Did the 1987 Supernova Produce A Pulsar?

Last week marked the 35^th anniversary of a remarkable event that revolutionised the field of astrophysics and caught the world’s attention. A massive star, located in the relatively nearby galaxy  (the Large Magellanic Cloud or  LMC), detonated in what would be the closest observed supernova since Kepler’s some 400 years earlier.

At first, a handful of underground observatories – all designed to detect fundamental particles that move at extreme velocities registered, an unusually high count of high-energy events. Nobody noticed much at the time, but the data was recorded.

Around the same time, amateur and professional astronomers in the southern hemisphere were taking photos of the LMC, unaware of the events unfolding at the underground detectors in the prior hours before they turned their telescopes to the sky. This included astronomers at the Siding Spring Observatory in NSW, astrophotographers in New Zealand, and at the Las Campanas Observatory in Chile.

Eventually, when they developed their images, something caught their eye. A new bright star appeared, literally overnight, where there was no bright star before.

Supernova.

Very quickly, astronomers from both sides of the Tasman Sea jumped onto the case and started imaging the LMC, capturing data from this rare event that was said to be bright enough to be seen by the naked eye from dark sky regions.

News quickly spread across the astronomy community, which then trickled down into media news outlets, causing headlines around the world to spread – a new star had just died and for the first time since the invention of the telescope, scientists were going to be able to study it in detail.

And study it, they did. For 35 years, our telescopes – across all wavelengths of the electromagnetic spectrum, have been observing the first supernova of 1987 (known as SN1987A) – including both terrestrial and space-based telescopes. For those with powerful backyard telescopes, the region of the LMC has also become a favourite for southern astrophotographers.

But there remains a mystery at the heart of SN1987A.

Namely – what did it leave behind? We know that when massive stars explode, they usually leave behind exotic objects, like black holes or neutron stars. But in the case of SN1987A, even 35 years later, scientists are still searching for the answer.

Supernovae events can be classified into two main categories – those that are caused by thermonuclear explosions, and those that are caused by the collapsing core of a massive star. The first type (thermonuclear events) involves a binary system of stars, where at least one is a compact white dwarf. The latter (core-collapse events) involves a single massive star that has reached the end of an evolutionary phase, and either destroys itself or produces a new compact remnant object. In both cases, the light from a supernova is extremely luminous and can often outshine the entire host galaxy it is in.  

Stars are massive engines that produce heavier elements through the process of nuclear fusion in their cores (formally known as stellar nucleosynthesis). As the most abundant elements in the Universe are hydrogen and helium (created during the Big Bang), it follows that many stars are predominately also made with hydrogen and helium.

During the course of the main part of a star’s lifetime (known as its ‘main sequence phase’), the core of the star is fusing hydrogen into helium, with these reactions producing vast quantities of energy that allow an outward radiation pressure to be exerted against the inward pull of gravity that the star’s mass produces. This harmonious balance lasts for millions of years.

But then, as these stars age and start to consume their hydrogen fuel loads, they undergo a number of variable phases of fusing, as they start to produce new elements. First helium is converted into carbon (which takes hundreds of thousands of years), then carbon into oxygen (another 600 or so years), then a suite of heavier elements like neon, magnesium, silicon, sulfar, argon (taking months) and so on, until the element iron is reached.

At this point, when iron starts to form in the core, the star only has one day left up its sleeve, as iron starts to absorb energy away from the reactions. This then reduces the outward radiation pressure that kept gravity at bay, causing the star to collapse in on itself and triggering the supernova detonation.

As part of this process, the event releases an enormous amount of sub-atomic particles, known as neutrinos, which have extremely small masses and are electrically neutral. Because of these properties, neutrinos normally pass through regular matter without interaction. At this very moment, there are billions of neutrinos passing through you, generated by the Sun’s nuclear processes and distant supernovae events. And because of this ability, neutrinos from a supernova event can pass through the star and into space much quicker than the shockwave that triggers the observable explosion. In other words, neutrinos from a supernova will be detected first, before the visual light from the explosion can be observed.

This was exactly the case in 1987. Three different underground neutrino detectors first registered the neutrino burst, several hours prior to anyone noticing the star had even exploded. To date, SN1987A remains the only event in which we have experienced this to occur.

Supernovae events are excellent demonstrations of the cyclic nature of our Universe – the death of a massive star produces many elements that are then spread into the local space environment by the explosion, to only later coalesce and become new stars, planets, moons and in one special part of the Universe (that we know about), even humans. But they can additionally leave behind objects, known as massive compact remnants.

What gets left behind, really depends on the mass of the progenitor star prior to the supernova explosion. For example, stars like our Sun (about 1 – 8 solar masses), are considered average in mass and live about 10 billion years. Once they get to the later stages of nucleosynthesis, they start to produce oxygen, carbon and other heavier elements as well. However, they are not massive enough to produce an iron core, and thus do not experience violent supernova events. Instead, they just puff off their outer layers into beautiful planetary nebulae and leave behind a central compact remnant object known as a white dwarf, the hot cinder core of the former star. These objects are about the size of Earth but contain 1.4 solar masses within them.

