Our ever-more-detailed study of the Universe has showcased that the latter seems not to adhere to our man-made definitions and classifications of celestial objects in distinct and different categories, but prefers continuity, as evidenced by the recent discoveries of asteroids with comet-like tails, planet-like brown dwarfs, as well as the detection of water ice sublimation on dwarf planet Ceres—a process which is the hallmark of comets, as the latter approaches close to the Sun. A similar continuity can be found among other astrophysical objects as well, like high-energy novae and gamma-ray sources, and among the different types of active galactic nuclei. Now, a new study by an international team of astronomers makes another addition to the list, that of an ultra-luminous, X-ray source which packs the power of a massive black hole, yet is a pulsar.
The dawn of the space age allowed astronomers to observe the high-energy Universe for the first time, with the help of space-based observatories that were sensitive to X-ray and gamma-ray wavelengths which are absorbed by the Earth’s atmosphere. The advent of X-ray astronomy in particular led to the discovery of hundreds of thousands of previously unobservable luminous X-ray cosmic sources which have helped to revolutionize our understanding of the extreme physics behind some of the most violent and energetic phenomena in the Universe.
Among the brightest of these X-ray cosmic sources are the accretion disks that surround black holes—regions in space-time where gravity is so strong that nothing, not even light, can escape. Probably the most fascinating and mysterious objects in the Universe, black holes fall into two categories: the stellar-mass black holes which are usually found in binary star systems and have masses up to a few dozen times that of the Sun, and the supermassive black holes which lie in the centers of galaxies and can be several million or even billion times more massive. These celestial monsters often grow by accreting matter from their surroundings, which in the case of smaller black holes in stellar binaries comes from their companion stars, while in supermassive black holes it is usually the gas and dust that surrounds the centers of galaxies. When this matter is caught by the black hole’s immense gravity, it begins to spiral down toward its center, similar to the way water spirals down a drain, forming an accretion disk. As it approaches ever closer to the black hole, the material inside the disk is accelerated to very high speeds, which cause it to heat up to extremely high temperatures that can reach several million Kelvin and glow brightly in X-ray wavelengths.
Astronomers have long hypothesized that a third class of black holes should also exist, ranging in mass between 100 to 1 million solar masses, essentially forming the missing link between the smaller stellar-mass black holes that are found in X-ray binary star systems, and the gigantic supermassive ones that lie in the heart of galaxies. Although the presence of these intermediate-mass black holes hasn’t been confirmed yet, scientists nevertheless think that among the likeliest such candidates are a class of enigmatic astrophysical X-ray sources, called ultra-luminous X-ray sources, or ULXs, that have been observed in several nearby galaxies. ULXs exhibit X-ray luminosities which are less than that of supermassive black holes, but are greater than that of stellar-mass ones. The reason that scientists think that ULXs are powered by black holes in the first place is because the observed energies from these X-ray sources can only result from objects that have a very high mass and gravity. The more massive the objects, the more matter they will be able to accrete around them and the more powerful the latter will emit in X-ray wavelengths. The only known astrophysical objects that fit the bill are black holes.
Yet, as is often the case in astronomy and astrophysics, many theoretical predictions are ultimately overturned by new observations which reveal the presence of new and surprising phenomena. A new study, which was published in the Oct. 9 issue of the journal Nature by a research team led by Dr. Matteo Bachetti, an astrophysicist at the University of Toulouse, France, reports the discovery of a ULX source that emits the energy of 10 million Suns, yet it is powered from an inconspicuous pulsar—the dead remnant of a massive star which ended its life in a supernova explosion. The unexpected discovery was made with the help of NASA’s Nuclear Spectroscopic Telescope Array, or NuSTAR X-ray space telescope, which studies the Universe in the high-energy part of X-rays that are beyond the observing capabilities of other space-based X-ray observatories like Chandra and XMM-Newton. Launched aboard a Pegasus XL rocket in June 2012, NuSTAR has already returned several important scientific results in its two first years of operations, including the more precise measurements to date of the spin-rates of black holes, the observation of a record-setting gamma-ray burst’s X-ray afterglow in great detail, and the production of the first-ever map of the radioactive material inside a supernova remnant.
