For nearly 10 years, NASA’s NuSTAR (Nuclear Spectroscopic Telescope Array) X-ray space observatory has studied some of the most energetic objects in the universe, such as the collision of dead stars and huge black holes. feasting on hot gases. During this time, scientists had to contend with stray light streaming in from the sides of the observatory, which can interfere with observations much like external noise can drown out a phone call.
But now, team members have figured out how to use this X-ray stray light to learn more about objects in NuSTAR’s peripheral vision while performing normal focused observations. This development has the potential to multiply the information provided by NuSTAR. A new scientific paper in the Astrophysical Journal describes the first use of NuSTAR lens flare observations to learn more about a cosmic object — in this case, a neutron star.
Nuggets of matter left behind after a star collapses, neutron stars are among the densest objects in the universe, second only to black holes. Their strong magnetic fields trap the gas particles and direct them towards the surface of the neutron star. As the particles are accelerated and energized, they release high-energy X-rays that NuSTAR can detect.
The new study describes a system called SMC X-1, which consists of a neutron star orbiting a living star in one of two small galaxies orbiting the Milky Way (the home galaxy of the Earth). The brightness of the X-ray output from SMC X-1 appears to vary wildly when viewed through telescopes, but decades of direct observations by NuSTAR and other telescopes have revealed a pattern of fluctuations. Scientists have identified several reasons why SMC X-1 changes in brightness when studied by X-ray telescopes. For example, X-ray brightness decreases as the neutron star dips behind the living star each orbit. According to the article, the lens flare data was sensitive enough to detect some of these well-documented changes.
“I think this paper shows that this stray light approach is reliable, because we’ve seen luminosity fluctuations in the neutron star in SMC X-1 that we’ve already confirmed with direct observations,” McKinley said. Brumback, an astrophysicist at Caltech in Pasadena, California. , and lead author of the new study. “In the future, it would be great if we could use lens flare data to look at objects when we don’t already know if they are regularly changing brightness and potentially use this approach to detect changes.”
Form and function
The new approach is possible thanks to the shape of NuSTAR, which resembles a dumbbell or dog bone: it has two bulky components at each end of a 33-foot-long (10-meter-long) narrow structure called a mast. deployable, or boom. Typically, researchers point one of the bulky ends — which contains the optics or hardware that collects the X-rays — at the object they want to study. The light travels along the arrow to the detectors, located at the other end of the spacecraft. The distance between the two is necessary to focus the light.
But stray light also reaches the detectors by penetrating through the sides of the pole, bypassing the optics. It appears in NuSTAR’s field of view with light from any object the telescope observes directly, and is often quite easy to identify with the naked eye: it forms a circle of faint light emerging from the sides of the picture. (Unsurprisingly, stray light is a problem for many other space and ground-based telescopes.)
A group of NuSTAR team members have spent the past few years separating stray light from various NuSTAR observations. After identifying known bright x-ray sources at the periphery of each sighting, they used computer models to predict how much stray light should appear based on what bright object was nearby. They also looked at almost all of the NuSTAR observations to confirm the telltale sign of lens flare. The team created a catalog of about 80 objects for which NuSTAR had collected lens flare observations, naming the collection “StrayCats.”
“Imagine sitting in a quiet movie theater, watching a drama and hearing the explosions in the action movie playing next door,” said Brian Grefenstette, principal investigator at Caltech and member of the NuSTAR team leading StrayCats work. “In the past, that’s what lens flare looked like – a distraction from what we were trying to focus on. We now have the tools to turn that extra noise into useful data, opening up a whole new way to use NuSTAR to study the universe.
Of course, flare data cannot replace direct NuSTAR observations. Besides the fact that the stray light is not focused, many objects that NuSTAR can directly observe are too faint to appear in the stray light catalog. But Grefenstette said several Caltech students combed through the data and found instances of rapidly brightening peripheral objects, which could be any number of dramatic events, such as thermonuclear explosions on the surface of neutron stars. Observing the frequency and intensity of a neutron star’s changes in brightness can help scientists decipher what is happening to these objects.
“If you’re trying to look for a pattern in the long-term behavior or brightness of an X-ray source, stray light observations could be a great way to check more often and establish a baseline,” said said Renee Ludlam, an Einstein Fellow in NASA’s Hubble Fellowship Program at Caltech and a member of the StrayCats team. “They could also allow us to detect strange behavior in these objects when we least expect them or when we normally wouldn’t be able to point NuSTAR directly at them. Lens flare observations are not a substitute for direct observations, but more data is always good.
Learn more about the mission
NuSTAR was launched on June 13, 2012. A Small Explorer mission led by Caltech and managed by JPL for NASA’s Science Mission Directorate in Washington, it was developed in partnership with the Danish Technical University (DTU) and the Italian Space Agency (ASI). The telescope’s optics were built by Columbia University, NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and DTU. The spacecraft was built by Orbital Sciences Corp. in Dulles, Virginia. NuSTAR’s mission operations center is at the University of California, Berkeley, and the official data archive is at NASA’s High Energy Astrophysics Science Archive Research Center. ASI provides the mission ground station and mirror data archive. Caltech manages the JPL for NASA.
For more information on NuSTAR, visit: