Never-Before-Seen Features Found Around A Neutron Star

An unusual infrared light emission from a nearby neutron star detected by NASA’s Hubble Space Telescope could indicate new features never before seen. One possibility is that there is a dusty disk surrounding the neutron star; another is that there is an energetic wind coming off the object and slamming into gas in interstellar space the neutron star is plowing through.

Although neutron stars are generally studied in radio and high-energy emissions, such as X-rays, this study demonstrates that new and interesting information about neutron stars can also be gained by studying them in infrared light, say researchers.

The observation, by a team of researchers at Pennsylvania State University, University Park, Pennsylvania; Sabanci University, Istanbul, Turkey; and the University of Arizona, Tucson, Arizona, could help astronomers better understand the evolution of neutron stars — the incredibly dense remnants after a massive star explodes as a supernova. Neutron stars are also called pulsars because their very fast rotation (typically fractions of a second, in this case 11 seconds) causes time-variable emission from light-emitting regions.

A paper describing the research and two possible explanations for the unusual finding appears Sept. 17, 2018, in the Astrophysical Journal.

“This particular neutron star belongs to a group of seven nearby X-ray pulsars — nicknamed ‘the Magnificent Seven’ — that are hotter than they ought to be considering their ages and available energy reservoir provided by the loss of rotation energy,” said Bettina Posselt, associate research professor of astronomy and astrophysics at Pennsylvania State and the lead author of the paper. “We observed an extended area of infrared emissions around this neutron star — named RX J0806.4-4123 — the total size of which translates into about 200 astronomical units (approximately 18 billion miles) at the assumed distance of the pulsar.”

This is the first neutron star in which an extended signal has been seen only in infrared light. The researchers suggest two possibilities that could explain the extended infrared signal seen by Hubble. The first is that there is a disk of material — possibly mostly dust — surrounding the pulsar.

“One theory is that there could be what is known as a ‘fallback disk’ of material that coalesced around the neutron star after the supernova,” said Posselt. “Such a disk would be composed of matter from the progenitor massive star. Its subsequent interaction with the neutron star could have heated the pulsar and slowed its rotation. If confirmed as a supernova fallback disk, this result could change our general understanding of neutron star evolution.”

The second possible explanation for the extended infrared emission from this neutron star is a “pulsar wind nebula.”

“A pulsar wind nebula would require that the neutron star exhibits a pulsar wind,” said Posselt. “A pulsar wind can be produced when particles are accelerated in the electrical field that is produced by the fast rotation of a neutron star with a strong magnetic field. As the neutron star travels through the interstellar medium at greater than the speed of sound, a shock can form where the interstellar medium and the pulsar wind interact. The shocked particles would then emit synchrotron radiation, causing the extended infrared signal that we see. Typically, pulsar wind nebulae are seen in X-rays and an infrared-only pulsar wind nebula would be very unusual and exciting.”

Using NASA’s upcoming James Webb Space Telescope, astronomers will be able to further explore this newly opened discovery space in the infrared to better understand neutron star evolution.

New X-Ray Pulsar Found Near Center of Milky Way

Using NuSTAR spacecraft and NICER instrument, an international team of astronomers has found a new accreting millisecond X-ray pulsar. The newly discovered object, designated IGR J17591−2342, is the newest addition to a still short list of known accreting millisecond X-ray pulsars. The finding is reported in a paper published August 30 on the arXiv pre-print server.

Astronomers noted that IGR J17591−2342 is located near the center of our Milky Way galaxy, some 28,000 light years away from the Earth. The estimated accretion rate was found to be about 0.52 billionth of one solar mass per year. IGR J17591−2342 is so far the 22nd known AMXP. The authors of the paper underlined that their discovery enriches the census of these objects that are essential for the understanding of the late stages of stellar evolution.

X-ray pulsars exhibit strict periodic variations in X-ray intensity, which can be as short as a fraction of a second. Accreting millisecond X-ray pulsars (AMXPs) are a peculiar type of X-ray pulsars in which short spin periods are caused by long-lasting mass transfer from a low-mass companion star through an accretion disc onto a slow-rotating neutron star. Astronomers perceive AMXPs as astrophysical laboratories that could be crucial in advancing our knowledge about thermonuclear burst processes.

