The core of the Milky Way Galaxy (aka. Galactic Center), the region around which the rest of the galaxy revolves, is a strange and mysterious place. It is here that the Supermassive Black Hole (SMBH) that powers the compact radio source known as Sagittarius A* is located. It is also the most compact region in the galaxy, with an estimated 10 million stars within 3.26 light-years of the Galactic Center.
X-ray astronomy helps scientists study neutron stars, binary star systems, and supernova remnants, and even helps detect black holes. But even if human eyes had the ability to see X-rays, we couldn’t just look up at the night sky and see these amazing objects since Earth’s atmosphere absorbs and blocks X-rays. So, thank goodness for space telescopes! And the newest X-ray instrument in space has just produced a breathtaking view of the Universe, and is the deepest X-ray view of the sky we’ve ever seen.
Hundreds of millions of light years away, a supermassive black hole sits in the center of a galaxy cluster named Ophiuchus. Though black holes are renowned for sucking in surrounding material, they sometimes expel material in jets. This black hole is the site of an almost unimaginably powerful explosion, created when an enormous amount of material was expelled.
Astronomers have found a supermassive black hole (SMBH) with an unusually regular feeding schedule. The behemoth is an active galactic nucleus (AGN) at the heart of the Seyfert 2 galaxy GSN 069. The AGN is about 250 million light years from Earth, and contains about 400,000 times the mass of the Sun.
A rogue star is one that has escaped the gravitational pull of its home galaxy. These stars drift through intergalactic space, and so are sometimes called intergalactic stars. Sometimes, when a rogue star is ejected from its galaxy, it drags its binary pair along for the ride.
In the coming years, thousands of satellites, several next-generation space telescopes and even a few space habitats are expected to be launched into orbit. Beyond Earth, multiple missions are planned to be sent to the lunar surface, to Mars, and beyond. As humanity’s presence in space increases, the volume of data that is regularly being back sent to Earth is reaching the limits of what radio communications can handle.
For this reason, NASA and other space agencies are looking for new methods for sending information back and forth across space. Already, optical communications (which rely on lasers to encode and transmit information) are being developed, but other more radical concepts are also being investigating. These include X-ray communications, which NASA is gearing up to test in space using their XCOM technology demonstrator.
It’s relatively easy for galaxies to make stars. Start out with a bunch of random blobs of gas and dust. Typically those blobs will be pretty warm. To turn them into stars, you have to cool them off. By dumping all their heat in the form of radiation, they can compress. Dump more heat, compress more. Repeat for a million years or so.
Eventually pieces of the gas cloud shrink and shrink, compressing themselves into a tight little knots. If the densities inside those knots get high enough, they trigger nuclear fusion and voila: stars are born.
A team of researchers in the UK have observed matter falling into a black hole at 30% the speed of light. This is much faster than anything previously observed. The high velocity is a result of misaligned discs of material rotating around the black hole.
It’s not exactly an organ donor, but a star in the direction of the hyper-populated core of the Milky Way donating some of its mass to a dormant neighbor. The result? The dormant neighbor sprung back to life with an X-ray burst captured by the ESA‘s INTEGRAL (INTErnational Gamma-Ray Astrophysics Laboratory) space observatory.
“INTEGRAL caught a unique moment in the birth of a rare binary system” – Enrico Bozzo, University of Geneva.
The neighbors have likely been paired together for billions of years, which is not in itself noteworthy: stars often live in binary pairs. But the pair spotted by INTEGRAL on August 13th 2017 is very unusual. The donor star is a red giant, and the recipient is a neutron star. So far, astronomers only know of 10 of these pairs, called ‘symbiotic X-ray binaries’.
To understand what’s happening between these neighbors, we have to look at stellar evolution.
The donor star is in its red giant phase. That’s when a star in the same mass range as our star reaches the later stage of its life. As its mass is depleted, gravity can’t hold the star together in the same way it has for the early part of its life. The star expands outwards by millions of kilometers. As it does so, it sheds stellar material from its outer layers in a solar wind that travels several hundreds of km/sec.
