Testing a Recipe for a Star Within a Star

A cartoon illustrating the proposed phases of Thorne–Żytkow object formation. From top left to bottom right, a neutron star accreting matter from its stellar companion is engulfed and migrates to the companion star's core. Credit: AAS Nova/Kerry Hensley

A Thorne–Żytkow object is a star within a star — a star with a neutron star at its core. These objects are theorized to form in close binary systems, but new research reveals complications in this proposed formation pathway.

A Star Within a Star

The term “star” encompasses a wide variety of objects, from our familiar Sun to roiling supergiants dozens of times as massive and hundreds of times as wide. Certain types of stars are only theorized, like those containing huge amounts of dark matter or with cores composed of strange quarks. One such theorized star — and the subject of today’s article — is a Thorne–Żytkow object, also known as a hybrid star.

After being engulfed by its companion, the neutron star is thought to sink to the star’s core. There, it is hypothesized to energize the surrounding star through accretion and nuclear fusion, creating a curious mix of elements that distinguishes a Thorne–Żytkow object from an ordinary star.

An artist’s impression of an X-ray binary, in which a compact object accretes material from a companion star and emits X-rays during intermittent outbursts. Credit: ESO/L. Calçada; CC BY 4.0

Diverging Paths

At least, that’s the theory. But as a recent research article led by Tenley Hutchinson-Smith (University of California, Santa Cruz; University of Copenhagen) shows, more work is needed to understand whether X-ray binary systems could truly evolve into Thorne–Żytkow objects. At the core of this question is how the inspiraling neutron star affects the companion that has engulfed it. Does the engulfing star hold on to its extended gaseous envelope, or does it lose its atmosphere and diverge from the Thorne–Żytkow object path? And how long would the Thorne–Żytkow phase last — could the neutron star remain at the center of its companion indefinitely, or does the neutron star eventually gain mass and collapse into a black hole?

The team used as the basis of their exploration the X-ray binary system LMC X-4, which contains a 1.57-solar-mass neutron star and an 18-solar-mass primary star. The stars are locked in a tight gravitational embrace, separated by only 14 solar radii and orbiting one another every 1.4 days

Simulation screenshots showing the density of gas as the neutron star (white circle) spirals in toward the core of its companion star. Credit: Adapted from Hutchinson-Smith et al. 2024

Comparison of the luminosity and duration of the gamma-ray burst produced by the collapse of the neutron star in LMC X-4 with the properties of long gamma-ray bursts (LGRBs) and ultra-long gamma-ray bursts (ULGRBs). Credit: Hutchinson-Smith et al. 2024

Total Collapse of the Heart

Using a three-dimensional fluid dynamics simulation, Hutchinson-Smith and collaborators followed the evolution of LMC X-4 as the primary star engulfed the neutron star. As the neutron star spiraled inward, the energy released ejected only a small amount of gas, and the neutron star accreted only a small amount of matter from the companion star. At that point, the formation of a Thorne–Żytkow object seemed inevitable, but the merger of the neutron star with the companion star’s core set the system on a different course.

As the neutron star melded with the companion’s core, it imparted angular momentum to the core. This created an accretion disk that fed the neutron star until it collapsed into a black hole. The collapse launched a relativistic jet and powered gamma-ray emission that was about as bright and as long-lasting as an ultra-long gamma-ray burst. Feedback from accretion onto the black hole ejected nearly all of the gaseous envelope, definitively halting the short-lived Thorne–Żytkow phase.

Thus, Hutchinson-Smith’s team has demonstrated that a Thorne–Żytkow object is unlikely to result from the evolution of an X-ray binary system like LMC X-4 — though this evolution may provide a path to powering ultra-long gamma-ray bursts. This suggests that accretion and feedback leading to the collapse of the neutron star and the ejection of the stellar envelope must be taken into consideration when exploring the formation of Thorne–Żytkow objects.

