Title: Reheated Sub-40000 Kelvin Neutron Stars at the JWST, ELT, and TMT
Authors: Nirmal Raj, Prajwal Shivanna, Gaurav Niraj Rachh
First Author’s Institution: Centre for High Energy Physics, Indian Institute of Science, C. V. Raman Avenue, Bengaluru 560012, India
Status: Accepted for publication in Physical Review D [closed access]
Neutron stars are the remnants of massive stars formed when the star explodes as a supernova. The resulting neutron star is an incredibly dense object. Newly formed neutron stars are extremely hot, reaching temperatures of hundreds of billions of kelvin. Eventually, they cool down over time by radiating heat away, which can even be via neutrino emissions. Interestingly, some observations suggest that neutron stars could undergo a reheating period.
Neutron stars are exotic objects with extreme conditions – their densities can exceed those of atomic nuclei. It is impossible to replicate such conditions on Earth to study the properties of neutron stars, so physicists rely heavily on theoretical models and observations to study their structure and composition. In addition to theoretical models, neutron star cooling offers crucial insights into the physical and chemical processes within the star’s interior, making it essential to observe them as they are cooling down.
Using telescopes as thermometers!
All we need now is a giant thermometer to measure the temperature of neutron stars as they cool! Luckily, we have large telescopes that can double as thermometers. Taking images in infrared light, in particular, will help detect cooling neutron stars, as the energy from the neutron star will be seen optimally at this wavelength range. In this study, the authors determine if infrared instruments on current and future next-generation telescopes, such as JWST, the Extremely Large Telescope (ELT), and the Thirty Meter Telescopes (TMT), could detect neutron stars at various cooling temperatures.
The authors begin by determining the energy emitted (also called spectral flux density) by the neutron star to be captured by the telescopes. They assume the neutron star behaves like a perfect blackbody at a particular temperature. The blackbody spectrum of any object largely depends on its temperature, and this relationship is characterized by Wien’s displacement law. This model allows the authors to assign a temperature to the neutron star and have a reasonable determination of its energy output. Additionally, the spectral flux density will depend on the distance of the neutron star from the telescope. Hence, the authors determine a range of theoretical spectral densities based on a combination of the neutron star temperatures and distances.
Getting a strong signal!
While taking observations from any astronomical object, the signal from the object must be sufficiently distinguishable from the background noise (that can be caused by several factors, including the telescope itself!). This is characterized by a term known as the signal-to-noise ratio (SNR). The longer you look (or expose) at an object, the higher the SNR you will obtain, allowing you to cleanly identify the signal from the observed target, especially if the signal is pretty weak and likely to get buried under the noise. Although spending as much time as possible on any target would be great, telescope times are expensive! The trick is to determine what a reasonable exposure time is so that you can safely obtain the required signal.
Nearly every telescope has an ‘exposure time calculator’ that lets the observer determine the exposure time or how long they should observe the target to get the required SNR. A significant input necessary for the calculation is the energy expected from the neutron star, which the authors determined. They then use exposure time calculators from the various telescopes to determine if they can observe neutron stars at multiple temperatures, assuming an SNR of 2 (the minimum value you can consider to obtain a decent signal). The resulting calculations determined that the required exposure times range from a few hours to months (see Figure 1). While neutron stars with lower temperatures need to be close to be detected, those with higher temperatures can be observed from greater distances, as their higher energy output is more likely to fall within the minimum detection threshold of infrared instruments. Consequently, the authors find that more neutron stars can be detected at higher temperatures than those with lower temperatures.
More neutron stars are waiting to be discovered!
Overall, the authors determine the maximum distances to neutron stars with temperatures between 2000 and 40000K that can be detected by all of the next-generation telescopes with reasonable exposure times for several different filters. Several of the predicted distances are in the solar neighborhood. This would make them some of the closest and coldest neutron stars ever observed, enabling astronomers to study their properties with great detail. The capacity of next-generation telescopes to observe neutron stars as they cool will significantly enhance our understanding of them!
Astrobite edited by Abbé Whitford
Featured image credit: NASA, ESA, CSA, STScI, C. Fransson (Stockholm University), M. Matsuura (Cardiff University), M. J. Barlow (University College London), P. J. Kavanagh (Maynooth University), J. Larsson (KTH Royal Institute of Technology)
