To date, we have detected only a small and certainly non-representative sample of the vast population of neutron stars that our galaxy is thought to contain. With the new Nancy Grace Roman Space Telescope, scheduled to launch this coming September, we will discover and determine the mass of many more. In fact, a team of researchers maintains that, once it becomes operational, Roman will open a new window into the study of these ultra-dense stellar remnants and help us discover the first significant sample of neutron stars that, immediately after their formation, were ejected at enormous speeds and have since been wandering alone through the vastness of space.
Neutron stars are the rapidly rotating, ultra-dense stellar cores that remain as remnants after the explosive death of massive stars. Neutron stars pack more mass than the Sun into a sphere just a few kilometers in diameter and are among the most extreme objects in the Universe. Their study is important because they serve as a unique natural laboratory for studying matter under conditions of extreme pressure and density. Furthermore, we still do not know much, not only about the mechanism by which they form, but also about how many of them are ejected into space at enormous speeds after their formation.
Astrophysicists, finally, wish to thoroughly investigate the so-called “mass gap” that exists between the most massive neutron stars and the least massive black holes. Given, in other words, that the heaviest neutron star discovered to date does not exceed approximately 2.35 solar masses, and the lightest stellar black hole we know of has a mass about 5 times that of the Sun, the question remains: Are there objects that “bridge” this difference, and if so, how are they formed and what properties do they have? Or perhaps the mass gap does not actually exist and is simply due to our still incomplete observations?
According to theoretical estimates, our Milky Way may contain several hundred million neutron stars; however, to date we have discovered only a few thousand. One reason for this is that most neutron stars are billions of years old, which means that many of them have slowed down their initially high rotation speed so much and have already lost their initially enormous temperature, making them extremely difficult to detect.
Furthermore, most of the neutron stars we have discovered are pulsars, which emit beams of radiation from their magnetic poles. In other words, when the magnetic poles of a neutron star are not aligned with its axis of rotation, the radiation cones it emits also rotate, sweeping across space, just like the rotating “eye” of a lighthouse. Of all the pulsars in our galaxy, however, we can theoretically detect with our telescopes only those whose radiation passes through Earth on its way.
Until now, the mass of a neutron star could be calculated accurately only if it was part of a binary star system. The new study, however, suggests that the Nancy Grace Roman Space Telescope, which NASA plans to launch as early as this coming September, could detect them—and calculate their mass—using a different method. Using detailed simulations of our galaxy and future observations by Roman, the researchers behind the study demonstrated that this new space telescope will be able to detect dozens of isolated neutron stars, as well as determine their mass, using the phenomenon of gravitational microlensing. This study, led by Zofia Kaczmarek of the University of Heidelberg, was published in the scientific journal Astronomy and Astrophysics.
When a neutron star passes in front of another distant star, its gravity amplifies and bends the light from the star in the background. The phenomenon of gravitational microlensing, in other words, causes the star to appear brighter and somewhat shifted from its actual position in the sky. While many telescopes can detect this temporary increase in the brightness of a distant star, Roman can measure both the increase in its brightness (photometry) and the minute shift in its position (astrometry) with exceptional precision. In fact, the greater the apparent shift in a star's position, the greater the mass of the neutron star that caused it.
“What’s really impressive about using microfocus is that you can obtain direct mass measurements,”, explained study co-author Peter McGill of Lawrence Livermore National Laboratory. “Our photometric data show that something passed in front of the star, butwhat tells us how massive this object is is the magnitude of the shift we observe in the star’s position.” By measuring this minuscule shift in the sky, we can directly calculate the mass of an object that would otherwise remain invisible.
If the predictions of the relevant study are confirmed, the Roman space telescope could provide us with the first large sample of isolated neutron stars, which will have been discovered solely through their gravity, revealing a hidden population of these ultra-dense stellar remnants that has remained inaccessible until now.
Source: NASA's Roman Set to Transform the Search for Elusive Neutron Stars | STScI
Photo: Artist's rendering of an isolated neutron star [Credit: NASA, STScI, Ralf Crawford (STScI)]