To date, only 21 AMXPs have been discovered, with spin periods ranging from 1.7 to 9.5 milliseconds. In order to expand the list of this peculiar objects, the scientific community is still actively searching for such sources using space observatories like NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) telescope.

Recently, a group of researchers led by Andrea Sanna of the University of Cagliari, Italy, has used NuSTAR to identify a new AMXP. The source, named IGR J17591−2342, was initially classified as an X-ray transient by European Space Agency’s INTernational Gamma-Ray Astrophysics Laboratory (INTEGRAL) during galactic center scanning on August 10, 2018. The team observed this source with NuSTAR, which revealed evidence about the nature of this object. Additional observations of the pulsar were conducted using the Neutron Star Interior Composition Explorer (NICER) onboard the International Space Station (ISS).

“In this letter, we describe a coherent timing analysis of the NuSTAR and NICER observations that provided the pulsar spin period and binary ephemeris,” the researchers wrote in the paper.

The team detected coherent X-ray pulsations around 527.4 Hz (1.9 milliseconds) in the NuSTAR and NICER observations performed almost 25 days from the beginning of the outburst, with a pulse fraction of 15 percent.

According to the paper, IGR J17591−2342 has an orbital period of about 8.8 hours. The mass of the neutron star was calculated to be approximately 1.4 solar masses, while the minimum mass of the companion is most likely 0.42 solar masses.

How Phosphorus Made It’s Way to Earth

Phosphorus, which is vital to life but somewhat rare, condensed inside asteroids in the outer Solar System before moving back towards the Sun, where some of it ended up on Earth, according to new research.

Phosphorus is one of the six main elements that make up the human body, and is a necessary building block for other organisms. However, unlike hydrogen, oxygen, carbon, nitrogen and calcium, phosphorus is rare. It is even more scarce in the rest of the Solar System.

Astrobiologists are tracking phosphorus, hoping that it leads them to signs of other life.

Many meteorites contain phosphorus, and knowing how phosphorus is distributed through the Solar System could help scientists determine where the meteorites came from, depending on the amount and type of phosphorus they contain.

“Phosphorus is one of the key elements in biology,” says Matthew Pasek, an astrobiologist and geochemist at the University of South Florida.

Unlike the other elements essential for life, phosphorus is mainly found in solid form, whereas the likes of hydrogen, oxygen and nitrogen are often found as a gas. “Studying phosphorus keeps us grounded in actual hard rock samples. Unlike the others, there is no obvious gas form, so has to come from rock sources,” Pasek says. “We hope to tie that eventually to biology and life.”

His recent paper in scientific journal Icarus investigates the distribution of gaseous (or “volatile”) phosphorus in the early Solar System, and what this means for the current distribution of phosphorus.

Phosphorus is thought to form in the heart of exploding stars or supernovae. In the case of our early Solar System, everything close to the sun was vaporized, explains Pasek. Then, as the elements moved away from the sun, they grew colder and began to condense into solids.

Meteorites containing phosphorus could have also brought the element, which is essential for life, to Earth. Credit: David A. Aguilar (CfA).
The question behind the paper was, “If phosphorus doesn’t react to form a solid at these high temperatures, then maybe it can form a different type of solid in the cold reaches,” says Pasek.

The gas phosphine (PH3) is the principal volatile phase of phosphorus at low temperatures. Two different groups have proposed that phosphine could play an active role in ice chemistry on the outer edges of the Solar System.

Pasek’s paper aims to determine how quickly phosphorus would react with solids – “very fast”, Pasek quips – and then how long it would take to cool and be attracted back toward warmer environments. Ultimately, the goal was to determine the distribution of volatile forms of phosphorus, such as phosphine, and how they were distributed throughout the Solar System.

According to a theoretical model, which combined thermodynamics, rates at which phosphorus reacts with metals, and gas diffusion models, Pasek’s research found that most phosphorus should be in a solid form everywhere in the Solar System, out to about Saturn. “Phosphorus was depleted as a volatile throughout the developing Solar System, and volatile forms of phosphorus would have been minimal, even in the colder regions of the solar nebula,” the paper says.

