The Neutron Star That Thinks It’s a Black Hole

Black holes and neutron stars are among the odder denizens of the cosmic zoo. They’re both dense collections of matter and, except for supermassive black holes, are the end states of massive stars. Fundamentally, they’re two different types of objects that are detectable via the activity in the accretion disks that form around them. Astronomers recently observed an object that acted like a black hole but turned out to be a neutron star. The clues lay in the accretion disk surrounding it.

Where do accretion disks come from? And, why do they brighten up with flares and outbursts? In the case of binary systems, that material comes from companion stars that feed them as much (or more) than they can eat. The accretion disks respond in various ways, often forming hotspots and blasting out jets of radiation. Yet, in many respects, understanding some of the disk activities had to wait for extensive, multi-wavelength studies of their outbursts.

Monitoring Odd Behavior in an Accretion Disk

When astronomers first saw a weirdly bright object called J1858.6-0814, they first assumed it was a black hole with a companion star. It gave off a series of flares that extended across the electromagnetic spectrum, from radio to x-rays. After a lot of observations and head-scratching, the team members realized that this thing is a neutron star with a solid surface. There’s an accretion disk around it created by a companion star. The bright flares come from instability in that accretion disk and high-speed ejections of material away from the objects.

According to IAC researcher Federico Vincentelli, the lead scientist studying this object, the instabilities affect the behavior of the accretion disk around the neutron star. “Matter from the disk is launched within an outflow or a jet,” he said. “Some may still fall onto the black hole or neutron star. However, the exact fraction of matter which goes either way is still unknown. This dramatic process remains poorly understood and, until now, has only been observed in detail in a system in which the compact object is a black hole.”

Understanding X-ray Binary J1858.6-0814

X-ray binaries are basically pairs of objects—usually a black hole or a neutron star, plus a companion star—that are bright in x-rays. J1858.6-0814 is in a special class called a low-mass x-ray binary. Stars in such pairs have masses below around 1.5 solar masses. The neutron star has a mass somewhere between 10 and 25 solar masses. There’s a great deal of interactivity between the members as the star shares its material with its companion. When that happens at a neutron star, the material doesn’t fall straight onto the surface. It spirals around into the accretion disk. As it does, it undergoes friction heating, loses its potential energy, and eventually contributes to those instabilities Vincentelli describes.

After its 2018 discovery, astronomers noticed that J1858.6-0814 showed a great deal of flaring activity across the entire electromagnetic spectrum. It quieted down in 2020, but not before giving off Type-1 X-ray bursts and exhibiting what’s called an x-ray eclipse. Essentially, the system undergoes eclipses as one member of the binary pair eclipses the other. For this system, that implies an almost edge-on viewing angle from our vantage point on Earth. Astronomers also determined that this pair is in an approximately 21.3-hour orbit.

At first, astronomers weren’t sure about the origin of this binary pair’s “cosmic fireworks”. They were so bright that everybody assumed that the compact object must be a black hole since that would be powerful enough to spark such intensely bright flares. However, in 2020, observers discovered thermonuclear explosions that could only have happened if the material were falling onto the surface of a neutron star. That finding spurred the call for new observations.

So, What’s Happening on this Neutron Star?

Although astronomers observed J1858.6-0814 extensively over the years since its discovery, Vincentelli and a team of astronomers put together a new observing campaign aimed at tracking the specifics of the outbursts. They used the Cosmic Origins Spectrograph on Hubble Space Telescope, a fast readout wide-field camera called RISE on the Liverpool Telescope on La Palma, Spain, a near-infrared instrument called HAWK on the Very Large Telescope in Chile, and radio observations with the Karl G. Jansky Very Large Array in New Mexico. From those observations, they compared the outbursts with another, similar-type object called GRS 1915+105. It’s actually a black hole in a binary pair that astronomers use for comparison studies. What they found is exotic behavior in the accretion disk very much like the one around the neutron star in J1858.6-0814.

The Black Hole and Neutron Star Experience Similar Disk Activities

In essence, these systems are telling astronomers that almost-identical processes are happening at both. “We realized that we could explain the complex phenomenology of both objects with three ingredients: an unstable accretion disk, which produces extremely variable x-ray emission as the inner parts of the disk cyclically empty and fill; repeated ejections of matter (produced after the emptying of the disk), which can be seen in radio waves and infrared; and bright echoes of these internal variations in the outermost areas of the disk, which can be observed from infrared to ultraviolet”, said Vincentelli.

It turns out that instabilities in accretion disks happen as material exits the inner part of the disk. Then, new material flows in at high speed. It may well be a very fundamental process in the weird physics that happen near neutron stars and black holes. That finding now provides astronomers with new facets to the accretion disks around neutron stars and black holes, and how their instabilities happen. “We now plan to extend this type of study to other very luminous systems to shed light on black holes and neutron stars when they incorporate matter at extreme speeds,” said Vincentelli.

For More Information

The Outburst of a Neutron Star Reveals the Nature of Phenomena Only Observed in Black Holes
A Shared Accretion Instability for Black Holes and Neutron Stars
arXiv pre-print

A Persistent Ultraviolet Outflow from an Accreting Neutron Star Transient
Low-Mass X-ray Binaries

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