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 near the speed of light, that create intricate structures of charged particles and magnetic fields known as “pulsar 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.
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 supports 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 Journal, and a preliminary version is available online. The other authors of the work are Barbara Olmi and Fabrizio Bocchino, also from INAF-Palermo; Shigehiro Nagataki and Masaomi Ono from the Big Bang Astrophysical Laboratory, RIKEN in Japan; Akira Dohi of the University of Kyushu in Japan and Giovanni Peres of the University of Palermo.
NASA’s Marshall Space Flight Center administers the Chandra program. The Smithsonian Astrophysical Observatory’s Chandra X-ray Center 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 agency’s 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). The NuSTAR 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 ground station and a mirror file. JPL is a division of Caltech.