Phosphorus should also exist in a form called schreibersite, which is a mineral containing nickel, iron and phosphorus, he says. “We find it in meteorites all the time, and in the more cometary forms. It does imply that pretty much all the meteorites we collect, which have a small amount of phosphide, have to form in this region… This study implies that phosphorus for life comes from solid form, rather than phosphorus from ice.”

Mikhail Zolotov, a research professor at Arizona State University who specializes in volatile elements on other planets, notes that both the abundance and species of phosphorus could affect biological activity.

It was clear from previous studies of meteorites that phosphorus is mainly present in minerals rather than gases. “Earlier models for condensation of hot solar-composition gas indicated formation of phosphorus-bearing minerals observed in meteorites,” says Zolotov.

While he says that Pasek’s paper is a “decent piece of work”, it is contentious that gas movement toward the sun, which was not modeled in the paper, could be faster than the diffusion of gas away from the sun. “Available meteorite data do not indicate phosphorus depletion away from the sun… [and this hypothesis] remains to be confirmed by data from outer Solar System materials such as comets,” he says.

For Pasek, the next step in this research is to experiment with phosphine in the laboratory, and move it into a more practical realm. “We’re going to take pieces of metal and expose them to phosphorus-bearing gas and see how long it takes to make these rocks,” he says. He will then feed that data back into his models.

Almost 500 Explosions Found in Galaxy Cores

Apart from a billion Milky Way stars, ESA’s Gaia spacecraft also observes extragalactic objects. Its automated alert system notifies astronomers whenever Gaia spots a transient event. A team of astronomers have found out that by tweaking the existing automated system, Gaia can be used to detect hundreds of peculiar transients in the centres of galaxies. They found about 480 transients over a period of about a year. Their new method will be implemented in the system as soon as possible allowing astronomers to determine the nature of these events. The findings will be published in the November issue of the Monthly Notices of the Royal Astronomical Society.

In 2013, ESA launched its Gaia spacecraft to measure the location of a billion stars in the Milky Way and tens of millions of galaxies. Each position on the sky enters Gaia’s view once every month, for a total of about 70 times during the mission. This allows the spacecraft to spot transient events, such as supermassive black holes ripping stars apart or stars exploding as a supernova. Gaia will notice a change in brightness when it returns to the same patch of sky a month later. A team of astronomers from SRON, Radboud University and the University of Cambridge now report nearly 500 transients occurring in the centres of galaxies over a period of one year.

Astronomers Zuzanna Kostrzewa-Rutkowska, Peter Jonker (both affiliated with SRON and Radboud University), Simon Hodgkin and others searched the Gaia database for transient events around the nuclei of galaxies in the period between July 2016 and June 2017. They used a galaxy catalogue—from the Sloan Digital Sky Survey Release 12— and a custom-made mathematical tool. The new tool allows the researchers to identify rare luminous events coming from galactic centers. They dug up 480 events, of which only five were picked up before by the alert system.

Rapidly alerting the astronomical community is key for many of the events found. For about one hundred transients nothing out of the ordinary was observed by Gaia the month before and the month after detection, indicating that the event leading to the enhanced emission of light was short. ‘Such events have great value because they could allow astronomers to study for a brief period previously invisible supermassive black holes,’ says Jonker. ‘Especially the short-duration events could point us to the location of the so far elusive intermediate-mass black holes ripping stars apart.’

The leading explanation for most events is that supermassive black holes residing in the nuclei of galaxies suddenly become much more active as the amount of gas falling into the black hole surges and lights up the close environment of the black hole. This fresh fuel may be extracted from a star which is ripped apart by the enormous gravitational pull of the black hole.

New Era of Astronomy Uncovers Clues about Particles and Waves

For most in the history of astronomy, scientists primarily studied signals transmitted by one messenger, electromagnetic radiation. These waves, which move through space and time, are described by their wavelengths or the amount of energy found in their particles.

Radio waves have photons with the lowest amount of energy and the longest wavelengths, followed by infrared and optical light at intermediate energies and wavelengths. X-rays and gamma-rays have the shortest wavelengths and the highest energy.

Multimessenger astronomy is a natural evolution of astronomy. Scientists need more data to put together a complete picture of the objects they study and match the theories they develop with their observations.

