1. ATCA monitoring of Sgr A* at 3 millimeter Juan Li Shanghai Astronomical Observatory 2009/10/22 Collaborators: Z.Q. Shen (Shao.), A. Miyazaki (NAOJ), L. Huang (USTC), R. J. Sault (Univ. Melbourne), M. Miyoshi (NAOJ), M. Tsuboi & T. Tsutsumi (NAOJ)

2. Outline Introduction: IDV of of Sgr A* at mm wavelengths Observations: monitoring of Sgr A* with the ATCA Results: Two IDV (IntraDay Variability) events detected Discussion: two possible scenarios –Expanding plasmon model –Orbiting hot spot model Summary

3. Introduction: IDV at mm wavelengths Sgr A*: the closest massive black hole candidate temporally variable over all the EM bands IDV (Intraday Variability): –NMA@2- & 3-mm (Miyazaki et al. 2004) –OVRO@3-mm (Mauerhan et al. 2005) –SMA @1-mm (Marrone et al. 2006) … 2000 March 7 at 140 GHz flux increased by 30% from 3.5 to 4.7 Jy in 30 minutes twofold timescale  ~ 1.5 hrs (Miyazaki et al. 2004) Nobeyama Millimeter Array (NMA: 6x10m)

4. Observations: ATCA Australia Telescope Compact Array (ATCA) –6x22m telescopes –~10 hr per day at elevations >30 deg for Sgr A* –Available at 3mm since 2005 October Australia Telescope Compact Array (ATCA: 6x22m)

5. Observations: ATCA Search for IDVs with the ATCA –Frequency: 94/96 GHz, 86/89 GHz, ~120 hr in total from 2005 –Control sources: SiO maser (OH2.6-0.4) & other nearby continuum sources

6. Results (2005-2007) 2005: elevation-dependent gain effect ? 2006: two IDV events 2007: one IDV event IDV

7. 2 IDV events (Li et al. 2009, ApJ, 700, 417) –2006 August 12: 33% fractional variation in ~2.5 hr –2006 August 13: 2 peaks separated by ~4 hr, and maximum variation of 22% within ~2 hr Modulation Indices

8. Expanding plasmon model Flare at a given frequency is produced through the adiabatic expansion of an initially optically thick blob The initial rise due to the increase of blob’s surface area the effect of synchrotron cooling on this model (van der Laan 1966, Yusef-Zadeh et al. 2006, 2008, 2009; Eckart et al. 2008, 2009) the discrepancy increases with increasing magnetic field Take into account the synchrotron cooling Expanding plasma model

9. Expanding plasmon model Flare at a given frequency is produced through the adiabatic expansion of an initially optically thick blob The initial rise is related to the increase of blob’s surface taking into account the synchrotron cooling particle energy spectral index =2.2; quiescent flux density=1.4 Jy Expanding velocity: 0.004c, 0.003c Blob’s radius: 1.8 Rg, 2.6 Rg Ne: 2.56x10^7, 5.12x10^7 /cm^3 B: 19, 7 Gauss Expanding velocity: 0.005c, 0.005c, 0.005c Blob’s radius: 4.2 Rg, 2.8 Rg, 2.3 Rg Ne: 1.6x10^6, 2.56x10^7, 5.12x10^7 /cm^3 B: 23, 15, 13 Gauss (van der Laan 1966, Yusef-Zadeh et al. 2006, 2008, 2009; Eckart et al. 2008, 2009)

10. Orbiting hot spot model Rise/decay due to the Doppler shift and beaming of its orbital motion (quasi) periodic variation (e.g. NIR 17 min) In the accretion disk, the gas rotates with a sub-Keplerian angular velocity as a result of the viscous shear (Narayan et al. 1997) assume the rotation of hot spot is sub-Keplerian (a=0.4) (Broderick & Loeb 2005, 2006; Meyer et al. 2006a, 2006b, Eckart et al. 2008, Trippe et al. 2007) Hot spot radius: 4.1 Rg Ne: 6x10^6 /cm^3 B: 6 Gauss distance: 11.4 Rg Hot spot radius: 6.5, 8.0 Rg Ne: 4x10^6, 4x10^6 /cm^3 B: 3, 1 Gauss distance: 10, 12 Rg

11. Which model is better? Both models seem to be able to fit the IDV data The fitting parameters are consistent with other work Hot spot model is successful in explaining the quasi-periodical variation with a symmetric profile Expanding model can explain the time lag (~20 min) between the flare peaks observed at 13 and 7 mm  require multi-frequency monitoring Yusef-Zadeh et al. 2006

12. Simultaneous Observations (preliminary results) 2008 Oct 3: quasi-simultaneous observation of Sgr A* at 3- and 7-mm DCF: 3-mm leading 7-mm emission by 1 +/- 0.5 hr ( consistent with Yusef- Zadeh et al. 2009: ~1hr ) can be fitted simultaneously with expanding plasmon model Require more observations! Expanding velocity: 0.011c, 0.008c Blob’s radius: 6 Rg, 2 Rg Ne: 4x10^5, 1.024x10^7 /cm^3 B: 46, 15 Gauss

13. Summary We have performed monitoring observations of of Sgr A* at 3- mm with the ATCA since 2005 Sgr A* is quite active, clearly showing IDVs Both the expanding plasmon model and the orbiting hot spot model can reasonably fit the IDV light curves Future simultaneous multi-frequency monitoring is needed to discriminate between two flare emission models