Neutron Stars/X-ray Binaries

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 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.

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.

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.

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.

NASA'S Chandra X-ray Observatory has taken deep exposures of two nearby energetic pulsars flying through the Milky Way galaxy. The shape of their X-ray emission suggests there is a geometrical explanation for puzzling differences in behavior shown by some pulsars.

Pulsars - rapidly rotating, highly magnetized, neutron stars born in supernova explosions triggered by the collapse of massive stars- were discovered 50 years ago via their pulsed, highly regular, radio emission. Pulsars produce a lighthouse-like beam of radiation that astronomers detect as pulses as the pulsar's rotation sweeps the beam across the sky.

Michael McCollough

We are pleased to welcome Dr. Michael McCollough as our guest blogger. Dr. McCollough is the first author of a paper that is the subject of our latest press release. He has spent the last 30 years working with and analyzing data from astronomical radio, optical, X-ray, and gamma-ray telescopes. Currently, he serves as an Archival Astrophysicist at the Chandra X-ray Center in Cambridge, Mass.

Throughout my career I have been a multi-wavelength astronomer. To fully understand astronomical objects, one must look across the electromagnetic spectrum (from radio waves to gamma-rays). Also throughout my career I have been doing spacecraft operations. Starting with NASA’s Hubble Space Telescope (before, during, and after launch), ROSAT, NASA’s Compton Gamma-Ray Observations with the Burst and Transient Source Experiment (BATSE), and currently with NASA’s Chandra X-Ray Observatory. It was when I was working with BATSE that I was introduced to Cygnus X-3. Discovering that high-energy X-rays (as seen by BATSE) were correlated with emissions in the radio.

Using NASA's Chandra X-ray Observatory and other X-ray observatories, astronomers have found evidence for what is likely one of the most extreme pulsars, or rotating neutron stars, ever detected. The source exhibits properties of a highly magnetized neutron star, or magnetar, yet its deduced spin period is thousands of times longer than any pulsar ever observed.

A new look at the debris from an exploded star in our galaxy has astronomers re-examining when the supernova actually happened. Recent observations of the supernova remnant called G11.2-0.3 with NASA's Chandra X-ray Observatory have stripped away its connection to an event recorded by the Chinese in 386 CE.

We are pleased to welcome Bailey Tetarenko as our guest blogger. She is the lead author on a paper featured in our latest press release about a possible new population of black holes in the Galaxy. Bailey received her undergraduate degree in Astrophysics at the University of Calgary and then a master’s in Physics at the University of Alberta in 2014. She is now two years into her Ph.D. in Physics at the University of Alberta, where she is studying the black hole population of the Milky Way.

From right to left Bailey Tetarenko, Dr. Arash Bahramian and Dr. Craig Heinke and Dr. Greg Sivakoff. Credit: John Ulan

For fans of black holes, we live in exciting times. Nearly all of our empirical knowledge about stellar mass black holes – that is, black holes weighing about 5 to 35 times the mass of the sun – comes from black hole X-ray binary systems. In these systems a black hole pulls in material from a nearby companion star, causing the system to become very bright in X-rays. But, recently gravitational waves have been detected from pairs of distant black holes that emit no electromagnetic radiation (a.k.a. all forms of light). And now, my team's work suggests that there are many black hole X-ray binaries in our own Milky Way that emit relatively little X-rays.