NICER’s Night Moves Trace the X-ray Sky

Credits: NASA/NICER.

In this image, numerous sweeping arcs seem to congregate at various bright regions. You may wonder: What is being shown? Air traffic routes? Information moving around the global internet? Magnetic fields looping across active areas on the Sun?

In fact, this is a map of the entire sky in X-rays recorded by NASA’s Neutron star Interior Composition Explorer (NICER), a payload on the International Space Station. NICER’s primary science goals require that it target and track cosmic sources as the station orbits Earth every 93 minutes. But when the Sun sets and night falls on the orbital outpost, the NICER team keeps its detectors active while the payload slews from one target to another, which can occur up to eight times each orbit.

The map includes data from the first 22 months of NICER’s science operations. Each arc traces X-rays, as well as occasional strikes from energetic particles, captured during NICER’s night moves. The brightness of each point in the image is a result of these contributions as well as the time NICER has spent looking in that direction. A diffuse glow permeates the X-ray sky even far from bright sources.

This image of the whole sky shows 22 months of X-ray data recorded by NASA's Neutron star Interior Composition Explorer (NICER) payload aboard the International Space Station during its nighttime slews between targets. NICER frequently observes targets best suited to its core mission (“mass-radius” pulsars) and those whose regular pulses are ideal for the Station Explorer for X-ray Timing and Navigation Technology (SEXTANT) experiment. One day they could form the basis of a GPS-like system for navigating the solar system.  Credits: NASA/NICER. Download full-resolution images from NASA Goddard’s Scientific Visualization Studio 

The prominent arcs form because NICER often follows the same paths between targets. The arcs converge on bright spots representing NICER’s most popular destinations — the locations of important X-ray sources the mission regularly monitors.

“Even with minimal processing, this image reveals the Cygnus Loop, a supernova remnant about 90 light-years across and thought to be 5,000 to 8,000 years old,” said Keith Gendreau, the mission’s principal investigator at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “We’re gradually building up a new X-ray image of the whole sky, and it’s possible NICER’s nighttime sweeps will uncover previously unknown sources.”

NICER’s primary mission is to determine the size of dense remains of dead stars called neutron stars — some of which we see as pulsars — to a precision of 5%. These measurements will finally allow physicists to solve the mystery of what form of matter exists in their incredibly compressed cores. 

Pulsars, rapidly spinning neutron stars that appear to “pulse” bright light, are ideally suited to this “mass-radius” research and are some of NICER’s regular targets.

Other frequently visited pulsars are studied as part of NICER's Station Explorer for X-ray Timing and Navigation Technology (SEXTANT) experiment, which uses the precise timing of pulsar X-ray pulses to autonomously determine NICER’s position and speed in space. It’s essentially a galactic GPS system. When mature, this technology will enable spacecraft to navigate themselves throughout the solar system — and beyond.

By Francis Reddy
NASA's Goddard Space Flight Center, Greenbelt, Md.


Media contact:  

Claire Andreoli
NASA's Goddard Space Flight Center, Greenbelt, Md.


Editor: Rob Garner

Source: NASA/NICER

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NASA’s NICER Mission Maps ‘Light Echoes’ of New Black Hole

In this illustration of a newly discovered black hole named MAXI J1820+070, a black hole pulls material off a neighboring star and into an accretion disk. Above the disk is a region of subatomic particles called the corona. Credit: Aurore Simonnet and NASA’s Goddard Space Flight Center. Hi-res image

Scientists have charted the environment surrounding a stellar-mass black hole that is 10 times the mass of the Sun using NASA’s Neutron star Interior Composition Explorer (NICER) payload aboard the International Space Station. NICER detected X-ray light from the recently discovered black hole, called MAXI J1820+070 (J1820 for short), as it consumed material from a companion star. Waves of X-rays formed “light echoes” that reflected off the swirling gas near the black hole and revealed changes in the environment’s size and shape.