The newly discovered ULX source was discovered in the nearby starburst galaxy M82, also known as the Cigar galaxy, which lies approximately 11.5 million light-years away in the direction of the Ursa Major constellation. M82 has been the object of intense study by NuSTAR and other space telescopes ever since the detection of a supernova that had exploded there earlier this year. During a series of seven observations between 23 January and 6 March, NuSTAR observed an intensely bright X-ray signal that was slightly offset from the galaxy’s center, whose energy was equivalent to that of a black hole with a few hundred times the mass of the Sun. Due to its very high luminosity, the research team that led the study initially thought that this new-found ULX source, called M82 X-2, was a perfect candidate for the long-sought-for intermediate-mass black holes. Yet further analysis of the X-ray signal’s variability from M82 X-2 revealed that the latter was rapidly pulsating with a period of 1.37 seconds and exhibited an orbital modulation period of 2.5 days. Since black holes cannot produce such a signal, this could only mean one thing: M82 X-2 was a pulsar inside an X-ray binary system. “We took it for granted that the powerful ULXs must be massive black holes,” says Bachetti. “When we first saw the pulsations in the data, we thought they must be from another source.”
Pulsars are rapidly-rotating neutron stars with masses between 1.4 and about 3 times that of the Sun, which emit beams of electromagnetic radiation that when sweep through our line of sight here on Earth they are seen as pulses. But the pulsar discovered in M82 X-2 is truly an oddball: it is accreting matter from its companion star at an extreme rate, while producing the energy equivalent of 10 million Suns, making it 10 times more luminous than any other known X-ray pulsars. “This dead star [inside M82 X-2], called a neutron star, packs about the mass of the whole Sun into a region the size of San Fransisco, yet, this little ‘Mighty Mouse’ pulsar packs the power of a much bigger black hole,” said Dr. Fiona Harrison, principal investigator for NuSTAR at the California Institute of Technology, during the announcement of the results at a NASA teleconference. “This discovery is astonishing, because no object like this has ever been observed to be even remotely this bright. Theorists didn’t think that it was possible.”
The discovery of the ultra-luminous M82 X-2 puts a new twist to the mystery of ULX sources, overturning the established theoretical predictions concerning the nature of these mysterious objects. In addition, a nearby ULX source, called M82 X-1, which was previously discovered by NASA’s now defunct Rossi X-ray Timing Explorer satellite, had been recently determined to harbor an intermediate-mass black hole candidate with a mass approximately 428 times that of the Sun, while exhibiting the same energy output with M82 X-2. “In the news recently, we have seen that another source of unusually bright X-rays in the M82 galaxy seems to be a medium-sized black hole,” says Jeanette Gladstone, an astrophysicist at the University of Alberta, Canada, who was not involved with the study. “Now, we find that the second source of bright X-rays in M82 isn’t a black hole at all. This is going to challenge theorists and pave the way for a new understanding of the diversity of these fascinating objects.”
The high luminosity of M82 X-2 presents a puzzle for scientists, who are not sure as to how exactly pulsars can become that bright. According to one theory, the high accretion rate of M82 X-2 is caused by the pulsar’s super-charged magnetic field, which temporarily pulls in more matter from the companion star, increasing the pulsar’s luminosity beyond predicted levels for brief periods of time. Yet many more observations will be needed in order to determine whether M82 X-2 represents a fluke or a new class of objects. “It’s possible that [M82 X-2] is an oddball or it might be possible that there are other [like it]” says Bachetti. “The next step is to look for pulsars in other ultra-luminous X-ray sources.”
If the history of astronomical research is any indication, the answer to the mystery of the ultra-luminous pulsars will lead to new, exciting insights and many more questions regarding the amazing Universe we live in.