Science: NuSTAR and Relativistic Jets
Studying nature's most powerful cosmic accelerators
Many galaxies – including our own – harbor immense black holes with masses millions to billions times greater than our Sun. Through much of cosmic history, these supermassive black holes will be quiescent with their surrounding material in stable orbits, much like the Earth is in a stable orbit about the Sun. However, episodes of activity occur during which surrounding gas is perturbed and accretes onto the supermassive black holes, causing intense radiation across the electromagnetic spectrum. While the brightest objects in the night sky at visual, or optical, wavelengths are stars in our own Milky Way Galaxy, the brightest objects visible to NuSTAR will be accreting supermassive black holes out to distances of several billion light-years.
Two of the primary goals of NuSTAR are to (1) take a cosmic census of galactic activity, and (2) to understand what is physically happening in the extreme environments near the accreting black holes to cause this intense radiation. We discuss the latter here.
We are fairly confident that accretion takes part via a rotating, disk-like structure encircling the black hole, and is often associated with powerful jets aligned with the rotation axis of the disk and black hole. Such structures have clearly been seen in several sources, such as M87 and Centaurus A. The jets contain particles moving at very high, or relativistic, speeds, and are strong astronomical sources from the radio to energies even higher than that probed by NuSTAR. If a jet points directly at us, relativistic effects (“Doppler boosting”) will cause it to look even more luminous. Such aligned sources have several unique properties and are known as “blazars.”
NuSTAR offers several new capabilities for studying and understanding blazars and the jets associated with active supermassive black holes. Our unprecendented sensitivity will allow us to probe the high energy spectrum of a large number of systems at high fidelity. Importantly, blazars are known to rapidly vary in brightness on time scales of days to years, likely related to changes in the feeding rate onto the supermassive black hole. NuSTAR will, for the first time, be able to study the high energy emission from these systems in detail while they are in both their actively accreting and quiescent states.
The time scale of the variability, and how the variability varies across the elecromagnetic spectrum gives important clues to the physical structure of the accretion disks and jets. For instance, the rapid variability immediately teaches us that the source size scale is quite small, only light-hours to light-days across. Lags between the high energy and optical variability teach us that the high energy emission is coming from closer to the black hole.
One of the primary science goals of NuSTAR is to measure variability of jet radiation in the hard X-ray band. These observations will be done in coordination with radio and optical observatories on the ground, as well as with NASA’s Fermi Gamma-Ray Space Telescope in orbit. Such data will allow us to assess the role of magnetic fields in the accretion disks, the structure and composition of the relativistic jets, as well as the energetics of the emission from these rare, extremely luminous systems.
