NICER Science Results

NICER Publications and Other Notable Items

Recent Science from NICER on the ISS

Key results depend on sophisticated numerical simulations

This week, The Astrophysical Journal Letters accepted for publication a paper by the NICER science team (Bogdanov et al. 2019, Paper II) that describes a key method that we use to extract neutron star radius and mass measurements from NICER's X-ray observations of millisecond pulsars. This method, which drove the NICER design and mission concept, makes use of the gravitational bending of light expected due to the high density of neutron stars.

Many neutron stars appear to pulse because hot spots on their surfaces rotate in and out of our view as seen from the Earth. When the star's rotation places a hot spot on the side of the star nearer to the Earth, it appears to brighten. As the star rotates and the hot spot moves to the far side, as seen from the Earth, it fades. The millisecond pulsars that NICER observes rotate hundreds of times each second, leading to pulses every few milliseconds.

In strong gravity, Einstein's theory of general relativity predicts that light bends. The denser the object, the higher the degree of gravitational light bending. The effect with pulsars is that as a hot spot rotates out of our field of view, we continue to see some of its emission, depending on the ratio of the star's mass to its radius. This affects the depth of modulation of the pulses.

The visibility of antipodal hotspots changes with mass and radius
    of the neutron star due to relativistic effects

Figure: A rotating neutron star with a pair of antipodal hotspots as seen by a distant observer without light-bending (left) and for strong gravity (M/R = 0.25, right). The visibility of the far-side hotspot provides a measure of M/R through light curve analysis with NICER. (Adapted from Nollert et al. 1989.)

The paper describing our detailed numerical modeling of this effect will be published alongside NICER's first definitive results on neutron star mass and radius in the coming weeks.

Latest News

  • An X-ray Flare from eta Carinae as seen by NICER (03 Dec 2019)
    NICER monitoring of the massive colliding wind binary eta Carinae as the stars approach periastron passage has detected a sudden X-ray brightening. This X-ray flare is similar to the X-ray flaring seen at a similar orbital phase in previous binary orbital cycles. X-ray flaring should continue until the star reaches it X-ray maximum before the decline to the X-ray minimum on 2020 February 13.
  • NICER detection of 9.29 s pulsations from RX J0209.6-7427 (25 Nov 2019)
    NICER observations of the hard X-ray transient MAXI J0206-749 = HMXB RX J0209.6-7427 revealed a strong 9.29 s X-ray periodicity, making this system only the second confirmed pulsar in the Magellanic Cloud bridge.
  • NICER detection of a strong X-ray flare from GRS 1915+105 (25 Nov 2019)
    NICER detected a strong flare from the black hole binary system GRS 1915+105 during the source's current low flux state. The NICER flare was the largest yet seen from this system. This X-ray flare may have been the precursor to a strong radio flare seen from the source a short time after the NICER observation.
  • NASA's NICER Catches Record-setting X-ray Burst (07 Nov 2019)
    NICER detected an intense burst of X-rays at about 10:04 p.m. EDT on Aug. 20 from a massive thermonuclear flash on the surface of the pulsar SAX J1808.4-3658. This is the brightest burst seen by NICER so far. The burst shows a remarkable two-stage evolution in flux, along with and burst oscillations at the known pulsar spin frequency.
  • NICER Background Estimator Tools (30 Oct 2019)
    Software tools to estimate background in NICER observations are under development, based on analysis of deep NICER exposures of "blank-sky" fields. "Alpha-release" versions of these tools are now available to help NICER observers better estimate backgound in observations and proposals. Understanding of the NICER background is still evolving so the estimates provided by these new tools should be used with caution.

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