Neutron Stars/X-ray Binaries

Sagittarius A* Swarm: Black Hole Bounty Captured in the Milky Way Center

Astronomers have discovered evidence for thousands of black holes located near the center of our Milky Way galaxy using data from NASA's Chandra X-ray Observatory.

This black hole bounty consists of stellar-mass black holes, which typically weigh between five to 30 times the mass of the Sun. These newly identified black holes were found within three light years — a relatively short distance on cosmic scales — of the supermassive black hole at our Galaxy's center known as Sagittarius A* (Sgr A*).

Theoretical studies of the dynamics of stars in galaxies have indicated that a large population of stellar mass black holes — as many as 20,000 — could drift inward over the eons and collect around Sgr A*. This recent analysis using Chandra data is the first observational evidence for such a black hole bounty.

A black hole by itself is invisible. However, a black hole — or neutron star — locked in close orbit with a star will pull gas from its companion (astronomers call these systems "X-ray binaries"). This material falls into a disk and heats up to millions of degrees and produces X-rays before disappearing into the black hole. Some of these X-ray binaries appear as point-like sources in the Chandra image.

A Crab Walks Through Time

Next year marks the 20th anniversary of NASA's Chandra X-ray Observatory launch into space. The Crab Nebula was one of the first objects that Chandra examined with its sharp X-ray vision, and it has been a frequent target of the telescope ever since.

There are many reasons that the Crab Nebula is such a well-studied object. For example, it is one of a handful of cases where there is strong historical evidence for when the star exploded. Having this definitive timeline helps astronomers understand the details of the explosion and its aftermath.

ULX in M51: Beaming with the Light of Millions of Suns

In the 1980s, scientists started discovering a new class of extremely bright sources of X-rays in galaxies. These sources were a surprise, as they were clearly located away from the supermassive black holes found in the center of galaxies. At first, researchers thought that many of these ultraluminous X-ray sources, or ULXs, were black holes containing masses between about a hundred and a hundred thousand times that of the sun. Later work has shown some of them may be stellar-mass black holes, containing up to a few tens of times the mass of the sun.

In 2014, observations with NASA's NuSTAR (Nuclear Spectroscopic Telescope Array) and Chandra X-ray Observatory showed that a few ULXs, which glow with X-ray light equal in luminosity to the total output at all wavelengths of millions of suns, are even less massive objects called neutron stars. These are the burnt-out cores of massive stars that exploded. Neutron stars typically contain only about 1.5 times the mass of the sun. Three such ULXs were identified as neutron stars in the last few years. Scientists discovered regular variations, or "pulsations," in the X-ray emission from ULXs, behavior that is exhibited by neutron stars but not black holes.

Now, researchers using data from NASA's Chandra X-ray Observatory have identified a fourth ULX as being a neutron star, and found new clues about how these objects can shine so brightly. The newly characterized ULX is located in the Whirlpool galaxy, also known as M51. This composite image of the Whirlpool contains X-rays from Chandra (purple) and optical data from the Hubble Space Telescope (red, green, and blue). The ULX is marked with a circle.

Chandra Makes First Detection of X-rays from a Gravitational Wave Source: Interview with Chandra Scientist Daryl Haggard

Daryl Haggard
Chandra Scientist Daryl Haggard

Daryl Haggard is an Assistant Professor of Physics at McGill University. She is an observational astronomer and received her PhD at the University of Washington (Seattle, WA). She and her group study Sagittarius A*, the supermassive black hole at the heart of our Milky Way Galaxy. Haggard's team uses radio, submillimeter, near infrared, and X-ray telescopes to probe Sgr A*'s exotic environment, where strong gravity plays a key role. Her team also studies the interplay between distant growing supermassive black holes, or active galactic nuclei, and their host galaxies. And when opportunity knocks, she and her group search for X-ray flashes emitted from neutron stars and black holes when they collide. These collisions send ripples through space-time, gravitational waves, now being detected by LIGO and Virgo.

What are gravitational waves? What are neutron stars?

