Results of a search of ultra-high energy neutrinos with the Pierre Auger Observatory from the powerful blazar TXS0506+056 at a redshift of 0.34 have been recently presented in a Journal article. The implications of the non-observation of neutrinos are compared to results obtained with the IceCube neutrino detector for three different search periods.
On September 22nd 2017, the detection of a 23.7 TeV through-going muon by IceCube stirred a flurry of observational activities that ended up in the identification of TXS0506+056 as a likely source of neutrinos implying hadronic particle acceleration, a notable highlight in multimessenger astronomy. Within a few days, FERMI-LAT detected a flaring activity in gamma rays between 100 MeV and 1 TeV from this source with a position compatible with the muon arrival direction and with a chance coincidence disfavored at the level of 3σ. Follow-up studies in neutrinos and in practically all bands of the electromagnetic spectrum were then made. Data from the MAGIC telescope confirmed the flaring activity also in gamma rays between 80 and 400 GeV days after the IceCube event. Moreover, the subsequent analysis of the archival data of through-going muons at IceCube revealed an excess of neutrinos over a period of about four months between December 2014 and February 2015 from compatible directions. However, during this period, no flaring activity in this direction was reported in gamma-rays.
The search of neutrinos of energies above 100 PeV in the Pierre Auger Observatory data is routinely done by looking for inclined showers with large electromagnetic component corresponding to showers that develop deep in the atmosphere. The total sensitivity of the Observatory for diffuse neutrino fluxes is comparable to that of IceCube, but for a particular point source it is highly dependent on the source declination. Sensitivity plots have been obtained by calculating the constant flux from such a source (assuming an E-2 spectrum) that would result in one event being detected with the Auger Observatory over a period of half a year, which is the excess period that was reported by the IceCube Collaboration. The same analysis was done for the period between January 1, 2004 and August 31, 2018, the Observatory lifetime, to be compared with that obtained by IceCube over its lifetime (Figure 1).
The non-observation of Ultra High Energy photons with the Pierre Auger Observatory from this source in the same half year period analogously leads to a photon sensitivity flux also shown in the figure.
Figure 1: UHE flux reference that would give one expected neutrino event at the Pierre Auger Observatory over a period of half a year (March 22 - September 22, 2017) for a spectrum dN/dE ∝ E−2 in comparison to the flux that would produce on average one detection like IceCube-170922A event over the same period (solid red and black lines). Flux references are also shown for the Pierre Auger Observatory period of ∼15 years during which it has taken data (January 1, 2004 - August 31, 2008) and for a period of 7.5 years for IceCube (dashed red and black lines). The average VHE and UHE photon fluxes measured with Fermi-LAT and MAGIC around September 22, 2017 and the archival photon measurement from Fermi-LAT, as well as the UHE photon flux this direction that would give one expected photon event in half a year at the Pierre Auger Observatory, are also shown for comparison.
We have also compared the sensitivity of the Auger Observatory to the neutrino flux observed by IceCube between October 19, 2014 and February 6, 2015. We calculate the 1σ and 2σ bands of the average flux obtained from the reported fluence assuming an activity period of 110 days as obtained from the IceCube data analysis. The bands are obtained varying the normalization and the spectral index in the allowed parameter space at 68% and 95% confidence levels in the IceCube analysis. The extreme values of the spectral index are γ ∼ 1.75 and γ ∼ 2.45 (∼ 1.5 and ∼ 2.7) for the 68% (95%) CL contour plot.
The results obtained indicate that the Pierre Auger Observatory could only be expected to have detected a signal if the flux extrapolated to the EeV regime follows spectral indices harder than γ ∼ 1.5.
Figure 2: UHE neutrino flux sensitivities for the Pierre Auger Observatory (one event expected) assuming a constant flux during a period of 110 days from October 19, 2014 to February 6, 2015 in comparison to the measured photon flux (Padovani et al. 2018) and to the neutrino flux inferred with IceCube during the same period with a spectral index of γ = 2.1 ± 0.2. The band shown for IceCube is obtained using the extreme values of γ (∼1.75, ∼2.45) from the given one-sigma contour plot and (∼ 1.5 and ∼ 2.7) from the two-sigma contour.
Related paper:
A search for ultra-high energy neutrinos from TXS 0506+056 using the Pierre Auger Observatory
The Pierre Auger Collaboration, The Astrophysical Journal, 902:105, 2020
[arxiv.org/abs/2010.10953] [doi: 10.3847/1538-4357/abb476]
