Elucidating Extreme Entities

by Adam Hadhazy

The cosmic creatures astrophysicists study defy everyday reckoning

MIT astronomers mapped the “disk winds” associated with the accretion disk around Hercules X-1, a system in which a neutron star is drawing material away from a sun-like star, represented as the teal sphere. The findings may offer clues to how supermassive black holes shape entire galaxies. Credits: Credit: Jose-Luis Olivares, MIT. Based on an image of Hercules X-1 by D. Klochkov, European Space Agency

The Author

Adam Hadhazy

Astrophysicists sure get to study some weird-and-wild specimens. For instance, Kavli Astrophysics Institute-affiliated researchers have recently reported on making new insights into how the ultra-dense remnants of massive stars, called neutron stars, truly operate. These objects have unimaginable densities, with a teaspoon packing in as much as 4 billion tons, according to NASA. The strangeness doesn't stop there with two other strange beasts, black holes and dark matter, having come to the fore in recent research. Black holes even trump neutron stars' uber-densities by being so-called singularities which, by definition, have zero volume, and thus infinite or at least immeasurable density. As for dark matter, well, since we don't know definitely what it is yet, its properties remain essentially ineffable. Our well-evidenced understandings of the material indicate it outnumbers normal matter universe-wide about five times over. Yet the recent progress on all these astrophysical fronts suggests deeper understandings of the cosmic creatures we share the universe with will indeed someday be within our grasp.

Digging deep into pulsar physics with IXPE

How do the rapidly spinning neutron stars known as pulsars generate their characteristic beams of radiation that sweep through space? A recent highlight from Josephine Wong, a PhD student at the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) at Stanford University, explains that a first-of-its-kind instrument is helping researchers get to the bottom of this mystery. In late 2021, NASA launched the Imaging X-ray Polarimetry Explorer. IXPE is the first spacecraft dedicated to observing polarized x-rays from pulsars, black holes, and other extreme cosmic objects. "Polarization" refers to the vibration of the x-rays' electric fields in a single plane. Capturing this polarization in turn reveals the geometry of the magnetic fields of cosmic objects because of the intrinsic relation between electric and magnetic fields comprising light; because the fields oscillate perpendicularly to each other, knowing one reveals the other. Mapping the magnetic environment of pulsars in this way allows for the testing of theories about how pulsar beams form. Wong is working on ways of better extracting pulsar polarization from other polarized light streaming from the nebulae surrounding pulsars. Doing so should unveil heretofore hidden goings-on of pulsars.

Map of neutron star's gales could reveal the source of such winds

For the first time, researchers have a created a map of the winds blowing off of a neutron star. The "winds" consist of outflows of superhot plasma from an area of material swirling around the neutron star, known as an accretion disk. How these winds are generated, whether by magnetic fields or neutron star radiation that heats the plasma, remains poorly understood. To help solve this puzzle, researchers at the Massachusetts Institute of Technology (MIT) Kavli Institute for Astrophysics and Space Research (MKI) observed an unusual neutron star arrangement. Called Hercules X-1, it consists of a neutron star pulling material off of a Sun-like star. From our vantage point at Earth, the resulting accretion disk around the neutron star appears to wobble over time. That wobbling provides a varying perspective that has let astrophysicists plot the heights of the winds over time. Now the researchers intend to compare their observations with models describing wind generation to see if a convergence between the two can be made.

Dark matter, clumpy and clandestine

Researchers have published a new measurement of the "clumpiness" of dark matter in the universe. Dark matter is a theoretical substance that interacts with the rest of the cosmos' matter through the force of gravity but is otherwise undetectable and its properties unknown. Scientists at the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) at the University of Tokyo are part of the new effort. The findings hinge on detecting the miniscule effect known as weak gravitational lensing, wherein light from background galaxies is ever so slightly distorted by the mass of foreground galaxies. Statistically compiling these weak lensing instances across millions of galactic objects can speak to the effects of dark matter and its distribution. The data for the study came from three years-worth of observations by the Hyper Suprime-Cam on the Subaru Telescope in Hawaii. Overall, the clumpiness measurement turned out to be consistent with other such measurements in the modern universe. Intriguingly, however, these values do not agree with those obtained from the early universe. The findings thus further bolster an enduring conundrum at the heart of cosmology, which suggests crucial information remains missing in our paradigm of the evolution of the universe.

A deep dive into dark matter with the universe's oldest light

Speaking of dark matter, another new study has presented a map of dark matter out to the largest distances yet across a quarter of the sky. A preprint of the results is now out and boasts researchers from four Kavli astrophysics institutes, including the Kavli Institute for Cosmology, Cambridge (KICC), KIPAC, Kavli IPMU, and the Kavli Institute for Cosmological Physics (KICP) at Chicago University. Blake Sherwin from KICC is a leader on the project, which has leveraged the abilities of the Atacama Cosmology Telescope (ACT) in Chile. Until its observing run ended in 2022, ACT captured distortions in the oldest light in the universe, the cosmic microwave background, as it filtered to us past millions of galaxies. Those measurements revealed the clumpiness of dark matter and are in agreement with other early universe-derived readings. Looking more broadly, the findings fit with models of the universe consistently governed by general relativity, the extremely well-supported theory of gravity that Albert Einstein put forth in 1915. Continuing measurements of the ancient and modern universe will continue to inform the other and, hopefully in time, reach deeper concordance.

Closest instance of a black hole shredding a star

A tidal disruption event, or a TDE, is a rather euphemistic term for when the intense gravity from black hole rips an unlucky passing star to pieces. The process unleashes a tremendous flash of light visible from great distances. To date, about 100 TDEs have been spotted, all in fairly far-off galaxies, and nearly all seen in the optical as well as high-energy x-ray and ultraviolet bands of light. Along comes an exception, dubbed WTP14adbjsh. Researchers at MKI uncovered this TDE in old data captured by NASA's NEOWISE spacecraft. The data came in the form of infrared observations in 2014-2015, which captured a rapid brightening in a galaxy only 137 million light-years from Earth. Subsequent analysis showed the event to have the telltale signs of a TDE, and in a galaxy one-quarter of the distance of the previous recordholder for the nearest TDE. Finding evidence of the event in archived infrared imagery suggests that other such events could be similarly uncovered or even maybe detected in ongoing observing campaigns.