As the first outcome of a targeted survey, South African researchers using MeerKAT have found nine millisecond pulsars, the majority of them in uncommon and occasionally unusual binary systems. 79 unidentified pulsar-like sources were chosen by an international team from observations made by NASA’s Fermi Gamma-ray Space Telescope and MeerKAT, with significant contributions from AEI (Hannover) and MPIfR (Bonn).
Fermi LAT image with bright, diffuse glow from the central plane of the Milky Way.
Many of the bright sources along the plane are pulsars. Credit: NASA/DOE/Fermi LAT Collaboration
In comparison to earlier surveys, using this tried-and-true technique with a next-generation telescope array had many advantages. Nine rapidly rotating neutron stars were found by the team, the majority of which had uncommon characteristics.
“Our TRAPUM survey used MeerKAT, a relatively new and superbly sensitive radio telescope, together with dedicated analysis software to observe a selection of very promising pulsar-like sources,”
said Colin Clark, lead author and group leader at the Max Planck Institute for Gravitational Physics in Hannover.
Fermi Gamma-ray Space Telescope Catalog
To find new millisecond pulsars, the team followed a tried-and-true procedure. Eight years of observations with NASA’s Fermi Gamma-ray Space Telescope have been compiled into the Fermi Large Area Telescope catalog of gamma-ray sources.
The positions of the sources in the sky, the energies of their gamma rays, and changes in their gamma-ray brightness over time are all described in this catalog. To determine the pulsar-likeliness of all Fermi catalog sources that were not connected to known celestial objects, the team used machine learning algorithms.
“After we had identified the most pulsar-like sources in the Fermi catalog, we whittled down our target list to those sources which would most likely be detectable by our survey. We observed 79 sources with MeerKAT,”
explained Clark.
MeerKAT Antenna Array
Artist impression of MeerKAT and ASKAP dishes at night. Credit: SKA Organisation CC-BY
MeerKAT is a 64-dish antenna array with an effective diameter of 13.5 meters in South Africa’s Karoo. MeerKAT has unprecedented sensitivity to sources in the southern celestial hemisphere, detecting sources that are approximately five times fainter than those found with the next most powerful southern hemisphere telescope.
This sensitivity is used by the TRansients and Pulsars Using MeerKAT (TRAPUM) Large Survey Project to look for new pulsars in the areas of the sky where they are most likely to be found: unidentified gamma-ray sources, globular clusters, nearby galaxies, and supernova remnants. This necessitated the development of dedicated computing hardware capable of combining data from the MeerKAT antennas into a single virtual large radio telescope capable of simultaneously observing nearly 500 closely spaced sky positions.
The extra sensitivity provided by MeerKAT allowed this TRAPUM survey of Fermi sources to reduce observation times to just 10 minutes, much shorter than the hour-long observations previously required to find pulsars in these sources.
TRAPUM Pulsar Survey
There are numerous advantages to making brief observations: In a limited amount of time, more sources can be targeted. Sources can be observed repeatedly, increasing the likelihood of discovering a new radio pulsar that was not detectable during the first survey pass.
Each source was observed twice by the TRAPUM pulsar survey. Analyzing short observations requires less computational power than analyzing longer observations.
Finally, orbital motion in binary systems can make radio pulsar detection more difficult. Because the pulsar’s motion is almost constant during the short observing times, the negative effect of changing orbital motion is mitigated.
Aside from sheer sensitivity, the MeerKAT array has one additional advantage over other single-dish telescopes. Its 8-kilometer footprint enables it to precisely pinpoint the location of new sources, allowing for rapid follow-up studies at other wavelengths.
Millisecond Pulsar Candidates Confirmed
eROSITA all-sky survey image of 4FGL J1803.1−6708 indicated in blue, and the GAIA position overlaid on the eROSITA counterpart of PSR J1803−6707 marked in red. The image has been spatially binned to a pixel size of 10 arcsec and smoothed with a 15 arcsec kernel.
Credit: Monthly Notices of the Royal Astronomical Society CC-BY
Searching for pulsars in large amounts of data obtained during TRAPUM observations necessitates a large amount of computing power as well as a quick turnaround to free up storage space for future observations.
“We ran purpose-built data analysis pipelines on 120 graphics processing units (GPUs) in a dedicated computing cluster to sift through our MeerKAT survey observations,”
said Ewan Barr, TRAPUM Project Scientist and group leader at the Max Planck Institute for Radio Astronomy, agreed. The team quickly identified nine millisecond pulsar candidates, which were all confirmed by additional MeerKAT observations.
