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Register a new userUsing space-VLBI to probe gravity around Sgr A*
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Christian Michael Fromm
Publication date
2021
fields
Physics
and research's language is
English
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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.
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The Space VLBI 2020: Science and Technology Futures meeting was the second in The Future of High-Resolution Radio Interferometry in Space series. The first meeting (2018 September 5--6; Noordwijk, the Netherlands) focused on the full range of science applications possible for very long baseline interferometry (VLBI) with space-based antennas. Accordingly, the observing frequencies (wavelengths) considered ranged from below 1~MHz (> 300 m) to above 300~GHz (< 1 mm). For this second meeting, the focus was narrowed to mission concepts and the supporting technologies to enable the highest angular resolution observations at frequencies of 30~GHz and higher (< 1 cm). This narrowing of focus was driven by both scientific and technical considerations. First, results from the RadioAstron mission and the Event Horizon Telescope (EHT) have generated considerable excitement for studying the inner portions of black hole (BH) accretion disks and jets and testing elements of the General Theory of Relativity (GR). Second, the technologies and requirements involved in space-based VLBI differ considerably between 100~MHz and 100~GHz; a related consideration is that there are a number of existing instruments or mission concepts for frequencies of approximately 100~MHz and below, while it has been some time since attention has been devoted to space VLBI at frequencies above 10~GHz. This conference summary attempts to capture elements of presentations and discussions that occurred.
Scalar-tensor theories of gravity generally violate the strong equivalence principle, namely compact objects have a suppressed coupling to the scalar force, causing them to fall slower. A black hole is the extreme example where such a coupling vanishes, i.e. black hole has no scalar hair. Following earlier work, we explore observational scenarios for detecting strong equivalence principle violation, focusing on galileon gravity as an example. For galaxies in-falling towards galaxy clusters, the supermassive black hole can be offset from the galaxy center away from the direction of the cluster. Hence, well resolved images of galaxies around nearby clusters can be used to identify the displaced black hole via the star cluster bound to it. We show that this signal is accessible with imaging surveys, both ongoing ones such as the Dark Energy Survey, and future ground and space based surveys. Already, the observation of the central black hole in M~87 places new constraints on the galileon parameters, which we present here. $mathcal{O}(1)$ matter couplings are disfavored for a large region of the parameter space. We also find a novel phenomenon whereby the black hole can escape the galaxy completely in less than one billion years.
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.
While many aspects of general relativity have been tested, and general principles of quantum dynamics demand its quantization, there is no direct evidence for that. It has been argued that development of detectors sensitive to individual gravitons is unlikely, and perhaps impossible. We argue here, however, that measurement of polarization of the Cosmic Microwave Background due to a long wavelength stochastic background of gravitational waves from Inflation in the Early Universe would firmly establish the quantization of gravity.
The Galactic Center black hole Sagittarius A* is a variable NIR source that exhibits bright flux excursions called flares. The low-flux density turnover of the flux distribution is below the sensitivity of current single-aperture telescopes. We use the unprecedented resolution of the GRAVITY instrument at the VLTI. Our light curves are unconfused, overcoming the confusion limit of previous photometric studies. We analyze the light curves using standard statistical methods and obtain the flux distribution. We find that the flux distribution of SgrA* turns over at a median flux density of (1.1pm0.3)mJy. We measure the percentiles of the flux distribution and use them to constrain the NIR K-band SED. Furthermore, we find that the flux distribution is intrinsically right-skewed to higher flux density in log space. Flux densities below 0.1mJy are hardly ever observed. In consequence, a single powerlaw or lognormal distribution does not suffice to describe the observed flux distribution in its entirety. However, if one takes into account a power law component at high flux densities, a lognormal distribution can describe the lower end of the observed flux distribution. We confirm the RMS-flux relation for Sgr~A* and find it to be linear for all flux densities in our observation. We conclude that Sgr~A* has two states: the bulk of the emission is generated in a lognormal process with a well-defined median flux density and this quiescent emission is supplemented by sporadic flares that create the observed power law extension of the flux distribution.
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