Pulsars are rapidly spinning stars that emit beams of radiation. These beams sweep across the cosmos, like cosmic lighthouses scattered across a vast ocean of stars, blinking rhythmically. For decades, scientists have been charting the steady ticking of these stellar clocks, relying on their predictable pulses to understand the extreme physics of space. But just like any clock, sometimes things go a little awry.
In a recent study, researchers from the Indian Institutes of Science Education and Research (IISER) Kolkata and Tata Institute of Fundamental Research (TIFR) delved into the curious case of SXP 138, a pulsar in a binary star system. They discovered that its cosmic clock is not ticking as predictably as expected. In fact, it's slowing down, but in a rather complicated and fascinating way, revealing new insights into the dynamic lives of these extreme celestial objects.
SXP 138, a special type of pulsar called a Be X-ray binary pulsar. Be stars are typically B-type stars that are significantly more massive, ranging from 2-15 times the sun's mass, and hotter, burning at temperatures of 10,000 K to 33,000 K, compared to the sun. Be stands for Balmer emission lines that are characteristic of the light spectrum of these stars. Be stars are whirlwinds, spinning incredibly fast and flinging material off their equator, forming a disk of gas swirling around the star, much like a flattened donut. This disk is the hallmark of Be stars and is responsible for many of their unique properties, including the characteristic 'emission lines' seen in their light, unlike the Sun's light which is mostly absorption lines. Because of their unstable nature and the dynamic disk, Be stars are also often variable in brightness, unlike the steady output of our Sun.
A Be X-ray binary involves a super-dense, rapidly spinning neutron star—the pulsar—locked in orbit with a much larger, hotter, faster-spinning Be star. Now, the neutron star, with its immense gravity, slowly siphons off material from the disk around the Be star. As the stellar debris falls onto the neutron star, it heats up to millions of degrees and emits powerful X-rays, which telescopes like NuSTAR can detect from Earth.
To understand SXP 138's faulty clock, the researchers used data from the NuSTAR (Nuclear Spectroscopic Telescope Array), an space telescope that sees the universe in high-energy X-rays. They examined four separate observations taken between 2016 and 2017. They observed X-ray pulses from SXP 138 flash over several nights, carefully timing each blink. By analysing the timing of these pulses, they found that the pulsar's spin period or the time it takes to complete one rotation was increasing, a phenomenon called spin-down.
The slowdown wasn't a simple, steady decline but non-linear, meaning the rate of slowing changed over time. The team used a mathematical tool called the Lomb-Scargle periodogram to measure the spin period from the X-ray signals precisely. They also described this non-linear spin-down mathematically, showing that a simple linear slowdown wasn’t enough to explain the observations.
Pulsars generally slow down because they lose energy as they spin and emit radiation. In the case of SXP 138, being part of a binary system adds a layer of complexity. The researchers believe SXP 138 is in the "propeller regime." Imagine the neutron star as a cosmic propeller sucking in the gas from the Be star's disk. But, the neutron star’s strong magnetic field and rapid spin push away much of the material trying to fall onto it. This "propeller" action prevents efficient accretion, the process of matter falling onto the neutron star, and causes the pulsar to lose rotational energy, leading to spin-down. The changing spin-down rate might indicate fluctuations in the amount of material being pushed away or subtle shifts in the magnetic interaction.
Beyond the spin, the team also investigated the shape of the X-ray pulses, known as pulse profiles, and the energy distribution of the X-rays, called the energy spectra. The pulse profiles were like fingerprints, revealing details about the pulsar’s emission regions. They found that the pulse profiles consistently showed two prominent and two smaller peaks.. These features suggest that the X-rays are likely emitted from two "hotspots" near the pulsar's magnetic poles, regions where the intense magnetic field channels the infalling material. The double peaks might arise from a combination of “pencil beam" and "fan beam" emissions, like different types of light emanating from the hotspots, further warped and bent by the intense gravity around the neutron star – a phenomenon known as relativistic light bending.
The energy spectra, on the other hand, acted like a cosmic thermometer and chemical analyser. By fitting the spectra with models that included blackbody and power-law components, the scientists found that the temperature of the X-ray emitting region seemed to increase over the first three observations, from about 1.8 keV to 2.5 keV, before possibly decreasing slightly in the last observation. This temperature change is linked to the amount of matter accreting onto the neutron star. Higher accretion rates mean hotter temperatures. Furthermore, they observed changes in the photon index, which is related to a process called Compton scattering, where X-ray photons gain energy by bouncing off energetic electrons. These spectral changes suggest that as the accretion rate changes, so do the physical processes in the hot gas around the neutron star.
This research significantly improves our understanding of SXP 138. This work is the first comprehensive analysis of its timing and spectral properties using NuSTAR observations. It bridges a gap in our knowledge, providing a much clearer picture of its accretion processes and emission mechanisms. However, the researchers also note a limitation. The temperature measurement in the final observation has higher uncertainty, which makes it harder to conclude definitively about the source's state at that time. Future observations would be helpful in confirming the trends seen. Moreover, the research was published by the open-access publisher arXiv and has not yet been peer reviewed.
Ultimately, extreme objects like SXP 138 are cosmic laboratories for fundamental physics. Neutron stars pack more mass than our Sun into a sphere the size of a city, creating conditions of gravity, density, and magnetism that are impossible to replicate on Earth. By deciphering the signals from these cosmic lighthouses, we learn about how matter behaves in extreme environments, test the limits of our physical laws, and gain a deeper appreciation for the dynamic and often surprising workings of the universe.
This research article was written with the help of generative AI and edited by an editor at Research Matters.