A Hadronic Scenario for the Galactic Ridge
Daniele Gaggero, Dario Grasso, Antonio Marinelli, Alfredo Urbano, Mauro Valli
AA Hadronic Scenario for the Galactic Ridge
Daniele Gaggero
SISSA and INFN, via Bonomea 265, I-34136 Trieste, ItalyE-mail: [email protected]
Dario Grasso
Dipartimento di Fisica “E. Fermi" , Pisa University and I.N.F.N., Largo B. Pontecorvo 3,I-56127 Pisa, ItalyE-mail: [email protected]
Antonio Marinelli ∗ † Dipartimento di Fisica “E. Fermi", Pisa University and I.N.F.N., Largo B. Pontecorvo 3,I-56127 Pisa, ItalyE-mail: [email protected]
Alfredo Urbano
SISSA and INFN, via Bonomea 265, I-34136 Trieste, ItalyE-mail: [email protected]
Mauro Valli
SISSA and INFN, via Bonomea 265, I-34136 Trieste, ItalyE-mail: [email protected]
Several observations from Fermi-LAT, up to few hundred GeV, and from H.E.S.S., up to ∼ γ -ray emission from the inner part of the Galactic plane. After thesubtraction of point-like contributions, the remaining γ -ray spectrum can provide important hintsabout the cosmic-ray (CR) population in that region. In particular, the diffuse spectrum measuredby both Fermi-LAT and H.E.S.S. in the Galactic Ridge is significantly harder with respect tothe rest of the Galaxy. These results were recently interpreted in terms of a comprehensive CRtransport model which, adopting a spatial dependent diffusion coefficient and convective velocity,reproduces Fermi-LAT results on the whole sky as well as local CR spectra. We showed asthat model predicts a significantly harder neutrino diffuse emission compared to conventionalscenarios: The predicted signal is able to account for a significant fraction of the astrophysicalflux measured by IceCube. In this contribution, we use the same setup to calculate the expectedneutrino flux from several windows in the inner Galactic plane and compare the results withIceCube observations and the sensitivities of Mediterranean neutrino telescopes. In particular, forthe ANTARES experiment, we compare the model expectations with the upper limits obtainedfrom a recent unblinded data-analysis focused on the galactic ridge region. Moreover, we alsoshow the expectations from the galactic ridge for the future KM3NeT observatory, whose positionis optimal to observe this portion of the sky. The 34th International Cosmic Ray Conference,30 July- 6 August, 2015The Hague, The Netherlands c (cid:13) Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/ a r X i v : . [ a s t r o - ph . H E ] A ug amma-ray and Neutrino Galactic emissions above the TeV Antonio Marinelli
1. Introduction
In 2006, the High Energy Stereoscopic System (H.E.S.S.) reported the observation of a γ -ray emission [1] from the Galactic Ridge (GR) region: − . ◦ < l < . ◦ , | b | < . ◦ in Galacticcoordinates. After the subtraction of the point-like component associated to Sagittarius A [2] andthe SNR G0.9+0.1[3], the spectrum of the diffuse emission was fitted using a power-law with index Γ = − . ± . stat ± . sys . The hard slope of the emission and its positional coincidence withthe Central Molecular Zone (CMZ) cloud complex in the central 200 pc of the Milky Way ledthe H.E.S.S. collaboration to argue in favour of its hadronic origin. Nevertheless, the spectrummeasured by H.E.S.S. is significantly harder than expected from π -decay if the CR spectral shapein the GC region is assumed to be similar to the locally observed one, as assumed in all conventionalCR transport models. In principle such discrepancy may be just a local effect due to the proximitybetween the dense molecular gas with some CR sources located in the complex GC region [1],although no compelling evidences have been provided so far in favour of that interpretation. Onthe other hand, a leptonic origin of the GR emission cannot be excluded a priori, as it may arisefrom bremsstrahlung of ultra relativistic electrons onto the molecular gas [4, 5].We recently showed that the Fermi-LAT data suggest a different interpretation. Our scenario isbased on a new phenomenological model proposed in Ref. [6] and implemented with the DRAGON code [7]. That model assumes a radial dependence for both the rigidity scaling index δ of theCR diffusion coefficient and the convective wind. This setup was introduced in order to solvethe discrepancy between conventional models predictions and the spectrum of the γ -ray diffuseemission measured by Fermi in the inner GP region [8]. Since the model assumes that δ increaseswith the galactocentric radius R , it predicts a hardening of the CR propagated spectrum hence ofthe γ -ray diffuse emission in the inner Galaxy.In Ref. [9] we showed that, if the CR hardening measured by PAMELA and AMS-02 at about250 GV is taken into account in this setup (hereafter, KRA γ model), it is possible to consistentlyreproduce both the γ -ray measurements by Fermi-LAT and those taken by several ground experi-ments above the