By Kerry Hensley

Citation

“Rethinking Thorne–Żytkow Object Formation: The Fate of X-Ray Binary LMC X-4 and Implications for Ultra-Long Gamma-Ray Bursts,” Tenley Hutchinson-Smith et al 2024 ApJ 977 196. doi:10.3847/1538-4357/ad88f3

Source: American Astronomical Society/AAS-Nova

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NuSTAR Probes Black Hole Jet Mystery

This artist's concept shows a black hole with an accretion disk -- a flat structure of material orbiting the black hole - and a jet of hot gas, called plasma. Credit: NASA/JPL-Caltech.  › Larger view

Black holes are famous for being ravenous eaters, but they do not eat everything that falls toward them. A small portion of material gets shot back out in powerful jets of hot gas, called plasma, that can wreak havoc on their surroundings. Along the way, this plasma somehow gets energized enough to strongly radiate light, forming two bright columns along the black hole's axis of rotation. Scientists have long debated where and how this happens in the jet.

Astronomers have new clues to this mystery. Using NASA's NuSTAR space telescope and a fast camera called ULTRACAM on the William Herschel Observatory in La Palma, Spain, scientists have been able to measure the distance that particles in jets travel before they "turn on" and become bright sources of light. This distance is called the "acceleration zone." The study is published in the journal Nature Astronomy.

Scientists looked at two systems in the Milky Way called "X-ray binaries," each consisting of a black hole feeding off of a normal star. They studied these systems at different points during periods of outburst -- which is when the accretion disk -- a flat structure of material orbiting the black hole -- brightens because of material falling in.

One system, called V404 Cygni, had reached nearly peak brightness when scientists observed it in June 2015. At that time, it experienced the brightest outburst from an X-ray binary seen in the 21st century. The other, called GX 339-4,was less than 1 percent of its maximum expected brightness when it was observed. The star and black hole of GX 339-4 are much closer together than in the V404 Cygni system.

Despite their differences, the systems showed similar time delays - about one-tenth of a second -- between when NuSTAR first detected X-ray light and ULTRACAM detected flares in visible light slightly later. That delay is less than the blink of an eye, but significant for the physics of black hole jets.

"One possibility is that the physics of the jet is not determined by the size of the disc, but instead by the speed, temperature and other properties of particles at the jet's base," said Poshak Gandhi, lead author of the study and astronomer at the University of Southampton, United Kingdom.

The best theory scientists have to explain these results is that the X-ray light originates from material very close to the black hole. Strong magnetic fields propel some of this material to high speeds along the jet. This results in particles colliding near light-speed, energizing the plasma until it begins to emit the stream of optical radiation caught by ULTRACAM.

Where in the jet does this occur? The measured delay between optical and X-ray light explains this. By multiplying this amount of time by the speed of the particles, which is nearly the speed of light, scientists determine the maximum distance traveled.

This expanse of about 19,000 miles (30,000 kilometers) represents the inner acceleration zone in the jet, where plasma feels the strongest acceleration and "turns on" by emitting light. That's just under three times the diameter of Earth, but tiny in cosmic terms, especially considering the black hole in V404 Cygni weighs as much as 3 million Earths put together.

"Astronomers hope to refine models for jet powering mechanisms using the results of this study," said Daniel Stern, study co-author and astronomer based at NASA's Jet Propulsion Laboratory, Pasadena, California.

Making these measurements wasn't easy. X-ray telescopes in space and optical telescopes on the ground have to look at the X-ray binaries at exactly the same time during outbursts for scientists to calculate the tiny delay between the telescopes' detections. Such coordination requires complex planning between the observatory teams. In fact, coordination between NuSTAR and ULTRACAM was only possible for about an hour during the 2015 outburst, but that was enough to calculate the groundbreaking results about the acceleration zone.

The results also appear to connect with scientists' understanding of supermassive black holes, much bigger than the ones in this study. In one supermassive system called BL Lacertae, weighing 200 million times the mass of our Sun, scientists have inferred time delays millions of times greater than what this study found. That means the size of the acceleration area of the jets is likely related to the mass of the black hole.

"We are excited because it looks as though we have found a characteristic yardstick related to the inner workings of jets, not only in stellar-mass black holes like V404 Cygni, but also in monster supermassive ones," Gandhi said.

The next steps are to confirm this measured delay in observations of other X-ray binaries, and to develop a theory that can tie together jets in black holes of all sizes.

