The second fastest known spinning radio pulsar is also a gamma-ray pulsar. Multi-messenger observations look closely at the system and raise new questions.
An international research team of the Max Planck Institute for Gravitational Physics (AEI) in Hannover has found that the radio Pulsar J0952-0607 also transmits pulsed Gamma radiation off. J0952-0607 rotates 707 times in one second and ranks # 2 and the list of fast-spinning neutron stars. By analyzing 8.5-year NASA Fermi Gamma-Ray Space Telescope data, LOFAR radio observations from the past two years, observations from two large optical telescopes, and gravitational-wave data from the LIGO detectors The team used a multi-messenger approach to examine in detail the pulsar's binary system and its lightweight companion. Their study, published in the Astrophysical Journal, shows that extreme pulsar systems are published in the Fermi catalogs and in the Astrophysical Journal today, that extreme pulsar systems are hidden in the Fermi catalogs and motivate further search queries. Although the analysis is very extensive, it also raises new unanswered questions about this system.
Pulsars are the compact remnants of stellar explosions that have strong magnetic fields and spin quickly. They radiate like a cosmic lighthouse and can be observed as radio and / or gamma pulsars depending on their orientation towards the earth.
The Fastest Pulsar Outside Globular Cluster
PSR J0952-0607 (the name means the position) in the sky) was first discovered in 2017 by radio observations from a source identified by the Fermi Gamma-Ray Space Telescope as possibly a pulsar. The Large Area Telescope (LAT) data on board Fermi did not detect any pulsations of the gamma rays. Observations with the radio telescope array LOFAR identified a pulsating radio source and, together with observations with optical telescopes, made it possible to measure some properties of the pulsar. It circles the common center of gravity in 6.2 hours with a companion star that weighs only one fiftieth of our sun. The pulsar rotates 707 times in a second, making it the fastest spin in our galaxy outside the dense star environments of globular clusters.
In Search of Extremely Weak Signals
Using this previous information on the binary pulsar system Lars Nieder, a doctoral candidate at the AEI Hannover, has investigated whether the pulsar also emits pulsed gamma rays. "This search is extremely difficult, as the Fermi gamma-ray telescope has registered only about 200 gamma rays of the weak pulsar during the 8.5 years of observation. During this time, the pulsar itself turned 220 billion times. In other words, only once every one billion revolutions was a gamma ray observed! "Explains Nieder. "For each of these gamma rays, the search must specify exactly when it was transmitted every 1.4 milliseconds."
For this purpose, the data must be combed with very fine resolution in order not to overlook possible signals. The required computing power is enormous. The very sensitive search for faint gamma-ray pulsations would have taken 24 years on a single computer core. It was finished in just two days with the Atlas computer cluster at the AEI Hannover.
A strange first discovery
"Our search found a signal, but something was wrong! The signal was very weak and not exactly where it should be. The reason: Our detection of gamma rays from J0952-0607 had resulted in a positional error in the initial observations with optical telescopes that we used for our analysis. Our discovery of gamma-ray pulsations has uncovered this error, "explains Nieder. "This mistake was corrected in the publication about the discovery of the radio pulsar. A new and expanded gamma ray search resulted in a rather weak but statistically significant discovery of gamma ray pulsars at the corrected position.
After detecting and confirming the presence of pulsed gamma rays from the pulsar, the team went back to the Fermi data and used the full 8.5 years from August 2008 to January 2017, physical parameters of the pulsar and its binary system to determine. Since the gamma radiation of J0952-0607 was so weak, they had to improve their previously developed analytical method to enclose all the unknowns correctly.
Another surprise: no gamma-ray pulsations before July 2011
The derived solution contained another surprise as it was impossible to capture gamma-ray pulsations from the pulsar in the data from before July 2011. The reason why the Pulsar only seems to show pulsations after this date is unknown The amount of gamma rays it emits may be a cause, but the pulsar is so weak that it has not been possible to test this hypothesis with sufficient accuracy . Changes in the pulse train observed in similar systems could also provide an explanation, but there was not even a hint in the data that this happened.
Optical observations raise further questions.
The team also used observations with ] ESO 's New Technology Telescope on La Silla and the Gran Telescopio Canarias on La Palma investigate the companion star of the pulsar. It is most likely tidal to the pulsar, like the moon to the earth, so that one side always points to the pulsar and heats up by its radiation. As the Companion orbits the center of mass of the binary system, its hot "day" side and the cooler "night" side are visible from Earth, and the observed brightness and color vary.
These observations raise another mystery. While the radio observations suggest a distance of about 4,400 light-years to the pulsar, the optical observations imply an approximately three times greater distance. If the system were relatively close to the earth, it would have an unprecedented extremely compact companion with high density, while larger distances would be compatible with the densities of known similar pulsar companions. One explanation for this discrepancy could be the presence of shock waves in the wind of particles of the pulsar, which could lead to a different warming of the companion. Further gamma-ray observations with Fermi-LAT observations should help to answer this question.
Search for continuous gravitational waves
Another research group at the AEI Hannover looked for continuous gravitational wave emissions of the pulsar using LIGO data from the first (O1) and second (O2) observation runs. Pulsars can emit gravitational waves when they have tiny hills or bumps. The search did not detect any gravitational waves, which means that the shape of the pulsar must be very close to a perfect sphere with highest peaks less than a fraction of a millimeter.
Fast rotating neutron stars
It is important to understand fast rotating pulsars because they are probes of extreme physics. How fast neutron stars can rotate before they detach from the centrifugal forces is unknown and depends on unknown nuclear physics. Millisecond pulsars such as J0952-0607 spin so fast because they were spun by their companion by the deposition of matter. It is believed that this process buries the magnetic field of the pulsar. In long-term observations with gamma rays, the research team showed that J0952-0607 has one of the ten lowest magnetic fields ever measured for a pulsar, which is in line with theoretical expectations.
[email protected] looks for test cases of extreme physics
"We will continue to study this system with gamma ray, radio and optical observatories, as there are still questions left. This discovery shows once again that extreme pulsar systems are hidden in the Fermi LAT catalog, "says Prof. Bruce Allen, Nieder's PhD supervisor and director at the AEI Hannover. "With our Citizen Science Distributed Computing Project [email protected] we are also looking for binary gamma-ray pulsar systems in other Fermi-LAT sources and are confident we will make more exciting discoveries in the future."
Neutron stars are compact remnants of Supernova explosions and consist of exotic, extremely dense matter. They have a diameter of about 20 kilometers and weigh more than our sun. Due to their strong magnetic fields and fast rotation, they emit radiant radio waves and energetic gamma rays, similar to a cosmic lighthouse. When these rays point to the earth during the rotation of the neutron star it becomes visible as a pulsating radio or gamma ray source – a so-called pulsar.
Millisecond Pulsar is produced when a pulsar is spun by the accumulation of matter from a companion star. The influx of material from the partner star can accelerate the pulsar to hundreds of revolutions in one second. After completing the accretion, the fast-rotating neutron star can be observed as a millisecond pulsar.