Science: The Milky Way Galaxy
Identifying high energy sources in our Galaxy
NuSTAR is poised to make several important studies of our own Milky Way Galaxy with unprecedented sensitivity. Whereas low-energy X-rays are subject to strong absorption from gas, the high energy X-rays observed by NuSTAR pass through undiminished. This, together with a more than 100 times greater sensitivity compared to previous telescopes in this energy range, implies that NuSTAR will open a new window on the the densest regions and sources in the Galaxy.
NuSTAR will make deep maps of the inner few hundred parsecs around the Galactic center. This region contains roughly 1% of the entire Galaxy's stellar mass, up to 10% of its most massive, young stars, and, at the very center, a supermassive black hole which has a mass four million times that of our Sun. The black hole, known as Sagittarius A* (or Sgr A*), is one of the most puzzling objects known. Despite its immense mass, Sgr A* is very faint. In fact, it is the most underluminous supermassive black hole known. Much of our detailed knowledge of Sgr A* comes from indirect observations of stars orbiting around it. NuSTAR observations of Sgr A* flares, thought to result from instabilities in the flow of matter into the black hole, can test models of this inflow. NuSTAR will also illucidate the nature of mysterious high-energy emission seen by the INTEGRAL satellite very near Sgr A*. Indeed, NuSTAR observations of the Galactic Center region are likely to discover entirely new source populations, or, at the very least, increase the numbers of known objects dramatically
One rare, but exciting, class of Galactic object which NuSTAR will study is magnetars. Magnetars are young, isolated, ultra-magnetized neutron stars, and are now widely believed to be powered by the decay of their intense internal magnetic fields, the largest fields known in the Universe. Stresses from this decay cause a variety of observable, often violent, magnetic activity, which includes variability on many time scales, from milliseconds to years. The physics of magnetars is still poorly understood, as is their origin, and over the past decade they have provided astronomers with many surprises. Indeed, one of the unexpected discoveries made by the INTEGRAL satellite is that magnetars are very luminous sources in the hard X-ray band. The origin of this high energy emission is unknown, although several models have been proposed. NuSTAR will do a comprehensive high-energy study of magnetars, first by monitoring bright sources in the soft and hard X-ray ranges to see if the respective emission mechanisms are correlated, as is predicted in some models. Second, NuSTAR will measure precise hard X-ray spectra for practically all known magnetars. If we are lucky and there is a major magnetar outburst during NuSTAR's lifetime, we will have the opportunity to study magnetar bursts, and perhaps illucidate the nature of mysterious spectral features reported in some magnetar bursts by NASA's Rossi X-ray Timing Explorer, a possible uniquely direct signature of the high magnetic field.
Finally, NuSTAR will also be important for the study of "Pulsar Wind Nebulae", generally spectacular nebular objects powered by the central young and rapidly rotating neutron star as it slows down. Such neutron stars, observed as "pulsars" thanks to a misalignment of their rotation and magnetic axes (the analogous phenomenon on Earth results in the the magnetic North pole not coinciding with the geographic North Pole), also somehow manages to produce a powerful, relativistic matter/anti-matter wind. This wind, upon interaction with the surrounding medium, sets up the dramatic nebula, whose features often include torii and jets. Understanding this wind and its interaction with its surroundings is a top priority in high-energy astrophysics, as it likely will help us understand the nature of astrophysical jets more generally. Thanks to its unprecendented angular resolution, NuSTAR will be able to study the morphology of several Pulsar Wind Nebulae for the first time in the hard X-ray band, as well as determine accurate nebular spectra, thereby constraining the properties of the relativistic wind. NuSTAR observations are also important for understanding the origin of very high energy gamma-ray emission discovered recently from many pulsar wind nebulae by the High Energy Stereoscopic System (HESS) telescope, among others.