Astronomers using the Dutch Low-Frequency Array radio telescope, including the UvA’s Jason Hessels, have found two fast-spinning pulsars. The researchers did this by looking at unknown sources of gamma radiation in the universe.
The first pulsar, PSR J1552+5437, rotates 412 times per second. The second pulsar, PSR J0952-0607, rotates 707 times per second, which makes it the fastest-spinning pulsar in our galaxy and the second-fastest pulsar ever detected.
Pulsars are rotating neutron stars, the remnants of massive stars that died in supernova explosions. They emit radio waves from their magnetic poles. Since pulsars rotate, we can see their electromagnetic radiation from the earth, coming at us in pulses. They act like highly precise cosmic lighthouses that hurl large volumes of radiation through the universe.
Neutron stars are much smaller than our sun, only about 20km in diameter, but they are also much heavier. For this reason, scientists use them to study the behaviour of materials under extreme circumstances. By studying the fastest-spinning pulsars, astronomers hope to find out more about the internal structure of neutron starts and the extremities of the universe.
Pulsars shine brightest in radio waves with low radio frequencies. Since the Dutch Low-Frequency Array radio telescope (LOFAR) receives low-frequency radio waves, it is an ideal telescope with which to study pulsars. ‘Unfortunately, using LOFAR to locate pulsars is difficult, since the gases and dust between the stars interfere with low-frequency radio waves,’ said Cees Bassa of ASTRON, the Netherlands Institute for Radio Astronomy. For this reason, astronomers tend to look for pulsars at higher radio frequencies.
Bassa, Hessels and others have found a way around this problem. ‘We have developed a new technique that allows us to analyse the data garnered by LOFAR using graphics cards (which were originally designed for computer games) at the large DRAGNET computer cluster in Groningen.’ This computer cluster was funded by means of an ERC starting grant awarded to the University of Amsterdam’s Jason Hessels.
Ziggy Pleunis, a collaborator of Bassa and Hessels’, was the first person to test this new technique with LOFAR in 2016. He hit the jackpot when he detected PSR J1552+5437, a pulsar with a rotational period of 2.43 milliseconds, which means it rotates 412 times per second. This was the first millisecond pulsar detected using LOFAR.
‘Since millisecond pulsars emit both high-energy gamma rays and radio waves, we specifically looked at unknown sources in the universe that emit gamma radiation,’ stated Pleunis, who is now a PhD candidate at McGill University in Montreal, Canada. He demonstrated that the gamma rays emitted by the millisecond pulsar arrived at the same moment as the radio pulses, which presumably means they are generated in the same way.
Encouraged by the success of the test study, Bassa, Hessels and Pleunis continued to look for millisecond pulsars using LOFAR. They soon found another pulsar, which rotated even faster. This pulsar, called PSR J0952-0607, rotates around its axis 707 times per second. This makes it the fastest-spinning pulsar we know in our galaxy, second only to a pulsar in a densely populated star cluster outside the Milky Way that rotates 716 times per second.
According to Bassa, ‘PSR J0952-0607 is considerably closer to us than the pulsar in the star cluster, so we are able to study it in much greater detail.’ Using the Isaac Newton Telescope at La Palma, the astronomers have been able to locate a faint double star that rotates around the pulsar. Using this double star, they have already been able to calculate PSR J0952-0607’s distance from Earth. Future observation of the double star will help the astronomers determine the mass and composition of the fast-spinning pulsar.
Both pulsars (J1552+5437 and J0952-0607) are unexpectedly bright at low radio frequencies and become considerably less bright at higher radio frequencies. This means that they probably could not have been detected at a higher radio frequency, which is where most radio telescopes used to look for pulsars. This implies that there might be a hitherto unknown population of fast-spinning millisecond pulsars in our galaxy, just waiting to be discovered.
Jason Hessel pointed out that ‘We are increasingly finding evidence that the fastest-spinning pulsars shine brightest at low radio frequencies, and that there may be a link with the production of high-energy gamma rays.’ If this is indeed the case, LOFAR will probably be able to locate more fast-spinning millisecond pulsars, possibly some that spin even faster than the two already found. The rotational speeds of these pulsars will provide astronomers with a deeper insight into the internal structure of neutron stars.
Two publications about Pleunis’ en Bassa’s discovered pulsars appeared in the Astrophysical Journal Letters on 5 September 2017.