100012. Abstract. We report here the results of VLBA observations of Sagittarius ... center of mass of the Milky Way (Reid et al. 1999). Since its ... at the Galactic center. We observed .... The apparent velocity of the jet eruption is 0.5c assuming the distance of ... We thank Dr. K. Kameno and Dr. Y. Hagiwara, Mr. K. Asada and ...
Future Directions in High Resolution Astronomy: The 10th Anniversary of the VLBA ASP Conference Series, Vol. 340, 2005 J. D. Romney and M. J. Reid (eds.)
VLBA Observations of an Intraday Flare of Sagittarius A* Makoto Miyoshi National Astronomical Observatory Japan, 2-21-1 Osawa, Mitaka, Tokyo, Japan 181-8588 Hiroshi Imai Department of Physics, Kagoshima University, Korimoto 1-21-35, Kagoshima Japan, 890-0065 Junichi Nakashima Department of Astronomy, University of Illinois at Urbana-Champaign 1002 West Green Street, Urbana, IL 61801, USA Shuji Deguchi Nobeyama Radio Observatory, NAOJ, Minamimaki, Minamisaku, Nagano Japan, 384-1305 Zhi-Qiang Shen Academia Sinica Institute of Astronomy and Astrophysics, P.O. Box 23-141, Taipei 106 Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030 National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012 Abstract. We report here the results of VLBA observations of Sagittarius A* (Sgr A*) at 43GHz, when Sgr A* showed an intraday variation. The flux density increased from 1Jy to 3Jy, and the duration is about 104 seconds which is quite similar to its X-ray flaring that Chandra detected. VLBA snapshot mappings show that Sgr A* flared and erupted jets in two opposite directions (north-south) with nearly half the speed of light. The lengths of jets reached about 15AU at most.
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Introduction
Sgr A* is now the most convincing super massive black hole, whose mass is 2.6 − 3.7×106 M⊙ (Ghez et al. 2000, Sch¨ odel et al. 2002) and is located at the center of mass of the Milky Way (Reid et al. 1999). Since its discovery (Balick and Brown 1974) Sgr A* has long been recognized to be very quiet though there exists evidence that Sgr A* was active about 300 to 1000 years ago (Murakami et al. 2001). However recent detections of the rapid flaring of its X-ray, radio and infrared emissions (Baganoff et al. 2001, Miyazaki et al. 2003, Genzel et al. 2003) and the periodic variation of radio flux density in a 106 day cycle (Zhao et al. 2001) are evidence of AGN-like activity of Sgr A*. 258
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We found that a radio intraday variation occurred at our observing time and the flux density increased when the jet eruption occurred. Previous VLBA observations already suggested a jet with north-south direction for intrinsic structure of Sgr A* (Lo et al. 1998). The first jet model is proposed by Falcke et al.(1993, 2000). Accretion flow models are proposed by Rees (1982), Melia (1992), and Narayan et al.(1995). Yuan et al.(2002) explain the spectrum of Sgr A* using jet and ADAF model. 2.
