by Giovanni Cozzolongo
About today’s guest author: Giovanni Cozzolongo earned his Bachelor’s and Master’s degrees in Physics from the University of Pisa and is pursuing his PhD in Physics at the University of Erlangen-Nuremberg, focusing on high-energy astrophysics. He contributes to cutting-edge research as a member of both NASA’s Fermi-LAT space telescope and the ground-based H.E.S.S. telescope collaborations. Beyond traditional research, Giovanni applied his expertise at Team Tumbleweed, a startup selected for ESA’s Business Incubation Centre in Austria, where he helped develop technologies for Martian rovers. A TEDx speaker, he is passionate about bridging the gap between complex science and public understanding, actively engaging in science communication initiatives throughout Italy and internationally.
Title: A phase-resolved Fermi-LAT analysis of the mode-changing pulsar PSR J2021+4026 shows hints of a multipolar magnetosphere
Authors: A. Fiori, M. Razzano, A. K. Harding, M. Kerr, R. P. Mignani, and P. M. Saz Parkinson
First Author’s Institution: Università di Pisa and Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, Italy
Status: Published in Astronomy & Astrophysics, 2024
A pulsar is a rapidly rotating neutron star, that is the collapsed remnant of a massive star’s core after a supernova explosion. These incredibly dense objects spin at remarkable speeds, beaming radiation from their magnetic poles. Like cosmic lighthouses, these beams sweep through space as the star rotates, producing regular pulses when they intersect our line of sight. Among the thousands of pulsars known, PSR J2021+4026 is a unique enigma. Located within the Gamma Cygni supernova remnant, it’s the only gamma-ray pulsar known to regularly switch between different emission states. Before proceeding, it’s worth clarifying some technical terms used throughout this article. A ”spin down” occurs when a pulsar’s rotation slows over time as it loses energy. The ”spin-down rate” specifically measures how quickly this slowdown happens – it is the rate of change of the pulsar’s rotation frequency.
A Peculiar Pulsar
Most pulsars maintain steady emission patterns over decades. However, PSR J2021+4026 breaks this rule by changing states about every 3.5 years. During these transitions, two things occur simultaneously: the gamma-ray brightness changes by 13% to 20%, and the spin- down rate changes by approximately 2% to 6%. Specifically, when the brightness decreases, the pulsar’s rotation slows down more rapidly, while an increase in brightness corresponds to a slower rate of spin-down. These changes occur relatively quickly, completing within timescales of up to 100 days. Note that no sudden spin changes (an increase or decrease in the pulsar’s rotational frequency) are observed during these mode changes. This behavior distinguishes PSR J2021+4026’s mode changes from pulsar glitches, which are characterized by a sudden change in spin.
Using NASA’s Fermi Space Telescope, researchers tracked PSR J2021+4026 for over a decade, witnessing four complete state transitions (see Figure 1). They found that when the pulsar dims, its gamma-ray emission becomes “softer”—meaning it produces fewer high-energy photons. Think of it like a special light bulb that, as it dims, shifts its color slightly toward the red end of the spectrum. The pulsar’s gamma-ray signal shows two main peaks in each rotation, like a lighthouse beam sweeping past twice per turn. Intriguingly, these peaks respond differently to the state changes – one peak varies dramatically while the other remains nearly constant, suggesting that different regions of the pulsar are affected differently.
A Magnetic Puzzle
The key to understanding this behavior lies in the pulsar’s magnetic field. While most pulsar models assume a simple magnetic field like a bar magnet (dipole), the researchers propose that PSR J2021+4026 has a more complex field with an additional component called a quadrupole. It’s like adding a second, more complicated magnet to the mix. They suggest that this quadrupole component shifts orientation slightly during state transitions while the main dipole field stays stable. This explains why some emission features change while others remain constant – different regions of the magnetic field affect different areas where electromagnetic radiation is produced.
This model gained strong support from X-ray observations, which showed that the timing of X-ray pulses relative to gamma-ray pulses changed significantly during one transition. X- rays originate from the pulsar’s polar caps—hot regions on the surface where magnetic field lines extend into space and charged particles heat the surface. When the quadrupole component shifts orientation, it changes the location of these polar caps. Meanwhile, the outer field where gamma rays are produced remains more stable because it is shaped mainly by the stable dipole component.
Why It Matters
This study suggests that pulsar magnetic fields can be more dynamic and complex than previously thought, potentially affecting how they evolve and lose energy over time. The regular pattern of these changes is particularly intriguing to the authors. It hints at some process within the pulsar – perhaps periodic shifts in its crust or internal structure – that triggers these magnetic field reconfigurations. Thus, these researchers plan to continue monitoring this pulsar with both gamma-ray and X-ray telescopes, hoping to catch more details of these transitions in action. Understanding the mechanisms behind them could provide new insights into the extreme physics of these dense stellar remnants.
Edited by Diana Solano-Oropeza
Featured image credit: NASA/CXC/ASU/J. Hester et al., HST/ASU/J. Hester et al.