Small but Mighty: Extreme Luminosities from Neutron Stars

Ever since the first time we could detect x-ray sources in space, we have been fascinated by the question of what could be generating such intense radiation. It turns out that accretion – particles falling towards a dense object like a black hole, neutron star, or white dwarf – is the main engine that produces x-rays. But the story doesn’t end there. Second-year PhD student Nabil Brice tells us more.

When we looked out at other galaxies and measured the amount of x-ray radiation being produced by these sources, we found that some of them were producing far beyond the worked-out theoretical limit. The theoretical model of accretion discs had worked wonders for the x-ray sources inside our galaxy and even for most of the sources we found outside of our galaxy. So it couldn’t be a problem with the basic description.

Ultra-luminous x-ray sources (or ULXs for short) are point-like sources outside of galactic nuclei, which emit above the theoretical luminosity limit for black holes that are born from stars (called stellar mass black holes). X-ray emission comes primarily from a binary system, in which particles from the donor companion accrete onto a compact object (such as a white dwarf, neutron star, or black hole). As the particles fall towards the compact object, they release some of their gravitational potential energy as x-rays, which we then observe. A prevailing mystery has been exactly what type of compact object would be powering such extreme luminosities.

For a while, there was some hope that ULXs might powered by (the missing) intermediate mass black holes – these are black holes which have masses 10 – 1000 times larger than stellar mass black holes but still smaller than supermassive black holes found in galactic nuclei. The idea was that since the luminosity limit is directly proportional to the mass of the compact object, a more massive compact object would result in a higher luminosity limit. There was also the added benefit that if ULXs were powered by intermediate mass black holes, then we would have a clue as to how supermassive black holes are formed. However, as more observations were collected, it became clearer that ULXs were unlikely to be powered by such massive black holes.

Image of a pulsating ultra-luminous x-ray source. Credit: NASA/ JPL-Caltech

A paradigm shift came in 2014 when regular pulsations were discovered in the ULX M82 X-2 (Bachetti et al. 2014). Such pulsations have been observed in other non-ULX neutron star x-ray binary systems. This clearly indicated the M82 X-2 system contained a neutron star as the compact object. But the luminosity emitted from M82 X-2 was 100 times larger than the theoretical luminosity limit for neutron stars. So the question became: how can a neutron star emit much higher than its theoretical limit, as we observe in M82 X-2?

One possible explanation, which is detailed in an upcoming paper, is related to super strong magnetic fields which neutron stars can possess. In this scenario, the accreting particles become more transparent to the radiation when close to the surface of the neutron star, where the magnetic fields are strongest. This effect allows for more radiation to escape and so the theoretical limit is overcome.

However, the community is not yet at a consensus. Further observation and theoretical work is needed to get to a full understanding of what exactly is going on in these extreme accretion regime.

Featured image credit: X-ray- NASA/ CXC/Caltech/M. Brightman et al; Optical – NASA/STScI.

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