Astronomers using Australia’s ASKAP radio telescope have identified two new pulsars whose signals are profoundly distorted, offering a rare opportunity to map the turbulent and magnetic regions of our galaxy. The discovery was made by searching for circularly polarized light in images from a large-scale survey of the sky, a novel technique that has proven effective at locating pulsars missed by conventional search methods.
These celestial objects, the rapidly spinning remnants of massive stars, are fundamentally important tools for astrophysics. Their lighthouse-like beams of radiation sweep across space, and when these beams intersect with the interstellar medium—the diffuse gas and dust between stars—the signals are altered. By studying pulsars like the two newly found, scientists can probe the structure of the Milky Way and the fundamental laws of physics with greater precision.
A Novel Search Method
The breakthrough came from the Australian SKA Pathfinder (ASKAP) Variables and Slow Transients (VAST) survey. Instead of looking for the repeating signals that are the hallmark of pulsars, the research team focused on identifying sources that exhibited strong circular polarization in the telescope’s wide-field images. This property of light is a known characteristic of pulsars but is not the basis of typical discovery algorithms, which can be overwhelmed by signal degradation.
Once candidate sources were flagged by ASKAP, the team used the powerful Murriyang radio telescope at Parkes to conduct follow-up observations. These targeted observations confirmed that two of the sources were indeed previously unknown pulsars, validating the image-based search technique as a powerful tool for finding objects that are otherwise hidden in plain sight. This method is particularly adept at finding pulsars whose signals are heavily scattered.
Introducing Two New Cosmic Lighthouses
The newly discovered pulsars have been cataloged as PSR J1646−4451 and PSR J1837−0616. Both exhibit characteristics common to the general pulsar population but with notable extremes that make them scientifically valuable. They are classified as normal pulsars, distinct from the ultra-fast millisecond pulsars.
PSR J1646−4451
This pulsar has a spin period of 217 milliseconds and a high dispersion measure (DM) of 928 cm⁻³ pc. The dispersion measure indicates that the pulsar’s signal has traveled through a significant amount of free electrons on its way to Earth, which suggests it is either very distant or located behind a dense patch of interstellar gas.
PSR J1837−0616
The second pulsar, PSR J1837−0616, spins even faster, with a rotation period of 118 milliseconds. It also has a high dispersion measure of 793 cm⁻³ pc. The combination of their spin periods and spin-down rates places both objects in a region of diagnostic diagrams typically occupied by young pulsars, some of which are associated with the remnants of the supernovae that created them.
Extreme Signal Broadening
The most significant feature of these two pulsars is the extreme scattering of their radio signals. As the pulses travel through the inhomogeneous interstellar medium, they are broadened in time, a phenomenon that can smear the signal so much that it becomes undetectable by searches for sharp, repeating pulses. These two objects are among the most heavily scattered pulsars discovered to date.
Astronomers measured the pulse broadening for PSR J1646−4451 at 290 milliseconds at an observing frequency of 1.8 GHz, while PSR J1837−0616 showed a broadening of 343 milliseconds at 1.9 GHz. When these measurements are extrapolated to a standard reference frequency of 1 GHz, the scattering timescales become even more extreme: 2,479 ms and 2,154 ms, respectively. This degree of scattering makes their detection a remarkable achievement and highlights why they were missed by previous surveys.
Mapping the Interstellar Void
The discovery is more than just a catalog entry for two new objects; it provides powerful new probes for studying the Milky Way’s interstellar medium. The scattering and dispersion of a pulsar’s signal contain detailed information about the density, turbulence, and magnetic fields of the galactic material the signal has traversed. Highly scattered pulsars like these are exceptionally valuable because their signals have interacted intensely with this medium.
By analyzing these signals, researchers can create detailed maps of the structure of the gas and magnetic fields within our galaxy. This helps build a more complete picture of the galactic ecosystem, from star-forming regions to the diffuse plasma that fills the void between stars. These pulsars offer a unique line of sight into regions of the galaxy that may be particularly turbulent or dense.
Implications for Future Discoveries
This work successfully demonstrates that hunting for pulsars in wide-field continuum images based on their polarization is a highly effective strategy. The findings underscore the potential for future surveys with next-generation radio telescopes to uncover a much larger population of extreme pulsars. Scientists believe many more of these objects—including those that are intrinsically faint, highly scattered, or located in binary systems—are waiting to be found.
As facilities like ASKAP and the future Square Kilometre Array continue to scan the skies with unprecedented sensitivity and field-of-view, such novel techniques will be crucial. Finding more of these scattered sources will provide an even denser network of probes to map the interstellar medium, potentially leading to new insights into the evolution of our galaxy and the physics of pulsars themselves.
