What Are ULXs or Ultra Luminous X-Ray Sources?

Updated on October 9, 2019
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Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.

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It seems like astronomy offers new surprises to challenge our understanding of the Universe. For every new phenomena that is explained, a mystery develops to further the intrigue. Ultraluminous X-ray sources (ULXs) are no different. They offer challenges to known astronomical processes and seem to violate the norms our theories predict should be there. So let’s look into ULXs and see how they too add to the challenge of mastery over the heavens.

Black Holes?

Two main theories exist for what ULXs could be: Either pulsars or black holes. Infalling matter around a black hole gets heated up by friction and gravitational forces as it spins around the black hole. But not all of this material ends up being consumed by the black hole, for that heat causes light to radiate out provides sufficient radiation pressure to remove material from the vicinity of the black hole before it is consumed. This causes a restriction in the amount a black hole can eat, and is known as the Eddington limit. For ULXs to work, this limit has to be exceeded, for the amount of x-rays being generated can only come from plenty of material being accelerated. What can account for this? (Rzetelny “Possible,” Swartz)

It could be that the size of the black hole is wrong – and therefore means we have a greater Eddington limit. Intermediate black holes, the bridge between stellar and supermassive in terms of mass, and therefore can have a greater area in which to bend the limit. Several studies have shown a clustering of the luminosities of the ULXs that would match the known mass of intermediate black holes. It could however be that we don’t fully understand the mechanics of black hole dining etiquette and that something may allow stellar black holes to achieve the output ULXs have been seen to have. Environmental issues such as star-forming regions may provide further complications, for we cannot rule out the mass of stellar black holes in these situations. But intermediates are a possibility still. Several ULXs including NGC 1313 X-1 and NGC 5408 X-1 have been spotted with high winds around their discs that have high x-ray outputs themselves, sometimes as fast as a quarter the speed of light. This can help scientists understand the eating habit of the ULXs and refine their models (Rzetelny “Possible,” ESA, Swartz, Miller).

ULX in Whirlpool Galaxy
ULX in Whirlpool Galaxy | Source

Clues

We can learn more about them though if we can look through multiple wavelengths besides x-rays. This is challenging though because ULXs are weak in other portions of the spectrum, especially optical waves. These objects just lack the angular resolution we require for distinct measurements. But with the right technology and perfect targets to remove background noise from, scientists were surprised to see that the spectrums of ULXs optically matched supergiant and luminous blue variable stars. The emission spectrums showed ionized iron, oxygen, and neon, some elements one would expect to see from an accretion disc. This hints at a binary nature to ULXs, for something has to be constantly feeding the object. But this isn’t unusual, for many black hole detections are a result of binaries, especially active in the x-ray spectrum. What makes this unusual is the intensity that is way too high according to modeling. Is it the type of object at play that causes the distinction? (Rzetelny “Possible,” (Rzetelny “Strange,” Swartz)

Further research showed that the characteristics of the ULXs when compared to their less eventual brethren were similar in terms of “spectral shapes, colors, time series, and (radial) positions within the host galaxies. This implies that since less excitable events come from several different sources like supernova remnants and black holes, ULXs may also come from a wide range of options. ULXs also seem to naturally fit onto a spectrum of x-ray luminous objects in the Universe, also implying that they are just the high end of a known process (Swartz).

Pulsars?

But what about that pulsar model? Their magnetic field could direct x-rays to a high concentration, but is it enough? AO538-66, SMC X-1, and GRO J1744-28 all seem to point to yes, for their highest X-ray outputs put them at the lower end of possible ULXs. How did we know they weren’t those black holes? Scientists spotted cyclotron resonance scattering which involves orbiting charged particles, a phenomena that can only happen in a magnetic field that black holes don’t possess. The pulsars spotted were in nearly circular orbits with their binary companions, indicating a high-torque situation that could provide additional energy needed to kick the X-rays emanating from them so long at their geometry lines up with the magnetic fields present. This isn’t a likely outcome, so something unknown to scientists is likely driving the ULXs here (Rzetelny “Strange,” Bachetti, Masterson, O’Niell).

Some ULXs have even been spotted with flaring activity, implying a repeating process. Sources like NGC 4697, NGC 4636, and NGC 5128 all have been spotted with repeating high x-rays. This also isn’t unusual behavior for binary systems, but to repeatedly do such an intensity every couple of days is nuts. The severity of the event should knock out all the material around the source yet the process continues (Dockrill).

NGC-925
NGC-925 | Source

Something New?

It could simply be a case of a brand new type of object unknown to astronomy. NGC 925 ULX-1 and ULX-2 were spotted in galaxy NGC 925 (located 8.5 mega-parsecs AWAY) by Fabio Pintore and the team at ISAF using data from XMM-Newton and the Chandra Space Telescope. ULX-1 was able to achieve a peak luminosity of 40 deodecillion ergs each second (that’s 40 followed by 39 zeros!). The rest of the spectrum did not match what a black hole would have around it for either of them, and yet they also didn’t match a binary situation (Nowakowski).

Stay tuned, folks. The answer is sure to be interesting.

Works Cited

Bachetti, M. et al. “An Ultraluminous X-ray Source Powered by An Accreting Neutron Star.” arXiv:1410.3590.

Dockrill, Peter. “Astronomers say these mysterious flaring objects could be an entirely new phenomena.” Sciencealert.com. Science Alert, 20 Oct. 2016. Web. 20 Nov. 2018.

ESA. “Powerful winds spotted from mysterious X-ray binaries.” Astronomy.com. Kalmbach Publishing Co., 29 Apr. 2016. Web. 19 Nov. 2018.

Masterson, Andrew. “Neutron star that defies all the rules discovered.” Cosmosmagazine.com. Cosmos, 27 Feb. 2018. Web. 30 Nov. 2018.

Miller, J.M. et al. “A Comparison of Intermediate Mass Black Hole Candidate ULXs and Stellar-Mass Black Holes.” arXiv:astro-ph/0406656v2.

Nowakowski, Tomasz. “Researchers investigate two Ultraluminous X-ray sources in the galaxy NGC 925.” Phys.org. Science X Network, 11 Jul. 2018. Web. 30 Nov. 2018.

O’Neill, Ian. “Tiny Yet Mighty: Neutron Stars May Be Ravenous X-ray Dazzlers.” Science.howstuffworks.com. How Stuff Works, 27 Feb. 2018. Web. 30 Nov. 2018.

Rzetelny, Xaq. “Possible identity for mysteriously bright x-ray-emitting objects.” Arstechnica.com. Conte Nast., 09 Jen. 2015. Web. 19 Nov. 2018.

---. “Strange X-ray sources are shooting ions at us at 20 percent of light speed.” Arstehcnica.com. Conte Nast., 05 May 2016. Web. 20 Nov. 2018.

Swartz, Douglas A et al. “The Ultra-Luminous X-Ray Source Population from the Chandra Archive of Galaxies.” arXiv:astro-ph/0405498v2.

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    © 2019 Leonard Kelley

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