An article published in the journal “Nature Astronomy” describes a research that offers new confirmations that the anomalous gamma-ray detected for the first time in 2009 by NASA’s Fermi gamma-ray space telescope come from millisecond pulsars. The first hypothesis was that they were collisions of dark matter particles but millisecond pulsars that are found in the Milky Way nucleus with emissions mixed up in the signal detected by Fermi seem more and more probable.
An anomalous source at the center of the Milky Way that matched no known gamma-ray source was discovered after creating a gamma-ray sky map with energies higher than 1 GeV. It was simply called gamma-ray excess by astronomers and raised their interest because according to a dark matter model it’s possible that the particles that make it collide generating electromagnetic emissions in the form of gamma rays. However, it was also possible that the gamma-ray excess was generated by stars in the bulge, to be precise by pulsar, remains of dead stars, which spin in periods between 1 and 10 milliseconds and therefore called millisecond pulsars.
Various studies are favoring the millisecond pulsar hypothesis. The most recent was described in an article published in March 2018 again in the journal “Nature Astronomy” which concluded that the gamma-ray excess was due to the presence of millisecond pulsar in the nucleus of the Milky Way, in the area called in jargon bulge. It was a conclusion based on theoretical models, not on direct observations since the abundance of interstellar dust filters many electromagnetic frequencies, so the question remained more than ever open with the need for further independent studies.
Now another team of researchers from the Dutch University of Amsterdam and the French Centre National de la Recherche Scientifique examined data collected over 8 years using the Fermi space telescope’s Large Area Telescope (LAT) instrument. An analysis tool called Sky Factorization with Adaptive Constrained Templates (SkyFACT) was created by the researchers themselves.
The image (Courtesy Christoph Weniger, University of Amsterdam/FermiLAT) shows the map of gamma-ray emissions from the galactic disk. The insets show to the right the profile that researchers expect if the gamma-ray excess is caused by interactions of dark matter particles and to the left the profile they expect if the excess is generated by stars.
Combining physical models with image analysis, SkyFACT clearly indicated that the gamma-ray emissions come from regions of the bulge where there’s also a significant amount of stellar masses. It also showed a consistency in the light-to-mass ratio in the bulge. In very simple words, it indicated a clear correlation between gamma-ray emissions and the presence of stellar masses.
Richard Bartels and Christoph Weniger, two members of this team, already participated in a previous research published in February 2016 in the journal “Physical Review Letters” which supported the millisecond pulsar hypothesis so they’re researchers who accumulated a lot of experience on the subject. Francesca Calore, another member of the team that conducted the research published in “Nature Astronomy”, explained that the results will also be useful to try to observe the hidden pulsars with next-generation radio telescopes capable of capturing frequencies that are not filtered by the interstellar dust.
The MeerKAT radio telescope was recently inaugurated, while for the gigantic SKA radio telescope, the one that’s supposed to revolutionize radio astronomy, will only come into operation in a few years. At the moment, the ALMA radio telescope could also offer the possibility of discovering some pulsars in the Milky Way’s bulge to provide definitive answers on the gamma-ray excess and to obtain further information on millisecond pulsars.