Gravitational waves are “jiggles” in the fabric of space-time. They are like sound waves traveling through the air around me while I talk, but gravitational waves race through the Universe at the speed of light and carry LOTS of energy away from their source.

Meanwhile, neutron stars are basically big balls of neutrons. They are born during supernova explosions when the gravity on the inside of a star is so intense it forces the electrons into the protons, making neutrons. Actually, the fusion that happens during these supernovas is how atoms like oxygen, iron, all the stuff we need for life, gets built. Neutron stars weigh two or three times the mass of our Sun, but are only the size of a modest city, like Boston or Montreal. They are incredibly dense objects. For example, if you took the Earth’s whole human population and squished it into an object the size of a sugar cube, we’d all become a neutron star.

Chandra Makes First Detection of X-rays from a Gravitational Wave Source: Interview with Chandra Scientist Wen-fai Fong

Wen-fai Fong
Chandra Scientist Wen-fai Fong

Originally from Rochester, NY, Wen-fai Fong received double Bachelor's degrees in Physics and Biology at the Massachusetts Institute of Technology, and earned her Ph.D. in Astronomy & Astrophysics from Harvard University. She was subsequently awarded an Einstein Postdoctoral Fellowship, which she took to the University of Arizona's Steward Observatory. She is currently a Hubble Postdoctoral Fellow at Northwestern University and will begin her appointment as Assistant Professor there in Fall 2018. Wen-fai is excited about unraveling the mysteries enshrouding cosmic explosions, including gamma-ray bursts and gravitational wave sources.

What are gravitational waves? What are neutron stars?

Gravitational waves are best described as ripples in space-time. To envision these merging compact objects, I always try to think of two round objects on a very flexible trampoline, rolling and rolling around each other. For the most flexible of trampolines, they will create some sort of pattern outward, while also spiraling toward each other and eventually colliding. It’s an oversimplified version, but that is how I best imagine what is going on.

In reality, the specific properties of the system — the masses, spins, orbital orientation, and distance — determine the very special pattern of the gravitational waves that are radiated from a system. Scientists then match that pattern against a gigantic bank of patterns by the gravitational wave experts who are able to determine very specific properties of the system. So it is a very neat and elegant problem that is made possible by many years of hard work.

Chandra Makes First Detection of X-rays from a Gravitational Wave Source: Interview with Chandra Scientist Raffaella Margutti

Raffaella Margutti
Chandra Scientist Raffaella Margutti

Raffaella Margutti obtained a PhD degree in Physics and Astronomy from the University of Milano Bicocca, Italy, in 2010, working on the broad-band (radio to gamma-ray) emission from relativistic jets in gamma-ray bursts within the Swift team. She then worked as a postdoctoral fellow at the Institute for Theory and Computation (ITC) at Harvard University, and then moved in 2015 for one year to New York University as James Arthur Fellow. Raffaella began a faculty position at Northwestern University (Physics and Astronomy) in 2016. She has been working in the field of Astronomical transients for more than a decade, with a wide range of expertise including, Stellar Explosions, Gamma-Ray bursts, Tidal Disruption Events, Stellar Outburst, and now, counterparts to GW.

What are gravitational waves?

Gravitational Waves are ripples in space-time that become particularly strong when very violent event in our Universe happen, like the merge of two very peculiar stars that we call neutron stars (NS). NS are what get left behind after a big star like 10 times the Sun ends its life with a big explosion.

Chandra Makes First Detection of X-rays from a Gravitational Wave Source: Interview with Chandra Scientist Eleonora Nora Troja

Eleonora Troja
Chandra Scientist Eleonora Nora Troja

Eleonora Nora Troja was born and raised in Palermo, Sicily. After completing her PhD at the University of Palermo, she moved to NASA Goddard Space Flight Center as a NASA Postdoctoral Program Fellow. Her main research interest is the study of gamma-ray bursts, and in particular the connection between short duration gamma-ray bursts, neutron star mergers and gravitational waves. Beside her studies, she also leads the Swift Guest Investigator Program and serves as co-chair for the Athena mission working groups.

What are gravitational waves?

