An isolated neutron star may have been found in a famous supernova

On the left, data from NASA’s Chandra X-ray Observatory shows a portion of the debris from an exploded star known as supernova 1987A. On the right, an illustration of what might be at the center of the supernova remnant, a structure known as a “pulsar wind nebula.” Credit: NASA / CXC

What is left of the star that exploded on the outskirts of our galaxy in 1987? The debris has obscured scientists’ view, but two of NASA’s X-ray telescopes have revealed new clues.

Ever since astronomers captured a bright star explosion on February 24, 1987, researchers have been searching for the crushed stellar core that should have been left behind. A group of astronomers who used data from NASA space missions and ground-based telescopes may have finally found it.

As the first supernova visible to the naked eye in about 400 years, Supernova 1987A (or SN 1987A for short) sparked great enthusiasm among scientists and soon became one of the most studied objects in the sky. The supernova is located in the Large Magellanic Cloud, a small companion galaxy to our own Milky Way, only about 170,000 light-years from Earth.

As astronomers watched the debris explode from the detonation site, they also searched for what should have been left of the star’s core: a neutron star.

Data from NASA’s Chandra X-ray Observatory and unpublished data from NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR), in combination with data from the ground-based Atacama Large Millimeter Array (ALMA) reported last year, now present an intriguing collection of evidence for the presence of the neutron star in the center of SN 1987A.

“For 34 years, astronomers have been examining the stellar debris of SN 1987A to find the neutron star that we hope is there,” said study leader Emanuele Greco, of the University of Palermo in Italy. “There have been many indications that have turned out to be dead ends, but we think our latest results could be different.”

This computer model from a paper by Orlando et al. Shows the remnant in 2017, incorporating data taken by Chandra, ESA’s XMM-Newton, and Japan’s Advanced Satellite for Cosmology and Astrophysics (ASCA). Credit: INAF-Osservatorio Astronomico di Palermo / Salvatore Orlando

When a star explodes, it collapses in on itself before the outer layers are thrown into space. Compressing the core makes it an extraordinarily dense object, with the Sun’s mass compressed into an object only about 10 miles in diameter. These objects have been called neutron stars because they are made up almost exclusively of densely packed neutrons. They are laboratories of extreme physics that cannot be duplicated here on Earth.

Rapidly rotating and highly magnetized neutron stars, called pulsars, produce a beacon-like beam of radiation that astronomers detect as pulses as their rotation sweeps the beam across the sky. There is a subset of pulsars that produce winds from their surfaces, sometimes almost at the speed of light, creating intricate structures of charged particles and magnetic fields known as “pulsars wind nebulae.”

Using Chandra and NuSTAR, the team found relatively low-energy X-rays from the SN 1987A debris hitting surrounding material. The team also found evidence of high-energy particles using NuSTAR’s ability to detect more energetic X-rays.

An isolated neutron star may have been found in a famous supernova

Supernova 1987A exploded more than 30 years ago and is still surrounded by debris. The energy environment has been imaged by NASA’s Nuclear Spectroscopic Telescope Array, or NuSTAR (shown in blue) and the Chandra X-ray Observatory (shown in red), which has finer resolution. Credit: NASA / CXC

There are two possible explanations for this energetic X-ray emission: a pulsar wind nebula or particles that are accelerated to high energies by the blast wave from the explosion. The latter effect does not require the presence of a pulsar and occurs at much greater distances from the center of the explosion.

The latest X-ray study backs up the case for the pulsar wind nebula, meaning the neutron star must be there, arguing on a couple of fronts against the blast wave acceleration scenario. First, the brightness of the higher-energy X-rays remained about the same between 2012 and 2014, while the radio emission detected with the Australia Telescope Compact Array increased. This goes against expectations for the shock wave scenario. Next, the authors estimate that it would take nearly 400 years to accelerate the electrons to the highest energies seen in the NuSTAR data, which is more than 10 times the age of the remnant.

“Astronomers have wondered if enough time has passed for a pulsar to form, or even if SN 1987A created a black hole,” said co-author Marco Miceli, also from the University of Palermo. “This has been a constant mystery for a few decades, and we are very excited to bring new information to the table with this result.”

Data from Chandra and NuSTAR also support a 2020 result from ALMA that provided possible evidence for the structure of a pulsar wind nebula in the millimeter wavelength band. While this “blob” has other possible explanations, its identification as a pulsar wind nebula could be substantiated by the new X-ray data. This is further evidence to support the idea that a neutron star remains.

If it is a pulsar in the center of SN 1987A, it would be the youngest ever found.

“Being able to observe a pulsar essentially from birth would be unprecedented,” said co-author Salvatore Orlando of the Palermo Astronomical Observatory, a research center at the National Institute of Astrophysics (INAF) in Italy. “It could be a once-in-a-lifetime opportunity to study the development of a baby pulsar.”

The center of SN 1987A is surrounded by gas and dust. The authors used state-of-the-art simulations to understand how this material would absorb X-rays at different energies, allowing for a more accurate interpretation of the X-ray spectrum – that is, the amount of X-rays at different energies. This allows them to estimate what the spectrum of the central regions of SN 1987A is without the obscuring material.

As is often the case, more data is needed to strengthen the case for the pulsar wind nebula. An increase in radio waves accompanied by an increase in relatively high-energy X-rays in future observations would argue against this idea. On the other hand, if astronomers observe a decrease in high-energy X-rays, then the presence of a pulsar wind nebula will be corroborated.

The stellar debris surrounding the pulsar plays an important role by largely absorbing its lower-energy X-ray emission, rendering it undetectable today. The model predicts that this material will disperse in the coming years, reducing its absorbent power. Therefore, the emission from the pulsar is expected to emerge in about 10 years, revealing the existence of the neutron star.

An article describing these results will be published this week in The Astrophysical Journaland a preprint is available online.

Kes 75: the youngest pulsar in the Milky Way exposes the secrets of the disappearance of the stars

More information:
Indication of a Pulsar wind nebula in hard X-ray emission from SN 1987A, arXiv: 2101.09029 [astro-ph.HE]

Citation: A reclusive neutron star may have been found in a famous supernova (2021, February 23) retrieved February 23, 2021 from -famous-supernova.html

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