Author: Xaq Rzetelny

Researchers are gaining ground in the struggle to understand the mysterious objects known as UltraLuminous X-ray sources (ULXs). These objects, named for their extreme brightness at X-ray wavelengths, are thought to be dense, compact objects like black holes or neutron stars. Their luminosity (which extends to other wavelengths) arises as they actively draw matter from an orbiting companion.

“We think these ‘ultra-luminous X-ray sources’ are somewhat special binary systems, sucking up gas at a much higher rate than an ordinary X-ray binary,” said Ciro Pinto from the Institute of Astronomy in Cambridge, UK, an author of a recent study. “Some host highly magnetised neutron stars, while others might conceal the long-sought-after intermediate-mass black holes, which have masses around 1000 times the mass of the Sun. But in the majority of cases, the reason for their extreme behaviour is still unclear.”

It’s been difficult to study them in detail because we've lacked the sensitivity needed to identify the emission lines and/or absorption lines created by specific elements. When light passes through material such as gas, certain wavelengths are absorbed by elements in the gas, leaving a blank line in the light source’s spectrum. Emission lines are light emitted by the element itself.

The composition of the dust between stars in our galaxy provides a window into some of the material that went into forming our Solar System. The local dust left behind from this process has been through many shake-ups in its history that have changed its composition; interstellar dust should be relatively pristine. For a long time, however, our efforts to understand interstellar dust have relied largely on inferences, as it’s difficult to directly observe the dim, diffuse material using telescopes.

Luckily, there is a way to get a direct measurement. There’s a cloud of interstellar dust near our Solar System, known as the Local Interstellar Cloud (LIC, sometimes called the “Local Fluff”). Some of it is streaming into our Solar System. This stream of LIC material was first observed by the Ulysses spacecraft in 1993, and grains of dust were captured by the Stardust spacecraft in the early 2000s and analyzed by a citizen science project.

The inward dust flow also passes by Saturn, where NASA happens to have a spacecraft with a dust collection system. This is Cassini, which was outfitted with that capability with the intention of capturing dust native to Saturn's rings. It just happened to be in the right place with the right tools to catch some interstellar dust.

Up to a certain point, the elements of the periodic table are largely formed in the hearts of stars. But for elements that are heavy enough (heavier than zinc typically), fusing two lighter nuclei just won’t do it. To form those elements, another process is needed: neutron capture.

Neutrons get captured when they collide with an atomic nucleus and get stuck together, creating a heavier nucleus. Neutrons can undergo these collisions at lower energies because they’re electrically neutral so they won’t be repelled, unlike protons. If the resulting nucleus is unstable, however, one of its neutrons can decay into a proton, creating a heavier element.

A lot of heavy elements are formed by neutron capture, but the details of how it happens haven't been well worked out. That's in part because there are two kinds of neutron capture processes: the slow process (s-process) and rapid process (r-process). Each of these accounts for about half of the elements produced by neutron capture in our Universe.

The source of fast radio bursts (FRBs)—an extremely brief flash of radio waves coming from space—remains unknown despite new observations. These events, of which only 17 have been observed, are largely mysterious. Until now, there’s been little indication where in the Universe they take place, and consequently there’s been no way to know what physical process is causing the flashes.

But in quick succession, two papers have suggested that the FRBs either come from a single object that can create repeated bursts or that the bursts come from the catastrophic destruction of a neutron star and thus can't possibly repeat. Just as quickly, the second results, which placed the source outside our galaxy, have been called into question.

As far back as the first FRB’s discovery in 2007, there was some indication that the source was outside the Milky Way. There was a difference in the arrival time at different wavelengths. The shortest radio waves arrived slightly earlier than their longer wavelength counterparts.

The most common rocks that fall to Earth are called chondritic meteorites, or chondrites. In fact, the Earth is probably made of them. These are some of the oldest rocks in the Solar System, some dating to its very origin at just over four and a half billion years old. And some of their internal material has remained largely untouched since that time.

That makes them extraordinary time capsules, since the Solar System underwent churning differentiation and other processes to reach its current form. It’s a bit like finding a still-living velociraptor: an opportunity to study a bygone era, largely uncontaminated by exposure to the interceding time.

