NICER Science Results

NICER Publications and Other Notable Items

• NICER Astronomer's Telegrams
Automated ADS query for Astronomer's Telegrams related to NICER
• NICER Refereed Papers
  
• Present NICER Science
• Use or Predict NICER Results


• Automated ADS Query
  A list of NICER publications including refereed and unrefereed articles (ATels, etc.) from July 2012 to present.
  
  
• NICER Selected Popular Publications
  
  
• NICER in the News
  
  
• NICER Science Nuggets
  High-level NICER science results, with a new science item posted every week. Listing also available by science topic.

Recent Science from NICER on the ISS

A vampire star's reflection

Binary star systems with short orbital periods are fertile ground for accretion, the transfer of matter from one star to another, which typiically produces copious X-ray emission. Key distinctions among such systems are the nature of the accretor (e.g., neutron star or black hole) and the size and chemical makeup of the donor star. When the orbital period is less than about 80 minutes, the separation between the stars is too small to accommodate a regular, hydrogen-rich "main sequence" star; only collapsed objects, such as white dwarfs rich in helium, carbon, or oxygen, can fit. Some four dozen of these ultracompact X-ray binaries (UCXBs) are known or suspected, all of which appear to host neutron-star accretors. A final useful feature of many UCXBs is that a relatively large fraction of them are persistently "on" - unlike most accreting systems, which are transient and only visible in episodic outbursts - enabling long-term studies of the accretion process.

The binary 4U 0614+091, with an orbital period of 50 minutes, has been well studied for nearly 45 years, but NICER's archive of 51 separate observations over 5 years represents a unique trove of data. A peer-reviewed paper led by D. Moutard (U. of Michigan) and collaborators, published this week in The Astrophysical Journal, compiles this dataset for a study of the evolving properties, on short and long timescales, of 4U 0614's X-ray emitting components: an accretion disk that produces a thermal glow and a hot electron "corona" cloud that generates higher-energy photons. The team used reflection modeling, spectroscopic analysis that accounts for X-rays that are re-processed and re-emitted within the binary, carrying a signature of the chemical makeup of the accreted material. Specifically, high-energy coronal photons stimulate fluorescence of atoms in the disk; when both the direct emission and these "reflected" X-rays can be measured, models of the process can be applied to reveal details of accretion physics at work on inaccessible spatial scales. Using a specialized model known as xillverCO, tuned specifically to the chemical compositions found in UCXBs, the study's chief finding is that a quantity called the emissivity index (q) - which characterizes how the reflected emission varies across the disk's radial extent - changes as the system's overall X-ray brightness varies. Values of q less than about 3 indicate a relatively flat emission profile (all disk radii contributing about equally), while larger values imply that smaller radii are brighter. The team found that high emissivity index is associated with high overall brightness, suggesting that stimulating photons from the corona are more concentrated toward the innermost disk during these intervals. This means, in turn, that in high-soft accretion states the corona is likely contracting and moving closer to the neutron star, where strong gravity bends more of the coronal photons toward the inner disk. Conversely, in the low-hard states, the reflected disk emission has a flatter profile, suggesting more extended (either vertically or radially) illumination by the corona. This is consistent with trends seen in some black-hole accreting systems. Ultimately, because UCXBs are likely sources of gravitational waves detectable by future observatories, a deeper understanding of their nature and evolution provides predictions that can help guide the design and capabilities of these facilities.