H., Yuan, F., Biermann, P. L. 2002, AA, 379, L13 \bibitem[Moran 1998]{mor1998} Moran, J. M., 1998, in Proc. SPIE, Vol. 3357: Advanced Technology MMW, Radio \& Terahertz Telescopes, ed. Thomas, G. Phillips, p. 208 \bibitem[Narayan et al (1998)]{nar1998} Narayan, R., Mahadevan, R., Grindly, J., Popham, R., \& Cammie, C.,1998, ApJ, 492, 554 \bibitem[Serabyn et al (1997)]{ser1997} Serabyn, E., Carlstrom, J., Lay. O., Lis, D., Hunter, T., \& Lacy, J., 1997, ApJ, 490, L77 \bibitem[Tsuboi et al (1999)]{tsu1999} Tsuboi, M., Miyazaki, A. \& Tsutsumi, T. 1999, ASP Conf. Series 186, p105 %\bibitem[Tsutsumi etal (2002)]{tsu2002} Tsutsumi, T., Miyazaki, A. %\& Tsuboi, M., 2002, %AAS Meeting 200, \#44.09 \bibitem[Wright and Backer (1993)]{wri1993} Wright, M. and Backer, D. C., 1993, ApJ, 417, 560 \bibitem[Yuan et al (2002)]{yua2002_a} Yuan, F., Markoff, S., \& Falcke, H., 2002a, AA, 383, 854 \bibitem[Yuan et al (2002)]{yua2002_b} Yuan, F., Markoff, S., \& Falcke, H., Biermann, P. L. 2002b, AA, 391, 139 \bibitem[Zhao et al (1989)]{zha1989} Zhao, J.-H., Ekers, R.D., Goss, W.M., Lo, K.Y. \& Narayan R. 1989, IAU Symp. 136, 535. \bibitem[Zhao etal (1992)]{zha1992} Zhao, J.-H., Goss, W. M., Lo, K. Y. and Ekers, R. D. 1992, ASP Conf. Series 31, 295 \bibitem[Zhao and Goss (1993)]{zha1993} Zhao, J.-H. and Goss, W. M. 1993 Sub-arcsecond Radio Astronomy, R.J. Davis and R. S. Booth, Cambridge University Press, 38 \bibitem[Zhao et al (2001)]{zha2001} Zhao, J.-H., Bower, G. C., Goss, W. M., 2001, ApJ, 547, L29 \bibitem[Zhao (2002b)]{zha2002b} Zhao, J.-H., 2002, in The Newsletter of Galactic Center Research, Vol. 15, 4 \bibitem[Zhao et al (2002c)]{zha2002c} Zhao, J.-H., Pegg, J, Herrnstein, R. M., Bower, G. C., \& Goss, W. M., 2002, in preparation \bibitem[Zylka et al (1992)]{zyl1992} Zylka, R., Mezger, P. G., \& Lesch, H., 1992, AA, 261, 119 \end{thebibliography} %% Generally speaking, only the figure captions, and not the figures %% themselves, are included in electronic manuscript submissions. %% Use \figcaption to format your figure captions. They should begin on a %% new page. \clearpage %% No more than seven \figcaption commands are allowed per page, %% so if you have more than seven captions, insert a \clearpage %% after every seventh one. %% There must be a \figcaption command for each legend. Key the text of the %% legend and the optional \label in curly braces. If you wish, you may %% include the name of the corresponding figure file in square brackets. %% The label is for identification purposes only. It will not insert the %% figures themselves into the document. %% If you want to include your art in the paper, use \plotone. %% Refer to the on-line documentation for details. \figcaption[fig1.ps]{ A pseudo-color image of Sgr~A* (blue) and its vicinity (red) made from 3.5 hr observations using the four elements of the partially finished SMA on 2002 May 23. The total integration time on the source is about 1.5 hr. The r.m.s. noise is $\sim$30 mJy/beam. The FWHM beam is 7.4\arcsec$\times$2.3\arcsec (P.A. = 7 \arcdeg). The data reduction was done in the AIPS environment following a procedure described in a SMA Technical Memorandum (Zhao 2002). \label{fig1}} \figcaption[fig2.ps]{ The SMA light curves at 1.3 mm observed in the period between 2001 March and 2002 May for the calibrators, OV 236 and NRAO 530 (panel a) and Sgr~A* (panel b). The solid curves in panel a are the quadratic fits to the secular variations of the flux density of OV 236 and NRAO 530. The secular trends of the flux density from NRAO 530 and OV 236 are the same as those observed at 3 mm at other observatories ({\it e.g.} M. Yun 2002, private communication). The densely sampled radio light curves at 1.3 and 2 cm observed with the VLA is shown in panel c (Herrnstein \etal~2002). \label{fig2}} \figcaption[fig3.ps]{(a) A mean spectrum of Sgr~A* made from the observations near the peak of Flare 1. The flux densities of $6.7\pm1.5$ and $3.7\pm0.7$ Jy at 0.87 and 1.3 mm are derived from the SMA observations. The flux density of $1.3\pm0.6$ Jy at 3 mm is averaged from the measurements made with the Nobeyama Millimeter Array (NMA) (Tsutsumi, Miyazaki, \& Tsuboi~2002). The data ($1.2\pm0.2$, $1.15\pm0.17$, and $1.04\pm0.18$ Jy) at 0.7, 1.3 and 2 cm is derived from the VLA monitoring observations (Herrnstein, {\it et. al}~2002, in preparation). The curves represent a resultant spectrum of two power-law components (straight lines) for various sets of spectral indices corresponding to different emission processes for the sub-millimeter component (see section 3.3). (b) A spectrum of Sgr A* made from the observations on the same date (2001 June 17) during a minimum of the Sgr A* light curves (see Fig. 2). The flux density of $1.1 \pm0.15$ at 1.3 mm is measured with the SMA. The rest of the measurements ($0.72\pm0.09$, $0.69 \pm 0.07$, and $0.65 \pm 0.06$ Jy at 0.7, 1.3 and 2 cm) are made from the VLA observations. \label{fig3}} \figcaption[fig4.ps]{ The z-transformed discrete cross-correlation function (ZDCF) of the SMA data at 1.3 mm with the VLA data at 1.3 cm. \label{fig4}} %%%UCP%%% \newpage \plotone{f1.eps} \newpage \plotone{f2.eps} \newpage \plotone{f3.eps} \newpage \plotone{f4.eps} \end{document} %% %% End of file`sample.tex'.