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Register a new userGoing with the flow: using gas clouds to probe the accretion flow feeding Sgr A*
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The massive black hole in our galactic center, Sgr A*, accretes only a small fraction of the gas available at its Bondi radius. The physical processes determining this accretion rate remain unknown, partly due to a lack of observational constraints on the gas at distances between ~10 and ~10$^5$ Schwarzschild radii (Rs) from the black hole. Recent infrared observations identify low-mass gas clouds, G1 and G2, moving on highly eccentric, nearly co-planar orbits through the accretion flow around Sgr A*. Although it is not yet clear whether these objects contain embedded stars, their extended gaseous envelopes evolve independently as gas clouds. In this paper we attempt to use these gas clouds to constrain the properties of the accretion flow at ~10$^3$ Rs. Assuming that G1 and G2 follow the same trajectory, we model the small differences in their orbital parameters as evolution resulting from interaction with the background flow. We find evolution consistent with the G-clouds originating in the clockwise disk. Our analysis enables the first unique determination of the rotation axis of the accretion flow: we localize the rotation axis to within 20 degrees, finding an orientation consistent with the parsec-scale jet identified in x-ray observations and with the circumnuclear disk, a massive torus of molecular gas ~1.5 pc from Sgr A*. This suggests that the gas in the accretion flow comes predominantly from the circumnuclear disk, rather than the winds of stars in the young clockwise disk. This result will be tested by the Event Horizon Telescope within the next year. Our model also makes testable predictions for the orbital evolution of G1 and G2, falsifiable on a 5-10 year timescale.
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We study the dynamical evolution of the putative gas clouds G1 and G2 recently discovered in the Galactic center. Following earlier studies suggesting that these two clouds are part of a larger gas streamer, we combine their orbits into a single trajectory. Since the gas clouds experience a drag force from background gas, this trajectory is not exactly Keplerian. By assuming the G1 and G2 clouds trace this trajectory, we fit for the drag force they experience and thus extract information about the accretion flow at a distance of thousands of Schwarzschild radii from the black hole. This range of radii is important for theories of black hole accretion, but is currently unconstrained by observations. In this paper we extend our previous work by accounting for radial forces due to possible inflow or outflow of the background gas. Such radial forces drive precession in the orbital plane, allowing a slightly better fit to the G1 and G2 data. This precession delays the pericenter passage of G2 by 4-5 months relative to estimates derived from a Keplerian orbital fit; if it proves possible to identify the pericenter time observationally, this enables an immediate test of whether G1 and G2 are gas clouds part of a larger gas streamer. If G2 is indeed a gas cloud, its closest approach likely occurred in late summer 2014, after many of the observing campaigns monitoring G2s anticipated pericenter passage ended. We discuss how this affects interpretation of the G2 observations.
Millimeter polarimetry of Sgr A* probes the linearly polarized emission region on a scale of $sim 10$ Schwarzschild radii ($R_S$) as well as the dense, magnetized accretion flow on scales out to the Bondi radius ($sim 10^5 R_S$) through Faraday rotation. We present here multi-epoch ALMA Band 6 (230 GHz) polarimetry of Sgr A*. The results confirm a mean rotation measure, ${rm RM} approx -5 times 10^5 {rm rad m^{-2}}$, consistent with measurements over the past 20 years and support an interpretation of the RM as originating from a radiatively inefficient accretion flow (RIAF) with $dot{M} approx 10^{-8} { rm M_{odot} y^{-1} }$. Variability is observed for the first time in the RM on time scales that range from hours to months. The long-term variations may be the result of changes in the line of sight properties in a turbulent accretion flow. Short-term variations in the apparent RM are not necessarily the result of Faraday rotation and may be the result of complex emission and propagatation effects close to the black hole, some of which have been predicted in numerical modeling. We also confirm the detection of circular polarization at a mean value of $-1.1 pm 0.2 %$. It is variable in amplitude on time scales from hours to months but the handedness remains unchanged from that observed in past centimeter- and millimeter-wavelength detections. These results provide critical constraints for the analysis and interpretation of Event Horizon Telescope data of Sgr A*, M87, and similar sources.
