Artist's impression of a Star feeding a black hole. Credit: ESO/L. Calçada
Black holes are famous for being inescapable. Within the event horizon of these celestial objects, matter and even light enter and then disappear forever. However, beyond the event horizon, black holes are known to form accretion disks from which light can escape. In fact, this is how astronomers are able to confirm the presence of black holes and determine their properties (i.e. mass, spin rate, etc.)
However, according to a recent NASA-funded study led by researchers from the California Institute of Technology (Caltech), there is evidence that not all light emanating from a black hole’s disk simply escapes. According to their observations, some of the light escaping from the disk is pulled back in by the black hole’s gravity and reflected off the disk again. These observations confirm something astronomers have theorized for about forty years.
Composite image of Circinus X-1, which is about 24,000 light-years from Earth in the constellation Circinus. Credit: X-ray: NASA/CXC/Univ. of Wisconsin-Madison/S. Heinz et al; Optical: DSS; Radio: CSIRO/ATNF/ATCA
Circinus X-1 may look like a serene place from a distance, but in reality this gassy nebula is quite a busy spot. Embedded in the nebula is the neutron star that is also a leftover of the supernova that produced the gas. Not only that, but the neutron star is still locked on to a companion and is in fact “cannibalizing” it, astronomers said.
The “glowing wreck of a star”, as the team called it, is exciting because it demonstrates what systems look like in the first stages after an explosion. The nebula is an infant in cosmic terms, with an upper limit to its age of just 4,500 years. To put that in human terms, that’s around the time of the first civilizations (such as in Mesopotamia).
“The fact that we have this remnant along with the neutron star and its companion means we can test all kinds of things,” stated Sebastian Heinz, an astronomy professor at the University of Wisconsin-Madison who led the research.
“Our observations solve a number of puzzles both about this object and the way that neutron stars evolve after they are born. For example, the unusual elliptical orbit on which these two stars swing around each other is exactly what you would expect for a very young X-ray binary.”
X-ray binaries are typically made up of a black hole or a neutron star that is locked on to a “normal” companion star such as that of our sun. That star won’t stay normal forever, however, as it’s being subject to very intense gravity from the black hole or neutron star. Its starstuff is being pulled off, heated, and then emitting radiation in X-rays that are easily trackable across the universe.
While X-ray binaries have been spotted before, seeing one along with a nebula is something special. By comparison, the gas cloud doesn’t stick around for very long — just 100,000 years or so — while the stars can be there for a while longer.
Checking out this star system could not only teach scientists about stellar evolution, but about the nature of neutron stars. One thing puzzling the team right now is why the neutron star has a faint magnetic field, which stands against established theory. Further study will be required to figure out why it isn’t as strong as expected.
A binary X-ray system with a black hole (right) and companion star. Credit: ESO/L. Calçada
This high-resolution view from NASA’s Chandra X-Ray Telescope and the Australia Telescope Compact Array, however, has revealed some new things.
“I have been perplexed by the unusually strong evolution of the orbit of Circinus X-1 since my graduate-school days,” stated Niel Brandt, an astronomer at Pennsylvania State University who is on the team. “The discovery now of this system’s youth provides a satisfying explanation for why its orbit evolves so strongly — because the system likely still is settling down after its violent birth.”
You can read more in the Dec. 4 publication in The Astrophysical Journal or, in prepublished form, on Arxiv.
The Hulk (Bruce Banner), as portrayed in The Avengers. Credit: Marvel & Subs
When Bruce Banner gets angry, he gets big and green and strong and well, vengeful. The Hulk is the stuff of comic book legend and as Mark Ruffalo recently showed us in The Avengers, Banner’s/Hulk’s personality can transform on a dime.
Turns out rapid transformations are the case in astronomy, too! Scientists found a peculiar neutron star that can change from radio pulsar, to X-ray pulsar, back and forth. In the Hulk’s case, a big dose of gamma rays likely fuelled his ability to transform. This star’s superpowers, however, likely come from a companion star.
“What we’re seeing is a star that is the cosmic equivalent of ‘Dr. Jekyll and Mr. Hyde,’ with the ability to change from one form to its more intense counterpart with startling speed,” stated Scott Ransom, an astronomer at the National Radio Astronomy Observatory.
“Though we have known that X-ray binaries — some of which are observed as X-ray pulsars — can evolve over millions of years to become rapidly spinning radio pulsars, we were surprised to find one that seemed to swing so quickly between the two.”
A neutron star and its companion flipping between accretion (when it emits X-rays) and when accretion has stopped (when it emits radio pulses). Credit: Bill Saxton; NRAO/AUI/NSF. Animation by Elizabeth Howell
The star’s double personality came to light after astronomers made an accidental double-discovery. IGR J18245-2452, as the star is called, was flagged as a millisecond radio pulsar in 2005 using the National Science Foundation’s Robert C. Byrd Green Bank Telescope. Then this year, another team found an X-ray pulsar in the same region of the star cluster M28.
It took a little while to sort out the confusion, we’re sure, but eventually astronomers realized it was the same object behaving differently. That said, they were mighty confused: “This was particularly intriguing because radio pulses don’t come from an X-ray binary and the X-ray source has to be long gone before radio signals can emerge,” stated lead researcher Alessandro Papitto, who is with of Institute of Space Sciences in Catalunya (Institut d’Estudis Espacials de Catalunya) in Spain.
The key, it turns out, comes from the interplay with the star’s companion. Material doesn’t flow continuously, as astronomers previously believed is true of these system types, but in bunches. Starting and stopping the flow then led to swings in the behavior, making the star alternate between X-ray and radio emissions.
– A neutron star that has a normal star nearby forms an X-ray binary, which happens when the neutron star poaches starstuff off its companion. When the material hits the neutron star, the stuff gets really hot and emits X-rays.
– When the material stops, magnetic fields on the neutron produce radio waves. These appear to blink on and off from the perspective of Earth, as the neutron rotates super-fast (several times a second).
In the case of IGR J18245-2452, it behaved like an X-ray binary star for about a month, stopped suddenly, and then sent out radio waves for a while before flipping back again. (A month is less than a blink in astronomical terms, when you recall the universe is 13.8 billion years old.)
To take the longer view, astronomers used to believe that X-ray binaries could evolve into radio emitters over time. Now, though, it appears a star can be these two things at almost the same time.
“During periods when the mass flow is less intense, the magnetic field sweeps away the gas and prevents it from reaching the surface and creating X-ray emission,” NASA stated. “With the region around the neutron star relatively gas free, radio signals can easily escape and astronomers detect a radio pulsar.”
A whole suite of telescopes in Earth and space contributed to this discovery, but of note: the X-ray source was first spotted with the International Gamma-Ray Astrophysics Laboratory (INTEGRAL). You can read more details in the paper published in Nature.
