What are the curved lines in this X-ray image of the sky? This image was created using the NICER instrument on the ISS.

Source:

https://www.nasa.gov/feature/goddard/2019/nicer-s-night-moves-trace-the-x-ray-sky



What are the big curves between the X-ray sources?
GIFs too big to post here directly: 1, 2

NICER is all about time resolution and energy resolution and is used to collect the time and energy-resolved X-ray spectra from pulsars. It isn't an imaging telescope and so can't record "patches of the sky" like other devices can.

The curved lines are the paths that the narrow field of view (FOV) of the NICER telescope has traced out across the celestial sphere during this 22 month (intermittent) observation period.

According to this page the FOV is only about 30 square arc minutes:


Large effective area: ~1900 cm2 at 1.5 keV
Broad Bandpass: 0.2 &lt; E &lt; 12.0 keV
Absolute timing precision of &lt; 300 ns
Moderate spectral Resolution: 6 &lt; E/ΔE &lt; 80 from 0.5 keV to 8 keV
Restricted field of view: 30 arcmin²


With limited observing time and effectively a narrow single pixel field, it can only trace out thread-like arcs across the sky.

NICER is mounted on the constantly moving International Space Station and has its own pointing mechanisms on top of that. 

When you see several arcs meet at a certain point, this is an interesting X-ray source, usually a neutron star (the primary purpose of the device and the "N" in NICER), and the telescope passes over it several times in order to collect more data. 

Watch the COOL YouTube video NASA'S NICER Does the Space Station Twist

Watch the COOL YouTube video ISS NICER - First Deployment &amp; Range of Motion Checks

Watch more NICER videos here: https://heasarc.gsfc.nasa.gov/docs/nicer/nicer_gallery.html

Watch the YouTube video Unlocking Secrets of Neutron Stars with NICER

Read the presentation NICER Science Overview

You can see that it is a number of grazing-incidence curved crystals used as concentrators, with "single-pixel" X-ray detectors behind them in the photo below (from How will NavCube (actually) be important for the XCOM testing and demonstration?):

below: NICER X-ray timing telescope/concentrator array, from About NICER.