“NICER has allowed us to measure light echoes closer to a stellar-mass black hole than ever before,” said Erin Kara, an astrophysicist at the University of Maryland, College Park and NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who presented the findings at the 233rd American Astronomical Society meeting in Seattle. “Previously, these light echoes off the inner accretion disk were only seen in supermassive black holes, which are millions to billions of solar masses and undergo changes slowly. Stellar black holes like J1820 have much lower masses and evolve much faster, so we can see changes play out on human time scales.”

A paper describing the findings, led by Kara, appeared in the Jan. 10 issue of Nature and is available online.

Watch how X-ray echoes, mapped by NASA’s Neutron star Interior Composition Explorer (NICER) revealed changes to the corona of black hole MAXI J1820+070. Credits: NASA’s Goddard Space Flight Center. Download this video and related multimedia in HD formats from NASA Goddard's Scientific Visualization Studio

J1820 is located about 10,000 light-years away toward the constellation Leo. The companion star in the system was identified in a survey by ESA’s (European Space Agency) Gaia mission, which allowed researchers to estimate its distance. Astronomers were unaware of the black hole’s presence 

until March 11, 2018, when an outburst was spotted by the Japan Aerospace Exploration Agency’s Monitor of All-sky X-ray Image (MAXI), also aboard the space station. J1820 went from a totally unknown black hole to one of the brightest sources in the X-ray sky over a few days. NICER moved quickly to capture this dramatic transition and continues to follow the fading tail of the eruption.

“NICER was designed to be sensitive enough to study faint, incredibly dense objects called neutron stars,” said Zaven Arzoumanian, the NICER science lead at Goddard and a co-author of the paper. “We’re pleased at how useful it’s also proven in studying these very X-ray-bright stellar-mass black holes.”

A black hole can siphon gas from a nearby companion star into a ring of material called an accretion disk. Gravitational and magnetic forces heat the disk to millions of degrees, making it hot enough to produce X-rays at the inner parts of the disk, near the black hole. Outbursts occur when an instability in the disk causes a flood of gas to move inward, toward the black hole, like an avalanche. The causes of disk instabilities are poorly understood.

Above the disk is the corona, a region of subatomic particles around 1 billion degrees Celsius (1.8 billion degrees Fahrenheit) that glows in higher-energy X-rays. Many mysteries remain about the origin and evolution of the corona. Some theories suggest the structure could represent an early form of the high-speed particle jets these types of systems often emit.

Astrophysicists want to better understand how the inner edge of the accretion disk and the corona above it change in size and shape as a black hole accretes material from its companion star. If they can understand how and why these changes occur in stellar-mass black holes over a period of weeks, scientists could shed light on how supermassive black holes evolve over millions of years and how they affect the galaxies in which they reside.

One method used to chart those changes is called X-ray reverberation mapping, which uses X-ray reflections in much the same way sonar uses sound waves to map undersea terrain. Some X-rays from the corona travel straight toward us, while others light up the disk and reflect back at different energies and angles.

X-ray reverberation mapping of supermassive black holes has shown that the inner edge of the accretion disk is very close to the event horizon, the point of no return. The corona is also compact, lying closer to the black hole rather than over much of the accretion disk. Previous observations of X-ray echoes from stellar black holes, however, suggested the inner edge of the accretion disk could be quite distant, up to hundreds of times the size of the event horizon. The stellar-mass J1820, however, behaved more like its supermassive cousins.  

As they examined NICER’s observations of J1820, Kara’s team saw a decrease in the delay, or lag time, between the initial flare of X-rays coming directly from the corona and the flare’s echo off the disk, indicating that the X-rays traveled shorter and shorter distances before they were reflected. From 10,000 light-years away, they estimated that the corona contracted vertically from roughly 100 to 10 miles — that’s like seeing something the size of a blueberry shrink to something the size of a poppy seed at the distance of Pluto.

