When surrounded by a transparent emission region, black holes are expected to reveal a dark shadow caused by gravitational light bending and photon capture at the event horizon. To image and study this phenomenon, we have assembled the Event Horizon Telescope, a global very long baseline interferometry array observing at a wavelength of 1.3 mm. This allows us to reconstruct event-horizon-scale images of the supermassive black hole candidate in the center of the giant elliptical galaxy M87. We have resolved the central compact radio source as an asymmetric bright emission ring with a diameter of 42±3 μas, which is circular and encompasses a central depression in brightness with a flux ratio 10:1. The emission ring is recovered using different calibration and imaging schemes, with its diameter and width remaining stable over four different observations carried out in different days. Overall, the observed image is consistent with expectations for the shadow of a Kerr black hole as predicted by general relativity. The asymmetry in brightness in the ring can be explained in terms of relativistic beaming of the emission from a plasma rotating close to the speed of light around a black hole. We compare our images to an extensive library of ray-traced general-relativistic magnetohydrodynamic simulations of black holes and derive a central mass of M=(6.5±0.7)×10 9 M e . Our radiowave observations thus provide powerful evidence for the presence of supermassive black holes in centers of galaxies and as the central engines of active galactic nuclei. They also present a new tool to explore gravity in its most extreme limit and on a mass scale that was so far not accessible.

We summarize our current knowledge of neutron star masses and radii. Recent instrumentation and computational advances have resulted in a rapid increase in the discovery rate and precise timing of radio pulsars in binaries in the last few years, leading to a large number of mass measurements. These discoveries show that the neutron star mass distribution is much wider than previously thought, with 3 known pulsars now firmly in the 1.9−2.0 M⊙ mass range. For radii, large, high quality datasets from X-ray satellites as well as significant progress in theoretical modeling led to considerable progress in the measurements, placing them in the 9.9 − 11.2 km range and shrinking their uncertainties due to a better understanding of the sources of systematic errors. The combination of the massive neutron star discoveries, the tighter radius measurements, and improved laboratory constraints of the properties of dense matter has already made a substantial impact on our understanding of the composition and bulk properties of cold nuclear matter at densities higher than that of the atomic nucleus, a major unsolved problem in modern physics.

We present measurements of the properties of the central radio source in M87 using Event Horizon Telescope data obtained during the 2017 campaign. We develop and fit geometric crescent models (asymmetric rings with interior brightness depressions) using two independent sampling algorithms that consider distinct representations of the visibility data. We show that the crescent family of models is statistically preferred over other comparably complex geometric models that we explore. We calibrate the geometric model parameters using general relativistic magnetohydrodynamic (GRMHD) models of the emission region and estimate physical properties of the source. We further fit images generated from GRMHD models directly to the data. We compare the derived emission region and black hole parameters from these analyses with those recovered from reconstructed images. There is a remarkable consistency among all methods and data sets. We find that >50% of the total flux at arcsecond scales comes from near the horizon, and that the emission is dramatically suppressed interior to this region by a factor >10, providing direct evidence of the predicted shadow of a black hole. Across all methods, we measure a crescent diameter of 42±3 μas and constrain its fractional width to be <0.5. Associating the crescent feature with the emission surrounding the black hole shadow, we infer an angular gravitational radius of GM/Dc 2 =3.8±0.4 μas. Folding in a distance measurement of -+ 16.8 Mpc 0.7 0.8 gives a black hole mass of =  ´ | | M M 6.5 0.2 0.7 10 stat sys 9. This measurement from lensed emission near the event horizon is consistent with the presence of a central Kerr black hole, as predicted by the general theory of relativity.

The Event Horizon Telescope (EHT) has mapped the central compact radio source of the elliptical galaxy M87 at 1.3 mm with unprecedented angular resolution. Here we consider the physical implications of the asymmetric ring seen in the 2017 EHT data. To this end, we construct a large library of models based on general relativistic magnetohydrodynamic (GRMHD) simulations and synthetic images produced by general relativistic ray tracing. We compare the observed visibilities with this library and confirm that the asymmetric ring is consistent with earlier predictions of strong gravitational lensing of synchrotron emission from a hot plasma orbiting near the black hole event horizon. The ring radius and ring asymmetry depend on black hole mass and spin, respectively, and both are therefore expected to be stable when observed in future EHT campaigns. Overall, the observed image is consistent with expectations for the shadow of a spinning Kerr black hole as predicted by general relativity. If the black hole spin and M87's large scale jet are aligned, then the black hole spin vector is pointed away from Earth. Models in our library of non-spinning black holes are inconsistent with the observations as they do not produce sufficiently powerful jets. At the same time, in those models that produce a sufficiently powerful jet, the latter is powered by extraction of black hole spin energy through mechanisms akin to the Blandford-Znajek process. We briefly consider alternatives to a black hole for the central compact object. Analysis of existing EHT polarization data and data taken simultaneously at other wavelengths will soon enable new tests of the GRMHD models, as will future EHT campaigns at 230 and 345 GHz.

We present the first astrophysical measurement of the pressure of cold matter above nuclear saturation density, based on recently determined masses and radii of three neutron stars. The pressure at higher densities are below the predictions of equations of state that account only for nucleonic degrees of freedom, and thus present a challenge to the microscopic theory of neutron star matter.PACS numbers: 97.60. Jd, 26.60.Kp, Neutron stars probe the dense QCD phase diagram at lower temperatures and higher baryon densities, in contrast to the higher temperature-lower density regime in the early universe and in ultrarelativistic heavy-ion collisions [1,2]. The baryon density, ρ, in stellar interiors may reach an order of magnitude beyond nuclear saturation density, ρ ns ≃ 2.7 × 10 14 g cm −3 ≃ 0.16 fm −3 , and cross into a regime where quark degrees of freedom are excited or matter undergoes a meson condensation phase transition. The pressure of matter at these high densities can, as we show here, be extracted from current neutron star mass-radius determinations [3], and crucially constrains calculations of high density neutron star matter.The equation of state (EoS) of supranuclear matter determines the dividing line between neutron stars and black holes, and directly impacts the mechanism as well as outcomes of supernova explosions and the numbers of neutron stars and black holes in the Galaxy [4,5]. In certain models, it affects the mechanism and duration of gamma-ray bursts [6]. Accurate evolutions of inspiraling neutron-star binaries and the collapse to black holes, needed to calculate gravitational wave signals, depend sensitively on the assumed EoS [7].Microscopic calculations of the EoS of neutron-star matter have been based on a variety of inputs. The approach most firmly founded on experiment in the region of ρ ns is to determine two-body potentials from nucleonnucleon scattering data below 350 MeV and properties of light nuclei, supplemented by a three-body potential [8,9]. Such calculations, accurate in the neighborhood of ρ ns , have fundamental limitations. Beyond a few times ρ ns the forces between particles can no longer be described via static few-body potentials; since the characteristic range of the nuclear forces is ∼ 1/2m π , where m π is the mass of the pion, the parameter measuring the relative importance of three and higher body forces is ∼ ρ/(2m π ) 3 ∼ 0.35ρ/ρ ns . Thus, at ρ ≫ ρ ns a well defined expansion in terms of two-, three-, or more, body forces no longer exists. EoS based on nucleons alone do not take into account the rich variety of hadronic degrees of freedom that enter with increasing density. In addition, pion condensates [8,10,11] or kaon condensates [12,13] can enter at higher densities. Field-theoretic models based on nucleons interacting via meson exchange include, e.g., Ref [14]; see Ref.[15] for a general summary of EoS. However, one cannot assume that matter at higher densities can even be described in terms of well-defined "asymptotic" laboratory particles. More realist...