Journal of Physics: Conference Series Simultaneous Multi-Wavelength Spectrum of Sgr A* and Long Wavelength Cutoff at λ > 30 cm To cite this article: T An et al 2006 J. Phys.: Conf. Ser. 54 381 Related content - Cold Dust in Cas A: Metallic Needles Eli Dwek - Multiwavelength Observations of Sgr A* T. An, W. M. Goss, Jun-Hui Zhao et al. - Topography measurements for determining the decay factors in surface replication J Song, P Rubert, A Zheng et al. View the article online for updates and enhancements. This content was downloaded from IP address 148.251.232.83 on 17/03/2019 at 02:45
Institute of Physics Publishing Journal of Physics: Conference Series 54 (2006) 381 385 doi:10.1088/1742-6596/54/1/060 Galaxy Center Workshop Simultaneous Multi-Wavelength Spectrum of Sgr A* and Long Wavelength Cutoff at λ>30 cm An T. 1,2,GossW.M. 3, Zhao Jun-Hui 2, Hong X.Y. 1, Roy S. 4, Rao A.P. 5 and Shen Z.-Q. 1 1 Shanghai Astronomical Observatory, China 2 Harvard-Smithsonian CfA, the USA 3 NRAO-AOC, Socorro, the USA 4 ASTRON, the Netherlands 5 National Center for Radio Astrophysics, Pune University Campus, India E-mail: antao@shao.ac.cn Abstract. We reviewed the multiple-wavelength spectrum of Sgr A*. The recent continuum spectrum was determined using data obtained from the Very Large Array (VLA), the Giant Metrewave Radio Telescope (GMRT), the Submillimeter Array (SMA) and the Keck II 10m telescope on the same date (2003 June 17). The nearly simultaneous spectrum (S λ α ) covers a broad wavelength (λ) range from 90 cm to 3.8 micron. From about 2 cm to 1 mm, the source shows a rising spectrum with a spectral index of α =0.43 ± 0.04. However, the IR measurements suggested that the flux density of Sgr A* must drastically drop towards the shorter wavelengths and the turnover likely occurs in the submillimeter regime. The spectrum at wavelengths between 47 and 3.6 cm is flat with α =0.11 ± 0.03. The flux density of Sgr A* is 0.22±0.06 Jy at 90 cm on this date in 2003. Compared with measurement at 47 cm, the flux density at 90 cm has decreased by a factor of two (or 4σ), suggesting a cutoff at wavelength longer than 47 cm. The fits using free-free absorption opacity to the spectrum suggest a cutoff wavelength at 100 cm. The cutoff wavelength appears to be three times longer than that of 30 cm determined by Davies, Walsh, & Booth [1] three decades ago based on observations in 1974 and 1975. A model interpreting the variation of the long wavelength cutoff using stellar winds (An et al. [2]) is summarized in this paper. 1. Introduction The compact radio source Sgr A* in the Galactic center (GC) is now believed to be associated with a supermassive black hole (SMBH) with a mass of M 4 10 6 M ([3, 4, 5, 6]). Owing to its proximity to the earth (1 corresponds to 0.04 pc at a distance of 8 kpc,[7,6]), Sgr A* can be studied in a great detail. Soon after the discovery of Sgr A* ([8]), Davies, Walsh, & Booth ([1]) observed the Galactic center at 0.408, 0.960 and 1.660 GHz using the early MERLIN interferometer. Sgr A* was only detected at the two higher frequencies. The measurements showed a low frequency cutoff around 1 GHz in the spectrum of Sgr A* ([1] and references therein). The authors attributed the decrease in flux densities below 1 GHz to free-free absorption from ionized gas in Sgr A West. Recently, Roy & Rao ([9]) measured the Galactic center region at 620 MHz using the GMRT, and detected Sgr A* with a flux density of 0.5±0.1 Jy (angular resolution 9 ); while at 330 MHz, Nord et al. ([10]) detected Sgr A* using the VLA with a flux density of 0.33±0.12 Jy. The two measurements indicate that the 2006 IOP Publishing Ltd 381
382 spectrum below 1 GHz may indeed have a pronounced cutoff. However both the measured flux densities of Sgr A* show a significant excess compared to the expected values from Davies et al. s free-free absorption model. On 2003 June 17, we observed Sgr A* at multiple wavelengths using the VLA and the GMRT with the purpose of discerning the low frequency spectrum of Sgr A*. The observations and data reduction were reported in An et al. ([2]). We reviewed the spectrum covering wavelength range from 90 cm to 3.8 μm, with the data obtained from VLA, GMRT, SMA ([11, 12, 13]) and Keck II 10m telescope observations on the same date (2003 June 17). In this paper, we will further discuss the model interpreting the variation of the long wavelength cutoff. SgrA* Figure 1. The continuum image of Sgr A* at 90 cm. The resolution is 10.9 6.8 (PA= 10 ). The r.m.s. noise in the image is 12 mjy/beam. The position and expected scattering size of Sgr A* at 90 cm are marked with a red cross in the image center ([14, 15]. 