The VHE neutrino (and gamma-ray) flux from the galactic plane

The VHE neutrino (and gamma-ray) flux from the galactic plane F.L. Villante Universita’ dell’Aquila and INFN-LNGS Mostly based on work done in collaboration with: G. Pagliaroli, V. Vecchiotti, L. Espinosa Castro , C. Evoli The interaction of HE cosmic rays (CRs) with the gas contained in the galactic disk is a guaranteed source of HE neutrinos (and gammas) → Diffuse emission HE neutrinos (and gammas) can be also produced by freshly accelerated particles within or close to acceleration site → Source component The VHE neutrino (and gamma-ray) flux from the galactic plane Neutrino telescopes observe the sum of the two componets: - Diff+USE (unresolved source emission) emission model The interaction of HE cosmic rays (CRs) with the gas contained in the galactic disk is a guaranteed source of HE neutrinos (and gammas) → Diffuse emission HE neutrinos (and gammas) can be also produced by freshly accelerated particles within or close to acceleration site → Source component Hadronic interactions imply a strict relation between neutrinos and gammas However: - bright sources are resolved by gamma-ray detectors while they cannot be resolved by neutrino telescopes - gamma-rays can be absorbed either in source or in their path to Earth; - gamma-rays can be also produced by leptonic interactions (both in sources and in interstellar medium) The VHE neutrino (and gamma-ray) flux from the galactic plane Neutrino telescopes observe the sum of the two componets: - Diff+USE (unresolved source emission) emission model The HE galactic diffuse gamma (and neutrino) fluxes The diffuse HE neutrinos and gammas from the Galactic plane can be calculated as: where: N.B. At E > 20 TeV, gamma-ray absorption should be also included (see back-up slides). nucleon-nucleon cross section [Kelner & Aharonian, PRD 2008, 2010] Gas density – same as Galprop [http://galprop.stanford.edu] Differential CR flux - See next slides (e: : ) ≃ (1:1:1) because of -flavour oscill. The CR flux in the Galaxy The local determination has to be related to the CR flux in all the regions of the Galaxy where the gas density is not negligible. where: - CR flux at the Sun position The CR flux in the Galaxy The local determination has to be related to the CR flux in all the regions of the Galaxy where the gas density is not negligible. where: Solution of 3D (isotropic) diffusion equation - It takes into account the effect of sources distribution 𝑓 𝑆𝑟 ; -𝑅 = Diffusion radius; - Normalized to 1 at the Sun position - CR flux at the Sun position Standard Case e.g. Galprop The CR flux in the Galaxy The local determination has to be related to the CR flux in all the regions of the Galaxy where the gas density is not negligible. where: Solution of 3D (isotropic) diffusion equation - It takes into account the effect of sources distribution 𝑓 𝑆𝑟 ; -𝑅 = Diffusion radius; - Normalized to 1 at the Sun position - It introduces a position-dependent variation Δ 𝑟 of the CR spectral index (Gaggero et al. 2015, Acero et al. 2016, Yang et al, 2016, Pothast et al. 2018); -Δ0= 0.3 represents the difference between CR spectral index at the Sun position (𝛼⊙≃ 2.7 at 𝐸 = 20 𝐺𝑒𝑉) and its value close to the galactic center - CR flux at the Sun position Standard Case e.g. Galprop Hardening Case e.g. KRA, Dragon (+ Hermes), etc. The diffuse  and  fluxes in different scenarios Gamma-ray flux Neutrino flux Standard (Factorized) Hardening (Non factorized) R = ∞ R = 1 kpc (b=0) (b=0) No hardening (standard scenario): (Angle-integrated  -ray flux a 1 TeV) Hardening (non factorized): •The angle integrated flux increase by a factor ~1.2 •More significant increase in the central region (factor ~2 in the direction of the Galactic center) 𝜑𝐼𝐶𝐸𝐶𝑈𝐵𝐸 •The angle integrated diffuse neutrino flux is globally ~ few % of the isotropic flux observed by IceCube •it provides a dominant contribution in the central region (−60° ≤ 𝑙 ≤ 60°) •Small but not negligible. Potentially observable (with HESE in IceCube) Pagliaroli et al, JCAP 2016 Pagliaroli et al, JCAP 2018 The gamma (neutrino) flux at E=1 TeV (E=100 TeV) is determined by CR flux at: The source component The source component includes the contribution of all the Galactic hadronic sources that can be either resolved or unresolved by gamma-ray detectors Cataldo et al. Astrophys.J. 904 (2020) TeV gamma-ray source population study based on the H.E.S.S. Galactic Plane Survey (HGPS) The luminosity function of the TeV gamma-ray source population is inferred by fitting the flux, longitude and latitude distribution of brightest sources in the HGPS catalog. Φ𝛾,𝑆 →cumulative gamma-ray flux produced by the entire population in the 1-100 TeV energy range (comparable to or larger than CR diffuse emission) Sources catalogs 31 sources All sources are resolved Cumulative distribution of HGPS sources Comparing theoretical predictions with IceCube 4.5 detection Standard Case IceCube results