"Research is formalized curiosity. It is poking and prying with a purpose."
-Zora Neale Hurston
NuSTAR (Nuclear Spectroscopic Telescope Array) is the first focusing high energy X-ray mission, opening the hard X-ray sky for sensitive study for the first time. NuSTAR is searching for black holes, mapping supernova explosions, and studying the most extreme active galaxies. NuSTAR is a NASA small explorer mission, managed by the Jet Propulsion Laboratory with science operations conducted at the California Institute of Technology. NuSTAR was successfully launched on June 13th 2012 from the Reagan test site at Kwajalein Atoll and is now orbiting the Earth at an altitude of 630km. NuSTAR has completed observatory commissioning and is now a fully functional space telescope. NuSTAR has been cited in approximately 15,000 academic papers (a list of many of them can be viewed here).
NuSTAR Detectors and Hardware
In order to observe a range of astronomical phenomena, NuSTAR uses custom-built detectors that count individual photons, recording the energy, time of arrival, and position of each with great precision. We continue to test and develop detectors for hard X-ray astronomy by characterizing different semiconducting materials, including Cadmium Telluride (CdTe) and Cadmium Zinc Telluride (CZT), and designing new readout electronics with smaller pixels for improved performance. In the future, these detectors may help to observe the X-ray sky with a wider bandpass and greater spectral resolution than has been possible with NuSTAR.
Galactic Accreting Compact Objects
Adapted from original artwork by NASA/JPL-Caltech/R. Hurt (IPAC).
Galactic accreting compact objects such as black holes and neutron stars are among the most interesting objects in the universe. They bend and twist space-time, or contain degenerate matter with unique properties. One of the best ways to study them is by observing their interaction with their surrounding media. We use X-ray telescopes such as NuSTAR (sometimes in combination with other telescopes for softer energies) to observe the high-energy emission from Galactic compact objects in binary systems (a.k.a., X-ray binaries), in which the compact object is gravitationally tied to a stellar companion. Accretion of matter from the star onto the compact object produces copious energetic radiation (X- and Gamma-rays). Implementing sophisticated techniques developed by our group to analyze the variability and the energy spectrum, we study the properties of black hole X-ray binaries such as the physical origin of the hard X-ray emission (the “corona”), the composition and geometry of the accretion disk, and the spins of black holes. We also study neutron star X-ray binaries, and their pulsating sub-class known as pulsars, using many of these same analysis techniques to probe the structure of the accretion disk, the emission properties of the pulsar beam, and the radius of the neutron star. As an added bonus, most of the physics uncovered in these systems can be directly applied to their much larger cousins, the supermassive black holes in the center of active galactic nuclei (AGN).
Many of the brightest sources in the X-ray sky are compact objects (neutron stars and black holes) accreting material from a companion star. Many of these systems have been known for decades, but undergo unpredictable flaring events or “state transitions” where the source get brighter or fainter in the X-ray band observed by NuSTAR. These events can be associated with changes in how much or how fast material is falling onto the compact object. In addition to the NuSTAR focused observations of these sources, light can also shine directly onto NuSTAR’s X-ray cameras via “stray light”. This provides a unique set of serendipitous observations of a variety of X-ray sources, especially of observations near the galactic plane when the density of X-ray sources is high and in the LMC/SMC.
The stray light team has produced the StrayCats catalog of sources, and has resulted in several science projects over the last few years studying nuclear burning on the surface of neutron stars, the long-term evolution of pulsars, and the evolution of black hole X-ray binary systems.
Active Galactic Nuclei (AGN), located at the center of galaxies, host supermassive black holes. These objects accrete the surrounding matter powering some of the most energetic persistent phenomena in the Universe. The emitted radiation covers the entire electromagnetic spectrum from gamma rays to radio wavelength. X-ray radiation allows astrophysicists to study the closest regions to the central black hole, probing the properties of these extreme compact objects and testing general relativity. In our group, we study the mechanism behind the emitting radiation in order to constrain the characteristics of the accreting matter and the hosting galaxy. We observe local, low-redshift, AGN with X-ray telescopes, such as NuSTAR, to analyse the spectral energy distribution with state-of-the-art models of the system developed in our group. This procedure allows us to investigate the geometry of these systems, the composition of the accretion disk, and to measure the spin of black holes.
Multi-wavelength observations are crucial to study the interplay between AGN and the local and cosmic environment (AGN feedback). We value the synergy between NuSTAR and different wavelength instruments, making use of the access to optical facilities such as the Palomar Observatory and Keck Observatory. We also study high-redshift AGN, within the first billion years of the Universe, to understand their evolution. Observing these objects gives the possibility to learn how these objects were formed and grew, with substantial cosmological implications on the evolution of the Universe.
Ultraluminous X-Ray Sources (ULXs)
Ultraluminous X-ray Sources (ULXs) are X-ray binaries in other galaxies undergoing such extreme accretion that they are some of the most luminous X-ray sources in the nearby Universe outside of the centers of galaxies. NuSTAR has played a critical role in establishing many of these sources as black holes or neutron stars undergoing accretion far above the theoretical Eddington limit, both by constraining the broadband spectra, which indicate a mode of accretion very different to those usually seen in moderately accreting X-ray binaries in our own Galaxy, and by detecting pulsations from some ULXs, which firmly identifies the accretors as highly super-Eddington neutron stars. We use X-ray and multiwavelength data to learn more about the geometry of these extreme systems, the powerful outflowing winds they produce, and the magnetic field properties of the neutron stars.
Matteo Bachetti, Ten years of High-Energy Universe in Focus: NuSTAR 2022