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Bower G.C.,Academia Sinica Institute of Astronomy and Astrophysics | Dexter J.,Max Planck Institute for Extraterrestrial Physics | Markoff S.,University of Amsterdam | Gurwell M.A.,Harvard - Smithsonian Center for Astrophysics | And 2 more authors.
Astrophysical Journal Letters | Year: 2015

We analyze the light curves of 413 radio sources at submillimeter wavelengths using data from the Submillimeter Array calibrator database. The database includes more than 20,000 observations at 1.3 and 0.8 mm that span 13 years. We model the light curves as a damped random walk and determine a characteristic timescale τ at which the variability amplitude saturates. For the vast majority of sources, primarily blazars and BL Lac objects, we find only lower limits on τ. For two nearby low-luminosity active galactic nuclei, M81 and M87, however, we measure and , respectively ( errors). Including the previously measured for Sgr A∗, we show an approximately linear correlation between τ and black hole mass for these nearby low-luminosity AGNs (LLAGNs). Other LLAGNs with spectra that peak in the submillimeter are expected to follow this correlation. These characteristic timescales are comparable to the minimum timescale for emission processes close to an event horizon and suggest that the underlying physics may be independent of black hole mass, accretion rate, and jet luminosity. © 2015. The American Astronomical Society. All rights reserved.

Bower G.C.,Academia Sinica Institute of Astronomy and Astrophysics | Deller A.,ASTRON | Demorest P.,NRAO | Brunthaler A.,Max Planck Institute for Radio Astronomy | And 16 more authors.
Astrophysical Journal | Year: 2015

We measure the proper motion of the pulsar PSR J1745-2900 relative to the Galactic center massive black hole, Sgr A∗, using the Very Long Baseline Array (VLBA). The pulsar has a transverse velocity of 236 ± 11 km s-1 at position angle 22 ± 2 deg east of north at a projected separation of 0.097 pc from Sgr A∗. Given the unknown radial velocity, this transverse velocity measurement does not conclusively prove that the pulsar is bound to Sgr A∗; however, the probability of chance alignment is very small. We do show that the velocity and position are consistent with a bound orbit originating in the clockwise disk of massive stars orbiting Sgr A∗ and a natal velocity kick of ≲ 500 km s-1. An origin among the isotropic stellar cluster is possible but less probable. If the pulsar remains radio-bright, multiyear astrometry of PSR J1745-2900 can detect its acceleration and determine the full three-dimensional orbit. We also demonstrate that PSR J1745-2900 exhibits the same angular broadening as Sgr A∗ over a wavelength range of 3.6 cm to 0.7 cm, further confirming that the two sources share the same interstellar scattering properties. Finally, we place the first limits on the presence of a wavelength-dependent shift in the position of Sgr A∗, i.e., the core shift, one of the expected properties of optically thick jet emission. Our results for PSR J1745-2900 support the hypothesis that Galactic center pulsars will originate from the stellar disk and deepen the mystery regarding the small number of detected Galactic center pulsars. © 2015. The American Astronomical Society. All rights reserved.

Muoz D.J.,Harvard - Smithsonian Center for Astrophysics | Marrone D.P.,University of Chicago | Marrone D.P.,University of Arizona | Moran J.M.,Harvard - Smithsonian Center for Astrophysics | Rao R.,Academia Sinica Institute of Astronomy and Astrophysics
Astrophysical Journal | Year: 2012

