Deepest X-ray Image Ever Made Contains Mysterious Explosion

For over sixty years, astronomers have been exploring the Universe for x-ray sources. Known to be associated with stars, clouds of super heated gas, interstellar mediums, and destructive events, the detection of cosmic x-rays is challenging work. In recent decades, astronomers have been benefited immensely from by the deployment of orbital telescopes like the Chandra X-ray Observatory.

Since it was launched on July 23rd, 1999, Chandra has been NASA’s flagship mission for X-ray astronomy. And this past week (on Thurs. March 30th, 2017), the Observatory accomplished something very impressive. Using its suite of advanced instruments, the observatory captured a mysterious flash coming from deep space. Not only was this the deepest X-ray source ever observed, it also revealed what could be an entirely new phenomenon.

Located in the region of the sky known as the Chandra Deep Field-South (CDF-S), this X-ray emission source appeared to have come from a small galaxy located approximately 10.7 billion light-years from Earth. It also had some remarkable properties, producing more energy in the space of a few minutes that all the stars in the galaxy combined.

Artist illustration of the Chandra X-ray Observatory, the most sensitive X-ray telescope ever built. Credit: NASA/CXC/NGST

Originally detected in 2014 by a team of researchers from Penn State University and the Pontifical Catholic University of Chile in Santiago, Chile, this source was not even detected in the X-ray band at first. However, it quickly caught the team’s attention as it erupted and became 1000 brighter in the space of a few hours. At this point, the researchers began gathering data using Chandra’s Advanced CCD Imaging Spectronomer.

A day after the flare-up, the X-ray source had faded to the point that Chandra was no longer able to detect it. As Niel Brandt – the Verne M. Willaman Professor of Astronomy and Astrophysics at Penn State and part of the team that first observed it – described the discovery in a Penn State press release:

“This flaring source was a wonderful surprise bonus that we accidentally discovered in our efforts to explore the poorly understood realm of the ultra-faint X-ray universe. We definitely ‘lucked out’ with this find and now have an exciting new transient phenomenon to explore in future years.”

Thousands of hours of legacy data from the Hubble and Spitzer Space Telescopes was then consulted in order to determine the location of the CDF-S X-ray source. And though scientists were able to determine that the image of the X-ray source placed it beyond any that had been observed before, they are not entirely clear as to what could have caused it.

X-ray (left) and optical (right) images of the space around the X-ray source, made with Chandra and the Hubble Space Telescope, respectively. Credit: NASA/CXC/F. Bauer et al.

On the one hand, it could be the result of some sort of destructive event, or something scientists have never before seen. The reason for this has to do with the fact that X-ray bursts also come with a gamma-ray burst (GRB), which appears to be missing here. Essentially, GRBs are jetted explosions that are triggered by the collapse of a massive star or by the merger of two neutron stars (or a neutron star with a black hole).

Because of this, three possible explanations have been suggested. In the first, the CDF-S X-ray source is indeed the result of a collapsing star or merger, but the resulting jets are not pointed towards Earth. In the second, the same scenario is responsible for the x-ray source, but the GRB lies beyond the small galaxy. The third possible explanation is that the event was caused by a medium-sized black hole shredding a white dwarf star.

Unfortunately, none of these explanations seem to fit the data. However, these research team also noted that these possibilities are not that well understood, since none have been witnessed in the Universe. As Franz Bauer – an astronomer from the Pontifical Catholic University of Chile – said: “Ever since discovering this source, we’ve been struggling to understand its origin. It’s like we have a jigsaw puzzle but we don’t have all of the pieces.”

Not only has Chandra not observed any other X-ray sources like this one during the 17 years it has surveyed the CDF-S region, but no similar events have been observed by the space telescope anywhere in the Universe during its nearly two decades of operation. On top of that, this event was brighter, more short-lived, and occurred in a smaller, younger host galaxy than other unexplained X-ray sources.

Still image of the X-ray source observed by Chandra, showing the captured flare up at bottom Credit: NASA/CXC/Pontifical Catholic Univ./F.Bauer et al.