But when a progenitor star is approximately 8 – 25 solar masses, a supernova explosion does occur, and what’s left behind is a much more compact and denser object, known as a neutron star. These stars are much smaller in size, about the size of a city, but still contain about 1.4 solar masses and can reach a little higher than two solar masses in size. These objects are so dense that a single teaspoon of their material would equate to the mass of the entire human species being crushed into a sugar cube. It is during these events that a neutrino burst occurs.

Neutron stars also come in a variety of flavours that include the regular neutron stars, extremely magnetised versions known as magnetars and rapidly rotating types, known as pulsars. Pulsars are rather special – they emit beams of radio waves from their magnetic poles as they rotate, and as these beams sweep past the Earth (just like a lighthouse’s beam sweeps out to sea as it rotates), we see them ‘pulse’.

Over the last 35 years, astronomers have been keeping a careful eye on SN1987A, hoping to catch whatever remnant object has been produced, but to date, the conversation amongst astronomers continues as to what it could be.

We know that a compact remnant object, like a neutron star (of some flavour), must have formed as part of the SN987A event. For one, we know that the progenitor star (Sanduleak-69 202) was a blue supergiant star with a mass of approximately 20 solar masses. So it fits the size requirement. We also know that the SN1987A event triggered a neutrino burst, that was detected here on Earth – so a compact remnant object should have formed.

But unfortunately, we haven’t got enough evidence as yet to make a solid confirmation. There are however a number of papers that have argued the case (for and against) of a neutron star, or a pulsar being detected.

In 2014, Zanardo et al. reported that observations using the CSIRO ATCA as well as the ALMA instruments detected certain emissions that could possibly be attributed to a pulsar wind nebula - the stream of charged particles (plasma) moving at relativistic speeds, powered by the rotation of a pulsar and its powerful magnetic field. And if there is a pulsar wind nebula, then there has to be a pulsar.

Wang et al. (2017) later stated that they too also believe there is a pulsar that was formed as part of the supernova explosion of 1987A, but as part of the asymmetrical forces that occurred during the core collapse, the pulsar was given a natal high-speed kick, moving it away from the central location of the explosion and into the supernova remnant material. The authors state that as the pulsar moved into these dense regions, material is captured by its gravitational and magnetic fields, accumulated at the magnetic poles and thus blocking the ability for any radio waves to be emitted by the pulsar – hence why we don’t detect it as yet.

A year later Zhange et al. (2018) used the CSIRO Parkes radio telescope to search for any periodic or transient radio emissions from the region and came up short – no evidence of any pulsar was found in this survey.

Another study also came out in 2018 (Alp et al.) which looked at data collected by ALMA (radio telescopes), the VLT (optical telescope), the Hubble Space Telescope (optical and UV telescope) and Chandra (x-ray telescope) covering a large portion of the electromagnetic spectrum and came to the conclusion (by placing constraints and limits) that instead of a pulsar, the remnant object must be a regular neutron star, sitting inside a dense region of dust and ejecta from the explosion.

By 2020, further investigations were still left undecided. Page et al. noted that it could likely be a 30-year-old neutron star, due to the thermal power that was being emitted into the dust region of where the kicked object was expected to be. But they also stated that if the object was not where it should have been (post-natal kick) then it might be a pulsar spinning down and powering the region.

The pulsar theory really started to heat up in 2021, when astronomers (Greco et al.) used data collected by two space-based observatories (Chandra and NuSTAR) over a number of different years, which pointed to the likely scenario that a pulsar was present and generating a pulsar wind nebula. But a few months later, these ideas were challenged by another study (Alp et al. 2021) which concluded that there was no evidence for any compact remnant, aside from the ALMA study in 2018 which cited the possibility of a heated dust region, caused by a neutron star.

And so, the back and forth of this exciting science case continues. Is it a neutron star, a pulsar, or something else altogether? Will we be able to detect it (in any regime) in the not-too-distant future, or will we have to wait for hundreds of years until the debris and dust from the supernova clear?

From our human-minded perspective, thirty-five years seems like a very long time – almost half our lives. In astronomical terms, it is just a blink of an eye, and maybe we need more patience and time for SN1987A to reveal whatever secrets it keeps.

In any case, SN1987A has provided astronomers with so much new knowledge in astrophysics – from the birth of neutrino astronomy, to testing observation and detection methods of compact remnant objects. It has taught us about the types of massive stars that can explode in this manner and about the newer, heavier elements that are synthesised over the course of time from such events.

Or maybe we’ll get lucky. Maybe right now, there’s a neutrino burst headed our way – that will this time trigger off the alarms hours in advance. In that case, and with today’s technology, we’ll be well equipped to turn our telescopes towards the direction of origin and wait a few hours.

Suddenly, boom. A new star appears in the sky.

Video Credit: Chandra X-Ray Observatory