Title: Probing the Low-Velocity Regime of Non-Radiative Shocks with Neutron Star Bow Shocks
Authors: Stella K. Ocker and Maren Cosens
First Author’s Institution: Cahill Center for Astronomy and Astrophysics, California Institute of Technology
Status: Accepted to ApJL [open access]
Neutron stars are fascinating remnants of massive stars that have undergone a supernova explosion. These stellar remnants often move at incredible speeds through space, producing bow shocks, the regions where the fast-moving neutron star collides with interstellar gas. Imagine a cosmic wind so powerful that it creates a shockwave in space, much like a speedboat cutting through water. These powerful shockwaves hold clues to study non-radiative shocks, which play an important role in heating plasma and accelerating particles, such as cosmic rays. Today’s paper took a closer look at the properties of three neutron star bow shocks in unprecedented detail, revealing new insights into the hidden physics behind these cosmic collisions.
What are Bow Shocks?
A bow shock forms when a fast-moving object, like a neutron star, passes through a medium – in this case, the interstellar medium (ISM), the gas and dust that fills space between stars. The interaction between the neutron star’s wind and the ISM causes a shock wave to form, which resembles the bow wave that forms at the front of a boat moving through water (for example, see Figure 1).
In the context of neutron stars, the bow shock is non-radiative, meaning it does not emit much in the form of light or heat. However, the shock does produce a particular type of emission called Hα (Hydrogen alpha), which occurs when neutral hydrogen atoms in the ISM are excited and emit light at a specific wavelength in the optical. Observing this Hα emission is one of the main ways astronomers can study neutron star bow shocks.
Understanding the Shock’s Velocity and Structure
Today’s authors focused on three known neutron star bow shocks (see Figure 2): J0742−2822, J1741−2054, and J2225+6535 (also known as the “Guitar Nebula”). Using integral field spectroscopy (IFS), a technique that captures both the spatial and spectral information of an object, they were able to observe these bow shocks in detail. For their observations, they used the Keck Cosmic Web Imager (KCWI) on the Keck II Telescope in Hawaii. Unlike traditional spectroscopy, which provides a one-dimensional spectrum of light from a single region, IFS collects spectra across a two-dimensional field, allowing the astronomers to map the shock properties. This allows astronomers to study the shock shape, velocity structure, and Hα emission intensity in exquisite detail, giving a more complete picture of how these shocks behave.
Studying the relative contributions to the Hα emission is crucial to unlocking the detailed shock physics. There are two main components to the Hα emission: a narrow line that represents the ambient gas in the ISM, and a broad line produced by the shock itself. The ratio between these two lines, the broad-to-narrow line intensity ratio (Ib/In), provides crucial information about the velocity of the shock and the processes occurring within it, including the electron-ion temperature and the particle energy distribution.
The study revealed that the Ib/In values for all three neutron star bow shocks indicated low shock velocities, all below 200 kilometers per second. This is notably different from the much higher velocities seen in supernova remnants, where shocks can exceed 1,000 kilometers per second. These results suggest that neutron star bow shocks operate in a distinct low-velocity regime, and current models, which are designed for higher-velocity shocks, may not fully capture the behavior of these slower shocks. To better understand the temperature ratios between electrons and ions, as well as how particles are accelerated in this regime, new models are needed.
Why is The Low-Velocity Regime Important?
Understanding the low-velocity regime of non-radiative shocks is important for several reasons:
- Cosmic Ray Acceleration: Non-radiative shocks are believed to accelerate particles to very high speeds, contributing to the population of cosmic rays—high-energy particles that travel through space. Studying how these shocks operate at different velocities helps scientists understand how cosmic rays are produced and what role neutron stars might play in this process.
- Energy Transfer in Shocks: Non-radiative shocks are also key to understanding how energy is transferred between different types of particles, such as electrons and protons. In faster shocks, the temperature of electrons and protons can differ significantly, but in slower shocks, like those studied here, the temperatures might be more equal. Understanding this balance provides insight into the physics of shock waves and how they heat and accelerate particles.
- Astrophysical Modeling: Most models of non-radiative shocks are based on high-velocity shocks in supernova remnants. However, the findings from this study suggest that these models need to be expanded to include slower shocks, which behave differently and require new theoretical approaches.
This study provides critical new insights into the enigmatic nature of neutron star bow shocks, particularly in the unexplored low-velocity regime. By probing these slow shocks, we unlock a deeper understanding of how astrophysical plasmas are heated and how particles are accelerated to cosmic-ray speeds—shedding light on some of the most powerful processes in the universe. The findings challenge existing models of non-radiative shocks, emphasizing the need for new theory to capture the unique behavior of these slower shocks. As a result, this research not only reshapes our understanding of cosmic rays but also paves the way for exciting new directions in astrophysics, with potential breakthroughs on the horizon.
Astrobite edited by Megan Masterson
Featured image credit: NASA, Wikimedia Commons
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