Enormously powerful gravitational fields that warp the local fabric of space and time. Incomparably strong magnetic fields that can stretch atoms themselves into long spindles. Rotating radiation beams, like out-of-control cosmic lighthouses, spinning hundreds of times a second. These are just some of the exotic properties of compact objects, the main focus of Wednesday morning’s KIPAC@10 session and the topic of Tony Li's conversation with NASA Goddard's Alice Harding.
“Compact objects” is actually a catch-all term for several types of objects—white dwarfs, neutron stars, and black holes—each one denser than the last, all of which are possible remnants of a once-burning star. In particular, this session shined a spotlight on neutron stars and black holes. Scattered throughout our galaxy and beyond, these objects continue to serve as astrophysical laboratories that test the very limits of physics as we know it.
Indeed, compact objects are known to possess some of the most extreme physical properties ever observed. Neutron stars are known for their prodigious densities: a single teaspoonful of neutron star matter would weigh billions of tons. In this rare state, where the laws of both quantum mechanics and general relativity are important, matter behaves very differently than any known material on Earth.
The precise nature of this material's behavior, called the neutron star “equation of state,” is one of the outstanding mysteries of the field. No shortage of models has been proposed, but recent progress has been made constraining these models by inferring the masses of known neutron stars. Additionally, the inside of a neutron star is theorized to be a superfluid, a strange state of matter that has zero friction. However, because we cannot see beneath the surface of such dense objects, we have precious little direct evidence for the true equation of state of such highly dense matter. The main challenge here is in finding ways to directly probe the depths of a neutron star, and perhaps future and current observations will reveal a way forward.
Outside the surface of the neutron star, we have a somewhat better understanding of the physics at play. There, strong magnetic fields thread through and wrap around the star, charged plasmas and electric currents surround it, and outgoing beams of radiation rotating with the star broadcast its location to the cosmos. At the right alignment, these beams appear to us on Earth as regular pulses of radio, X-ray, or gamma radiation, and neutron stars thus detected are appropriately known as pulsars.
If a pulsar is magnetized enough, it is called a magnetar. The magnetic fields around these objects are the strongest known in the universe, able to tear living tissues simply through the magnetic properties of the water they contain. Historically, magnetars have been observed through their bursts or pulses of high-energy radiation—gamma rays and X-rays—and have been a separate class from radio pulsars, making high-energy observatories like Fermi and X-ray satellites essential to their study. However, new observations have suggested that the line between “standard” magnetars and “standard” radio pulsars is blurrier than once thought. Perhaps better data from advanced X-ray and gamma observatories, as well as more detections of such objects, will give us a better-unified view of all the types of neutron stars.
Finally, neutron stars and black holes are two of the most promising ways to observe gravitational waves. Long predicted by the theory of general relativity, gravitational waves have eluded direct detection to this very day. However, merging pairs of neutron stars and/or black holes are predicted to produce some of the strongest gravitational ripples in space, and simulations and theory have made excellent progress toward predicting what a detectable signal should look like. The search, then, to detect these ripples is on—not a trivial task! The predicted signals are so small they would deform a mile-long rod by less than one billionth of the width of an atom. Nonetheless, with preparations for the Advanced LIGO detector underway, a detection may be just on the horizon.
What does the future hold, then? To come any closer to truly understanding compact objects, more clues will need to come in from many approaches. Observing the sky at the highest energies, Fermi will continue to collect gamma ray data on pulsars. At least two additional missions, NuSTAR and NICER, will yield a wealth of data in the X-ray regime, with never-before-seen resolution and sensitivity. Arrays of radio receivers, like the upcoming Square Kilometre Array, will be used to collect precise timing data from many pulsars, looking for tiny shifts caused by passing gravitational waves. As mentioned, Advanced LIGO will hopefully detect merging compact objects, but it may even reveal premium information about the insides of neutron stars. Hopefully, the years to come will see some of the mysteries of compact objects answered, but perhaps new ones may emerge. Indeed, few if any objects naturally exist so close to the very extremes of physics as we understand it, so who knows how compact objects might yet surprise us?
You can watch all the talks in this session on the KIPAC youtube channel.
You can also read more about KIPAC@10 on the conference blog home page.