
Monthly Roundup: A Cluster of Cosmic Spiders
In the vast menagerie of cosmic critters, perhaps none is as impressively deadly as the spider pulsar. Spider pulsars are a subset of millisecond pulsars: tiny, extraordinarily fast-spinning remnants of dead massive stars. Like other types of pulsars, spider pulsars emit narrow beams of radio emission that can sweep across our field of view as the pulsar spins, generating the characteristic radio pulses from which pulsars get their name. Locked in a close embrace with a low-mass stellar companion, a spider pulsar roasts its companion with high-energy radiation and strips away the companion’s atmosphere with powerful particle winds. Eventually, like the ill-fated mates of certain spiders, the companion star may be entirely destroyed.
Just as there are many species of spiders, so too does it appear that there are different “species” of spider pulsars. So far, two categories are well established, and a third has started to emerge. These categories are based on the mass of the pulsar’s companion star and the orbital period of the system. Black widows have the lowest-mass companions, weighing in at just 5% of the mass of the Sun, and orbital periods less than one day. Redbacks, named after black-and-red spiders also known as Australian black widows, are similarly closely orbiting but have companions with masses ranging from 0.1 to 1.0 solar mass. Finally, the emerging class of huntsman pulsars, named for a large, long-legged type of spider, have low-mass red giant companions and relatively long orbits of 5–10 days.
Today, the Monthly Roundup will introduce three recent investigations of spider pulsars.
Discovery of a Second Huntsman
In 2016, researchers reported the discovery of a ~2-solar-mass millisecond pulsar in a 5.4-day orbit with a 0.33-solar-mass red giant companion stripped of its outer atmosphere. This pulsar — with its evolved stellar companion and long orbital period — was unlike any other spider pulsar known at the time and is now the prototype of a new class of spider pulsars nicknamed “huntsman” pulsars. A year later, a second system with similar properties was found, though the pulsar was not directly detected and remains a candidate.
Now, a team led by Jay Strader (Michigan State University) has announced the discovery of the second confirmed huntsman: the pulsar PSR J1947–1120. This source initially came to light in gamma-ray observations by the Fermi Gamma-ray Space Telescope. Millisecond pulsars are known to emit gamma rays, and many pulsars are first discovered in gamma rays. Using new and archival observations from the Neil Gehrels Swift Observatory (X-rays), XMM-Newton (X-rays), Gaia (optical), the Zwicky Transient Facility (optical), the Southern Astrophysical Research telescope (optical), and the Green Bank Telescope (radio), Strader’s team followed up on the gamma-ray detection and identified both the pulsar and its companion.

A spectrum of the PSR J1947–1120 system. The spectrum is consistent with a cool K-type star. Click to enlarge. [Strader et al. 2025]
Strader’s team outlined a potential formation mechanism for huntsman pulsars that involves a red giant companion in the “red bump” phase. During this phase of stellar evolution, red giants temporarily become smaller and less luminous, resulting in a pileup or “bump” in the Hertzsprung–Russell diagram. Accretion onto the pulsar also ceases during this period, matching what is seen in huntsman systems. Through stellar evolution modeling, the team showed that the properties of the two known huntsman pulsars can be attained by systems containing red giant stars in the red bump phase.
Why, then, have so few huntsman pulsars been found compared to other types of spider pulsars? The red bump phase of red giant evolution is brief, cosmically speaking, lasting on the order of tens of millions of years. Other types of spider pulsars arise in setups that are longer lived, on the order of billions of years, making it simply more likely to find a black widow or redback in the field than a huntsman.
Searching for Spiders That Will Consume Their Companions
Spider pulsars often exhibit eclipses: periods during which the pulsar’s radio signal passes through the ablated material of the companion star, causing the signal to drop out. While radio eclipses are a common feature of spider pulsars, not all spider pulsars experience this phenomenon. The inclination of the system and physical size of the companion star may both influence whether a spider pulsar exhibits eclipses.
The occurrence of radio eclipses may also be related to the mass-loss rate of the companion star, with higher mass-loss rates being associated with eclipses. The rate at which spider pulsars ablate their companions is a crucial piece of information for establishing spider pulsars as the missing link in the creation of solo millisecond pulsars.
Typically, millisecond pulsars like spider pulsars are thought to arise in binary systems. Accretion onto the pulsar from the binary companion spins up the pulsar, helping it achieve its extreme speed. While this mechanism explains the origins of millisecond pulsars in binary systems, it can’t explain solo millisecond pulsars — unless these singletons are spider pulsars that have entirely ablated their companions.
To explore this possibility, astronomers need to study the eclipses and mass-loss rates of a large sample of spider pulsars; so far, none of the known spider pulsars are blowing away their companions quickly enough for their companion stars to disappear within the universe’s current age.
In a recent research article, Sangita Kumari (National Centre for Radio Astrophysics, Tata Institute of Fundamental Research) and collaborators used the Giant Metrewave Radio Telescope (GMRT) to search for eclipses in 10 pulsars systems. In all, Kumari’s team observed eclipses for the first time in three systems and constrained the cutoff frequency — crucial to understanding the origin of the eclipses — in a further four systems for which eclipses had previously been observed. Three systems showed no eclipses.In addition to characterizing the eclipses they detected, the authors calculated the mass-loss rates for two of the pulsar’s companions. The mass-loss rates were too small to fully ablate the companion stars within the age of the universe, as has been the case for other spider pulsars under study. Additionally, the team found no correlation between the pulsar spin-down rates — thought to be related to the mass-loss rate — and the presence of eclipses, suggesting that other factors must play an important role in the creation of eclipses.
A Spider on the Dividing Line
J1908+2105 is a spider pulsar that was discovered in a search for counterparts to unidentified gamma-ray sources. The pulsar rotates every 2.56 milliseconds, and it’s in a 3.51-hour orbit with a companion star with a minimum mass of 0.055 solar mass, placing the pulsar between black widows and redbacks. As one of only a few known pulsars that may sit upon the dividing line between black widows and redbacks, studying J1908+2105 may help researchers understand whether these pulsars are evolving from one class to the other.

Average radio pulses at three different frequencies, showing the pulse behavior when not eclipsed. [Ghosh et al. 2025]
The team detected J1908+2105’s eclipses at frequencies up to 4 gigahertz, making this one of only a few pulsars with observable eclipses at such high frequency. They explored several possible sources for the eclipses, such as reaching the plasma frequency cutoff — the frequency at which radio waves are unable to travel through a plasma — and determined that synchrotron absorption is the likeliest cause.
Ghosh’s team measured the mass-loss rate from observations made at several frequencies, finding in the highest frequency band a mass-loss rate large enough for the companion to be ablated within 3 billion years. The authors caution that changes in the orbital properties of the system, spurred by the companion’s mass loss, likely mean that the rate will slow over time, and it may not be possible for the companion to be fully evaporated.
Citation
“PSR J1947−1120: A New Huntsman Millisecond Pulsar Binary,” Jay Strader et al 2025 ApJ 980 124. doi:10.3847/1538-4357/ada897
“Unveiling Low-Frequency Eclipses in Spider Millisecond Pulsars Using Wideband GMRT Observations,” Sangita Kumari et al 2025 ApJ 979 143. doi:10.3847/1538-4357/ad93ba
“Exploring Unusual High-Frequency Eclipses in MSP J1908+2105,” Ankita Ghosh et al 2025 ApJ 982 168. doi:10.3847/1538-4357/adb8e0