“It is great that we could also use the confirmation observations to refine the sky positions with MeerKAT’s capability to sample the sky in a dense grid. This is invaluable for follow-up studies at different wavelengths,”
he added.
PSR J1526−2744
PSR J15262744, one of the discoveries, was later closely studied. Following the discovery of this radio pulsar in a binary system, the researchers detected gamma-ray pulsations from the neutron star.
Using all available Fermi data, they were able to precisely study the orbital motion and determine the properties of the binary system. In less than five hours, the neutron star will most likely orbit the common center of mass with a light-weight white dwarf. This would make it the binary system of a pulsar and a white dwarf with the second shortest orbital period.
In addition, the team looked for continuous gravitational waves from PSR J15262744. When a neutron star is deformed, it emits gravitational waves at twice its rotational frequency.
The scientists used all of the Advanced LIGO data from the O1, O2, and O3 runs that were accessible to the general public. The research team attained the highest level of gravitational wave search sensitivity because they had a precise understanding of the pulsar’s motion in the binary system from the gamma-ray observations.
Perfect Axisymmetry
The team was unable to detect continuous gravitational waves from PSR J1526–2744, but they were still able to gauge the neutron star’s deviation from perfect axisymmetry.
“We now know that PSR J1526−2744 is very symmetric indeed. We showed that the neutron star’s equator cannot deviate from a perfect circle by much more than the width of a human hair,”
said Anjana Ashok, a Ph.D. student in the permanent independent Max Planck Research Group “Continuous Gravitational Waves” at the AEI Hannover who led the gravitational-wave search.
Redback Pulsar Systems
Orbital properties of known binary MSPs.
Credit: Monthly Notices of the Royal Astronomical Society CC-BY
Another two of the new pulsars, PSR J1036-4353 and PSR J18036707, are typical “redback” pulsar systems made up of neutron stars and companion stars weighing at least a quarter the mass of our Sun. These pulsars evaporate and destroy their companions over time, evoking the spidery namesake, Australian redback spiders, whose females consume the males after mating.
After using MeerKAT’s unique capabilities to rapidly and precisely pinpoint the pulsar positions, the astronomers identified their companions in the Gaia astrometry mission’s star catalog and studied them with dedicated optical observations using ESO’s New Technology Telescope’s ULTRACAM camera.
They also discovered X-rays from PSR J1803-6707 in data from the first eROSITA all-sky survey. The X-rays are typical of redback systems and are most likely caused by the energetic pulsar wind slamming into material evaporated from the companion.
More Potential Pulsar Targets
It is difficult to estimate the number of pulsars that have yet to be discovered in unrelated pulsar-like Fermi sources. Despite this, the astronomers are confident that future observations will reveal several more millisecond pulsars.
Several candidates on the target list are almost certainly pulsars. Radio-wave or gamma-ray pulsations, however, have not been discovered in any surveys to date.
One day, new telescopes, analysis techniques, and repeated observational attempts may reveal their pulsar nature. The underlying source catalog will expand as Fermi observation time increases, and more pulsar-like sources will emerge and become potential targets.
“Our results, which are only the first from TRAPUM’s survey of Fermi sources, already show the great potential of MeerKAT. With MeerKAT and dedicated software, we’re not only able discover, but also to rapidly and precisely localize new millisecond pulsars,”
said Clark.
MeerKAT observations are extremely beneficial for multi-wavelength follow-ups, catalog searches, and future observations, or in other words, for making millisecond pulsars gifts that keep on giving.
Reference:
C J Clark, R P Breton, E D Barr, M Burgay, T Thongmeearkom, L Nieder, S Buchner, B Stappers, M Kramer, W Becker, M Mayer, A Phosrisom, A Ashok, M C Bezuidenhout, F Calore, I Cognard, P C C Freire, M Geyer, J-M Grießmeier, R Karuppusamy, L Levin, P V Padmanabh, A Possenti, S Ransom, M Serylak, V Venkatraman Krishnan, L Vleeschower, J Behrend, D J Champion, W Chen, D Horn, E F Keane, L Künkel, Y Men, A Ridolfi, V S Dhillon, T R Marsh, M A Papa, The TRAPUM L-band survey for pulsars in Fermi-LAT gamma-ray sources, Monthly Notices of the Royal Astronomical Society, Volume 519, Issue 4, March 2023, Pages 5590–5606