The NICER instrument installed on the International Space Station, as captured by a high-definition external camera on Oct. 22, 2018. Credits: NASA

“NICER’s observations of J1820 have taught us something new about stellar-mass black holes and about how we might use them as analogs for studying supermassive black holes and their effects on galaxy formation,” said co-author Philip Uttley, an astrophysicist at the University of Amsterdam. “We’ve seen four similar events in NICER’s first year, and it’s remarkable. It feels like we’re on the edge of a huge breakthrough in X-ray astronomy.”

NICER is an Astrophysics Mission of Opportunity within NASA's Explorer program, which provides frequent flight opportunities for world-class scientific investigations from space utilizing innovative, streamlined and efficient management approaches within the heliophysics and astrophysics science areas. NASA's Space Technology Mission Directorate supports the SEXTANT component of the mission, demonstrating pulsar-based spacecraft navigation.

“This is the first time that we’ve seen this kind of evidence that it’s the corona shrinking during this particular phase of outburst evolution,” said co-author Jack Steiner, an astrophysicist at the Massachusetts Institute of Technology’s Kavli Institute for Astrophysics and Space Research in Cambridge. “The corona is still pretty mysterious, and we still have a loose understanding of what it is. But we now have evidence that the thing that’s evolving in the system is the structure of the corona itself.”

To confirm the decreased lag time was due to a change in the corona and not the disk, the researchers used a signal called the iron K line created when X-rays from the corona collide with iron atoms in the disk, causing them to fluoresce. Time runs slower in stronger gravitational fields and at higher velocities, as stated in Einstein’s theory of relativity. When the iron atoms closest to the black hole are bombarded by light from the core of the corona, the X-ray wavelengths they emit get stretched because time is moving slower for them than for the observer (in this case, NICER).

Kara’s team discovered that J1820’s stretched iron K line remained constant, which means the inner edge of the disk remained close to the black hole — similar to a supermassive black hole. If the decreased lag time was caused by the inner edge of the disk moving even further inward, then the iron K line would have stretched even more.

These observations give scientists new insights into how material funnels in to the black hole and how energy is released in this process.

By Jeanette Kazmierczak
NASA’s Goddard Space Flight Center, Greenbelt, Md.

Editor: Rob Garner

 Source: NASA/NICER

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NASA’s NICER Mission Finds an X-ray Pulsar in a Record-fast Orbit

The stars of IGR J17062–6143, illustrated here, circle each other every 38 minutes, the fastest-known orbit for a binary system containing an accreting millisecond X-ray pulsar. As they revolve, a superdense pulsar pulls gas from a lightweight white dwarf. The two stars are so close they would fit between Earth and the Moon. Credits: NASA’s Goddard Space Flight Center. Download this video in HD formats from NASA Goddard's Scientific Visualization Studio

Scientists analyzing the first data from the Neutron star Interior Composition Explorer (NICER) mission have found two stars that revolve around each other every 38 minutes — about the time it takes to stream a TV drama. One of the stars in the system, called IGR J17062–6143 (J17062 for short), is a rapidly spinning, superdense star called a pulsar. The discovery bestows the stellar pair with the record for the shortest-known orbital period for a certain class of pulsar binary system.

The data from NICER also show J17062’s stars are only about 186,000 miles (300,000 kilometers) apart, less than the distance between Earth and the Moon. Based on the pair’s breakneck orbital period and separation, scientists involved in a new study of the system think the second star is a hydrogen-poor white dwarf.

“It’s not possible for a hydrogen-rich star, like our Sun, to be the pulsar’s companion,” said Tod Strohmayer an astrophysicist at Goddard and lead author on the paper. “You can’t fit a star like that into an orbit so small.”

A previous 20-minute observation by the Rossi X-ray Timing Explorer (RXTE) in 2008 was only able to set a lower limit for J17062’s orbital period. NICER, which was installed aboard the International Space Station last June, has been able to observe the system for much longer periods of time. In August, the instrument focused on J17062 for more than seven hours over 5.3 days. Combining additional observations in October and November, the science team was able to confirm the record-setting orbital period for a binary system containing what astronomers call an accreting millisecond X-ray pulsar (AMXP).