From all of this, the only takeaway appears to be that the event was likely the result of a cataclysmic event, like a neutron star or a white dwarf being torn apart. But the fact that none of the more plausible explanations seem to account for it’s peculiar characteristics would seem to suggest that astronomers may have witnessed an entirely new kind of cataclysmic event.

The team’s study – “A New, Faint Population of X-ray Transient“- is available online and will be published in the June 2017 issue of the Monthly Notices of the Royal Astronomical Society. In the meantime, astronomers will be sifting through the data acquired by Chandra and other X-ray observatories – like the ESA’s XMM-Newton and NASA’s Swift Gamma-Ray Burst Mission – to see if they can find any other instances of this kind of event.

And of course, future surveys conducted using Chandra and next-generation X-ray telescopes will also be on the lookout for these kind of short-lived, high-energy X-ray bursts. It’s always good when the Universe throws us a curve ball. Not only does it show us that we have more to learn, but it also teaches us that we must never grow complacent in our theories.

Be sure to check out this animation of the CDF-S X-ray source too, courtesy of the Chandra X-ray Observatory:

Further Reading:  Chandra, PennState

Nearby Galaxy Holds First Ultraluminous X-Ray Source that is a Pulsar

A research team led by Caltech astronomers of Pasadena California have discovered an ultraluminous X-ray (ULX) source that is pulsating. Their analysis concluded that the source in a nearby galaxy – M82 – is from a rotating neutron star, a pulsar. This is the first ULX source attributed to a pulsar.

Matteo Bachetti of the Université de Toulouse in France first identified the pulsating source and is the lead author of the paper, “An ultraluminous X-ray source powered by an accreting neutron star” in the journal Nature. Caltech astronomer Dr. Fiona Harrison, the team leader, stated “This compact little stellar remnant is a real powerhouse. We’ve never seen anything quite like it. We all thought an object with that much energy had to be a black hole.”

What is most extraordinary is that this discovery places even more strain on theories already hard pressed to explain the existence of ultraluminous X-Ray sources. The burden falls on the shoulder of the theorists.

The NuStar Space Telescope launched into Earth orbit by a Orbital Science Corp. Pegasus rocket, 2012. The Wolter telescope design images throughout a spectral range from 5 to 80 KeV. (Credit: NASA/Caltech-JPL)
The NuStar Space Telescope launched into Earth orbit by a Orbital Science Corp. Pegasus rocket, 2012. The Wolter telescope design images throughout a spectral range from 5 to 80 KeV. (Credit: NASA/Caltech-JPL)

The source of the observations is the NuSTAR space telescope, a SMEX class NASA mission. It is a Wolter telescope that uses grazing incidence optics, not glass (refraction) or mirrors (reflection) as in visible light telescopes. The incidence angle of the X-rays must be very shallow and consequently the optics are extended out on a 10 meter (33 feet) truss. NuSTAR records its observations with a time stamp such as taking a video of the sky. The video recording in high speed is not in visible everyday light but what is called hard x-rays. Only gamma rays are more energetic. X-rays emanate from the most powerful sources and events in the Universe. NuStar observes in the energy range of X-Rays from 5 to 80 KeV (electron volt)while the famous Chandra space telescope observes in the .1 to 10 KeV range. Chandra is one NASA’s great space telescope, was launched by the Space Shuttle Columbia (STS-93) in 1999. Chandra has altered our view of the Universe as dramatically as the first telescope constructed by Galileo. NuSTAR carries on the study of X-rays to higher energies and with greater acuity.

ULX sources are rare in the Universe but this is the first pulsating ULX. After analysis, they concluded that this is not a black hole but rather its little brother, a spinning neutron star as the source. More specifically, this is an accreting binary pulsar; matter from a companion star is being  gravitationally attracted by and accreting onto the pulsar.

