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Reclusive neutron stars can be found in famous supernovae

Reclusive neutron stars can be found in famous supernovae

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

What’s left of the star that exploded just outside our galaxy in 1987? Waste has obscured scientists’ perception, but two of NASA’s X-ray telescopes have revealed new traces.

Since astronomers captured the bright explosion of a star on February 24, 1987, scientists have been searching for the shattered star nucleus that should have been left behind. A group of astronomers using data from NASA’s space missions and terrestrial telescopes may have finally found them.

As the first supernova visible to the naked eye in about 400 years, Supernova 1987A (or SN 1987A for short) sparked great excitement 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.

While astronomers saw debris blast outward from the detonation site, they also looked for what was left of the star’s nucleus: a neutron star.

Data from NASA’s Chandra X-ray Observatory and previously unpublished data from NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) combined with data from the terrestrial Atacama Large Millimeter Array (ALMA) reported last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

“For 34 years, astronomers have been sifting through the stellar waste of SN 1987A to find the neutron star we expect to be there,” said study leader Emanuele Greco at the University of Palermo in Italy. “There have been many tips that have proven to be dead ends, but we think our recent results could be different.”

This computer model from a paper from Orlando and partners shows the rest in 2017, containing data taken by Chandra, ESA’s XMM-Newton and Japan’s Advanced Satellite for Cosmology and Astrophysics (ASCA). Credit: INAF-Palermo Astronomical Observatory / Salvatore Orlando

When a star explodes, it collapses on itself before the outer layers explode into space. Compression of the core transforms it into an extraordinarily dense object, where the mass of the sun is forced into an object only about 10 miles across. These objects have been called neutron stars because they are made almost exclusively of tightly packed neutrons. They are laboratories with extreme physics that cannot be duplicated here on Earth.

Rapidly rotating and highly magnetized neutron stars, called pulsars, produce a lighthouse-like beam that astronomers detect as impulses as its rotation sweeps the beam across the sky. There are a subset of pulsars that produce wind from their surfaces – sometimes at almost the speed of light – creating intricate structures of charged particles and magnetic fields known as “pulsar wind bubbles.”

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

Reclusive neutron stars can be found in famous supernovae

Supernova 1987A exploded more than 30 years ago and is still surrounded by dirt. The energetic 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 a better resolution. Credit: NASA / CXC

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

The latest X-ray study supports the case of the pulsar wind nebula – which means the neutron star must be there – by arguing on a few fronts against the scenario of explosion wave acceleration. First, the brightness of higher-energy X-rays remained roughly the same between 2012 and 2014, while radio emissions detected with the Australia Telescope Compact Array increased. This is contrary to expectations for the explosion wave scenario. Next, authors estimate that it will take almost 400 years to accelerate the electrons up to the highest energies seen in NuSTAR data, which are over 10 times older than the rest of the age.

“Astronomers have wondered if not enough time has passed for a pulsar to form, or if SN 1987A created a black hole,” said co-author Marco Miceli, also from the University of Palermo. “This has been an ongoing mystery for a few decades and we are very excited to bring new information to the table with this result.”

The Chandra and NuSTAR data 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 potential explanations, its identification as a pulsar nebula could be substantiated with the new X-ray data. This is more evidence to support the idea that there is a neutron star left.

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

“Being able to see a pulsar essentially since its birth would be unprecedented,” said co-author Salvatore Orlando of the Palermo Astronomical Observatory, a National Institute for Astrophysics (INAF) research facility in Italy. “It may be an opportunity once in a lifetime to study the development of a baby pulsar.”

Center for SN 1987A is surrounded by gas and dust. The authors used advanced simulations to understand how this material would absorb X-rays at different energies, allowing 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 obscured material.

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

The stellar debris surrounding the pulsar plays an important role in strongly absorbing its X-ray emission with lower energy, making it undetectable at present. The model predicts that this material will disperse over the next few years, reducing its absorbent power. Thus, the pulsating emission is expected to occur in about 10 years and reveal the existence of the neutron star.

A paper describing these results will be published this week in The Astrophysical Journal, and a pre-print is available online.

Kes 75 – The Milky Way’s youngest pulsar reveals the secrets behind the star’s death

More information:
Indication of a pulsar fog in the hard X-ray emission from SN 1987A, arXiv: 2101.09029 [astro-ph.HE] arxiv.org/abs/2101.09029

Citation: Reclusive neutron stars can be found in famous supernova (2021, February 23) retrieved February 23, 2021 from https://phys.org/news/2021-02-reclusive-neutron-star-famous-supernova.html

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