When a massive star goes supernova, its core collapses into a black hole or a neutron star, which is small and superdense — around the size of a city but containing more mass than the Sun. Neutron stars are so hot the light they radiate passes red-hot, white-hot, UV-hot and enters the X-ray portion of the electromagnetic spectrum. A pulsar is a rapidly spinning neutron star.

The 2008 RXTE observation of J17062 found X-ray pulses recurring 163 times a second. These pulses mark the locations of hot spots around the pulsar’s magnetic poles, so they allow astronomers to determine how fast it’s spinning. J17062’s pulsar is rotating at about 9,800 revolutions per minute.

Hot spots form when a neutron star’s intense gravitational field pulls material away from a stellar companion — in J17062, from the white dwarf — where it collects into an accretion disk. Matter in the disk spirals down, eventually making its way onto the surface. Neutron stars have strong magnetic fields, so the material lands on the surface of the star unevenly, traveling along the magnetic field to the magnetic poles where it creates hot spots.

The constant barrage of in-falling gas causes accreting pulsars to spin more rapidly. As they spin, the hot spots come in and out of the view of X-ray instruments like NICER, which record the fluctuations. Some pulsars rotate over 700 times per second, comparable to the blades of a kitchen blender. X-ray fluctuations from pulsars are so predictable that NICER’s companion experiment, the Station Explorer for X-ray Timing and Navigation Technology (SEXTANT), has already shown they can serve as beacons for autonomous navigation by future spacecraft.

Over time, material from the donor star builds up on the surface of the neutron star. Once the pressure of this layer builds up to the point where its atoms fuse, a runaway thermonuclear reaction occurs, releasing the energy equivalent of 100 15-megaton bombs exploding over every square centimeter, explained Strohmayer. X-rays from such outbursts can also be captured by NICER, although one has yet to be seen from J17062.

The researchers were able to determine that J17062’s stars revolve around each other in a circular orbit, which is common for AMXPs. The white dwarf donor star is a “lightweight,” only around 1.5 percent of our Sun’s mass. The pulsar is much heavier, around 1.4 solar masses, which means the stars orbit a point around 1,900 miles (3,000 km) from the pulsar. Strohmayer said it’s almost as if the donor star orbits a stationary pulsar, but NICER is sensitive enough to detect a slight fluctuation in the pulsar’s X-ray emission due to the tug from the donor star.

“The distance between us and the pulsar is not constant,” Strohmayer said. “It’s varying by this orbital motion. When the pulsar is closer, the X-ray emission takes a little less time to reach us than when it’s further away. This time delay is small, only about 8 milliseconds for J17062's orbit, but it’s well within the capabilities of a sensitive pulsar machine like NICER.”

The results of the study were published May 9 in The Astrophysical Journal Letters.

NICER’s mission is to provide high-precision measurements to further study the physics and behavior of neutron stars. Other first-round results from the instrument have provided details about one object’s thermonuclear bursts and explored what happens to the accretion disk during these events.

“Neutron stars turn out to be truly unique nuclear physics laboratories, from a terrestrial standpoint,” said Zaven Arzoumanian, a Goddard astrophysicist and lead scientist for NICER. “We can’t recreate the conditions on neutron stars anywhere within our solar system. One of NICER’s key objectives is to study subatomic physics that isn’t accessible anywhere else.”

NICER is an Astrophysics Mission of Opportunity within NASA's Explorer program, which provides frequent flight opportunities for world-class scientific investigations from space utilizing innovative, streamlined, and efficient management approaches within the heliophysics and astrophysics science areas. NASA's Space Technology Mission Directorate supports the SEXTANT component of the mission, demonstrating pulsar-based spacecraft navigation.

By Jeanette Kazmierczak
NASA’s Goddard Space Flight Center, Greenbelt, Md.

