On the Hunt for a Hidden Neutron Star

Emanuele Greco

We are pleased to welcome Emanuele Greco as a guest blogger. Emanuele is the first author of a paper describing the possible discovery of a neutron star left behind by supernova 1987A. Emanuele received a master’s degree in Physics at the University of Palermo in 2017. He is now completing his PhD in Astrophysics at the same University, where he is expected to defend his thesis next June. He spent six months of his PhD at the Anton Pannekoek Instituut of the University of Amsterdam. Emanuele’s main research interests deal with the X-ray spectroscopy of supernova remnants and objects embedded within their shells, with a particular focus on the different processes that generate X-ray emission.

Imagine having a bright and small light bulb and putting it behind a thick wall made of elements like iron and silicon. No light stemming from the bulb would be observed, because it is completely obscured by the wall. This quite simple scenario is perfectly suited also for the elusive compact object of supernova (SN) 1987A, which was investigated by scientists from University of Palermo (UniPa), INAF-Observatory of Palermo (OAPa), Astrophysical Big Bang Laboratory (RIKEN) and University of Kyushu.

SN 1987A is the only naked-eye SN observed since telescopes were invented and offers a unique opportunity to watch a SN evolving into a supernova remnant (SNR) in this time of multi-wavelength and multi-messenger observatories simultaneously at work. This event was particularly important because neutrinos emitted from an exploding star were detected on Earth for the first time. This discovery implies that the core of the progenitor star must have collapsed producing a shock wave — similar to the sonic boom from a supersonic plane — that ejected part of the stellar material into the surrounding environment. As a result, a compact object such as a neutron star, a relic of the stellar core, should have formed in the very heart of SN 1987A. However, despite the continuous monitoring performed at almost all wavelengths since the SN was detected, no clear indication for this compact object has been found so far. Various hypotheses have been proposed to explain this non-detection, such as the formation of a black hole instead of a neutron star.

The most simple explanation to account for this non-detection leads us back to our obscured, small and bright light bulb: what if some material between us and the compact object is hiding its emission? We know that, because of the explosion, stellar fragments from the outer layers of the exploding star have been ejected (not by chance they are called ejecta) into space. This material is enriched of heavy elements (like iron and silicon) and is rapidly expanding in the ambient medium, but in the early phases of evolution it is very dense and cold. It acts as a cloudy wall for the radiation stemming from the compact object, preventing us from “seeing” a newly born neutron star. This is a reasonable solution to the “why don’t we detect the neutron star in SN 1987A?” puzzle, but we are more interested in solving the “how do we detect the neutron star in SN 1987A?” conundrum.

Since SN 1987A is about 168,000 light years from the Earth, we cannot really measure the absorbing power in loco, as we possibly could do for our light bulb. However, we can deduce it by taking into account three-dimensional (3D) computer simulations of SN 1987A that reproduce most of its observed properties. Thanks to this simulation (Orlando et al. 2020, A&A 636, id. A22; arXiv: 1912.03070), we estimated the quantity of cold and dense material that surrounds the putative neutron star, its chemical composition, and, therefore, also its absorbing power. We found that the ejecta absorption is so high that it can hide the energetic radiation expected for a neutron star with the age of SN 1987A up to an intermediate X-ray energy of roughly 10 keV. Therefore, it is not surprising that no radiation below this energy has been detected so far.

Supernova 1987A Pulsar Wind Nebula (Labeled):

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The follow-up question is then: what happens at X-ray energies above 10 keV? If our bulb would emit at such high energies, in principle its radiation should be detectable! We then looked at observations of SN 1987A performed between 2012 and 2014 by NASA’s Chandra X-ray Observatory and Nuclear Spectroscopic Telescope Array (NuSTAR), the latter being best suited for X-ray studies at energies higher than 10 keV. In fact, the model predictions suggest that these observations are the key to unlock the casket that holds the jewel sought for 34 years.

Our enthusiasm increased when we studied the hard energy part of the X-ray spectrum, which is the amount of X-rays at different energies. Beside the well-known component from supernova debris crashing into surrounding material, an additional component was needed to properly describe the data. This component originates from synchrotron radiation from charged particles spiraling around magnetic field lines. There are two main physical mechanisms possibly powering this synchrotron radiation: particles being accelerated to high energies by the shock front generated with the SN explosion, or a nebula produced by the wind of a pulsar, called a pulsar wind nebula. Both scenarios provide an accurate description of the observed X-ray spectra and under a merely statistic perspective, it is not possible to discriminate between the two scenarios.

However, the physical implications associated with each mechanism certainly makes the pulsar wind nebula scenario the most convincing As it is common in this kind of situations, future joint Chandra and NuSTAR observations will clarify the nature of the synchrotron radiation that we detected. If the pulsar wind nebula will be confirmed as the source of this emission, then we will finally have the possibility to study the youngest pulsar wind nebula ever observed; its monitoring will increase our knowledge on the dynamical evolution of these extreme objects. Moreover, since the ejecta are diluting with time due to the remnant expansion, the wall of clouds around the central object will be less opaque. According to our predictions, in about 10 years, our light bulb should become detectable also at energies lower than 10 keV, with observatories like Chandra or Lynx.