A new source of astrophysical neutrinos

 

A likely new source of astrophysical neutrinos has been identified!  In a paper published in Science, the IceCube Collaboration reported on the observation of neutrinos coming from an object called NGC1068, with 4.1 sigma significance.  The probability that this is a random statistical fluctuation is about 1.1 in 100,000.  This calculated significance accounts for the fact that this was part of a search for neutrinos from 110 likely sources that were studied - this is known as correcting for trial factors - essentially the number of experiments that were tried at once.  A version of the paper is also available here, on the Cornell arXiv preprint server).

NGC1068 (also sometimes known as M77) is an Active Galactic Nucleus (AGN) about 47 million light years from Earth.  AGNs are spiral galaxies with supermassive black holes at their center.  These black holes accrete (pull in) matter from the surrounding galaxy; in the process, some of the matter is ejected at relativistic velocities (speeds a fair fraction of the speed of light) in a collimated jet that follows the axis of rotation of the galaxy.  NGC1068 is one of the closer AGNs, so it is not too surprising that it should be one of the first (after TXS0605+56) high-energy astrophysical neutrino sources to be observed.  The signal was composed of about 80 neutrinos over the background, which was mostly  atmospheric neutrinos. The figure above shows the position of the hotspot (including contours corresponding to 68% and 90% positional uncertainty) compared with that of NGC1068, denoted by the red star.   The agreement is good, and there are no other likely sources within the error contours.

NGC1068 was first seen in an earlier IceCube paper, published in 2020 in Physical Review Letters  (also freely available here, from the Cornell arXiv).   That paper used 10 years of data, and observed NGC1068 with a significance of 2.9 sigma.  The higher significance came from a couple of factors: more new data (and some old partial-detector data was dropped), improved calibrations, and an improved analysis method.  Going from 2.9 sigma to  4.1 sigma is a pretty significant improvement for a re-analysis like this; it seems that the largest single factor was the improved calibrations.

Assuming that this holds up (it is not over the 5 sigma threshold ), this will be the second neutrino source seen by IceCube.  Both are AGNs, so those of us who are impatient can see a pattern developing.  That said, there are differences.  TXS0506 is much further away  - 5.6 billion light years.   It is a known source of gamma-rays, while NGC1068 is not - it's jet is surrounded by a dust cloud that would obscure X-rays and gamma-rays.  And, neutrino emission from TXS0506 was associated with a period exhibiting an increase in gamma-rays, while the NGC1068 source is steady state (at least we have not seen evidence for variability).