Cumulative neutrino background from quasar-driven outflows

Abstract

Quasar-driven outflows naturally account for the missing component of the extragalactic γ-ray background through neutral pion production in interactions between protons accelerated by the forward outflow shock and interstellar protons. We study the simultaneous neutrino emission by the same protons. We adopt outflow parameters that best fit the extragalactic γ-ray background data and derive a cumulative neutrino background of ∼ 10 −7 GeV cm −2 s −1 sr −1 at neutrino energies Eν 10 TeV, which naturally explains the most recent IceCube data without tuning any free parameters. The link between the γ-ray and neutrino emission from quasar outflows can be used to constrain the high-energy physics of strong shocks at cosmological distances.Introduction.-There is currently strong observational evidence for the existence of large-scale outflows driven by the active galactic nuclei (AGN), including the presence of broad absorption lines in quasars [3,4] and multiphase outflows in nearby ultraluminous infrared galaxies (ULIRGs) [1,2]. Semi-relativistic winds with a speed of ∼ 0.1 c are typically produced by quasars in the surrounding interstellar medium, driving a forward shock that accelerates a swept-up shell accompanied by a reverse shock that decelerates the wind itself [5,6].In a previous paper, we derived a detailed hydrodynamical model for the quasar outflow's interaction with the ambient medium [7] (see Supplemental Material), including a disk and a halo components for the host galaxy gas. The gas density profile was self-consistently determined by the halo mass and redshift. The continuous energy injection was assumed to be a fraction of the quasar's bolometric luminosity f kin L bol during the quasar's lifetime, which is of order the Salpeter time t sal ∼ 4 × 10 7 yrs for a radiative efficiency of 0.1 [8]. In the upper panel of Fig.2, we show the forward shock velocity v s as a function of radius R s for the outflow in a dark matter halo of mass ∼ 10 12 M ⊙ at a redshift of z ∼ 0.1. We find that v s 10 3 km s −1 within the galactic disk with a decline to few hundreds km s −1 when the outflow reaches the edge of the halo. In analogy with supernova (SN) remnants [9,10], protons should be accelerated via Fermi acceleration to relativistic energies in the forward outflow shock.The resulting proton number density per unit volume per unit energy can be expressed as a power-law with an exponential high-energy cutoff:

Ultrahigh energy cosmic rays from nonrelativistic quasar outflows

“…With ǫ nt ∼ 10%, f kin ∼ 1 − 5% and Γ p ∼ 2.3 − 2.4, constrained to fit the Fermi-LAT and IceCube data, we naturally explain the UHECR flux without additional parameter tuning, as shown in Figure 3. This is consistent with parameter values inferred from observations of outflows [9] as well as the branching ratio between secondary γ-rays and neutrinos, which sets an upper limit on the power-law index of the injection spectrum to be 2.2 − 2.4 [14,44]. Indeed, recent γ-ray observations suggest the existence of hadronic emission from an outflow in a nearby galaxy [43].…”

“…Secondary γ-ray photons and neutrinos are produced via pion decay from interaction between accelerated protons and ambient protons in the ISM. We adopt ǫ nt ∼ 0.1 similarly to the conditions in supernova remnants (SNRs) [17,18] and f kin ∼ 1 − 5% from fitting the resulting γ-rays and neutrinos to the EGB [13] and CNB [14], consistently with observations [39] and theoretical models [17] of supernova shocks. The maximum energy of the accelerated protons, E max , can be extrapolated from E max ≈ E sh ω c t dyn /3κ for shocks with an Alfvén Mach number M 100 [18], where…”

“…The resulting γ-ray photons can naturally account for the missing component of the EGB at E g 10 GeV as suggested by the most recent Fermi-LAT observation [13,39], while the associated neutrinos explain the CNB as observed by IceCube [14,40]. With ǫ nt ∼ 10%, f kin ∼ 1 − 5% and Γ p ∼ 2.3 − 2.4, constrained to fit the Fermi-LAT and IceCube data, we naturally explain the UHECR flux without additional parameter tuning, as shown in Figure 3.…”

“…[12] for details). Protons accelerated by the outflow shock to relativistic energies interact with the interstellar protons and produce secondary γ-ray photons and neutrinos via pion production that naturally account for the missing component of the extragalactic γ-ray background (EGB) [13], as well as the cumulative neutrino background (CNB) [14].…”

“…Using values of ǫ nt , f kin and Γ p constrained by fitting γ-rays to the EGB [13] (left section) and neutrinos to the CNB [14] (middle section), we derive the UHECR spectrum (right section) with Γ p ∼ 2.3 − 2.4 at E CR 10 18 eV without additional parameter tuning. For ǫ nt ∼ 10%, the best fit f kin ∼ 1 − 5% [13] is consistent with theoretical models…”