Observations and reduction
The purpose of our project was astrometry of SiO masers relative to Sgr A* at the Galactic center. We observed Sgr A* and SiO masers (J = 1 − 0, v=1 and 2, 43GHz) simultaneously in the single beam using VLBA (NRAO) from 0100 UT to 0800 UT on July 31 2001. The data of 8×4M Hz bands with 2-bit sampling were processed by FX correlator at Socorro NM to produce visibilities with 64ch/IF in the form of 1.04 sec accumulated fragments. Five correlation processings were done with changing tracking positions for Sgr A* and 4 SiO maser locations. We used the classic AIPS (NRAO) for data calibration and imaging. For delays and rates of all stations we adopted the FRING solutions with signal-to noise ratio greater than 4.5 from 1 minutes integrations. We put interpolated values for the data points without good FRING solutions in order to keep most of the data points. For getting fine solutions of complex gain errors, we performed CALIB and IMAGR iterations as usual. In this stage we abandoned the data points without good solutions from CALIB, and then made model images. At first we made one model map from the whole data, then we divided the data into shorter time subsets and made models and got CALIB solutions independently. We repeated the sequence, finally from 15 minutes integrations we got the maps as shown in Fig. 4 (top). After the conference we calculated the closure phases. Fig. 1 shows closure phases of triangles including the PT station, and of the largest triangle formed from HN, SC and MK. The integration time of each point is 10 minutes. The larger triangles show the larger deviations of closure phases from zero which suggests the shape of Sgr A* at this time was not symmetric. In Fig. 2 We also show the differences of closure phases between observed ones and calculated values from obtained images. The average of residual closure phases of the all triangles are 29.3 ◦ , the value from the inner 5 stations plus BR is 7.2 ◦ , and that from the inner 5 stations namely PT, FD, KP, LA, and OV is 4.1 ◦ . These small values suggest the images we obtained are consistent with the visibilities. Can we really make reliable maps of Sgr A* at 43GHz? It has been suggested that the uv coverage of the VLBA is not sufficient to image Sgr A* locating at δ = −30◦ . We made some simulations for checking the imaging performance of the VLBA and found that the uv coverage itself is not the problem. Only from 10 or 15 minutes integrations, it is possible to discriminate that the shape is a simple Gaussian or not. It is really an issue of the atmospheric (phase) fluctuations that damage the visibilities. The imaging of Sgr A* with mmwavelength VLBI is a task of difficulty, but is not impossible. We will show the details in the next paper.
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Results and Discussions
We show the cleaned maps of Sgr A* (top panel in Fig.4). Integration time of every map is 15min. We used the Gaussian restoring beam with 0.45 mas × 0.15 mas, P A = 0◦ , which is so called a super resolution for most maps in the figure. The maps show structural changing of Sgr A*. One of the explanations of the variation is the gravitational effect caused by MACHO at Galactic center. However judging from the time scale, such possibility is very low (Ohnishi, private communications). It is appropriate that a rapid intrinsic structural change of Sgr A* occurred at mm-wavelength emission during a few hours. From the beginning to 0330 UT the shape of Sgr A* was an ellipsoidal elongated to east-west direction with the flux density of about 1Jy, which is quite consistent to the shape obtained from previous VLBA observations (Lo et al. 1998, Bower et al. 1998). After 0345 UT a jet-like structure emerged towards north and south directions. At the same time the flux density began to increase and reached the peak of about 3.5Jy at 0530 UT (Fig.4, bottom left).
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Later the flux density began to decrease. The duration of the flare is about 2.5 hours or 104 seconds which is similar to the duration of the X-ray flare of Sgr A* (Baganoff et al. 2001). The bottom right in Fig. 4 shows the size variation of Sgr A*. We take the direction of P A = 13◦ as the jet axis, and measure the projected line length of contour level of 25mJy/B to the jet axis and defined them as jet length. The lengths of jets reached about 15 AU at most, which is about 300 Schwarzschild radii for the GC black hole (solid line in the bottom right panel of Fig. 4). The apparent velocity of the jet eruption is 0.5c assuming the distance of Sgr A* to be 8kpc. We also measured the width of the emission along the perpendicular direction, namely P A = 103◦ . It is slightly decreased during 7 hours as shown in the bottom right panel of Fig. 4 (dotted line). If accretion disk elongates perpendicularly to jet, the decrease of the width may indicate the variation of the radius of accretion disk in Sgr A*. Applying the calibrations of amplitudes and phases exactly the same as those used for visibilities of Sgr A*, we got the images of the v=1 and v=2, J=1-0 SiO masers in IRS10EE (Fig 3). The weightings to the uv points are also the same as those made in the Sgr A* imagings. The extent of SiO emission in IRS 10EE is consistent with the previous value obtained 5 years before by Reid et al. (2003). The v=2, J=1-0 SiO masers spread 3 mas in diameter, which is quite larger than the area of the v=1 SiO emissions. Also the flux density of the v=2 is stronger than that of the v=1, which has often been found in SiO masers around Mira type variables (Nakashima et al. 2003, Nyman et al. 1986, Deguchi et al. 2002). As suggested by Reid et al.(2003), IRS 10EE is likely to be a Mira type variable star.
Acknowledgments. We thank Dr. K. Kameno and Dr. Y. Hagiwara, Mr. K. Asada and Mr. T. Oyama for helpful discussions.
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