Physicists around the globe are working relentlessly to pin down the nature of dark matter. This enigmatic entity hides itself from our view as it does neither emit nor absorb any radiation. It only reveals itself through its gravitational interaction. With a new analysis of data from NASA’s gamma-ray large area telescope (LAT) on board the Fermi satellite, we have now come closer to test very light dark matter candidates.
Many regard the astrophysical evidence for dark matter as evidence for yet undiscovered fundamental particles. Well-motivated theories suggest that these particles should be quite heavy, probably a couple of dozen times as massive as the proton. If so, either their decay or the annihilation of two such particles could result in energetic light: gamma-ray emission that we could detect with the LAT. Also the LHC and direct detection experiments such as the recently inaugurated Xenon 1T are looking for traces of heavy dark-matter candidates.
However, dark-matter particles could also be extremely light, more than five hundred billion times lighter than the electron. They are often referred to as axion-like particles (ALPs). They are close relatives to the axion which was originally proposed in the late 70s to solve problem in the strong interactions. It was soon realized that this class of particles could also constitute the dark matter. Unfortunately, very light ALPs have lifetimes much longer than the age of the Universe and do not obliterate each other upon meeting. However, there’s a small chance that ALPs transform into ordinary light (and vice versa) in the presence of a magnetic field. This leads to oscillations between light and ALPs very much like neutrino oscillations.
We have used Fermi-LAT observations of an extremely bright gamma-ray source to search for traces of these oscillations: the central galaxy of the Perseus galaxy cluster. In its center, a spinning super-massive black hole powers a current stream of plasma that emits gamma-ray radiation. When the gamma rays propagate through the magnetic field present in the gas between the galaxies of the cluster, they could transform into ALPs and imprint distortions onto the galaxy’s spectrum.
We did not find any such features above the level of statistical fluctuations which enabled us to rule out ALP models that would contribute sub-dominantly to the overall dark-matter content. These are best constraints in the particular ALP mass region to date. Our limits turned out to be as sensitive as as the future “Any Light Particle Searches” (ALPS) experiment. In the ALPS experiment, it is planned to shoot a powerful laser beam immersed in a magnetic field onto a wall and look for light that might have made it through it in the form of ALPs.
Our analysis only marks the beginning for ALP searches with the Fermi-LAT. With the analysis of more sources we will be sensitive enough to probe ALPs that could make up all the dark matter in the Universe.
Caption for the picture in the cover: Composite image of the central galaxy in the Perseus cluster taken with the Hubble space telescope and the Chandra X-ray satellite.
Credits: X-ray: NASA/CXC/IoA/A.Fabian et al.; Radio: NRAO/VLA/G. Taylor; Optical: NASA/ESA/Hubble Heritage (STScI/AURA) & Univ. of Cambridge/IoA/A. Fabian
On 27 April, an incredible opportunity was given to GRB science detectives. As the spring was outbursting here in Stockholm the explosion of a distant star almost blinded the Gamma ray Burst Monitor (GBM) detectors on board the Fermi satellite. GRB130427 is the brightest GRB ever detected in the keV – MeV band and the longest lasting in the GeV energy range: Fermi Large Area Telescope (LAT) could detect it for hours after the trigger.
Gamma ray bursts (GRBs) are cosmological flashes of which the prompt emission, lasting for 0.01-100s, is in the gamma ray band. Their late emission can be detected at lower energy ranges like optical and radio. One or two GRBs per day are typically observed, but their origin and the particle acceleration mechanisms involved remain nowadays unknown. The favourite hypothesis on their origin is the collapse of a supermassive star, while there is not a leading hypothesis for the acceleration mechanisms involved in the outflow responsible for the prompt emission.
This burst was also detected by other experiments such as Swift and Integral which allowed a rapid and precise localization which enabled optical, infrared and radio follow-up observations. The redshift was measured within hours from the original trigger and revealed that the outbursting star was quite close (for this kind of objects): z= 0.34.
With nearly 1000 photons per second and square centimeter in the 10-1000 keV band and 14 photons per second per square meter in the 100 MeV – 10 GeV band (see attached figure), this burst is a unique occasion for the scientific community to probe models for particle acceleration and photon emission in the outflow.
Soon the Fermi and Swift collaborations will publish their papers and hopefully more news and more papers will follow. We expect to be able to take a step further in understanding the physics of the GRB thanks to this record breaker burst. Keep an eye on it!
The figure (source: arXiv:1303.2908) shows the fluence in two energy band of the Fermi LAT detected burst, the star indicate the position this burst would have in this plot.
The event fluence in the first 20 seconds in the 10-1000 keV band is (1.975 +/- 0.003) E-03 erg/cm^2, while in the fluence in the first 140s in the 100 MeV – 10 GeV band is (1.1 +/- 0.1)E-4 erg/cm^2.
Particles with TeV energies, like those produced at the LHC, seem exotic. But once outside the protection of our atmosphere, these “cosmic rays” (CR) become exceedingly common. The Fermi Telescope, for example, encounters a hundred thousand CR for every gamma ray it detects. These particles have an impressive scope of local effects, from damaging electronics and inhibiting manned space travel to possibly triggering lightning strikes. And although we have been aware of their existence since the early 1900ʼs, their exact origins remained unclear. Now a study by
the Fermi Collaboration claims to have solved the mystery.
The list of mechanisms capable of accelerating so many particles to high
energies was already a short one. Strong electromagnetic fields, like those surrounding pulsars, could do the job. So could the process for which the gamma-ray telescope got its name – Fermi acceleration. Less intuitive, it amounts to trapping particles in a region where they are repeatedly reflected by inhomogeneous magnetic fields. Each reflection supplies them with more energy, and the series of small enhancements adds up. One place there such conditions exist is at the boundary between slow and fast moving groups of particles, sometimes known as a “shock.” Supernova remnants (SNR), the
aftermath of massive astrophysical explosions, are expected to shock where the expanding sphere of stellar material encounters the surrounding gas. Continue reading Fermi observations proves Supernova Remnants produce Cosmic Rays→