NASA | Goddard Space Flight Center | Fermi | 2016 Aug 12
[img3=""]https://svs.gsfc.nasa.gov/vis/a010000/a ... s_only.gif[/img3][img3="Top: Gamma rays (magenta lines) coming from a bright source like NGC 1275 in the Perseus galaxy cluster should form a particular type of spectrum (right). Bottom: Gamma rays convert into hypothetical axion-like particles (green dashes) and back again when they encounter magnetic fields (gray curves). The resulting gamma-ray spectrum ((lower curve at right) would show unusual steps and gaps not seen in Fermi data, which means a range of these particles cannot make up a portion of dark matter.Dark matter, the mysterious substance that constitutes most of the material universe, remains as elusive as ever. Although experiments on the ground and in space have yet to find a trace of dark matter, the results are helping scientists rule out some of the many theoretical possibilities. Three studies published earlier this year, using six or more years of data from NASA's Fermi Gamma-ray Space Telescope, have broadened the mission's dark matter hunt using some novel approaches. ...
Credits: SLAC National Accelerator Laboratory/Chris Smith"]https://svs.gsfc.nasa.gov/vis/a010000/a ... ipoles.gif[/img3][hr][/hr][c][imghover=http://asterisk.apod.com/download/file.php?id=25057&t=1]http://asterisk.apod.com/download/file.php?id=25056&t=1[/imghover]The Small Magellanic Cloud (SMC), at center, is the second-largest satellite galaxy
orbiting our own. This image superimposes a photograph of the SMC with one half of
a model of its dark matter (right of center). Lighter colors indicate greater density
and show a strong concentration toward the galaxy's center. Ninety-five percent of the
dark matter is contained within a circle tracing the outer edge of the model shown. In
six years of data, Fermi finds no indication of gamma rays from the SMC's dark matter.
Credits: Dark matter, R. Caputo et al. 2016; SMC, Axel Mellinger, CMU[/c][hr][/hr][c][attachment=0]Fermi_blazars_3mo_7yr_LQ[1].gif[/attachment][/c][hr][/hr]
Among the new studies, the most exotic scenario investigated was the possibility that dark matter might consist of hypothetical particles called axions or other particles with similar properties. An intriguing aspect of axion-like particles is their ability to convert into gamma rays and back again when they interact with strong magnetic fields. These conversions would leave behind characteristic traces, like gaps or steps, in the spectrum of a bright gamma-ray source. ...
Another broad class of dark matter candidates are called Weakly Interacting Massive Particles (WIMPs). In some versions, colliding WIMPs either mutually annihilate or produce an intermediate, quickly decaying particle. Both scenarios result in gamma rays that can be detected by the LAT.
Regina Caputo at the University of California, Santa Cruz, sought these signals from the Small Magellanic Cloud (SMC), which is located about 200,000 light-years away and is the second-largest of the small satellite galaxies orbiting the Milky Way. Part of the SMC's appeal for a dark matter search is that it lies comparatively close to us and its gamma-ray emission from conventional sources, like star formation and pulsars, is well understood. Most importantly, astronomers have high-precision measurements of the SMC's rotation curve, which shows how its rotational speed changes with distance from its center and indicates how much dark matter is present. ...
In the third study, researchers led by Marco Ajello at Clemson University in South Carolina and Mattia Di Mauro at SLAC National Accelerator Laboratory in California took the search in a different direction. Instead of looking at specific astronomical targets, the team used more than 6.5 years of LAT data to analyze the background glow of gamma rays seen all over the sky.
The nature of this light, called the extragalactic gamma-ray background (EGB) has been debated since it was first measured by NASA's Small Astronomy Satellite 2 in the early 1970s. Fermi has shown that much of this light arises from unresolved gamma-ray sources, particularly galaxies called blazars, which are powered by material falling toward gigantic black holes. Blazars constitute more than half of the total gamma-ray sources seen by Fermi, and they make up an even greater share in a new LAT catalog of the highest-energy gamma rays. ...
Search for Spectral Irregularities due to Photon–Axionlike-Particle
Oscillations with the Fermi Large Area Telescope - Fermi-LAT Collaboration
- Physical Review Letters 116(16):1101 (22 Apr 2016) DOI: 10.1103/PhysRevLett.116.161101
arXiv.org > astro-ph > arXiv:1603.06978 > 22 Mar 2016 (v1), 21 Apr 2016 (v2)
the Small Magellanic Cloud with the Fermi Large Area Telescope - Regina Caputo et al
- Physical Review D 93(6):2004 (15 Mar 2016) DOI: 10.1103/PhysRevD.93.062004
arXiv.org > astro-ph > arXiv:1603.06978 > 03 Mar 2016 (v1), 29 Mar 2016 (v2)
- Physical Review Letters 116(15):1105 (15 Apr 2016) DOI: 10.1103/PhysRevLett.116.151105
arXiv.org > astro-ph > arXiv:1511.00693 > 02 Nov 2015