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Researchers Reveal a Hidden Pulsar

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Hubble Space Telescope image of the globular cluster NGC 6397
The dense core of the globular cluster NGC 6397 is an excellent place to search for stellar remnants interacting with other objects. [NASA, ESA, and T. Brown and S. Casertano (STScI); Acknowledgement: NASA, ESA, and J. Anderson (STScI)]

Astronomers have discovered a second millisecond pulsar — a rapidly spinning, ultra-dense remnant of a massive star — in one of the nearest globular clusters to Earth. The new observations might help explain the surprising rarity of millisecond pulsars discovered in dense globular clusters.

Close Encounters of the Stellar Kind

animation still of a millisecond pulsar

Still image from an animation of a millisecond pulsar accreting material from its companion. [NASA]

In the cores of globular clusters, where gravitational encounters between stars are common, compact remnants of massive stars form binary systems with a wide range of properties. This sets the stage for the formation of millisecond pulsars: tiny, dense, rapidly spinning stellar remnants composed entirely of neutrons. All pulsars spin incredibly fast, but millisecond pulsars are the fastest of them all; if you stood on the equator of the speediest known millisecond pulsar, you’d whirl around at 24% of the speed of light. Astronomers believe that most millisecond pulsars started out as more slowly rotating solo acts, but after gaining a stellar companion, pulsars accrete matter and get spun up to “millisecond” status.

Nearby globular cluster NGC 6397 — a glittering, spherical collection of 400,000 stars — is home to a curious binary system that has been detected at X-ray, optical, and ultraviolet wavelengths. Its X-ray emission flashes with the period of the binary orbit, and optical observations show a red star at the same location. Previous research has suggested that this system contains a millisecond pulsar, but the characteristic radio pulses have been elusive.

plot of the radio pulses of the newly confirmed pulsar

Left: Phase-folded radio observations from the Parkes Telescope, showing the characteristic radio signal of the newly confirmed pulsar. Right: Timing residuals as a function of orbital phase. The pulsar is not visible when the phase is between 0 and 0.5 and the pulsar is farthest from the observer. Click to enlarge. [Zhang et al. 2022]

In Pursuit of a Pulsar

In a new article, a team led by Lei Zhang (Chinese Academy of Sciences and Swinburne University of Technology, Australia) reports the results of their observations of the system made between 2019 and 2022 using the Parkes (Murriyang) radio telescope in Australia and the MeerKAT array in South Africa. Zhang and collaborators discerned faint but detectable radio pulses every 5.8 milliseconds, and the pulses were modulated with a period of 1.97 days — the same period as the orbital period of the X-ray-emitting binary system at the same location.

This confirms that the system contains a millisecond pulsar, dubbed NGC 6397B, and further analysis of the timing of the pulses suggests that the pulsar is also the source of the X-ray emission detected previously.

Implications of an Intermittent System

plot of orbital period versus companion mass for known millisecond pulsars

Orbital period and companion mass for millisecond pulsars (MSPs) discovered in globular clusters (filled circles) and in the field (empty circles). The black and gray symbols indicate whether the companion is a white dwarf (WD), main-sequence star (MS), or an ultralight or planet-mass object (UL). Click to enlarge. [Zhang et al. 2022]

Even after the team tracked down the elusive pulsar, it still managed to give them the slip; the radio pulses became undetectable for 14 months before reemerging in early 2022. The system’s on-again off-again radio emission could point to one of two possibilities: hot, ionized gas flowing out from the companion star could be blocking the radio emission from reaching us when the binary system swings into certain orientations, or the act of accreting material from the companion star — the process that generates the X-rays — could temporarily halt the pulsar’s radio emission.

Previous research has suggested that pulsars in binary systems should be common in globular clusters with exceptionally dense cores, like NGC 6397, but most known pulsars in so-called core-collapse clusters are singletons. The particulars of the newly discovered system may provide an explanation as to why pulsar-hosting binary systems have been elusive in these environments: pulsars in binary systems might have faint or intermittent radio emission, making them hard to track down.

Citation

“Radio Detection of an Elusive Millisecond Pulsar in the Globular Cluster NGC 6397,” Lei Zhang et al 2022 ApJL 934 L21. doi:10.3847/2041-8213/ac81c3