Here are the main messengers now being studied:

Cosmic rays: charged particles and nuclei travelling near the speed of light.
Neutrinos: uncharged particles that see most of the universe as transparent.
Gravitational waves: wrinkles in the very fabric of space and time.

While some fields in astronomy have explored these messengers for years, astronomers have only recently observed events from well beyond the Milky Way with more than one messenger at the same time. In just a few months, the number of sources where astronomers can piece together the signals from different messengers doubled.

Astronomers have combined different wavelengths of photons to piece together some of the mysteries of the universe. For example, the combination of radio and optical data played a major role in determining that the Milky Way is a spiral galaxy in 1951.

The cultures of astronomers and particle physicists represent different approaches to science. In multimessenger astronomy, these cultures collide.

Astronomy is an observational field and not an experiment. We study astronomical objects that change over time (time-domain astronomy), which means we often have only one chance to observe a transient astronomical event. Time-domain is the analysis of mathematical functions, physical signals or time series of economic or environmental data, with respect to time.

Until recently, most time-domain astronomers worked in small teams, on many projects at once. We use resources like The Astronomer’s Telegram or the Gamma-ray Coordination Network to rapidly communicate results, even before submitting scientific papers.

Particle physicists have led the way in creating large international collaborations to tackle their hardest problems, including the Large Hadron Collider, the IceCube Neutrino Observatory and the Laser Interferometer Gravitational-Wave Observatory (LIGO). Corralling hundreds to thousands of researchers to work towards common goals requires comprehensive identification of roles, strict communication guidelines and many teleconferences.

The need to respond to rapid changes in a multimessenger source and the huge effort to capture multimessenger signals means astronomy and particle physics must merge towards one another to elicit the best of both cultures.

The benefits of multimessenger astronomy

While multimessenger astronomy is an evolution of what astronomers and particle physicists have done for decades, the combined results are intriguing.

The detection of gravitational waves from merging neutron stars confirmed that these collisions made a large fraction of the gold and platinum on Earth (and throughout the universe). It also showed how these collisions give rise to (at least some) short gamma-ray bursts—the origin of these explosive events has been a huge open question in astronomy.

The first association of a neutrino with a single astronomical source provided a glimpse into how the universe makes its most energetic particles. Multimessenger astronomy is revealing details about some of the most extreme conditions in our universe.

The multimessenger perspective is already yielding more than the sum of its parts —and we can expect to see more surprising discoveries in the future. Elite teams are already contributing to the growth of this young field, and multimessenger astronomy promises to play a major role in our next decade of astronomical research across the world.

Galactic ‘Wind’ Stifling Star Formation Is Most Distant Yet Seen

For the first time, a powerful “wind” of molecules has been detected in a galaxy located 12 billion light-years away. Probing a time when the universe was less than 10 percent of its current age, University of Texas at Austin astronomer Justin Spilker’s research sheds light on how the earliest galaxies regulated the birth of stars to keep from blowing themselves apart. The research will appear in the Sept. 7 issue of the journal Science.

“Galaxies are complicated, messy beasts, and we think outflows and winds are critical pieces to how they form and evolve, regulating their ability to grow,” Spilker said.

Some galaxies such as the Milky Way and Andromeda have relatively slow and measured rates of starbirth, with about one new star igniting each year. Other galaxies, known as starburst galaxies, forge hundreds or even thousands of stars each year. This furious pace, however, cannot be maintained indefinitely.

To avoid burning out in a short-lived blaze of glory, some galaxies throttle back their runaway starbirth by ejecting — at least temporarily — vast stores of gas into their expansive halos, where the gas either escapes entirely or slowly rains back in on the galaxy, triggering future bursts of star formation.

Until now, however, astronomers have been unable to directly observe these powerful outflows in the very early universe, where such mechanisms are essential to prevent galaxies from growing too big, too fast.

Spilker’s observations with the Atacama Large Millimeter/submillimeter Array (ALMA), show — for the first time — a powerful galactic wind of molecules in a galaxy seen when the universe was only 1 billion years old. This result provides insights into how certain galaxies in the early universe were able to self-regulate their growth so they could continue forming stars across cosmic time.

Astronomers have observed winds with the same size, speed and mass in nearby starbursting galaxies, but the new ALMA observation is the most distant unambiguous outflow ever seen in the early universe.