But the amount of information the rocks provide really depends on whether their composition was typical of that of the early Solar System. In a new study, a group of researchers has found that the composition of the chondrites is probably a match for the cloud of gas and dust that condensed to create the Solar System.

It’s tempting to call the tetraneutron a theoretical particle, as its existence has yet to be confirmed. But that would imply that it’s a consequence of some existing theoretical model, that it’s predicted by some theory. The tetraneutron, however, contradicts the relevant theories—it should be impossible.

And yet, amidst all the (deserved) excitement for the detection of gravitational waves last week, an experiment quietly turned up the strongest evidence for a tetraneutron thus far. It’s not full confirmation yet, but if the new study’s conclusions are borne out, things are going to get weird.

The Story So Far

The troublesome particle may have first appeared in 2001 after decades of speculation and a few doubtful experiments. Researchers fired beryllium-14 atoms at a carbon target to observe the resulting chaos of particles, a relatively common practice.

Enlarge / The Enterprise, caught in the wake of a temporal vortex, witnesses the Earth, assimilated long ago, in the altered timeline. (credit: Paramount Pictures)

To continue our celebration of Star Trek's 50th anniversary, we've decided to resurface a few of our favorite Trek stories from the Ars archives. Contributor Xaq Rzetelny explored how time travel works across the various series in February 2016, so today his piece travels forward through the space-time continuum.

We're at the start of what should be a big year for Star Trek. The franchise will celebrate its 50^th anniversary this fall, but 2016 also brings a new movie (Star Trek Beyond), the recovery of long-lost documents belonging to Gene Roddenberry, and the development of (finally!) a new series set to launch in January 2017.

It’s no secret that we here at Ars love (and sometimes hate) our Star Trek. With so much time having elapsed since the original series first aired—and with so much time spent watching/reading/thinking through it all—we felt it was about, well, time to thoroughly explore one of our favorite Trek staples: time travel.

Seventy million years ago, some unknown force blasted a tremendous amount of gas out of our galaxy. Known as the Smith Cloud, that gas is now arcing back toward the Milky Way, pulled in by its gravity. Thirty million years from now, it will return to our galaxy once more.

The cloud and its trajectory were already well-known, but the new study confirms its origin was inside the Milky Way.

The Smith Cloud was originally an amorphous blob of gas, but the forces it has been subjected to have shaped it into a comet-like form. It’s a whopping 11,000 light-years long and 2,500 across. At that size and its current distance, if it could be seen in visible light, it would appear 30 times bigger than the Moon in the night sky.

Brightness can mean different things. A nearby candle is brighter than an identical one in the distance. To avoid confusion, astronomers use the word "luminosity" rather than brightness to indicate the total amount of light that an object puts out. By that measure, W2246-0526 is the brightest—the most luminous—galaxy in the observable Universe.

A group of researchers has now taken advantage of the abilities of the Atacama Large Millimeter Array (ALMA) to take a look inside W2246-0526 and see what’s going on there.

The cause of the brightness is not mysterious. The galaxy’s incredibly bright core, which outshines the rest of its stars by a factor of over 100, is home to a very active supermassive black hole (SMBH). While nearly every galaxy houses a SMBH, only the most active ones earn the title of quasar. ("Active" in this context means that the black hole is rapidly consuming a lot of matter, producing its incredible light output through friction as it does.)

Early in the Universe's history, something ionized most of the diffuse hydrogen gas that’s spread between galaxies. But until now, the source responsible for this ionization has been largely mysterious—a conundrum so persistent, the authors of a new paper call it “one of the key questions in observational cosmology.”

First, a little background: The Universe’s hydrogen started out ionized because the early Universe was too hot and energetic for electrons to settle down and pair with protons. This situation persisted for about 375,000 years after the Big Bang, at which time the Universe had cooled enough for neutral hydrogen to exist. Then, any light produced by interactions among these hydrogen atoms was at a wavelength where it was quickly re-absorbed by other hydrogen atoms—the Universe was opaque.

It wasn’t until a few hundred million years later that some of the hydrogen in the intergalactic medium (IGM) began to be ionized again by an unknown source of energy. This event is known as the epoch of reionization, and it’s the last major phase transition in the history of the Universe. It returned the Universe to a state where light could travel long distances.