"Global ground and space telescopes working together were key to this discovery. But this is only a peek, and much remains to be learned. The future is really bright for understanding the extreme physics of black holes," said Fiona Harrison, principal investigator of NuSTAR and professor of astronomy at Caltech in Pasadena.

NuSTAR is a Small Explorer mission led by Caltech and managed by JPL for NASA's Science Mission Directorate in Washington. NuSTAR was developed in partnership with the Danish Technical University and the Italian Space Agency (ASI). The spacecraft was built by Orbital Sciences Corp., Dulles, Virginia. NuSTAR's mission operations center is at UC Berkeley, and the official data archive is at NASA's High Energy Astrophysics Science Archive Research Center. ASI provides the mission's ground station and a mirror archive. Caltech manages JPL for NASA.

For more information on NuSTAR, visit: https://www.nasa.gov/nustar - http://www.nustar.caltech.edu/

News Media Contact

Elizabeth Landau
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6425
Elizabeth.landau@jpl.nasa.gov

Source: JPL-Caltech/News

Neutron star’s echoes give astronomers a new measuring stick

Residing in the plane of the Milky Way, where it cannot be observed by optical telescopes because of obscuring clouds of interstellar dust, Circinus X-1 is the glowing husk of a binary star system that exploded in a supernova event just 2,500 years ago. It consists of a very dense neutron star locked in the orbital embrace of a companion star. Hi-res Image

In late 2013, when the neutron star at the heart of one of our galaxy’s oddest supernovae gave off a massive burst of X-rays, the resulting echoes — created when the X-rays bounced off clouds of dust in interstellar space — yielded a surprising new measuring stick for astronomers.

Circinus X-1 is a freak of the Milky Way. Located in the plane of the galaxy, Circinus X-1 is the glowing husk of a binary star system that exploded a mere 2,500 years ago. The system consists of a nebula and a neutron star, the incredibly dense collapsed core of the exploded star, still in the orbital embrace of its companion star.

The system is called an X-ray binary because it emits X-rays as material from the companion star spirals onto the much denser neutron star and is heated to very high temperatures.

Sebastian Heinz

“In late 2013, the neutron star underwent an enormous outburst for about two months, during which it became one of the brightest sources in the X-ray sky,” explains University of Wisconsin-Madison astronomy Professor Sebastian Heinz. “Then it turned dark again.”

The flicker of X-rays from the odd binary system was monitored by a detector aboard the International Space Station. Heinz and his colleagues quickly mounted a series of follow-up observations with the space-based Chandra and XMM-Newton telescopes to discover four bright rings of X-rays, like ripples in a cosmic pond, all around the neutron star at the heart of Circinus X-1.

Their observations were reported June 23 in The Astrophysical Journal.

The rings are light echoes from Circinus X-1’s X-ray burst. Each of the four rings, says Heinz, indicates a dense cloud of dust between us and the supernova remnant. When X-rays encounter grains of dust in interstellar space they can be deflected, and if the dust clouds are dense they can scatter a noticeable fraction of the X-rays away from their original trajectory, putting them on a triangular path.

That phenomenon, Heinz and his colleagues recognized, could give astronomers an opportunity to use the geometry of the rings and a time delay between deflected and undeflected X-rays to calculate the distance to Circinus X-1, a measurement previously unobtainable because the supernova is hidden in the dust that permeates the plane of our galaxy

We can use the geometry of the rings and the time delay to do X-ray tomography,” Heinz explains. “Because the X-rays have traveled on a triangular path rather than a straight path, they take longer to get to us than the ones that were not scattered.”

Combining those measurements with observations of the dust clouds by Australia’s Mopra radio telescope, Heinz and his colleagues were able to determine which dust clouds were responsible for each of the four light echoes.

“Using this identification, we can determine the distance to the source accurately for the first time,” according to the UW-Madison astronomer. “Distance measurements in astronomy are difficult, especially to sources like Circinus X-1, which are hidden in the plane of the galaxy behind a thick layer of dust — which makes it basically impossible to observe them with optical telescopes.

“In this case, we used the dust that otherwise gets in the way to pioneer a new method of estimating distances to X-ray sources,” Heinz says.

