Black Holes

  • Variability of Sagittarius A*: Flares at 1 Millimeter

Abstract We report the results from recent observations of Sgr A* at short-/submillimeter wavelengths made with the partially finished Submillimeter Array (SMA) on Mauna Kea. A total of 25 epochs of observations were carried out over the past 15 months from 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 toward 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 Very Large Array data at 1 cm shows a global delay of t_delay > 3 days, suggesting that submillimeter 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 timescale (~1 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 minutes to 2.5 hr. The flares observed at the wavelengths between short-centimeter and submillimeter 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.

Zhao, Jun-Hui; Young, K.H.; Herrnstein1, R.M.; Ho, P. T. P.; Tsutsumi, T.; Lo, K. Y.; Goss, W. M. and Bower, G. C., 2003, ApJ, 586, L29

Zhao, Jun-Hui; Goss, W. M.; Lo, K.-Y.; Ekers, R. D., 1992, ASPC, 31, 295

Zhao, Jun-Hui; Bower, G. C.; Goss, W. M., 2001, ApJ, 547, L29

Zhao, Jun-Hui; Herrnstein, R. M.; Bower, G. C.; Goss, W. M.; Liu, S. M., 2004, ApJ, 603, L85

Herrnstein, Robeson M.; Zhao, Jun-Hui; Bower, Geoffrey C.; Goss, W. M., 2004, AJ, 127, 3399

Eckart, A.; Baganoff, F. K.; Schodel, R.; Morris, M.; Genzel, R.; Bower, G. C.; Marrone, D.; Moran, J. M.; Viehmann, T.; Bautz, M. W.; Brandt, W. N.; Garmire, G. P.; Ott, T.; Trippe, S.; Ricker, G. R.; Straubmeier, C.; Roberts, D. A.; Yusef-Zadeh, F.; Zhao, J. H.; Rao, R., 2006, AA, 450, 535

Marrone, D. P.; Baganoff, F. K.; Morris, M. R.; Moran, J. M.; Ghez, A. M.; Hornstein, S. D.; Dowell, C. D.; Munoz, D. J.; Bautz, M. W.; Ricker, G. R.; Brandt, W. N.; Garmire, G. P.; Lu, J. R.; Matthews, K.; Zhao, J.-H.; Rao, R.; Bower, G. C., 2008, ApJ, 682, 373

  • An Unambiguous Detection of Faraday Rotation in Sagittarius A*

Abstract The millimeter/submillimeter wavelength polarization of Sgr A* is known to be variable in both magnitude and position angle on timescales down to a few hours. The unstable polarization has prevented measurements made at different frequencies and different epochs from yielding convincing measurements of Faraday rotation in this source. Here we present observations made with the Submillimeter Array polarimeter at 227 and 343 GHz with sufficient sensitivity to determine the rotation measure at each band without comparing position angles measured at separate epochs. We find the 10-epoch mean rotation measure to be (-5.6+/-0.7) x 105 rad m-2 the measurements are consistent with a constant value. We conservatively assign a 3 sigma upper limit of 2 x 105 rad m-2 to rotation measure changes, which limits accretion rate fluctuations to 25%. This rotation measure detection limits the accretion rate to less than 2 x 10-7 Msun yr-1 if the magnetic field is near equipartition, ordered, and largely radial, while a lower limit of 2 x 10-9 Msun yr-1 holds even for a subequipartition, disordered, or toroidal field. The mean intrinsic position angle is 167+/-7deg and we detect variations of 31+18-9 deg. These variations must originate in the submillimeter photosphere, rather than arising from rotation measure changes.

Marrone, Daniel P.; Moran, James M.; Zhao, Jun-Hui; Rao, Ramprasad, 2007, ApJ, 654, L57

Marrone, Daniel P.; Moran, James M.; Zhao, Jun-Hui; Rao, Ramprasad, 2006, JPhCS, 54, 354

Marrone, Daniel P.; Moran, James M.; Zhao, Jun-Hui; Rao, Ramprasad, 2006, ApJ, 640, 308

Bower, Geoffrey C.; Backer, Donald C.; Zhao, Jun-Hui; Goss, Miller; Falcke, Heino, 1999, ApJ, 521, 582

