First ever mission to study neutron stars goes to ISS

Resupply cargo capsule Dragon, which is due to deliver food and instruments to ISS (launch was postponed to at least Saturday, because of bad weather), carries on board unique instrument devoted to study rapidly rotating neutron stars. This is Neutron Star Interior Composition Explorer, or NICER.

Image: NASA - NICER configuration: 56 X-rays mirrors

After delivery it will be installed on the external of the ISS and will begin observing neutron stars, the densest objects in the universe. 

Wiki:

 A neutron star is the collapsed core of a large (10–29 solar masses) star. Neutron stars are the smallest and densest stars known to exist.  Though neutron stars typically have a radius on the order of 10 km, they can have masses of about twice that of the Sun. They result from the supernova explosion of the massive star, combined with  gravitational collapse, that compresses the core past the white dwarf star density to that of atomic nuclei. Most of the basic models for these objects imply that neutron stars are composed almost entirely of neutrons...They are so dense that a normal-sized matchbox containing neutron-star material would have a mass of approximately 3 billion tonnes. .... As the star's core collapses, its rotation rate increases as a result of conservation of angular momentum, hence newly formed neutron stars rotate at up to several hundred times per second. Some neutron stars emit beams of electromagnetic radiation that make them detectable as pulsars.

Image: JIAPS

NICER will focus especially on pulsars—those neutron stars that appear to wink on and off because their spin sweeps beams of radiation.

During its one and a half year long mission, NICER will collect X-rays generated from the stars' tremendously strong magnetic fields and from hotspots located at their two magnetic poles. At these locations, the objects' intense magnetic fields emerge from their surfaces and particles trapped within these fields rain down and generate X-rays when they strike the stars' surfaces. On Earth these X-ray beams are observed as periodical flashes of radiation ranging from seconds to milliseconds, depending of rotating speed. 

Image: NASA - Artist's concept of a pulsar (blue-white disk in center) pulling in matter from a nearby star (red disk at upper right). The stellar material forms a disk around the pulsar (multicolored ring) before falling on to the surface at the magnetic poles. The pulsar's intense magnetic field is represented by faint blue outlines surrounding the pulsar

When British astrophysicist Jocelyn Bell discovered pulsars almost 50 years ago, there was a hypothesis that these objects are beacons of alien civilizations.

The mission has two primary goals: first is to gather information about internal structure of pulsars. Second is applied: to demonstrate X-rays navigation. Because these pulsations are predictable, they can be used as celestial clocks, providing high-precision timing, like the atomic-clock signals supplied through the Global Positioning System, also known as GPS. Using the same NICER hardware, the mission also plans to demonstrate the viability of autonomous X-ray or pulsar-based navigation, which has never been demonstrated before.

If an interplanetary mission were equipped with such a navigational device, it would be able to calculate its location autonomously, largely independent of NASA's Deep Space Network, which is considered to be the most sensitive telecommunications system in the world.

Another planned experiment - X-rays communications (XCOM) test. Modulated X-ray Source or MXS developed to calibrate detectors, generates X-rays with varying intensity, simulating pulsars. MXS will fly to the ISS perhaps in the next year and will be installed on the external palette of the ISS 50 meter from NICER receiver. During the experiment, the team would encode digital data in pulsed X-rays using the MXS and transmit the data to NICER's receivers. Potentially this technology could provide data transfer rate gigabits of data per seconds over interplanetary distances.

References: 

http://jiaps.org/article/neutron-stars.html

https://phys.org/news/2017-06-nasa-first-ever-neutron-star-mission.html