SpaceTime

This artist’s concept shows a black hole with an accretion disk -- a flat structure of material orbiting the black hole – and a jet of hot gas, called plasma.  Credits: NASA/JPL-Caltech Black holes are famous for being ravenous eaters, but they do not eat everything that falls toward them. A small portion of material gets shot back out in powerful jets of hot gas, called plasma, that can wreak havoc on their surroundings. Along the way, this plasma somehow gets energized enough to strongly radiate light, forming two bright columns along the black hole’s axis of rotation. Scientists have long debated where and how this happens in the jet. Astronomers have new clues to this mystery. Using NASA's NuSTAR space telescope and a fast camera called ULTRACAM on the William Herschel Observatory in La Palma, Spain, scientists have been able to measure the distance that particles in jets travel before they "turn on" and become bright sources of light. This distance is call...

Image: The spiral galaxy NGC 6814. Credit: ESA/Hubble & NASA . Acknowledgement: Judy Schmidt (Geckzilla) Spiral galaxies together with irregular galaxies make up approximately 60% of the galaxies in the local Universe. However, despite their prevalence, each spiral galaxy is unique - like snowflakes, no two are alike. This is demonstrated by the striking face-on spiral galaxy NGC 6814, whose luminous nucleus and spectacular sweeping arms, rippled with an intricate pattern of dark dust, are captured in this NASA/ESA Hubble Space Telescope image. NGC 6814 has an extremely bright nucleus, a telltale sign that the galaxy is a Seyfert galaxy. Seyfert galaxies account for about 10% of all galaxies and are some of the most intensely studied objects in astronomy, as they are thought to be powered by the same phenomena that occur in quasars, although they are closer and less luminous than quasars. These galaxies have supermassive black holes at their centers which are surro...

Image: This computer-simulated image shows gas from a star that is ripped apart by tidal forces as it falls into a black hole. Credits: NASA, S. Gezari (The Johns Hopkins University), and J. Guillochon (University of California, Santa Cruz) When a star in a galactic nucleus is deflected too close to the central supermassive black hole (BH), it can be torn apart by tidal forces. During this tidal disruption event (TDE), roughly half of the stellar debris remains bound to the BH, while the other half is flung outwards and unbound from the system. The bound material, following a potentially complex process of debris circularization accretes onto the BH, creating a luminous flare lasting months to years.

Picture: These images show the merger of two neutron stars simulated using a new supercomputer model. Redder colors indicate lower densities. Green and white ribbons and lines represent magnetic fields. The orbiting neutron stars rapidly lose energy by emitting gravitational waves and merge after about three orbits, or in less than 8 milliseconds. The merger amplifies and scrambles the merged magnetic field. A black hole forms and the magnetic field becomes more organized, eventually producing structures capable of supporting the jets that power short gamma-ray bursts. Credit: NASA/AEI/ZIB/M. Koppitz and L. Rezzolla The LIGO and Virgo Collaborations recently reported the first direct detection of a gravitational-wave (GW) signal and demonstrated that it was produced by the inspiral and coalescence of a binary black hole (BHBH) system.

Image: Quasar Pair Captured in Galaxy Collision. Credits: X-ray: NASA/CXC/SAO/P. Green et al. Optical: Carnegie Obs./Magellan/W. Baade Telescope/J.S. Mulchaey et al. Strong observational evidence suggests that every massive galaxy hosts a supermassive black hole in its nucleus. The central black hole (BH) is an important component of the galaxy, since the BH mass is correlated with the global properties of the host galaxy, e.g., dispersion velocity, bulge luminosity, or bulge mass.

Image: Merging black holes. Credit: NASA Recently it has been suggested that electromagnetic signals detected by Fermi GBM could be associated with the merger of the two black holes detected by LIGO ( GW150914 ).

Illustration of a black hole. Image Credit & Copyright: Alain Riazuelo The existence of the singularity is an intrinsic problem of the General Relativity (GR). At the fundamentally level, the resolution of the problem of the singularity lies with the expectation that under situations where quantum effects become strong, the behavior of gravity could possibly greatly deviate from that predicted by the classical theory of GR. Regular black hole solution are proposed with the same spacetime geometry outside the horizon as the traditional black hole, but bears no singularity inside. Whether or not black hole singularities should exist, they would be covered by the black hole horizon. The black hole horizon serves as an information curtain hindering outside observers from directly observing the interior structure of the black hole, and determining that whether or not the black hole singularity does really exist. A method is needed to check the correctness of the new constructions ...

Image: Illustration of a dusty supermassive black hole. Credit: ESA/NASA, the AVO project and Paolo Padovani Black holes could be bouncing stars as a consequence of quantum gravity: when the density of matter becomes high enough, quantum gravity effects generate sufficient pressure to compensate the matter's weight, the collapse ends, and matter bounces out. In a black hole, matter's collapse could stop before the central singularity is formed.

Image: Simulation of two colliding black holes. Animation created by SXS, the Simulating eXtreme Spacetimes (SXS) project (http://www.black-holes.org) - Caltech LIGO If the GW signal observed by LIGO is due to the merger of two isolated black holes (BHs) in vacuum, no electromagnetic counterparts are expected. However, Fermi observed a signal 0.4 s after LIGO in a region of space compatible with the GW source.