The galaxy, known as SPT2319-55, is more than 12 billion light-years away. It was discovered by the National Science Foundation’s South Pole Telescope.

ALMA was able to observe this object at such tremendous distance with the aid of a gravitational lens provided by a different galaxy that sits almost exactly along the line of sight between Earth and SPT2319-55. Gravitational lensing — the bending of light due to gravity — magnifies the background galaxy to make it appear brighter, which allows the astronomers to observe it in more detail than they would otherwise be able to. Astronomers use specialized computer programs to unscramble the effects of gravitational lensing to reconstruct an accurate image of the more-distant object.

This lens-aided view revealed a powerful wind of star-forming gas exiting the galaxy at nearly 800 kilometers per second. Rather than a constant, gentle breeze, the wind is hurtling away in discrete clumps, removing the star-forming gas just as quickly as the galaxy can turn that gas into new stars.

The outflow was detected by the millimeter-wavelength signature of a molecule called hydroxyl (OH), which appeared as an absorption line: essentially, the shadow of an OH fingerprint in the galaxy’s bright infrared light.

Molecular winds are an efficient way for galaxies to self-regulate their growth, the researchers note. These winds are probably triggered by either the combined effects of all the supernova explosions that go along with rapid, massive star formation, or by a powerful release of energy as some of the gas in the galaxy falls down onto the supermassive black hole at its center.

“So far, we have only observed one galaxy at such a remarkable cosmic distance, but we’d like to know if winds like these are also present in other galaxies to see just how common they are,” Spilker concluded. “If they occur in basically every galaxy, we know that molecular winds are both ubiquitous and also a really common way for galaxies to self-regulate their growth.”

Telescope Maps Cosmic Rays in Magellanic Clouds

A radio telescope in outback Western Australia has been used to observe radiation from cosmic rays in two neighboring galaxies, showing areas of star formation and echoes of past supernovae.

The Murchison Widefield Array (MWA) telescope was able to map the Large Magellanic Cloud and Small Magellanic Cloud galaxies in unprecedented detail as they orbit around the Milky Way. By observing the sky at very low frequencies, astronomers detected cosmic rays and hot gas in the two galaxies and identified patches where new stars are born and remnants from stellar explosions can be found.

The research was published today in Monthly Notices of the Royal Astronomical Society, one of the world’s leading astronomy journals.

International Centre for Radio Astronomy Research (ICRAR) astrophysicist Professor Lister Staveley-Smith said cosmic rays are very energetic charged particles that interact with magnetic fields to create radiation we can see with radio telescopes.

“These cosmic rays actually originate in supernova remnants from stars that exploded a long time ago,” he said.

“The supernova explosions they come from are related to very massive stars, much more massive than our own sun.

“The number of cosmic rays that are produced depends on the rate of formation of these massive stars millions of years ago.”

The Large and Small Magellanic Clouds are very close to our own Milky Way – less than 200,000 light years away and can be seen in the night sky with the naked eye.

ICRAR astronomer Dr. Bi-Qing For, who led the research, said this was the first time the galaxies had been mapped in detail at such low radio frequencies.

“Observing the Magellanic clouds at these very low frequencies between 76 and 227MHz – meant we could estimate the number of new stars being formed in these galaxies,” she said.

“We found that the rate of star formation in the Large Magellanic Cloud is roughly equivalent to one new star the mass of our sun being produced every ten years.

“In the Small Magellanic Cloud, the rate of star formation is roughly equivalent to one new star the mass of our sun every 40 years.”

Included in the observations are 30 Doradus, an exceptional region of star formation in the Large Magellanic Cloud that is brighter than any star formation region in the Milky Way, and Supernova 1987A, the brightest supernova since the invention of the telescope.

Professor Staveley-Smith said the results are an exciting glimpse into the science that will be possible with next-generation radio telescopes.

“It shows an indication of the results that we will see with the upgraded MWA, which now has twice the previous resolution,” he said.

Furthermore, the forthcoming Square Kilometre Array (SKA) will deliver exceptionally fine images.

“With the SKA the baselines are eight times longer again, so we’ll be able to do so much better,” Professor Staveley-Smith said.