The latter assumption is standard for the more familiar cases of axisymmetry and time-translation symmetry. Our prediction for the polarimetric image of M87* assumes that there is some physical process in its near-horizon region capable of producing photons whose redshifted energy would lie in the observation band of the EHT, and that is moreover invariant under the same emergent symmetries as the background geometry.
Local emissions near the horizon of a black hole have large gravitational redshift relative to a distant observer such as the EHT.
#Event horizon telescope cost series
An infinite series of subsidiary whorls also appear nearer the edge of the shadow, arising from photons that librate around the black hole multiple times before escaping to infinity. The polarization lines form a distinctive ‘whorl’ that spirals into a central point inside the shadow. Their signal lies entirely within the shadow and thus is not obscured by emissions from behind the black hole. The simplifying nature of the extremal limit allows us to perform our calculations analytically, and we obtain striking polarimetric images of the near-horizon emissions. In this paper, we assume that M87* is indeed in the high-spin regime, and exploit its emergent conformal symmetry to derive a universal prediction for the polarization profile of its near-horizon emissions. Black holes with pronounced jets have often been found to have high spins, and the spin of M87* was argued to be within 2% of criticality in. įortunately, there are some indications that M87* could be just such a high-spin black hole. Over the past several years, this symmetry has been judiciously exploited in order to analytically compute several otherwise intractable 1 astrophysically relevant processes. This conformal symmetry imposes strong constraints on fields and matter near the horizon with observable astrophysical consequences.
This occurs because general relativity dictates that a rapidly spinning black hole develops an emergent conformal symmetry in its near-horizon region. Certain features exhibit universal ‘critical’ behaviour that is independent of the detailed assumptions and parameters entering the model. However, in the special case of a high-spin black hole, a dramatic simplification takes place. The proliferation of tunable parameters and model-dependent assumptions can make it difficult to extract physical predictions or intuition from numerics. Instead, extensive numerical simulation is required in order to properly account for myriad physical ingredients. As a result, analytic computation is in most cases completely infeasible. This task is further complicated by the need to make numerous assumptions regarding the black hole’s environment, such as the surrounding matter distribution and its radiative properties. The determination of the optical and polarimetric image of a generic black hole is in general an arduous task, as one must account for a multitude of complex astrophysical effects. The present work concerns this polarimetric image, which has received comparatively less attention. The EHT will also measure the polarization of incident light, which is expected to carry important information about dynamics in the region surrounding the black hole. The optical appearance of the black hole is largely determined by the brightness of the surrounding emission region, and has been the focus of intense investigation. The data collected by the EHT will provide a wealth of information about the electromagnetic emissions from the black hole’s vicinity. This offers an unprecedented opportunity for theorists to make predictions: What will the images look like? As its capabilities improve, the EHT will eventually resolve the near-horizon regions of these black holes with a few dozen pixels at the horizon scale. Meanwhile, in the arena of electromagnetic astronomy, the event horizon telescope (EHT) has recently delivered the first-ever up-close pictures of the supermassive black hole at the centre of the galaxy Messier 87 (M87*) and will soon release images of the black hole at the centre of our own galaxy (Sagittarius A*). With its celebrated detection of gravitational waves from a binary black hole merger, the LIGO collaboration has provided scientists with a radically new tool to study black holes. Observational black hole astronomy is entering an exciting era of rapid progress.
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