Magnetar Pulses Seen in a New Light

New observations have captured pulses of radiation from a magnetic stellar remnant called a magnetar. What do these observations tell us about how magnetars and other neutron stars generate their beams of emission?

Rare Stellar Remnants

x-ray image of the vela pulsar

The Vela pulsar — seen emitting jets of fast-moving particles in this image — was the first pulsar to be detected at submillimeter wavelengths. Now, the first neutron star to pulsate in this wavelength range has been discovered. [NASA/CXC/Univ of Toronto/M. Durant et al.]

Neutron stars — ultra-dense, city-sized remnants of stars that exploded as supernovae — come in many flavors. Those that emit narrow beams of radio waves that sweep past Earth like the beacon of a lighthouse are called pulsars. Those that have extremely strong magnetic fields — 100 million times more intense than the strongest magnet ever made — are called magnetars. In rare cases, a neutron star can be both a pulsar and a magnetar!

It’s not yet clear how pulsars generate their beams of radio emission. One way to probe the generation mechanism is by studying the emission across a wide range of wavelengths, since some models predict that the emission should increase at a “turn-up” point somewhere between radio and infrared wavelengths. Previous observations have found tantalizing hints of this feature, but it has never been detected definitively. Can a new search at submillimeter wavelengths find the elusive turn-up point?

plot of submillimeter emission

Detection of XTE J1810−197 on 27 February 2020 at a wavelength of 0.85 mm. The panels show the target during a pulse (left) and not during a pulse (right) as well as the difference between the two states (bottom). Click to enlarge. [Adapted from Torne et al. 2022]

A Submillimeter Signal

A team of astronomers led by Pablo Torne (Institute of Millimeter Radio Astronomy, Spain; East Asian Observatory; and Max Planck Institute for Radio Astronomy, Germany) searched for signs of the turn-up point in observations of XTE J1810-197, one of only six neutron stars categorized as both a pulsar and a magnetar.

Torne and collaborators used telescopes across the globe to observe XTE J1810-197 over the course of 15 months. They detected a beam of emission swinging by for a few hundred milliseconds once per rotation period (5.54 seconds) at wavelengths ranging from 0.85 millimeters to 5.0 centimeters, marking the first time pulses from a neutron star have been detected at submillimeter wavelengths. However, they didn’t detect the pulses at 0.45 mm — the shortest wavelength searched in this study. What do these observations imply about the location of the turn-up point?

Where Will the Turn-up Turn Up?

plot of flux density versus frequency

Power-law (top) and broken power-law (bottom) spectral fits for XTE J1810−197. The downward pointing arrows indicate upper limits. [Torne et al. 2022]

Torne and collaborators found that XTE J1810-197’s emission is mostly flat across the wavelength range surveyed, with a potential downturn at longer wavelengths — and no sign of the turn-up point. If a turn-up is present, it might lie in the infrared or in the unexplored 0.37–3.00 cm (10–80 gigahertz) range.

However, searching these wavelength ranges may not be as simple as pointing the right kind of telescope at the target; magnetars are complicated, ever-changing objects that exhibit extremely energetic outbursts at short wavelengths and day-to-day variability across all wavelengths. This high level of variability can make determining the true shape of a neutron star’s spectrum challenging, since it might not be possible to compare measurements made at different times. (For example, astronomers have observed XTE J1810-197 in the infrared once before, but those observations were made when the magnetar was undergoing an explosive outburst.) However, with a little planning, simultaneous observations from radio to infrared could help us track down the turn-up point.


“Submillimeter Pulsations from the Magnetar XTE J1810-197,” Pablo Torne et al 2022 ApJL 925 L17. doi:10.3847/2041-8213/ac4caa