Its neighbor is in a different state. It’s a star that had an initial mass of about 25 to 30 times the Sun. When a star this big approaches the end of its life, it suffers a different fate. Stars this large live fast, and burn through their fuel quickly. Then, they explode as supernovae, in this case leaving a corpse behind. In the binary system captured by INTEGRAL, the corpse is a spinning neutron star with a magnetic field.
Neutron stars are dense. In fact, they’re some of the densest stellar objects we know of, packing as much mass as one-and-a-half of our Suns into an object that’s only about 10 km across.
When the red giant’s stellar wind met the neutron star, the neutron star slowed its rate of spin, and burst into life, emitting high-energy x-rays.
“INTEGRAL caught a unique moment in the birth of a rare binary system,” says Enrico Bozzo from University of Geneva and lead author of the paper that describes the discovery. “The red giant released a sufficiently dense slow wind to feed its neutron star companion, giving rise to high-energy emission from the dead stellar core for the first time.”
After INTEGRAL spotted the x-ray burst from the binary, other observations quickly followed. The ESA’s XMM Newton and NASA’s NuSTAR and Swift space telescopes got to work, along with ground-based telescopes. These observations confirmed what initial observations showed: this is a very peculiar pair of stars.
“…we believe we saw the X-rays turning on for the first time.” – Erik Kuulkers, ESA INTEGRAL Project Scientist.
The neutron star spins very slowly, taking about 2 hours to revolve, which is remarkable since other neutron stars can spin many times per second. The magnetic field of the neutron star was also much stronger than expected. But the magnetic field around a neutron star is thought to weaken over time, making this a relatively young neutron star. And a red giant is old, so this is a very odd pairing of old red giant with young neutron star.
One possible explanation is that the neutron star didn’t form from a supernova, but from a white dwarf. In that scenario, the white dwarf would’ve collapsed into a neutron star after a very long period of feeding on material from the red giant. That would explain the disparity in ages of the two stars in the system.
“These objects are puzzling,” says Enrico. “It might be that either the neutron star magnetic field does not decay substantially with time after all, or the neutron star actually formed later in the history of the binary system. That would mean it collapsed from a white dwarf into a neutron star as a result of feeding off the red giant over a long time, rather than becoming a neutron star as a result of a more traditional supernova explosion of a short-lived massive star.”
The next question is how long will this process go on? Is it short-lived, or the beginning of a long-term relationship?
“We haven’t seen this object before in the past 15 years of our observations with INTEGRAL, so we believe we saw the X-rays turning on for the first time,” says Erik Kuulkers, ESA’s INTEGRAL project scientist. “We’ll continue to watch how it behaves in case it is just a long ‘burp’ of winds, but so far we haven’t seen any significant changes.”
The INTEGRAL space observatory was launched in 2002 to study some of the most energetic phenomena in the universe. It focuses on things like black holes, neutron stars, active galactic nuclei and supernovae. INTEGRAL is a European Space Agency mission in cooperation with the United States and Russia. Its projected end date is December, 2018.
In addition to being the largest and most massive planet in our Solar system, Jupiter is also one of its more mysterious bodies. This is certainly apparent when it comes to Jupiter’s powerful auroras, which are similar in some ways to those on Earth. In recent years, astronomers have sought to study patterns in Jupiter’s atmosphere and magnetosphere to explain how aurora activity on this planet works..
For instance, an international team led by researchers from University College London recently combined data from the Juno probe with X-ray observations to discern something interesting about Jupiter’s northern and southern auroras. According to their study, which was published in the current issue of the scientific journal Nature – Jupiter’s intense, Jupiter’s X-ray auroras have been found to pulsate independently of each other.
As already noted, Jupiter’s auroras are somewhat similar to Earth’s, in that they are also the result of charged particles from the Sun (aka. “solar wind”) interacting with Jupiter’s magnetic field. Because of the way Jupiter and Earth’s magnetic fields are structured, these particles are channeled to the northern and southern polar regions, where they become ionized in the atmosphere. This results in a beautiful light display that can be seen from space.