Now astronomers know that Circinus X-1, one of the Milky Way’s most bizarre objects, is 30,700 light-years from Earth.

 Source: University of Wisconsin-Madison/News

The Workings of an X-ray Binary Star

A schematic image of the X-ray binary source LMC X-3 (not to scale). The disk around the black hole (on the right) is heated by accretion of material falling from the star (at the left) onto the disk, while some X-ray emission from the disk then heats the companion star. Astronomers were able to explain this process by modeling the time delay between the infrared and X-ray flares. Steiner, et al. 

The bright X-ray source known as LMC X-3 resides in the Large Magellanic Cloud, the dwarf galaxy that is the Milky Way’s nearest neighbor. Two decades ago astronomers discovered that the source is actually a binary system with a normal star rapidly orbiting a nearby black hole (whose mass is about 2.3 solar-masses) in only 1.7 days. In X-ray binary systems like this one, material from the normal star falls onto a disk around the black hole, causing it to glow and emit radiation – at X-ray wavelengths from the inner portion of the disk closest to the black hole, and at infrared wavelengths from the outer portions of the disk. The emission typically varies in time, presumably because the infalling matter arrives in clumps or in an uneven stream. The infrared and X-ray emissions also vary from one another, and astronomers have long thought that modeling their behaviors might lead to an enhanced understanding of black hole accretion processes.

CfA astronomers James Steiner and Jeff McClintock, along with a team of five colleagues, analyzed a ten-year collection of optical, infrared and X-ray data on LMC X-3. They discovered from the relative timing of the flares as seen in the two bands that the X-ray emission events lagged the infrared emission by about two weeks, and were able to develop a model that can successfully explain the processes at work. They considered the radiation as coming from three locations: the star itself (normal starlight dominates the emission), the disk (it is heated by accretion and emits in both X-rays and infrared), and other hot material in the disk and/or the star (it is heated by X-rays from the hot inner disk).

The scientists are able to conclude that the infrared probably arises from a narrow annular region of the disk, a somewhat surprising result because it had been thought that infrared would come from a much wider area. They also derive a more precise orbital period for the binary (1.704805 days) and key parameters of the disk. The authors note, however, that their model has about thirty parameters; their proposed scenario is the one that best fits the whole set of data. The new work is an impressive success at understanding a complex and dramatic extragalactic black hole system.

Reference(s): 

"Modeling the Optical–X-ray Accretion Lag in LMC X-3: Insights into Black-Hole Accretion Physics," James F. Steiner, Jeffrey E. McClintock, Jerome A. Orosz, Michelle M. Buxton, Charles D. Bailyn, Ronald A. Remillard, and Erin Kara, ApJ 783, 101, 2014.

Source: Smithsonian Astrophysical Observatory (SAO)

Circinus X-1: Supernova Blast Provides Clues to Age of Binary Star System

Circinus X-1

Credit: X-ray: NASA/CXC/Univ. of Wisconsin-Madison/S.Heinz et al; 

Optical: DSS; Radio: CSIRO/ATNF/ATCA 

JPEG (485.1 kb) - Large JPEG (3 MB) - Tiff (11 MB) - More Images
View on the Sky (WWT)

The youngest member of an important class of objects has been found using data from NASA's Chandra X-ray Observatory and the Australia Compact Telescope Array. A composite image shows the X-rays in blue and radio emission in purple, which have been overlaid on an optical field of view from the Digitized Sky Survey. This discovery, described in the press release, allows scientists to study a critical phase after a supernova and the birth of a neutron star.

Systems known as "X-ray binaries" are some of the brightest X-ray sources in the sky. They consist of either an ultra-dense star packed with neutrons --- a.k.a., a "neutron star" --- or a black hole that is paired with a normal star like the Sun. As these two objects orbit one another, the neutron star or black hole pulls material from the companion star onto it.

A new study shows that the X-ray binary called Circinus X-1 is less than 4,600 years old, making it the youngest ever seen. Astronomers have detected hundreds of X-ray binaries throughout the Milky Way and other nearby galaxies. However, these older X-ray binaries only reveal information about what happens later in the evolution of these systems.

