When astronomers refer to “compact objects,” they are generally referring to objects significantly more dense than a star or a planet. For example, white dwarfs or neutron stars are extremely dense stars that have collapsed, no longer able to produce a sufficient amount of pressure within to prevent their outer layers from falling into their centers. Under extreme conditions, these collapses can trigger the formation of a black hole – a region of space in which gravity is so strong that even light cannot escape.
Black Holes – Very Massive
Black holes with masses comparable to that of the Sun are scattered through the Milky Way and neighboring galaxies. Scientists also have found strong evidence of massive black holes – a million or more times more massive than the Sun – in the centers of many galaxies. In fact, one of these supermassive black holes sits at the heart of our very own Milky Way.
Detecting Black Holes
Compact objects are difficult to observe directly. Fortunately any ordinary matter falling toward them – or disappearing entirely into a black hole – tends to heat up and radiate in the process. Sometimes great streams, or “jets,” of matter and energy surge into space at velocities nearly equal to the speed of light. As matter falls onto a compact object, energy is often released in the form of X-rays and gamma-rays. Likewise, rotating neutron stars can produce copious amounts of high-energy radiation but because Earth's atmosphere absorbs most of this kind of radiation, observations must be made from space. Measurements have revealed that these phenomena are responsible for the highest energies yet detected in the universe.
Compact objects, celestial bodies characterized by very strong gravity such as neutron stars, pulsars and black holes, are subjects of intense research at KIPAC. KIPAC scientists study these various compact objects using data from the Fermi Gamma-ray Space Telescope. The gamma-ray emissions from pulsars allow researchers to study how their intense, pulsed radiation is produced. Researchers now believe that pulsars behave like powerful magnets in which the poles are not aligned with the axis of the star's rotation. The energetic particles emanating from a pulsar form a beacon — similar to the beam of the lighthouse — which periodically sweeps across our line of sight once or twice per revolution. By taking an accurate count of pulsars, KIPAC scientists have been able to estimate how often stellar collapses take place in the Milky Way.
Developing Complex Models
In addition to neutron stars and pulsars, the Fermi telescope has detected hundreds of black holes with powerful jets. To get a better idea of the structure of these jets and hone in on the nature of the radiating particles, KIPAC scientists are using the Fermi data as well as observations in the radio, visible, and X-ray bands to build computer models. The models are complex but KIPAC scientists have been among the very first to accurately describe the twisted magnetic field that occurs when matter falls into a black hole and deduce how the field determines the precise alignment of the jet.