  • Simultaneous Multiwavelength Observations of Sagittarius A*

Abstract We observed Sgr A* using the Very Large Array (VLA) and the Giant Metrewave Radio Telescope (GMRT) at multiple centimeter and millimeter wavelengths on 2003 June 17. The measured flux densities of Sgr A*, together with those obtained from the Submillimeter Array (SMA) and the Keck II 10 m telescope on the same date, are used to construct a simultaneous spectrum of Sgr A* from 90 cm to 3.8 micron. The simultaneous spectrum shows a spectral break at about 3.6 cm, a possible signature of synchrotron self-absorption of the strong radio outburst that occurred near epoch 2003 July 17. At 90 cm, the flux density of Sgr A* is 0.22+/-0.06 Jy, suggesting a sharp decrease in flux density at wavelengths longer than 47 cm. The spectrum at long cm wavelengths appears to be consistent with free-free absorption by a screen of ionized gas with a cutoff ~100 cm. This cutoff wavelength appears to be three times longer than that of ~30 cm suggested by Davies, Walsh, & Booth based on observations in 1974 and 1975. Our analysis suggests that the flux densities of Sgr A* at wavelengths longer than 30 cm could be attenuated and modulated by stellar winds from massive stars close to Sgr A*.

An, T.; Goss, W. M.; Zhao, Jun-Hui; Hong, X. Y.; Roy, S.; Rao, A. P.; Shen, Z.-Q., 2005, ApJ, 634, L49

Falcke, Heino; Goss, W. M.; Matsuo, Hiroshi; Teuben, Peter; Zhao, Jun-Hui; Zylka, Robert 1998, ApJ, 499, 731

  • Intrinsic Size and Shape of Sgr A*

Abstract By means of near-simultaneous multi-wavelength VLBA measurements, we determine for the first time the intrinsic size of Sgr A* to be 3.6 AU by < 1 AU or 72 Rsc(star) by < 20 Rsc(star), with the major axis oriented essentially north-south, where Rsc(star) ~7.5 times 1011 cm is the Schwarzschild radius for a 2.5 times 106 Msun black hole. Contrary to previous expectation that the intrinsic structure of Sgr A* is observable only at wavelengths shorter than 1 mm, we can discern the intrinsic source size at 7 mm because (1) the scattering size along the minor axis is half that along the major axis, and (2) the near simultaneous multi-wavelength mapping of Sgr A* makes it possible to extrapolate precisely the minor axis scattering angle at 7 mm. The intrinsic size and shape place direct constraints on the various theoretical models for Sgr A*.

Lo, K. Y.; Shen, Z.; Zhao, J.-H.; Ho, P. T. P., 1999, ASPC, 186, 72

Lo, K. Y.; Backer, D. C.; Kellermann, K. I.; Reid, M.; Zhao, J. H.; Goss, W. M.; Moran, J. M., 1993, Nature, 362, 38

Lo, K. Y.; Shen, Z.; Zhao, Jun-Hui; Ho, P. T. P., 1998, ApJ, 508, L61

Doeleman, S. S.; Shen, Z.-Q.; Rogers, A. E. E.; Bower, G. C.; Wright, M. C. H.; Zhao, J. H.; Backer, D. C.; Crowley, J. W.; Freund, R. W.; Ho, P. T. P.; Lo, K. Y.; Woody, D. P., 2001, AJ, 121, 2610

Bower, Geoffrey C.; Falcke, Heino; Herrnstein, Robeson M.; Zhao, Jun-Hui; Goss, W. M.; Backer, Donald C., 2004, Science, 304, 704

Zhi-Qiang Shen, K. Y. Lo, M.-C. Liang, Paul T. P. Ho & J.-H. Zhao, 2005, Nature, 438, 62

  • A transient radio source near the center of the Milky Way Galaxy

Abstract A new high-intensity radio source that appeared in late December 1990 near the center of the Galaxy is described. Following its first detection, the flux density of this Galactic-center transient (GCT) increased rapidly to a maximum a month later, and then declined gradually during the next 3 months. However, throughout its rising and decay phases, the GCT maintained a steep radio spectrum. Neutral-hydrogen-absorption and OH observations indicate that the GCT is either embedded in, or located behind, a molecular cloud moving with the velocity of +20 km/sec with respect to the local standard of rest. It is suggested that the GCT represents a transient radio emission from synchrotron-radiating plasma associated with an X-ray binary system.

Zhao, Jun-Hui; Roberts, D. A.; Goss, W. M.; Frail, D. A.; Lo, K. Y.; Subrahmanyan, R.; Kesteven, M. J.; Ekers, R. D.; Allen, D. A.; Burton, M. G., 1992, Science, 255, 1538