In the vast and mysterious reaches of the cosmos, few phenomena are as dynamic and extreme as the interplay between accreting neutron stars and their binary companions. Imagine a neutron star—a relic of a supernova explosion—locked in a relentless gravitational embrace with a neighboring star. This partnership is not a peaceful one; over time, matter from the companion star spirals onto the neutron star’s surface, setting off a chain reaction of extraordinary astrophysical events.
As this stellar material accretes, it imparts angular momentum to the neutron star, accelerating its rotation. These rapidly spinning neutron stars, often detected as radio pulsars, X-ray bursters, or even gamma-ray emitters, are known as ‘recycled pulsars’. Unlike their younger, solitary counterparts, these old pulsars have been rejuvenated through mass transfer from their binary companion. This accretion-driven spin-up process does more than just speed up rotation—it alters the very fabric of the neutron star’s structure.
Quaking Neutron Stars
Beneath the thin but incredibly dense outer shell of a neutron star lies a rigid crust, composed of exotic, ultra-dense material. As accretion continues, the infalling matter exerts immense stress on this crust, gradually pushing it to its breaking point. When the stress surpasses the crust’s ability to resist, a catastrophic event occurs: a starquake. Similar to earthquakes on Earth—but vastly more energetic—a starquake involves a sudden fracture and rearrangement of crustal material, dramatically altering the star’s shape and mass distribution.
A neutron star undergoing a starquake does not merely tremble in isolation—it sends ripples through spacetime itself. A deformed, rapidly spinning neutron star is a prime generator of continuous gravitational waves (GWs). The moment a starquake shifts the mass quadrupole moment of the star, the emitted GW signal changes. These shifts could provide a unique and detectable signature, offering astronomers a new way to probe the inner workings of recycled millisecond pulsars.
Magnetic Fields and the Cracking Crust
Neutron stars are not just spinning masses of nuclear matter; they are also profoundly magnetized. The intense magnetic field influences the way the crust cracks during a starquake. Cracking is more likely to occur along planes most aligned with the magnetic field lines, which can, in turn, affect the star’s spin-down torque. Such changes in magnetic field geometry or orientation add another layer of complexity to the already intricate physics of these objects.
Starquakes do not merely alter the crust—they can also release violent outbursts of energy. When cracks form in the crust, the pressure inside the newly formed vacuum fissures plummets to zero. If these cracks are filled with gas containing electron-positron pairs, the resulting energy release can rival the power of giant radio pulses (GPs) and even fast radio bursts (FRBs). This connection suggests that the mysterious, short-lived bursts of radio waves observed across the cosmos may be, in some cases, the cries of quaking neutron stars.
An Ongoing Debate: Are All Glitches Starquakes?
While starquakes offer a compelling explanation for many observed phenomena in neutron stars, not all researchers agree. Some observed pulsar glitches—sudden changes in rotation rate—do not fit neatly into a simple starquake model. Alternative theories suggest that internal superfluid dynamics or interactions between the neutron star’s core and crust may be responsible for some of these glitches. The precise mechanisms remain a topic of ongoing astrophysical investigation.
The energy released during a starquake does not vanish; much of it is converted into heat. A cold neutron star, once thought to be cooling steadily through neutrino emission, can be dramatically reheated by a crust-breaking event. A single starquake can elevate the internal temperature of the neutron star to several hundred million degrees Kelvin. At this extreme temperature, thermal photon emission begins to dominate the cooling process, replacing neutrino emission as the primary mechanism. This thermal evolution further influences the star’s behavior over time.
A Symphony of Forces
Accretion-driven starquakes in recycled millisecond pulsars represent an intricate and ongoing celestial symphony—one where gravitational forces, rotational dynamics, and magnetic fields work in concert to sculpt the universe’s most enigmatic remnants. The continuous flow of matter, the slow build-up of stress in the crust, the sudden and violent release of energy, and the resulting gravitational wave emissions all contribute to the rich and complex nature of these stellar remnants. With each new observation, astronomers draw closer to unravelling the full story of these quaking stars—revealing secrets that shake not only their crusts but our very understanding of the universe itself.
Illustration of gas falling into a Neutron Star. (Image Credit: NASA).
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