We report the first detections of circularly polarized emission at submillimeter wavelengths from the compact radio source and supermassive black hole candidate SgrA* at a level of 1.2% 0.3% at 1.3 mm wavelength (230GHz) and 1.6% 0.3% at 860 μm (345GHz) with the same handedness, left circular polarization (LCP), as observed at all lower frequencies (1.4-15GHz). The observations, taken with the Submillimeter Array in multiple epochs, also show simultaneous linear polarization (LP) at both wavelengths of about 6%. These properties differ sharply from those at wavelengths longer than 1 cm (frequencies below 30GHz), where weak circular polarization (CP) (0.5%) dominates over LP, which is not detected at similar fractional limits. We describe an extensive set of tests to ensure the accuracy of our measurements. We find no CP in any other source, including the bright quasar 1924-292, which traces the same path on the sky as SgrA* and therefore should be subject to identical systematic errors originating in the instrument frame. Since a relativistic synchrotron plasma is expected to produce little CP, the observed CP is probably generated close to the event horizon by the Faraday conversion process. We use a simple approximation to show that the phase shift associated with Faraday conversion can be nearly independent of frequency, a sufficient condition to make the handedness of CP independent of frequency. Because the size of the τ = 1 surface changes by more than an order of magnitude between 1.4 and 345GHz, the magnetic field must be coherent over such scales to consistently produce LCP. To improve our understanding of the environment of SgrA* critical future measurements includes determining whether the Faraday rotation deviates from a λ2 dependence in wavelength and whether the circular and linear components of the flux density are correlated. © 2012. The American Astronomical Society. All rights reserved.

Bower G.C.,Academia Sinica Institute of Astronomy and Astrophysics | Markoff S.,University of Amsterdam | Dexter J.,Max Planck Institute for Extraterrestrial Physics | Gurwell M.A.,Harvard - Smithsonian Center for Astrophysics | And 11 more authors.
Astrophysical Journal | Year: 2015

We report new observations with the Very Large Array, Atacama Large Millimeter Array, and Submillimeter Array at frequencies from 1.0 to 355 GHz of the Galactic Center black hole, Sagittarius A∗. These observations were conducted between 2012 October and 2014 November. While we see variability over the whole spectrum with an amplitude as large as a factor of 2 at millimeter wavelengths, we find no evidence for a change in the mean flux density or spectrum of Sgr A∗ that can be attributed to interaction with the G2 source. The absence of a bow shock at low frequencies is consistent with a cross-sectional area for G2 that is less than 2 × 1029 cm2. This result fits with several model predictions including a magnetically arrested cloud, a pressure-confined stellar wind, and a stellar photosphere of a binary merger. There is no evidence for enhanced accretion onto the black hole driving greater jet and/or accretion flow emission. Finally, we measure the millimeter wavelength spectral index of Sgr A∗ to be flat; combined with previous measurements, this suggests that there is no spectral break between 230 and 690 GHz. The emission region is thus likely in a transition between optically thick and thin at these frequencies and requires a mix of lepton distributions with varying temperatures consistent with stratification. © 2015. The American Astronomical Society. All rights reserved.

Tsai M.,National Central University | Hwang C.-Y.,National Central University | Matsushita S.,Academia Sinica, Taiwan | Matsushita S.,Academia Sinica Institute of Astronomy and Astrophysics | And 4 more authors.
Astrophysical Journal | Year: 2012

We present CO(3-2) interferometric observations of the central region of the Seyfert 2 galaxy NGC1068 using the Submillimeter Array, together with CO(1-0) data taken with the Owens Valley Radio Observatory Millimeter Array. Both the CO(3-2) and CO(1-0) emission lines are mainly distributed within 5arcsec of the nucleus and along the spiral arms, but the intensity distributions show differences: the CO(3-2) map peaks in the nucleus, while the CO(1-0) emission is mainly located along the spiral arms. The CO(3-2)/CO(1-0) ratio is about 3.1 in the nucleus, which is four times as large as the average line ratio in the spiral arms, suggesting that the molecular gas there must be affected by the radiation arising from the active galactic nucleus. On the other hand, the line ratios in the spiral arms vary over a wide range from 0.24 to 2.34 with an average value around 0.75, which is similar to the line ratios of star formation regions, indicating that the molecular gas is affected by star formation. Besides, we see a tight correlation between CO(3-2)/(1-0) ratios in the spiral arms and star formation rate surface densities derived from Spitzer 8 μm dust flux densities. We also compare the CO(3-2)/(1-0) ratio and the star formation rate at different positions within the spiral arms; both are found to decrease as the radius from the nucleus increases. © 2012. The American Astronomical Society. All rights reserved.

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