The Crab Nebula Pulsar, M1. Both are sequences of observations that show the expansion of shock waves emanating from the Pulsar interacting with the surrounding nebula. The Crab Pulsar actually pulsates 30 times per second a result of its rotation rate and the relative offset of the magnetic pole. Charndra X-Rays (left), Hubble Visible light (right). (Credit: NASA, JPL-Caltech)
The prime example of a pulsar – the Crab Nebula Pulsar, M1. These actual observations show the expansion of shock waves emanating from the Pulsar interacting with the surrounding nebula. The Crab Pulsar actually pulsates 30 times per second, not seen here, a result of its rotation rate and the relative offset of the magnetic pole. Charndra X-Rays (left), Hubble Visible light (right). (Credit: NASA, JPL-Caltech)

Take a neutron star and spin it up to anywhere from 700 rotations per second to a mere one  rotation every 10 seconds. Now you have a neutron star called a pulsar. Spinning or not, these are the remnants of supernovae, stellar explosions that can outshine a galaxy of 300 billion stars. Just one teaspoon of neutron star material weighs 10 million tons (9,071,847,400 kg). That is the same weight as 900 Great Pyramids of Giza all condensed to one teaspoon. As incredible a material and star that a neutron star is, they were not thought to be the source of any ultraluminous X-Ray sources. This view has changed with the analysis of observations by this research team utilizing NuSTAR. The telescope name – NuSTAR – stands for Nuclear Spectroscopic Telescope Array.

There is nothing run of the mill about black holes. Dr. Stephen Hawking only conceded after 25 years, in 2004 (the Thorne-Hawking Bet)  that Black Holes exist. And still today it is not absolutely certain. Recall the Universe Today weekly – Space Hangout on September 26 – “Do Black Holes exist?” and the article by Jason Major, “There are no such things as Black Holes.

Pulsars stars are nearly as exotic as black holes, and all astronomers accept the existence of these spinning neutron stars. There are three final states of a dying star. Stars like our Sun at the end of their life become very dense White Dwarf stars, about the size of the Earth. Neutron stars are the next “degenerate” state of a dying exhausted star. All the electrons have merged with the protons in the material of the star to become neutrons. A neutron star is a degenerate form of matter effectively made up of all neutron particles. Very dense, these stars are really small, the size of cities, about 16 miles in diameter. The third type of star in its final state is the Black Hole.

The Crab Nebula was first  observed in the 1700s and is catalogued Messier object, M1. The remant explosion of a SuperNova, Chinese astronomers observed in 1054 A.D and holds the second Pular discovered (1968).
The Crab Nebula was first observed in the 1700s and is catalogued Messier object, M1. The remant explosion of a SuperNova that Chinese astronomers observed in 1054 A.D, it holds the second Pulsar discovered (1968).

A spinning neutron star creates a magnetic field, the most powerful of such fields in the Universe. They are like a dipole of a bar magnet and because of how magnetic fields confine the hot gases – plasma – of the neutron star, constant streams of material flow down and light streams out from the magnetic poles.

Recently, the Earth has had incredible northern lights, aurora. These lights are also from hot gases — a plasma — at the top of our atmosphere. Likewise, hot energetic particles from the Sun are funneled down into the magnetic poles of the Earth’s field that creates the northern lights. For spinning neutron stars – pulsars – the extreme light from the magnetic poles are like beacons. Just like our Earth, the magnetic poles and the spin axis poles do not coincide. So the intense beacon of light will rotate around and periodically point at the Earth. The video of the first illustration describes this action.

Messier object - M82, the Cigar Nebula, nicknamed for the shape seen through telescopes of the 1800s. This is the location of the newly discovered Pulsar.
Messier object – M82, the Cigar Nebula, nicknamed for the shape seen through telescopes of the 1800s. This is the location of the newly discovered Pulsar.

The light beacons from pulsars are very bright but theory, until now, has been supported by observations. No ultraluminous X-ray sources should be pulsars. The newly discovered pulsar is outputting 100 times more energy than any other. Discoveries like the one by these astronomers utilizing NuSTAR is proof that there remains more to discover and understand and new telescopes will be conceived to help resolve questions raised by NuSTAR or Chandra.

Further reading: JPL