Image: Two black holes are entwined in a gravitational tango in this artist's conception. Credit: NASA A binary black hole (BBH) is a system consisting of two black holes in close orbit around each other. Binary black holes are often divided into stellar binary black holes, formed either as remnants of high-mass binary star systems or by dynamic processes and mutual capture, and binary supermassive black holes believed to be a result of galactic mergers.

Image: The sequence illustrates the macronova model for the formation of a short-duration gamma-ray burst. 1. A pair of neutron stars in a binary system spiral together. 2. In the final milliseconds, as the two objects merge, they kick out highly radioactive material. This material heats up and expands, emitting a burst of light called a macronova. 3. The fading fireball blocks visible light but radiates in infrared light. 4. A remnant disk of debris surrounds the merged object, which may have collapsed to form a black hole Credit: NASA, ESA, and A. Feild (STScI) A macronova (also called a 'kilonova' or an 'r-process supernova' ) occurs when two neutron stars or a neutron star and a black hole merge. It is a near-infrared/optical transient powered by the radioactive decay of heavy elements synthesized in the ejecta (~10 -4 -10 -1 M sun with velocities ~ 0.1-0.3c) of a compact binary merger. Strong electromagnetic radiation is emitted due to the decay of h...

Image: The magenta spots in this image indicate two black holes in the spiral galaxy called NGC 1313, the Topsy Turvy galaxy. Both black holes belong to a class called ultraluminous X-ray sources, or ULXs. The magenta X-ray data come from NASA's Nuclear Spectroscopic Telescopic Array (NuSTAR) and are overlaid on a visible image from the Digitized Sky Survey. ULXs consist of black holes actively accreting, or feeding, off material drawn in from a partner star. Astronomers are trying to figure out why ULXs shine so brightly with X-rays. NuSTAR's new high-energy X-ray data on NGC 1313 helped narrow down the masses of the black holes in the ULXs: the black hole closer to the center of the galaxy is about 70 to 100 times that of our sun. The other black hole is probably smaller, about 30 solar masses. The Topsy Turvy galaxy is located about 13 million light-years away in the Reticulum constellation. Credit: NASA's Goddard Space Flight Center/JPL-Caltech/IRAP An Ultra-Lum...

Image: This artist's impression illustrates how high-speed jets from supermassive black holes would look. These outflows of plasma are the result of the extraction of energy from a supermassive black hole’s rotation as it consumes the disc of swirling material that surrounds it. Credit: NASA, ESA, M. Chiaberge (STScI) The radiation from an active galactic nuclei (AGN) is believed to be a result of accretion of mass by a supermassive black hole at the centre of its host galaxy. In a recent paper (Hirotani et al., 2016 ApJ) the authors demonstrate that electrons and positrons created by photon collisions near a rotating black hole are accelerated in the opposite direction by the electric field present near the event horizon. This electric field acts as a particle accelerator (or gap) and dissipates a part of the hole’s rotational energy. The resultant gamma-ray luminosity increases with decreasing plasma accretion from the surroundings. The authors show that in the case o...

Image: Artistic illustration of a black hole divouring a star. Credit: NASA/Goddard Space Flight Center/CI Lab   A tidal disruption event occurs when a star gets close enough to a supermassive black hole's event horizon and is pulled apart by the black hole's tidal forces.

Supermassive black holes at the cores of galaxies blast radiation and ultra-fast winds outward, as illustrated in this artist's conception. Credit: NASA/JPL-Caltech Broad emission lines are a hallmark feature of type 1 active galactic nuclei (AGNs) and quasars. Many basic properties of the broad-line region (BLR), such as its basic geometry, dynamics, and physical connection to the accretion disk around the supermassive black hole (BH), remain illdefined. AGN spectra exhibit both tremendous diversity as well as discernable patterns of systematic regularity.

Image: X-rays from Chandra in blue and infrared emission from the Hubble Space Telescope in red and yellow. The inset shows a close-up view of Sgr A* in X-rays only, covering a region half a light year wide. The diffuse X-ray emission is from hot gas captured by the black hole and being pulled inwards. This hot gas originates from winds produced by a disk-shaped distribution of young massive stars observed in infrared observations. Credit: X-ray: NASA/UMass/D.Wang et al., IR: NASA/STScI Within the central tenth of a parsec in the middle of our galaxy there is a concentration of young stars (S-stars) that interact with a supermassive black hole Sagittarius A* (Sgr A*).

Image Credit: NASA/Goddard Space Flight Center Radio emission of radio-quiet quasars may be due to stars formation in the quasar host galaxy, to a jet launched by the supermassive black hole, or to relativistic particles accelerated in a wide-angle radiatively-driven outflow.

Image Credit: G. Vasilopoulos & M. Petropoulou. RGB image of the X-ray dust scattered rings observed by Swift/XRT. The Swift/XRT field-of-view is enclosed by the green circle, while a white circle of radius 9′ is overplotted for guiding the eye. The black hole is located at the center of the rings. First detection of X-ray dust scattered rings from the Galactic low mass X-ray binary V404 Cyg. The observation of the system with Swift/XRT on June 30 2015 revealed the presence of five concentric ring-like structures centred at the position of V404 Cyg.