The Event Horizon Telescope (EHT) will soon provide the first high-resolution images of the Galactic Centre supermassive black hole (SMBH) candidate Sagittarius A* (Sgr A*), enabling us to probe gravity in the strong-field regime. Besides studying the accretion process in extreme environments, the obtained data and reconstructed images could be used to investigate the underlying spacetime structure. In its current configuration, the EHT is able to distinguish between a rotating Kerr black hole and a horizon-less object like a boson star. Future developments can increase the ability of the EHT to tell different spacetimes apart. We investigate the capability of an advanced EHT concept, including an orbiting space antenna, to image and distinguish different spacetimes around Sgr A*. We use GRMHD simulations of accreting compact objects (Kerr and dilaton black holes, as well as boson stars) and compute their radiative signatures via general relativistic radiative transfer calculations. To facilitate comparison with upcoming and future EHT observations we produce realistic synthetic data including the source variability, diffractive and refractive scattering while incorporating the observing array, including a space antenna. From the generated synthetic observations we dynamically reconstructed black hole shadow images using regularised Maximum Entropy methods. We employ a genetic algorithm to optimise the orbit of the space antenna with respect to improved imaging capabilities and u-v-plane coverage of the combined array (ground array and space antenna and developed a new method to probe the source variability in Fourier space. The inclusion of an orbiting space antenna improves the capability of the EHT to distinguish the spin of Kerr black holes and dilaton black holes based on reconstructed radio images and complex visibilities.
We explore the MRI driven dynamo in a radiatively inefficient accretion flow (RIAF) using the mean field dynamo paradigm. Using singular value decomposition (SVD) we obtain the least squares fitting dynamo coefficients $alpha$ and $gamma$ by comparing the time series of the turbulent electromotive force and the mean magnetic field. Our study is the first one to show the poloidal distribution of these dynamo coefficients in global accretion flow simulations. Surprisingly, we obtain a high value of the turbulent pumping coefficient $gamma$ which transports the mean magnetic flux radially outward. This would have implications for the launching of magnetised jets which are produced efficiently in presence a large-scale poloidal magnetic field close to the compact object. We present a scenario of a truncated disc beyond the RIAF where a large scale dynamo-generated poloidal magnetic field can aid jet-launching close to the black hole. Magnitude of all the calculated coefficients decreases with radius. Meridional variations of $alpha_{phi phi}$, responsible for toroidal to poloidal field conversion, is very similar to that found in shearing box simulations using the `test field (TF) method. By estimating the relative importance of $alpha$-effect and shear, we conclude that the MRI driven large-scale dynamo, which operates at high latitudes beyond a disc scale height, is essentially of the $alpha-Omega$ type.
We present an analysis of the 101 ks, 2007 Suzaku spectrum of the LINER galaxy NGC 1052. The 0.5-10 keV continuum is well-modeled by a power-law modified by Galactic and intrinsic absorption, and it exhibits a soft, thermal emission component below 1 keV. Both a narrow core and a broader component of Fe K emission centered at 6.4 keV are robustly detected. While the narrow line is consistent with an origin in material distant from the black hole, the broad line is best fit empirically by a model that describes fluorescent emission from the inner accretion disk around a rapidly rotating black hole. We find no evidence in this observation for Comptonized reflection of the hard X-ray source by the disk above 10 keV, however, which casts doubt on the hypothesis that the broad iron line originates in the inner regions of a standard accretion disk. We explore other possible scenarios for producing this spectral feature and conclude that the high equivalent width (EW ~ 185 keV) and full-width-half-maximum velocity of the broad iron line (v ~ 0.37c) necessitate an origin within d ~ 8 gravitational radii of the hard X-ray source. Based on the confirmed presence of a strong radio jet in this galaxy nucleus, the broad iron line may be produced in dense plasma near the base of the jet, implying that emission mechanisms in the centralmost portions of active galactic nuclei are more complex than previously thought.
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