Circinus X-1 Infographic

Researchers have found that the neutron star in Circinus X-1 is less than 4,600 years old, making it much younger than any other X-ray binary known in the Milky Way. Credit: Univ. of Wisconsin-Madison/S.Heinz et al.

Astronomers were able to determine the age of Circinus X-1 by examining material around the orbiting pair. While the source itself has been known for decades, the neutron star is usually so bright that the glare from its X-ray light overwhelms any faint emission surrounding it. The new Chandra data were obtained while the neutron star was in a very faint state, which meant it was dim enough for astronomers to detect the faint afterglow created by the supernova explosion plowing through the surrounding interstellar gas. This, combined with characteristics of the radio emission, allowed the researchers to pinpoint the age of the supernova remnant. In turn, this information reveals the age of the neutron star since they were formed at the same time.

These results have been published in the December 4th issue of The Astrophysical Journal. In addition to those mentioned above, the other authors on this paper are Peter Jonker of the SRON Netherlands Institute for Space Research, Niel Brandt of Penn State University, Daniel Emilio Calvelo-Santos of the University of Southampton, Tasso Tzioumis of the Australia Telescope National Facility, Michael Nowak and Norbert Schultz of the Kavli Institute/MIT, Rudy Wijnands and Michiel van der Klis of the University of Amsterdam.

Fast Facts for Circinus X-1: 

Scale: Image is 10 arcmin across (about 76 light years) 
Category: Neutron Stars/X-ray Binaries 
Coordinates (J2000): RA 15h 20m 41.00s | Dec -51° 10' 00 
Constellation: Circinus 
Observation Date: 1 May 2009 
Observation Time: 27 hours 25 min (1 day 3 hours 25 min) 
Obs. ID: 10062 
Instrument: ACIS 

References: Heinz, S et al, 2013, ApJ accepted

Color Code: X-ray (Blue); Optical (Red, Green, Blue); Radio (Pink) D

Distance Estimate: About 26,000 light years

High mass X-ray binaries trace the Milky Way's spiral arms

 Artist's impression of a highly obscured high-mass X-ray binary
Credit: ESA/AOES Medialab
Hi-Res [jpg] 620.95 kb

  

Distribution of high-mass X-ray binary stars and star-forming complexes.  

From A. Coleiro and S. Chaty, ApJ 764:185, 2013

Copyright 2013 IOP Publishing

Hi-Rers [jpg] 211.84 kb

 High-mass X-ray binary systems in the Milky Way.

Credit: Image courtesy of A. Coleiro and S. Chaty; 

Image of spiral arms: NASA/Adler/U. Chicago/Wesleyan/JPL-Caltech.

Hi-Res [jpg] 150.59 kb.

Our Galaxy is littered with pairs of massive stars, many of which contain the remnants of supernova explosions. A new study of these X-ray emitting binary systems, using data from ESA's INTEGRAL space observatory, has made it possible to reconstruct the locations of the Milky Way's spiral arms many millions of years ago. 

High mass X-ray binaries (HMXBs) contain stars which consume their hydrogen and helium fuel so quickly that they explode as supernovas within a few tens of millions of years – the blink of an eye in the history of the Universe.

These short-lived stellar systems comprise an extremely dense, compact object (a neutron star or a black hole) which is pulling in matter ejected from a massive companion – a process known as accretion. The stellar companion is usually either a main sequence Be star or an evolved supergiant which is nearing the end of its life.

Be stars are rapidly rotating objects which are surrounded by a disc of gas that is ejected by the stars themselves. When the neutron star passes periodically through this disc, gas is strongly heated during accretion onto its surface, creating a blast of X-rays. In the case of supergiant X-ray binaries, the accreted material is derived from the massive companions ejecting large amounts of material in their stellar winds.

Dedicated X-ray observations of the sky, particularly with the INTEGRAL spacecraft, have quintupled the known population of high mass supergiant X-ray binaries in the Galaxy. Some 35 supergiant HMXBs are currently catalogued, out of a total of more than 200 HMXBs.

Until now, their locations within the Milky Way have not been fixed very precisely. However, most of the sources are seen to lie in the Galactic Plane and observations made with INTEGRAL over the last decade show that HMXBs seem to be associated with the spiral structure of the Galaxy.

