Mission

Dense matter and compact objects

NEBULA-Xplorer will investigate compact objects such as neutron stars and black hole X-ray binaries within our Milky Way. By investigating these objects, we can learn about fundamental physics such as strong gravity and the equation of state of neutron stars. We can also learn about accretion processes in extreme environments, unable to be reproduced in lab conditions. NEBULA-Xplorer’s coverage of the 0.5-12keV energy range will allow us to observe both the inner-most regions of these systems, where light is most affected by the gravity of compact objects and the magnetic environment of neutron stars.

By observing black holes in this energy range, we can also measure the spin of black holes by observing the relativistic smearing of emission resulting from the illumination of the disk by the corona or by measuring the inner radius of the accretion disk through broad continuum fitting. Understanding the spin population of black holes within X-ray binaries has major implications for stellar evolution in binary systems which are estimated to make up to 85% of star systems in our universe.

Time-domain Astrophysics

As a non-imaging X-ray timing observatory, NEBULA-Xplorer’s core science focuses on interpreting the variability of emission from the sources that it observes. This includes more traditional ways of thinking about variability such as looking for quasi-periodic signals in source light curve and phase-folded pulse profile modeling of neutron star surfaces but also includes topics such as “dipping” in the light curve produced by clumps of material produced by winds from both the accretion disk and companion star in these systems obscuring the central accretion engine.

Frequently, changes in variability are affected by multiple phenomena in the system that evolve on different time scales, making it difficult to understand how important each phenomenon is to driving transient outbursts or is linked to the fundamental process of accretion. NEBULA-Xplorer’s emphasis on performing continuous longer observations of individual sources will allow us to understand the short-term variability of emission and how that changes on timescales of days to weeks.

NEBULA-Xplorer’s moderate energy resolution and high timing resolution will also allow us to perform spectral-timing analysis of bright systems, like that of the NICER telescope. Spectral timing allows for the investigation of how variability changes with energy, which can be used to trace how the geometry of X-ray sources changes through time as well as understand the distances between different parts of the system through modeling of light travel time within the system.