Gravitational waves are ripples in the fabric of space and time. According to Einstein’s theory of general relativity, every moving object actually emits a tiny amount of gravitational waves, but the signal is usually too small for us to detect. In order for us to detect gravitational waves, we need to wait for catastrophic events like the collision of two neutron stars. This brings to the second question, what are neutron stars? Neutron stars are some of the most exotic and dense objects in our Universe. These stars weight as much as our Sun but they are much smaller in size. A neutron star can extend up to 15-17 miles across, this is more or less the size of Washington D.C.

IC 10: A Starburst Galaxy with the Prospect of Gravitational Waves

IC 10

In 1887, American astronomer Lewis Swift discovered a glowing cloud, or nebula, that turned out to be a small galaxy about 2.2 million light years from Earth. Today, it is known as the "starburst" galaxy IC 10, referring to the intense star formation activity occurring there.

More than a hundred years after Swift's discovery, astronomers are studying IC 10 with the most powerful telescopes of the 21st century. New observations with NASA's Chandra X-ray Observatory reveal many pairs of stars that may one day become sources of perhaps the most exciting cosmic phenomenon observed in recent years: gravitational waves.

Observatories Combine to Crack Open the Crab Nebula

Screening
The Crab Nebula

Astronomers have produced a highly detailed image of the Crab Nebula, by combining data from telescopes spanning nearly the entire breadth of the electromagnetic spectrum, from radio waves seen by the Karl G. Jansky Very Large Array (VLA) to the powerful X-ray glow as seen by the orbiting Chandra X-ray Observatory. And, in between, the Hubble Space Telescope's crisp visible-light view and the infrared perspective of the Spitzer Space Telescope.

The Crab Nebula, the result of a bright supernova explosion seen by Chinese and other astronomers in the year 1054, is 6,500 light-years from Earth. At its center is a super-dense neutron star, rotating once every 33 milliseconds, shooting out rotating lighthouse-like beams of radio waves and light — a pulsar. The nebula's intricate shape is caused by a complex interplay of the pulsar, a fast-moving wind of particles coming from the pulsar, and material originally ejected by the supernova explosion and by the star itself before the explosion.

This image combines data from five different telescopes: The VLA (radio) in red; Spitzer Space Telescope (infrared) in yellow; Hubble Space Telescope (visible) in green; XMM-Newton (ultraviolet) in blue; and Chandra X-ray Observatory (X-ray) in purple.

A White Dwarf and a Black Hole in a Tight Orbit

Dr. Arash Bahramian
Dr. Arash Bahramian

We are very happy to welcome Dr. Arash Bahramian as our guest blogger. Dr. Bahramian completed his graduate studies at University of Alberta, Canada with Dr. Craig Heinke on X-ray binaries in globular clusters. After defending his PhD in 2016, he moved to Michigan State University to work with Dr. Jay Strader on study of black holes in globular clusters. He is the first author of the paper featured in our most recent press release.

Stellar mass black holes are formed by the deaths of massive stars. Like other black holes, these objects do not emit any light of their own, and astronomers try to identify them from their interactions with their environment. For example, in a close binary with another star, the black hole's strong gravity pulls material from the companion star. This material falls towards the black hole through a disk called an accretion disk. The massive release of energy due to infall of matter towards the black hole plus friction between particles in the disk, makes this disk extremely hot (about a million degrees Kelvin, roughly 200 times hotter than the surface of the Sun). This temperature is high enough to make the disk bright in X-rays, and so X-ray observatories like NASA's Chandra X-ray Observatory have been used to identify and study these systems.

Over the last few decades, dozens of stellar mass black holes (and black hole candidates) in close binaries with another star have been identified throughout our Galaxy. However, none of these black holes were found in old dense stellar clusters known as globular clusters. This was surprising at first, as we would expect a lot of black holes (maybe around 1000 of them) in these clusters, because many massive stars should have turned into black holes. Furthermore, a crowded stellar environment like a cluster makes interactions between black holes and other stars more likely. For a long time, this absence of black holes in dense stellar clusters was thought to be a result of black holes getting kicked out of the cluster, due to their strong gravity and rapid movement after interacting with other stars and other black holes in the cluster.

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