Taking advantage of the much larger number of detected HMXBs, two French researchers decided to carry out a statistical analysis of their distribution in the Milky Way. The first step was to determine the distances of a sample of HMXBs. This was done by comparing their apparent magnitudes (brightness) with their light spectrum – a signature of their likely temperature and energy output. The spectrum of each HMXB was compared with theoretical models to compute the distance of each source.

"By using this novel technique, we were able to find a strong correlation between the positions of HMXBs and star-forming complexes in the Milky Way," said Alexis Coleiro, a PhD student at Université Paris Diderot, France, and lead author of the paper in the Astrophysical Journal.

Their statistical study showed that HMXBs are clustered with star formation complexes - the largest regions of star creation in galaxies. These clusters are typically 1000 light years across, with an average distance between the clusters of about 5000 light years.

"As expected, the current distribution of high mass X-ray binaries is closely linked to the stellar nurseries where they were born, some tens of millions of years ago, because the HMXBs have not been in existence long enough to have migrated very far from their birthplaces," said Coleiro.

The separation of the HMXBs and the complexes where they formed is largely due to the momentum supplied when the more massive star in the binary exploded as a supernova. Since such explosions tend to be asymmetrical, the dying star receives a kick in a particular direction. As long as the supernova's initial velocity is not too high, the binary stars are held together by their mutual gravitational attraction.

The scientists also compared the positions of current star forming regions in the Galaxy's spiral arms and a sample of 13 HMXBs. This enabled them to constrain the ages and migration distances of the HMXBs as a result of the velocity boost from the supernovas.

"We know that HMXBs are born in star forming complexes, which are usually located in the Galaxy's spiral arms," said Sylvain Chaty, a professor at Université Paris Diderot, and Alexis Coleiro's research supervisor. "Star formation in the spiral arms is triggered by density waves, regions of enhanced density where interstellar gas and dust are slowed down and compressed.

"We also know that HMXBs are short-lived, surviving for only a few tens of millions of years, so it ought to be possible to link them to their birthplaces. As a result, we decided to investigate the relationship between the binary systems and the spiral structure of the Milky Way."

By assuming that the density waves rotate at a different speed to the matter (stars, dust etc.) in the Galaxy, it was possible to calculate the expected HMXB locations relative to the positions of the spiral arms at certain times in the past. The researchers then compared these positions with the current locations of 13 HMXBs in order to determine the effects of the kicks they received from the supernova explosions.

Their calculations showed that the mean age of four supernova HMXBs was 45 million years, with a mean migration distance of about 325 light years. The mean age of nine Be-class HMXBs was 51 million years, with a mean migration distance of about 360 light years.

"For the first time, we have accurately derived the distances and distribution of a large sample of high mass X-ray binaries in our Galaxy, bringing new constraints on their formation and evolution," said  Coleiro.

"These new methods will allow us to assess the influence of the environment on these high energy objects with unprecedented reliability," said Chris Winkler, ESA's INTEGRAL project scientist.

"This study was made possible because INTEGRAL is the only observatory with the sensitivity to observe numerous HMXBs in the Milky Way."

Notes for editors

The results presented here are reported in the paper "Distribution of High Mass X-ray Binaries in the Milky Way", by A. Coleiro and S. Chaty, published in The Astrophysical Journal, 764, 185, 2013. DOI:10.1088/0004-637X/764/2/185

INTEGRAL is an ESA project with instruments and science data centre funded by ESA Member States (especially the Principal Investigator countries: Denmark, France, Germany, Italy, Spain, Switzerland) and Poland, and with the participation of Russia and the USA.

Contacts

Alexis Coleiro
Laboratoire AIM - CEA CNRS
Université Paris Diderot, France
Email: alexis.coleiro@cea.fr

Sylvain Chaty
Laboratoire AIM - CEA CNRS
Université Paris Diderot, France
and Institut Universitaire de France, Paris, France
Email:chaty@cea.fr

Chris Winkler
INTEGRAL Project Scientist
Research and Scientific Support Department
Directorate of Science and Robotic Exploration
ESA, The Netherlands
Email:cwinkler@rssd.esa.int

Phone: +31-71-5653591