2. Flux Densities Measurements and Spectrum of Sgr A* The measurements of the flux densities for Sgr A* at wavelengths shorter than 20 cm with the VLA in the A-configuration are straight-forward. The measurements have been published by An et al. ([2]). Because the apparent size increases as λ 2 and the confusion of the nonthermal emission from the surrounding sources also increases towards longer wavelengths, the measurements of the flux density at 90 cm are more difficult. Figure 1 shows the 90 cm continuum mapinwhichsgra*ismarkedasaredcrossintheimagecenterwithanexpectedscattering size of 11 6 (e.g. [15, 16, 17]). The supernova Sgr A East dominates the total flux density at this wavelength. The emission from the shell to the east appears to be significantly brighter than that to the west as pointed out earlier by other authors (e.g. [18]). However, at the position of Sgr A* a weak source is detected. The region surrounding this source has a lower flux density to the east, north and west. The deficiency in emission likely arises from absorption by ionized gas in Sgr A West. To determine the flux density at 90 cm, we fit two intensity slices using Gaussians along the expected major (PA =80 ) and minor (PA = 10 ) axes after subtracting the diffuse emission from the background. The measured flux density of Sgr A* at 90 cm is
383 Figure 2. Quasi-simultaneous spectrum of Sgr A* from 90 cm to 3.8 μm on 2003 June 17 ([2]). solid circle: VLA; diamond: GMRT; triangle: SMA; square: Keck II 10m. 0.22±0.06 Jy. The result is consistent with the measurement of Nord et al. ([10]) within 1σ at the same wavelength. Figure 2 shows the quasi-simultaneous spectrum of Sgr A* at wavelengths ranging from 90 cm to 3.8 μm, observed on 2003 June 17 ([2]). A number of interesting results are outlined here: (i) The source shows a rising spectrum in the range of wavelengths from 2cmto 0.89 mm. High angular resolution measurements of the simultaneous flux densities at 230 and 690 GHz show that the mean value of 690 GHz flux density observed from multiple epochs is less than that of 230 GHz, suggesting a turnover in the spectrum between these frequencies (Marrone et al., these proceedings). (ii) A break in the spectrum appears at 3.6 cm. The spectrum at shorter centimeter to millimeter wavelengths can be described by a power-law: S ν ν α with α 0.89mm 3.6cm = 0.43 ± 0.04. The spectrum between 3.6 and 47 cm is flat with a spectral index of α 3.6cm 47cm =0.11 ± 0.03. The difference in spectral indices between the two parts of power-law =0.32 ± 0.05, suggesting that the break is significant (> 6σ). spectra is α 0.89mm 3.6cm α3.6cm 47cm (iii) At 90 cm, the measured flux density of 0.22±0.06 Jy is well below the inferred 0.5 Jy extrapolated from power-law fitting (S ν ν 0.11±0.03 ). This significant decrease in flux density at 90 cm (larger than 4σ) suggests that the spectrum of Sgr A* may well have a long wavelength cutoff at λ>47cm on this date in 2003. In fact, Davies, Walsh, & Booth ([1]) detected a long wavelength cutoff at 30 cm in the spectrum of Sgr A* three decades ago. Figure 3 compares the observed spectra for Sgr A* at epochs 1975 ([1]) and 2003 ([2]) at wavelengths longer than 3.6 cm. The spectra at both epochs were fitted using a model of slowly rising power-law (S ν ν α ) along with a free-free absorption screen (S ν e τ ff) between the observer and Sgr A*, as proposed by Davies et al. ([1]). The comparison of the fits to the 1975 (Figure 3, dashed line) and 2003 data (Figure 3, solid line) suggest that the cutoff wavelength
