I show that the high energy neutrino flux predicted to arise from active galactic nuclei cores can explain the PeV neutrinos detected by IceCube without conflicting with the constraints from the observed extragalactic cosmic-ray and gamma-ray backgrounds

Stecker, Floyd W.

NASA Technical Reports Server

PeV neutrinos observed by IceCube from cores of active galactic nuclei
Floyd W. Stecker*
NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
(Received 3 June 2013; published 15 August 2013)
I show that the high energy neutrino ﬂux predicted to arise from active galactic nuclei cores can explain
the PeV neutrinos detected by IceCube without conﬂicting with the constraints from the observed
extragalactic cosmic-ray and -ray backgrounds.
DOI: 10.1103/PhysRevD.88.047301 PACS numbers: 95.85.Ry, 98.54.Cm
Recently, the IceCube Collaboration has reported
the ﬁrst observation of cosmic 2 PeV energy neutrinos
giving a signal 3 above the atmospheric background
[1]. More recently, 18 events 4 above the expected
atmospheric background were reported with energies
above 100 TeV. These neutrinos are likely to be of
cosmic origin; their angular distribution is consistent
with isotropy. The average spectral index for these neu-
trinos was approximately 2 over the energy range
between 100 TeV and 2 PeV [2].
In 1991 we proposed a model suggesting that very high
energy neutrinos could be produced in the cores of active
galaxies (AGN) such as Seyfert galaxies [3]. Using that
model, we gave estimates of the ﬂux and spectrum of high
energy neutrinos to be expected. In light of subsequent
AGN observations and the discovery of neutrino oscilla-
tions, the ﬂux estimates for this model were revised down-
ward [4], although the shape of the predicted neutrino
spectrum remained unchanged.
The new estimate was obtained by lowering the ﬂux
shown in the ﬁgure in Ref. [5] by a factor of 20. This
rescaling gives a value for the  ﬂux at 100 TeV
of E2ðEÞ  108 GeV cm2 s1 sr1 and a ﬂux of
5:6 108 GeV cm2 s1 sr1 at 1 PeV. The peak
ﬂux in these units occurs at an energy 1 PeV.
In our model protons are accelerated by shocks in the
cores of AGN in the vicinity of the black hole accretion
disk (see, e.g., Ref. [6]). Being trapped by the magnetic
ﬁeld, they lose energy dramatically by interactions with
the dense photon ﬁeld of the ‘‘big blue bump’’ of ther-
mal emission from the accretion disk (see, e.g., Ref. [7])
which is optically thick to protons [3]. The primary
interactions are those from photomeson production.
The primary neutrino producing channel, which occurs
near threshold, is
þ p! þ ! nþ þ; (1)
giving the pion an energy roughly  0:2 of that of the
emitting protons [8]. Since the secondary  rays from
þ ! pþ 0 followed by 0 ! 2 lose energy in the
source from electron-positron pair production and the
protons in our model do not reach ultrahigh energies,
losing energy in the source, there is no conﬂict with
ultrahigh energy cosmic-ray (UHECR) or extragalactic
-ray background constraints [9]. Full details of the
model may be found in Refs. [3,4] and references
therein.
The important decays leading to  production are
þ ! þ þ  followed by þ !  þ e þ eþ, with
all leptons carrying off about 1=4 of the pion energy. Thus
in our model we expect that with an assumed power-law
proton spectrum from shock acceleration, followed by
photomeson production, there will be both more and higher
energy e’s produced than e’s, reducing the effect of
Glashow resonance production at 6.3 PeV [10] in the
detector (see also Ref. [9]).
Given the effective area of IceCube for an isotropic ﬂux
of e’s at 1 PeV of 5 m2 and an exposure time of 615.9
days [1], and giving a total predicted  ﬂux at 1 PeV of
6 1014 ðcm2 s srÞ1, we would predict that IceCube
should see a total of 6 neutrino induced events.
The main uncertainty in the magnitude of the modeled
ﬂux is from the uncertainty in the number of AGN
per Mpc3. The neutrino spectrum was predicted to be
proportional to E2:1 between 1 and 10 PeV under the
assumption that the proton spectrum has an E2 depen-
dence from shock acceleration. The average slope of the
neutrino spectrum was predicted to be 3:1 between 1
and 100 PeV. Protons were assumed to be accelerated up
to a maximum energy of 2:5 104 PeV. It is, of course,
possible that the average proton spectrum can be steeper
than E2 and that the maximum energy to which protons
are accelerated is less than 2:5 104 PeV. Since the
sources are extragalactic, we expect that the observed
neutrinos will have a roughly isotropic angular distribu-
tion on the sky.
Given the uncertainties in the model parameters, the
general agreement with the AGN core model is signiﬁcant,
particularly the prediction of a peak neutrino energy ﬂux at
1 PeV. I conclude that, given the present IceCube results,
AGN cores may naturally account for the implied  ﬂux
and angular distribution without violating constraints from
-ray background and UHECR ﬂuxes. More IceCube data
should soon be forthcoming.
I thank Francis Halzen for helpful comments.*Floyd.W.Stecker@nasa.gov
PHYSICAL REVIEW D 88, 047301 (2013)
1550-7998=2013=88(4)=047301(2) 047301-1 Published by the American Physical Society
https://ntrs.nasa.gov/search.jsp?R=20140013086 2019-08-31T16:45:21+00:00Z
[1] M.G. Aartsen et al., Phys. Rev. Lett. 111, 021103 (2013).
[2] F. Halzen, Proceedings of the 33rd International Cosmic
Ray Conference, Rio de Janiero, 2013 (unpublished).
[3] F.W. Stecker, C. Done, M.H. Salamon, and P. Sommers,
Phys. Rev. Lett. 66, 2697 (1991).
[4] F.W. Stecker, Phys. Rev. D 72, 107301 (2005).
[5] F.W. Stecker, Phys. Rev. Lett. 69, 2738 (1992).
[6] D. Kazanas and D. C. Ellison, Astrophys. J. 304, 178
(1986).
[7] A. Laor, Mon. Not. R. Astron. Soc. 246, 369 (1990).
[8] F.W. Stecker, Phys. Rev. Lett. 21, 1016 (1968).
[9] E. Roulet, G. Sigl, A. van Vliet, and S. Mollerach, J.
Cosmol. Astropart. Phys. 01 (2013) 028.
[10] S. L. Glashow, Phys. Rev. 118, 316 (1960).
BRIEF REPORTS PHYSICAL REVIEW D 88, 047301 (2013)
047301-2


PeV Neutrinos Observed by IceCube from Cores of Active Galactic Nuclei

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server