Editor: Rob Garner

Source: NASA/NICER

NASA Continues to Study Pulsars, 50 Years After Their Chance Discovery

Pulsar

Credits: NASA’s Goddard Space Flight Center

Most known neutron stars are observed as pulsars, emitting narrow, sweeping beams of radiation. They squeeze up to two solar masses into a city-size volume, crushing matter to the highest possible stable densities. To explore these exotic states of matter, NICER measures X-ray emissions across the surfaces of neutron stars as they spin, ultimately confronting the predictions of nuclear physics theory. Credits: NASA’s Goddard Space Flight Center. Animation

Technology advances in the past half-century allowed scientists to study these compact stellar objects from space using different wavelengths of light, especially those much more energetic than the radio waves received by the Cambridge telescope. Several current NASA missions continue to study these natural beacons.

The Neutron star Interior Composition Explorer, or NICER, is the first NASA mission dedicated to studying pulsars. In a nod to the anniversary of Bell’s discovery, NICER observed the famous first pulsar, known today as PSR B1919+21.

NICER launched to the International Space Station in early June and started science operations last month. Its X-ray observations – the part of the electromagnetic spectrum in which these stars radiate both from their million-degree solid surfaces and from their strong magnetic fields – will reveal how nature’s fundamental forces behave within the cores of these objects, an environment that doesn’t exist and can’t be reproduced anywhere else. "What's inside a pulsar?" is one of many long-standing astrophysics questions about these ultra-dense, fast-spinning, powerfully magnetic objects.

The “stuff” of pulsars is a collection of particles familiar to scientists from over a century of laboratory studies on Earth – neutrons, protons, electrons, and perhaps even their own constituents, called quarks. However, under such extreme conditions of pressure and density, their behavior and interactions aren’t well understood. New, precise measurements, especially of the sizes and masses of pulsars are needed to pin down theories.

“Many nuclear-physics models have been developed to explain how the make-up of neutron stars, based on available data and the constraints they provide,” said Goddard’s Keith Gendreau, the principal investigator for NICER. “NICER’s sensitivity, X-ray energy resolution and time resolution will improve these by more precisely measuring their radii, to an order of magnitude improvement over the state of the art today.”

NICER is currently installed on the International Space Station. This turntable animation of the payload calls out the locations of NICER’s star tracker camera, electronics, space station attachment mechanism, 56 sunshields, pointing actuators and stow/deploy actuator. Credits: NASA’s Goddard Space Flight Center. Animation

The mission will also pave the way for future space exploration by helping to develop a Global Positioning System-like capability for the galaxy. The embedded Station Explorer for X-ray Timing and Navigation Technology, or SEXTANT, demonstration will use NICER’s X-ray observations of pulsar signals to determine NICER's exact position in orbit.

“You can time the pulsations of pulsars distributed in many directions around a spacecraft to figure out where the vehicle is and navigate it anywhere,” said Arzoumanian, who is also the NICER science lead. “That’s exactly how the GPS system on Earth works, with precise clocks flown on satellites in orbit.”

Scientists have tested this method using computer and lab simulations. SEXTANT will demonstrate pulsar-based navigation for the first time in space.

NICER-SEXTANT is the first astrophysics mission dedicated to studying pulsars, 50 years after their discovery. “I think it is going to yield many more scientific discoveries than we can anticipate now,” said Gendreau.

NICER-SEXTANT is a two-in-one mission. NICER is an Astrophysics Mission of Opportunity within NASA's Explorer program, which provides frequent flight opportunities for world-class scientific investigations from space utilizing innovative, streamlined, and efficient management approaches within the heliophysics and astrophysics science areas. NASA's Space Technology Mission Directorate supports the SEXTANT component of the mission, demonstrating pulsar-based spacecraft navigation.

More about NICER: https://www.nasa.gov/nicer/

Read about five famous pulsars from the past 50 years: https://nasa.tumblr.com/post/163637443034/five-famous-pulsars-from-the-past-50-years ­­­­

By Clare Skelly
NASA’s Goddard Space Flight Center in Greenbelt, Md.

Editor: Karl Hille

Source: NASA/Pulsars