When a star explodes, it collapses on itself before the outer layers are blown into space. The compression of the nucleus turns it into an extraordinarily dense object, with the mass of the sun in an object that is only about 20 kilometers transverse. These objects are called neutron stars because they consist almost exclusively of densely packed neutrons. These are laboratories for extreme physics that cannot be duplicated here on earth.
Rapidly rotating and strongly magnetized neutron stars, called pulsars, produce a lighthouse-like ray that astronomers observe as pulses when their rotation sweeps the ray across the sky. There is a subset of pulses that produce winds on their surfaces – sometimes at almost the speed of light – that create complex structures of charged particles and magnetic fields known as ‘pulsar wind nebula’.
With Chandra and NuSTAR, the team found relatively low-energy X-rays of SN 1987A’s debris that crashed into the surrounding material. The team also found evidence of high-energy particles using NuSTAR’s ability to detect more energetic X-rays.
There are two probable explanations for this energetic X-ray emission: either a pulsating wind nebula or particles are accelerated to high energies by the explosion wave. 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 for the pulsar wind nebula – which means that the neutron star must be there – by arguing on several fronts against the scenario of explosion wave acceleration. First, the brightness of the higher-energy X-rays remained approximately the same between 2012 and 2014, while the radio emission 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 to the highest energies seen in the NuSTAR data, which is more than ten times older than the remnant.
“Astronomers have wondered if not enough time had elapsed to form a pulsar, or even if SN 1987A created a black hole,” said co-author Marco Miceli, also of the University of Palermo. “This has been an ongoing mystery for several decades, and we are very excited to bring new information to light 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. Although this ‘stain’ has other potential explanations, its identification as a pulsating wind nebula can be confirmed with the new X-ray data. There is more evidence to support the idea that there is a neutron star left behind.
If it is indeed a pulsar in the middle of SN 1987A, it is the youngest one ever found.
“To be able to see a pulsar since its birth would be unprecedented,” said co-author Salvatore Orlando of the Palermo Astronomical Observatory, a National Institute of Astrophysics (INAF) in Italy. “This may 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 modern 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 enables them to estimate what the spectrum of the central regions of SN 1987A is without the obscuring material.
As usual, more data is needed to amplify the case of the pulsar wind nebula. Against this idea, an increase in radio waves may be accompanied by an increase in relatively high-energy X-rays in future observations. On the other hand, if astronomers observe a decrease in the high-energy X-rays, the presence of a pulsar wind nebula will be confirmed.
The stellar remnants around the pulsar play an important role by strongly absorbing the X-ray emission of its lower energy, which makes it undetectable at the moment. The model predicts that this material will spread over the next few years, which will reduce its absorbent strength. Thus, the pulsar emission is expected to emerge in about 10 years, revealing the existence of the neutron star.
A paper describing these results will be published in The Astrophysical Journal this week, and a pre-print is available online. The other authors of the article are Barbara Olmi and Fabrizio Bocchino, also from INAF-Palermo; Shigehiro Nagataki and Masaomi Ono of the Astrophysical Big Bang Laboratory, RIKEN in Japan; Akira Dohi of Kyushu University in Japan, and Giovanni Peres of Palermo University.
NASA’s Marshall Space Flight Center manages the Chandra program. The Chandra X-ray Center of the Smithsonian Astrophysical Observatory controls science from Cambridge Massachusetts and flight operations from Burlington, Massachusetts.
NuSTAR is a Small Explorer mission led by Caltech and managed by NASA’s Jet Propulsion Laboratory for the Science Mission Directorate in Washington. NuSTAR was developed in partnership with the Danish Technical University and the Italian Space Agency (ASI). The spacecraft was built by Orbital Sciences Corporation in Dulles, Virginia (now part of Northrop Grumman). NuSTAR’s mission operations center is at UC Berkeley, and the official data archive is at NASA’s High Energy Astrophysics Science Archive Research Center. ASI provides the mission’s ground station and a mirror archive. JPL is a division of Caltech.