Monday, July 14, 2014

Very High Energy Neutrinos from the Galactic Centre

A supermassive black hole (SMBH) with an estimated ~4 million times the Sun’s mass sits in the galactic centre of the Milky Way, in a particular region of space called Sagittarius A*, pronounced “Sagittarius A-Star”. In addition, the galactic centre also contains a high concentration of astrophysical oddballs. Rare elsewhere but not uncommon in the galactic centre, these objects include massive stars, intensely magnetised neutron stars and even intermediate mass black holes (IMBHs). Due to its unique environment, the galactic centre is a source of frequent highly energetic astrophysical events.

Figure 1: Schematic of the IceCube Neutrino Observatory at the South Pole.

In a recent paper by Peterson et al. (2014), the authors investigate whether a subset of very high energy (VHE) neutrinos observed by the IceCube Neutrino Observatory may have originated from Sagittarius A*. Neutrinos are ghostly subatomic particles that pass through normal matter virtually unimpeded. To be detectable, a neutrino has to interact with normal matter. However, the probability of this happening is vanishingly small. As a result, a neutrino detector needs to be enormously large to detect a significant number of neutrinos. The IceCube Neutrino Observatory is the world’s largest neutrino detector. It detects high energy neutrinos using a densely instrumented cubic kilometre of clear ice within the Antarctic ice sheet under the South Pole.

A three-year data set from the IceCube Neutrino Observatory shows the detection of 36 VHE neutrinos with energies in the 30 TeV to 2 PeV range. Of these 36 VHE neutrino detection events, 7 of them were found to occur within a 30° angular region of the galactic centre. These 7 VHE neutrino detection events exhibit both time and space clustering. Of the 7 events, events #14 and #15 took place within one day of each other. The probability that this would occur randomly is only 1.6 percent. Additionally, event #25 occurred only ~3 hours after the brightest X-ray flare of Sagittarius A* was observed by the Chandra X-ray Observatory and the likelihood of it occurring randomly is only 0.9 percent.

These correlated events indicate that Sagittarius A* might be a source of VHE neutrinos. Asteroids, comets, planets and stars that come too close to the SMBH in Sagittarius A* can become disrupted. Such an event would drive an energetic flare, possibly producing some of the VHE neutrinos detected by the IceCube Neutrino Observatory that appear to originate from the galactic centre. Other sources of high energy astrophysical events in the galactic centre region near Sagittarius A* that can generate VHE neutrinos include exotic objects such as SGR J1745-29, a neutron star with an extraordinarily powerful magnetic field.

Figure 2: Properties of the VHE neutrino detection events consistent with a galactic centre origin. Subsequent probability analysis excludes events #12 and #33. “Pos. Err” refers to the angular resolution of the measurement by the IceCube Neutrino Observatory. Peterson et al. (2014).

Figure 3: Time and positions of the 36 VHE neutrino detection events by the IceCube Neutrino Observatory. The 7 events consistent within 30° of the galactic centre, which fall within the inner blue band, show more time clustering than events away from the galactic centre. Peterson et al. (2014).

Peterson et al. (2014), “Neutrino Lighthouse at Sagittarius A*”, arXiv:1407.2243 [astro-ph.HE]