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Rao, Ramprasad

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Rao

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Ramprasad

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Rao, Ramprasad
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    An Unambiguous Detection of Faraday Rotation in Sagittarius A*
    (IOP Publishing, 2006) Marrone, Daniel P.; Moran, James; Zhao, Jun-Hui; Rao, Ramprasad
    The millimeter/submillimeter wavelength polarization of Sgr A* is known to be variable in both magnitude and position angle on time scales 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) × 10 5 rad m − 2; the measurements are consistent with a constant value. We conservatively assign a 3 σ upper limit of 2 × 10 5 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 × 10 − 7 M⊙ yr − 1 if the magnetic field is near equipartition, ordered, and largely radial, while a lower limit of 2 ×10 − 9 M⊙ yr − 1 holds even for a sub-equipartition, disordered, or toroidal field. The mean intrinsic position angle is 167 ◦ ± 7 ◦ and we detect variations of 31+18 −9 degrees. These variations must originate in the submillimeter photosphere, rather than arising from rotation measure changes.
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    The flare activity of Sagittarius A*
    (EDP Sciences, 2006) Eckart, A.; Baganoff, F. K.; Schödel, R.; Morris, M.; Genzel, R.; Bower, G. C.; Marrone, D.; Moran, James; 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, Jun-Hui; Rao, Ramprasad
    Context. We report new simultaneous near-infrared/sub-millimeter/X-ray observations of the SgrA* counterpart associated with the massive 3–4×106M⊙ black hole at the Galactic Center. Aims. The main aim is to investigate the physical processes responsible for the variable emission from SgrA*. Methods. The observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatory’s Very Large Telescope⋆ and the ACIS-I instrument aboard the Chandra X-ray Observatory as well as the Submillimeter Array SMA⋆⋆ on Mauna Kea, Hawaii, and the Very Large Array⋆⋆⋆ in New Mexico. Results. We detected one moderately bright flare event in the X-ray domain and 5 events at infrared wavelengths. The X-ray flare had an excess 2 - 8 keV luminosity of about 33×1033 erg/s. The duration of this flare was completely covered in the infrared and it was detected as a simultaneous NIR event with a time lag of ≤10 minutes. For 4 flares simultaneous infrared/X-ray observations are available. All simultaneously covered flares, combined with the flare covered in 2003, indicate that the time-lag between the NIR and X-ray flare emission is very small and in agreement with a synchronous evolution. There are no simultaneous flare detections between the NIR/X-ray data and the VLA and SMA data. The excess flux densities detected in the radio and sub-millimeter domain may be linked with the flare activity observed at shorter wavelengths. Conclusions. We find that the flaring state can be explained with a synchrotron self-Compton (SSC) model involving up-scattered sub-millimeter photons from a compact source component. This model allows for NIR flux density contributions from both the synchrotron and SSC mechanisms. Indications for an exponential cutoff of the NIR/MIR synchrotron spectrum allow for a straight forward explanation of the variable and red spectral indices of NIR flares.
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    Interferometric Measurements of Variable 340 GHz Linear Polarization in Sagittarius A*
    (IOP Publishing, 2006) Marrone, Daniel P.; Moran, James; Zhao, Jun‐Hui; Rao, Ramprasad
    Using the Submillimeter Array, we have made the first high angular resolution measurements of the linear polarization of Sagittarius A* at submillimeter wavelengths, and the first detection of intra-day variability in its linear polarization. We detected linear polarization at 340 GHz (880 µm) at several epochs. At the typical resolution of 1 . ′′ 4 × 2. ′′2, the expected contamination from the surrounding (partially polarized) dust emission is negligible. We found that both the polarization fraction and position angle are variable, with the polarization fraction dropping from 8.5% to 2.3% over three days. This is the first significant measurement of variability in the linear polarization fraction in this source. We also found variability in the polarization and total intensity within single nights, although the relationship between the two is not clear from these data. The simultaneous 332 and 342 GHz position angles are the same, setting a one-sigma rotation measure (RM) upper limit of 7 ×10 5 rad m − 2 . From position angle variations and comparison of “quiescent” position angles observed here and at 230 GHz we infer that the RM is a few ×10 5 rad m − 2, a factor of a few below our direct detection limit. A generalized model of the RM produced in the accretion flow suggests that the accretion rate at small radii must be low, below 10 − 6 − 10 − 7 M⊙ yr − 1 depending on the radial density and temperature profiles, but in all cases below the gas capture rate inferred from X-ray observations.