In June of 2017, NASA’s Neutron Star Interior Composition Explorer (NICER) was installed aboard the International Space Station (ISS). The purpose of this instrument is to provide high-precision measurements of neutron stars and other super-dense objects that are on the verge of collapsing into black holes. NICER is also be the first instrument designed to test technology that will use pulsars as navigation beacons.
Recently, NASA used data obtained from NICER’s first 22 months of science operations to create an x-ray map of the entire sky. What resulted was a lovely image that looks like a long-exposure image of fire dancers, solar flare activity from hundreds of stars, or even a visualization of the world wide web. But in fact, each bright spot represents an x-ray source while the bright filaments are their paths across the night sky.
The primary science goal of NICER requires that it target and track cosmic sources of x-rays and other energetic particles as the ISS orbits Earth every 93 minutes. However, the instrument’s detectors remain active even when it’s “nighttime” aboard the station, during which time the detectors will be wandering between targets.
It was this data, gathered during the “night moves” of the NICER instrument, that went into the creation of the image. Each arc traces the movements of particularly bright X-ray sources – which consists of pulsars, black holes, and distant galaxies (labeled in the image above) – relative to the ISS as it orbits the Earth.
The brightness of each point is the result of the time the NICER instrument spent looking directly at them, as well any additional energy that was picked up during its “night moves”. The image also reveals a diffuse glow that permeates the sky even far away from the bright sources, which corresponds to the X-ray background (XRB).
The prominent arcs, meanwhile, are due to the fact that NICER often follows the same paths between targets, the brightest of which are sources that NICER regularly monitors. Keith Gendreau, the mission’s principal investigator at NASA’s Goddard Space Flight Center, summarized the importance of NICER in a recent NASA press release:
“Even with minimal processing, this image reveals the Cygnus Loop, a supernova remnant about 90 light-years across and thought to be 5,000 to 8,000 years old. We’re gradually building up a new X-ray image of the whole sky, and it’s possible NICER’s nighttime sweeps will uncover previously unknown sources.”
NICER’s primary mission is to determine the size and dense of stellar remnants like neutron stars to within a 5% margin of error. Pulsars, which are rapidly-spinning neutron stars that appear pulse (hence the name), are among NICER’s regular targets because they are ideally suited to this type of “mass-radius” research.
These measures NICER gathers will help physicists to finally solve the mystery of what form matter takes inside the cores of these super-compressed objects. Aside from NICER, pulsars are the primary research focus of the Station Explorer for X-ray Timing and Navigation Technology (SEXTANT) experiment, which could aid in the development of cutting-edge navigation technology for space.
Like a GPS system, SEXTANT uses the precise timing of pulsar X-ray pulses to autonomously determine NICER’s position and speed in space. Coupled with NICER’s proven ability to use pulsars as timing sources, this technology could lead to the development of a deep-space navigation system that would allow for missions throughout the Solar System, and possibly even interstellar space.
Further Reading: NASA