Precision measurements provide clues to magnetars’ cosmic origin Ten National Science Foundation telescopes collaborate for three years to make exciting discoveries Magnetar Swift J1818.0-1617

Image credit: Image credit: NSF, AUI, NSF NRAO, S. Dagnello.

An international team of astronomers used an array of powerful radio telescopes to gain new insights into magnetars that are only a few hundred years old. Precise measurements of the magnetar’s position and velocity provide new information about its development. When a relatively massive star collapses at the end of its life and explodes as a supernova, it can leave behind an extremely dense star called a neutron star. The extreme forces during formation often cause neutron stars to spin so fast that they emit a beam of light like a lighthouse. When this beam is aligned so that it can be seen from Earth, the star is also called a pulsar. And when a neutron star forms with a rapid pulsar-like rotation and a magnetic field thousands of times stronger than a normal neutron star, it is called a magnetar. These stars pack about twice the mass of the Sun into a physical size of a few tens of kilometers, about the size of a city. Neutron stars, pulsars, and magnetars have many similarities, but astronomers remain puzzled as to what conditions cause these extreme stars to evolve in such different orbits. This time, a team of astronomers led by Hao Ding of the Mizusawa VLBI Observatory of the National Astronomical Observatory of Japan used the Very Long Baseline Array (VLBA) of the National Science Foundation’s (NSF) National Radio Astronomy Observatory (NRAO) to determine key properties of the newly discovered magnetar with unprecedented precision. Thirty magnetars are currently identified, but only eight of them are similar enough to be relevant for this study. Over the course of three years, Ding and his team used NSF’s VLBA to collect data on the position and speed of the Swift J1818.0-1617 magnetar, which was discovered in early 2020. Swift J1818.0-1617 is believed to be the youngest magnetar ever discovered and is the fastest rotating magnetar with a rotation period of 1.36 seconds. Swift J1818.0-1617 is in the constellation Sagittarius. It is located on the opposite side of the central galactic bulge, i.e. within the Milky Way, at a distance of only 22,000 light years. This means that it is relatively close to Earth; close enough that its three-dimensional position in the galaxy can be accurately determined using the parallax method. (The parallax method calculates distance based on the apparent change in an object’s position relative to known distant background objects.) The lifetime of magnetars is currently unknown, but astronomers estimate that Swift J1818.0-1617 has a lifespan of only a few hundred years. The emission of bright X-rays from magnetars requires a very high energy output mechanism. Only the rapid decay of its powerful magnetic field can explain the force behind these spectral features. But it is also an extreme process. Ordinary main sequence stars have very short lifetimes because they run out of fuel much faster than their yellow siblings. Although the physics of magnetars is different, magnetars probably also have short lifetimes compared to pulsars. “Magnetars are very young because they can’t release energy at this rate for long periods of time,” Ding explains. Moreover, magnetars can also show radiation at the lower end of the electromagnetic spectrum, in the radio wavelength range. For these, the radiation from the magnetar’s rapid rotation is thought to be the energy source. In synchrotron radiation, the plasma surrounding a neutron star is so densely packed on the star’s surface that it rotates at nearly the speed of light, producing radiation in the radio wavelength range. These radio emissions were detected through three years of observations by NSF’s VLBA. “VLBA provided us with excellent angular resolution to measure this small parallax,” Ding says. “The spatial resolution is unprecedented.” The results, published in August 2024, show that Swift J1818.0-1617’s parallax is the smallest of any neutron star and its so-called transverse velocity is the smallest of any magnetar, i.e. a new lower limit. Velocity in astronomy can be most easily described as consisting of two components or directions: Its radial velocity describes how fast it moves along the line of sight. In this case, that means moving along the radius of the galaxy. For magnetars like Swift J1818.0-1617, which is on the other side of the central bulge, there is too much other material to accurately determine the radial velocity. The transverse velocity, also called airspeed, describes the motion perpendicular to the galactic plane and is easier to detect. Astronomers are trying to understand the common and different formation processes of “normal” neutron stars, pulsars and magnetars, and hope to use precise measurements of the transverse velocity to analyze the conditions that cause stars to evolve along one of these three paths. Professor Ding said the study supports the theory that magnetars are unlikely to form under the same conditions as young pulsars, suggesting that magnetars form through a more exotic formation process. “We need to know how fast magnetars were moving when they were just born,” Ding said. “The formation mechanism of magnetars is still a mystery that we hope to understand.”

source: https://public.nrao.edu/news/precision-measurements-offer-clues-to-magnetars-cosmic-origin/