are compatible with gamma-rays Non-negligible source component allowed in the Standard scenario. -𝜉 < 0.40 𝐸𝑐𝑢𝑡 =500 𝑇𝑒𝑉 ; -𝜉 ∼ 0.20, ifwe require that Galactic sources accelerate particles up to the CR «knee». Vecchiotti et al., Astrophys.J.Lett. 956 (2023) 2, L44 Comparing theoretical predictions with IceCube 4.5 detection Standard Case Hardening Case IceCube results are compatible with gamma-rays Non-negligible source component allowed in the Standard scenario. -𝜉 < 0.40 𝐸𝑐𝑢𝑡 =500 𝑇𝑒𝑉 ; -𝜉 ∼ 0.20, ifwe require that Galactic sources accelerate particles up to the CR «knee». No space for a relevant source component in the Hardening case. Potentially problematic, because: -we expect PeVatrons in our Galaxy; - the Hardening case needs hadronic sources Truly diffuse emission only (no source comp.) Vecchiotti et al., Astrophys.J.Lett. 956 (2023) 2, L44 Latest improvements in gammas The LHAASO detector has probed both source and diffuse (after masking resolved sources) gamma-ray emission in the TeVPeV energy range. Vecchiotti et al, JCAP 2025: - Our source population (based on H.E.S.S. observations, at few TeVs) also accounts for LHAASO-KM2A observations at 50TeV; - The LHAASO data appear compatible with our baseline model in the outer Galactic region. In the inner region, the data show an excess with respect to the predictions below∼50 TeV, while at higher energies they are well described by our model. - Two plausible explanations for enhanced gamma-ray emission — unresolved sources and CR spectral hardening in the inner Galaxy — are likely suppressed by the LHAASO masking strategy, which excludes regions where both effects are expected to be most prominent. LHAASO detector has probed the local CR proton spectrum at PeV energies and above (most relevant for neutrino emission) Latest improvements in protons The neutrino flux at 𝐸𝜈= 100 𝑇𝑒𝑉 is determined by CR (nucleon) flux at 𝐸𝑛≃20 𝐸𝜈= 2 𝑃𝑒𝑉 (L. Espinosa Castro et al., MNRAS Lett. 2025) If we increase heavy element contribution at expenses of hydrogen, we obtain a smaller CR flux (since the flux decrease faster than E-2) Note that: Diffuse gamma and neutrino fluxes are determined by the total nucleon flux (that may depend on the assumed CR composition) LHAASO detector has probed the local CR proton spectrum at PeV energies and above (most relevant for neutrino emission) LHAASO protons versus LHAASO diffuse gamma-rays: A consistency check (L. Espinosa Castro et al. 2025) - Testing CR distribution in different galactic regions; - Discrepancy in both normalization and spectral shape (softer toward galactic center); - Possibly challenging conventional scenarios linking the local cosmic-ray sea to Galactic gamma-ray emission Latest improvements in protons The neutrino flux at 𝐸𝜈= 100 𝑇𝑒𝑉 is determined by CR (nucleon) flux at 𝐸𝑛≃20 𝐸𝜈= 2 𝑃𝑒𝑉 (L. Espinosa Castro et al., MNRAS Lett. 2025) Flux Ratios = Data/Predictions 15 < l < 125 125 < l < 250 The pictures that emerge from ANTARES and IceCube data do not seem completely consistent but differences may arise from limited statistics and/or assumptions in observational and theoretical analyses. The considered window for the exploration of the Galaxy just opened. We may hope that future data and/or detectors (e.g. km3net) may clarify the picture. We compared our predictions for the total neutrino galactic emission (including unresolved sources) with signals observed by ANTARES and IceCube. Conclusions Our analysis shows that constraints can be potentially obtained both for the truly diffuse emission and the source component. Thank you for your attention Additional slides The CR flux in the Galaxy CR Spectral hardening in the inner Galaxy was suggested by Gaggero et al. 2015 and then reported by two different model-independent analyses of (Acero et al. 2016) and (Yang et al. 2016) of Fermi-LAT data. More recent analysis (Pothast et al. 2018) reports the same behavior. → The spectral hardening is observed in different energy ranges and resilient wrt different prescriptions in the analysis SNR distribution 𝑓 𝑆𝑟 R=1 kpc R= ∞ Galprop The results that I have presented for HE diffuse photon and neutrino fluxes are from: - Pagliaroli et al, JCAP 1611 (2016), 004 - Pagliaroli et al, JCAP 1808 (2018), 035 - Cataldo et al, JCAP 12 (2019) 050 A similar (bottom-up) approach to ours was used by Lipari and Vernetto, PRD 2018 with different prescriptions for CR space and energy distribution. - Factorized flux → No hardening - Non-factorized flux → Hardening See also Schwefer et al, arXiv 2211.15607 - recent calculation (standard scenario, no CR spectral hardening; detailed comparison with local CR measurements) – KRA Dragon (+ Hermes), etc. - CR Propagation model with radially dependent transport properties, see e.g. Gaggero et al., APJ 2015, De la Torre Luque et al, 2022 Credits and comparisons … → There is generically a good agreement between different calculations (when performed with similar assumptions)