384 (τ ff (λ cutoff ) = 1) due to free-free absorption has changed from λ cutoff 100 cm in 1975 to λ cutoff 30 cm in 2003. If the free-free absorption model is correct, the variation in the long wavelength cutoff would suggest that the column density of the free-free absorption screen must have decreased significantly in the past 28 years. Quantitatively, the free-free absorption opacity τ ff would need to decrease by a factor of nine from 1975 to 2003. Assuming that the electron temperature of the ionized gas is constant, the inferred decrease in τ ff wouldthencorrespondtoadecreaseby a factor of nine in emission measure (EM). Such a large variation in EM would likely occur in a compact region, i.e. 0.1 10 displaced from Sgr A*. In this region, stellar winds originating from massive stars may have modified the local environment. The stellar wind mass flux captured by the SMBH potential is highly variable owing to orbital motions of stars as well as wind-wind collisions (a number of contributors to this volume have shown the nature of the complex region in the central parsec of the Galactic center). Most likely the source accounting for the long wavelength cutoff is free-free absorption from cool gas close to Sgr A*. The emission line measurements of the star S2 have shown that the surface temperature of this star is about 30,000K; the temperature was measured when S2 was only 100 AU from Sgr A* ([4]). This indicates that free-free absorption gas with temperature 10 4 K can exist very close to the SMBH in the Galactic center. In addition, recent simulations ([19, 20] and Cuadra et al. in these proceedings) have shown two phases of gas co-existing in the Galactic center, i.e., hot gas with a typical temperature of 10 7 K and cold gas with a temperature of 10 4 K. The cold but dense gas clumps originated from the stellar winds can be captured by the potential of the SMBH as have been suggested in the numerical simulations ([19, 20]). If a cold ionized gas component moves to the front of Sgr A*, it would attenuate the flux density of Sgr A* at long wavelengths. As an example, for a cold ionized gas component with a temperature of 10 4 Kand a size of 0.004 pc, the free-free absorption opacity is about unity at a wavelength 79 cm for an electron density of 10 4 cm 3, while the cutoff wavelength will be about 30 cm if the emission measure has increased by a factor of nine. In this scenario, the change of the long wavelength cutoff due to free-free absorption toward Sgr A* might be sensitive to fluctuations of stellar wind mass flux captured by the SMBH potential. Acknowledgments The work is supported by the NSFC (10503008, 10328306). The VLA is operated by the National Radio Astronomy Observatory, a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. The GMRT is a component of the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research. References [1] Davies, R.D., Walsh, D., & Booth, R.S., 1976, MNRAS, 177, 319 [2] An, T., Goss W.M., Zhao, J.-H., Hong, X.-Y., Roy, S. & Rao, A.P., Shen, Z.-Q., 2005, ApJ, 634, L49 [3] Schödel, R., Ott, T., Genzel, R., et al. 2002, Nature, 419, 694 [4] Ghez, A.M., Duchene, G., Matthews, K., et al., 2003, ApJ, 586, L127 [5] Reid, M.J., & Brunthaler, A., 2004, ApJ, 616, 872 [6] Eisenhauer, F., Genzel, R., Alexander, T., et al., 2005, ApJ, 628, 246 [7] Reid, M.J., 1993, ARA&A, 31, 345 [8] Balick, B, & Brown, R.L., 1974, ApJ, 194, 265 [9] Roy, S., & Rao, A.P., 2004, MNRAS, 349, L25 [10] Nord, M.E., Lazio, T.J.W., Kassim, N.E., et al. 2004, ApJ, 601, L51 [11] Moran, J.M., 1998, Proc. SPIE, 3357, 208 [12] Blundell, R., 2004, in 15th International Symposium on Space Terahertz Technology, ed. G.Narayanan (Amherst: U Mass), 3 (astro-ph/0508492) [13] Ho, P.T.P., Moran, J.M., & Lo, K.Y., 2004, ApJ, 616, L1 [14] Rogers, A.E.E., et al. 1994, ApJ, 434, L59
385 Figure 3. Comparison of the long wavelength spectra of Sgr A* observed between epochs 2003 (solid square, [2]) and 1975 (open circle, [1]). The dashed and solid lines represent fits using a free-free absorption opacity model applied to the spectrum at epochs 1975 and 2003, respectively. The free-free opacity decreases by a factor of nine at 30 cm (or 1 GHz) from 1975 to 2003. [15] Lo, K.Y., Shen, Z.-Q., Zhao, J.-H., & Ho, P.T.P., 1998, ApJ, 508, L61 [16] Bower, G.C., Falcke, H., Herrnstein, R.M., Zhao, J.-H., Goss, W.M., & Backer, D.C., 2004, Science, 304, 704 [17] Shen, Z.-Q., Lo, K.Y., Liang, M.-C., Ho, P.T.P., & Zhao, J.-H., 2005, Nature, 438, 62 [18] Pedlar, A., Anantharamaiah, K.R., Ekers, R.D., et al., 1989, ApJ, 342, 769 [19] Cuadra, J., Nayakshin, S., Springel, V., Di Matteo, T., 2006, MNRAS, 366, 358 [20] Cuadra, J., Nayakshin, S., Springel, V., Di Matteo, T., 2005, MNRAS, 360, L55