It’s called M82 X-2 and emits light equivalent to 10 million suns
The Universe is an inexhaustible source of wonder and knowledge. As in a gigantic detective game, astrophysicists try to unravel cosmic mysteries, and the laws that govern them, from clues that light leaves in its wake. We cannot recreate the conditions inside a star, simulate the brutal gravitational fields that surround them, and even with state-of-the-art technology we cannot achieve even a fraction of the forces at work in supermassive objects such as black holes. With the exception of the occasional probe that has managed to collect samples from an asteroid and with the expectation of a manned mission in the future to set foot on and study in situ a planet in our neighborhood, all we have left is the observation and careful study of the light collected by our observatories and telescopes, both on the ground and in space. Most of what we know about the Universe we have found out from light… but it represents only a tiny part of what we have yet to learn. That’s why we will always have exciting surprises.
One of these cosmic strangenesses is the protagonist of an interesting article by NASA researchers about an object called M82 X-2 that emits electromagnetic energy far beyond what our physical models would expect. The news is fascinating and to explain it in an accessible and entertaining way I will have the help of two great astrophysicists, Héctor Socas, research scientist at the Instituto de Astrofísica de Canarias and director of the Museo de la Ciencia y el Cosmos de Tenerife, and Miguel Santander, astrophysicist at the Observatorio Astronómico Nacional de Madrid.
Just over a decade ago, in 2012, NASA launched the NuSTAR telescope, (an acronym that stands for Joint Nuclear Spectroscopic Telescope) with the task of observing and obtaining data the most energetic objects in the Universe. Among these phenomena are the so-called ultra-luminous X-ray sources (ULX) whose luminosity is at the boundary of a physical law known as the Eddington limit. Roughly speaking, this limit expresses the maximum amount of luminosity that a body can emit uniformly according to its mass and that is why the most luminous events in the cosmos correspond to objects of enormous masses such as super-massive black holes or quasars. This Eddington Limit is one of the most used physical equations in astrophysics since knowing the luminosity of an object we can calculate the maximum mass it can have.
However, only a few months ago a scientific paper published in The Astrophysical Journal reported an unusual finding: M82 X-2 was the first ultra-bright X-ray source that did not correspond to a huge object, but was in fact a neutron star. This is a puzzling fact: For the first time, a simple neutron star, with a mass of about 1.4 times our Sun, was capable of producing a luminosity equivalent to ten million suns….
“One of the options that was considered was that there was a cloud of matter around this neutron star,” explains astrophysicist Miguel Santander. “It could happen that, from time to time, some of that matter fell into the neutron star and produced a kind of flash.
Neutron stars rotate very quickly (some of them can rotate hundreds of times on their axis in a single second) and can produce these flashes, as if they were flashes coming from a lighthouse on the coast. “This would solve the problem with the Eddington Limit since this limit assumes spherical symmetry and uniform luminosity in the object.”
Another option, related to the previous idea, is that “these pulsars or neutron stars produce a bipolar-shaped lighthouse, aligned with a certain axis, which is rotating and defines two light cones that sweep the sky at high speed,” Santander clarifies. “If it turns out that the Earth, or in this case the Telescope, is aligned with those cones, then you will observe pulses as if you were looking at a lighthouse, but the object is not radiating all that energy over the whole surface, over the whole sphere, but it is radiating very directly a lot of energy in that specific direction… and that is the direction in which it reaches you, so it is logical to assume that it shines more than it can physically shine.”
This hypothesis, based on the star focusing the light at a certain point, was attractive and solved the problem with the Eddington Limit… however, the study itself discards it after following the orbit of M82 X-2 for 7 years and confirming that there is an extreme mass transfer, “of more than 150 times the mass transfer limit set by the Eddington luminosity”. The authors point out that these strong data “are more than enough to justify its luminosity, without the need for focusing”… Back to square one.
If the “focusing” effect is not enough to explain this excessive luminosity, the scientists went on to consider a new factor: the magnetic field of M82 X-2. “In neutron stars the magnetic field is huge and it could be that we are seeing an amplified intensity in one direction and that this is not representative of what that object emits in all directions,” explains astrophysicist Hector Socas. “The magnetic field is our prime suspect, especially if we take into account that this Arthur Eddington equation was enunciated in the early 20th century and did not take into account magnetic fields, so we could say that they have the motive and the murder weapon.”
Still, that the powerful magnetic field of a neutron star could cause that object to exceed Eddington’s luminosity limit so far remains a controversial idea that will need further study, data and observations. For now, we can only affirm that, up there, 12 million light-years away, there is a strange object so luminous that it will need the work of our most “brilliant” astrophysicists…