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

Whither accretion's boundaries?

Discovered by JAXA's MAXI payload during an outburst in 2017, the transient neutron-star binary system MAXI J1807+132 underwent additional outbursts in 2019 and 2023. The last of these was intensively monitored by NICER, and a study of the resulting dataset appears in a peer-reviewed paper recently published by S. Rout (NYU) and collaborators in The Astrophysical Journal.

Compared to black-hole binaries, systems in which a neutron star is the accreting body - drawing matter from a low-mass companion star - involve complexities due to the presence of a hard surface (as opposed to an event horizon) and a magnetic field that, in some cases, is strong enough to affect the flow of matter just before its arrival at the surface. Even as overall accretion phenomena show strong similarities - such as transitions between "hard," "intermediate," and "soft" accretion states - the two additional factors relevant to neutron stars can reveal themselves in the time-dependent spectrum of emitted X-rays. A typical outburst begins and ends in the hard state, dominated by high-energy X-rays from a "corona" of plasma. At its brightest, however, the outburst's intermediate and soft states herald the filling-in of the accretion disk: as it achieves its closest approach to the accreting body, spectrum and variability measurements encode the radius of the innermost edge of the disk, and (for neutron star accretors) reveal emission from a region known as the boundary layer, where the fast flow may slow down to match the star's surface rotation speed, likely under the influence of a magnetic field.

The Rout et al. team fit NICER spectra at the nearly daily cadence of the observations with models that include contributions from these accretion structures (see Figure), based on their distinct signatures as having thermal (disk and boundary layer, glowing at different temperatures) vs. non-thermal (corona) origins. As expected, the 2023 outburst of MAXI J1807 began with emission dominated by the coronal component, but the accretion disk rapidly (within 2-3 days) became prominent, followed soon after by the presence of emission from the boundary-layer. (The change was not permanent, however, as state transitions were seen twice more, not including a re-flare after an initial decay to near-quiescence around Modified Julian Day 60157.) The model fits implied that the inner edge of the disk in the hard state was "truncated" (i.e., far from the neutron star surface) but reached the innermost stable radius during the intermediate and soft states; the independent diagnostic of variability in X-ray brightness was found to be consistent with this conclusion. The implied value of this innermost radius depends on the poorly known distance to MAXI J1807, but for a variety of assumed distances conclusions can be drawn about the strength of the neutron star's magnetic field, as the inner flow interacts with - may be channeled or even repelled by - the magnetosphere. Finally, Rout et al. draw inferences about the size and shape of the corona and investigate unusual aspects of the late re-flare.