In the past, auroras have been spotted around Jupiter’s poles by NASA’s Chandra X-ray Observatory and by the Hubble Space Telescope. Investigating this phenomena and the mechanisms behind it has also been one of the goals of the Juno mission, which is currently in an ideal position to study Jupiter’s poles. With every orbit the probe makes, it passes from one of Jupiter’s poles to the other – a maneuver known as a perijove.
For the sake of their study, Dr. Dunn and his team were forced to consult data from the ESA’s XMM-Newton and NASA’s Chandra X-ray observatories. This is due to the fact that while it has already acquired magnificent images and data on Jupiter’s atmosphere, the Juno probe does not have an X-ray instrument aboard. Once they examined the X-ray data, Dr. Dunn and his team noticed a difference between Jupiter’s northern and southern auroras.
Whereas the X-ray emissions at the north pole were erratic, increasing and decreasing in brightness, the ones at the south pole consistently pulsed once every 11 minutes. Basically, the auroras happened independently of each other, which is different from how auroras on Earth behave – i.e. mirroring each other in terms of their activity. As Dr. Dunn explained in a recent UCL press release:
“We didn’t expect to see Jupiter’s X-ray hot spots pulsing independently as we thought their activity would be coordinated through the planet’s magnetic field. We need to study this further to develop ideas for how Jupiter produces its X-ray aurora and NASA’s Juno mission is really important for this.”
The X-ray observations were conducted between May and June of 2016 and March of 2017. Using these, the team produced maps of Jupiter’s X-ray emissions and identified hot spots at each pole. The hot spots cover an area that is larger than the surface area of Earth. By studying them, Dr. Dunn and his colleagues were able to identify patterns of behavior which indicated that they behaved differently from each other.
Naturally, the team was left wondering what could account for this. One possibility they suggest is that Jupiter’s magnetic field lines vibrate, producing waves that carry charged particles towards the poles. The speed and direction of these particles could be subject to change over time, causing them to eventually collide with Jupiter’s atmosphere and generate X-ray pulses.
As Dr Licia Ray, a physicist from Lancaster University and a co-author on the paper, explained:
“The behavior of Jupiter’s X-ray hot spots raises important questions about what processes produce these auroras. We know that a combination of solar wind ions and ions of Oxygen and Sulfur, originally from volcanic explosions from Jupiter’s moon, Io, are involved. However, their relative importance in producing the X-ray emissions is unclear.”
And as Graziella Branduardi-Raymont- a professor from UCL’s Space & Climate Physics department and another co-author on the study – indicated, this research owes its existence to multiple missions. However, it was the perfectly-timed nature of the Juno mission, which has been in operation around Jupiter since July 5th, 2016, that made this study possible.
“What I find particularly captivating in these observations, especially at the time when Juno is making measurements in situ, is the fact that we are able to see both of Jupiter’s poles at once, a rare opportunity that last occurred ten years ago,” he said. “Comparing the behaviours at the two poles allows us to learn much more of the complex magnetic interactions going on in the planet’s environment.”
Looking ahead, Dr. Dunn and his team hope to combine X-ray data from XMM-Newton and Chandra with data collected by Juno in order to gain a better understanding of how X-ray auroras are produced. The team also hopes to keep tracking the activity of Jupiter’s poles for the next two years using X-ray data in conjunction with Juno. In the end, they hope to see if these auroras are commonplace or an unusual event.
“If we can start to connect the X-ray signatures with the physical processes that produce them, then we can use those signatures to understand other bodies across the Universe such as brown dwarfs, exoplanets or maybe even neutron stars,” said Dr. Dunn. “It is a very powerful and important step towards understanding X-rays throughout the Universe and one that we only have while Juno is conducting measurements simultaneously with Chandra and XMM-Newton.”
In the coming decade, the ESA’s proposed JUpiter ICy moons Explorer (JUICE) probe is also expected to provide valuable information on Jupiter’s atmosphere and magnetosphere. Once it arrives in the Jovian system in 2029, it too will observe the planet’s auroras, mainly so that it can study the effect these have on the Galilean Moons (Io, Europa, Ganymede and Callisto).