TeV. This is the case of the γ -ray emission measured by Milagro experiment in theregion (30 ◦ < l < ◦ , | b | < ◦ ) with a median energy of 15 TeV, solving a long standing problemfaced by conventional models [10]. In this work we show as the KRA γ model also consistently re-produces Fermi-LAT and H.E.S.S. data in the GR region (see also [11]). In both regions the originof the emission is dominated by hadronic scattering.The fact that the KRA γ scenario predicts a significantly harder CR spectrum in the innerGalaxy has also relevant consequences for neutrino astronomy and may change the standard predic-tions regarding the diffuse neutrino emission (see e.g. [12]). This is very interesting in light of therecent IceCube detection of 37 neutrino events with energy above 30 TeV corresponding to a 5 . σ excess on top of the atmospheric background [13]. The inferred flavour composition is compatiblewith a mixture of electronic, muonic and tauonic neutrino in equal amounts as expected if theirorigin were astrophysical. Recent analyses [14, 15] based on neutrino events with vertices con-tained in the detector allowed the IceCube collaboration to lower the energy threshold and to mea-sure the extraterrestrial diffuse neutrino spectrum: Φ ν = . + . − . × − (cid:0) E ν / GeV (cid:1) − . ± . ∗ Speaker. † A footnote may follow. amma-ray and Neutrino Galactic emissions above the TeV Antonio Marinelli
GeV − cm − sr − s − for 25 TeV < E ν < . γ model predicts between 10 and ∼
40 % of the 37cosmic neutrino events depending on the poorly known details of the CR spectrum/composition inthe knee region. This result is in accordance with a recent statistical analysis [16] that gives to thediffuse Galactic emission a maximum contribution of 50% to the total astrophysical signal mea-sured by IceCube.The fraction of Galactic neutrinos, however, grows considerably in the GC regionwhere the diffuse Galactic emission is expected to dominate. Unfortunately IceCube can only ob-serve downward shower-like events with a poor angular resolution ( ∼ ◦ ) from this region of thesky. This strongly limits the identification power of the source and the background discrimination.Neutrino telescopes on the North hemisphere, instead, can detect upward track-like events due to ν µ ’s coming from the GC with a much better resolution ( ∼ ◦ ), and may offer better perspectivesto reveal the emission expected for our scenario.In this contribution we compare our prediction with IceCube results and ANTARES upperlimits as computed with a recent unblinded analysis focused on the GR region. Some preliminaryexpectations for the future KM3NeT[17] experiment are also deduced.
2. The γ -ray emission in the Galactic ridge window Figure 1: In the left plot:
The computed γ -ray diffuse emission from the Galactic ridge region comparedwith Fermi-LAT and H.E.S.S. data. For each model the spectrum normalization was varied to minimize the χ against the data. The spectral components are shown for the KRA- γ model only. Fermi-LAT: 5 yearsof data, within the event class ULTRACLEAN according to Fermi tools v9r32p5. In the right plot:
Thecomparison of the KRA- γ spectrum and the Fermi-LAT data for the more extended region | l | < ◦ , | b | < ◦ .The 5 years Fermi-LAT data are computed with the same tools of the left plot. In this section we shortly summarize the main achievements (already reported in [9]) of theKRA γ model regarding the spectrum of the γ -ray diffuse emission in the inner GP and in the GR.3 amma-ray and Neutrino Galactic emissions above the TeV Antonio Marinelli
We compute the γ -ray spectrum starting form the CR proton and Helium densities all over theGalaxy computed with DRAGON . For each selected angular bin, we integrate along the line of sightthe γ -ray emissivity given in [18], accounting for the energy dependence of the pp inelastic crosssection (significant above the TeV). This is done using GammaSky , a dedicated code designed tocompute diffuse γ -ray maps. This code features, among other options, the gas maps included in thepublic GALPROP package (see e.g. [19, 8] and references therein). For the GR window we alsocross check our results with a more realistic gas distribution [20] in the inner Galaxy. We disregardthe γ -ray opacity due to the interstellar radiation field since it is negligible up to few tens of TeV.Our results for the GR window ( | l | < . ◦ , | b | < . ◦ ) are reported in Fig.1. As mentionedabove, a representative conventional model (KRA) with the same properties as the Fermi benck-march model (see Ref. [6]) cannot consistently match Fermi and H.E.S.S. data. A combined fitof Fermi-LAT data above 10 GeV and H.E.S.S. data is unsatisfactory for that model (its reduced χ is 2 . γ setup, instead, is more successful: The reduced χ is 1 .
79. Therefore weconclude that the KRA γ setup provide a satisfactory description of γ -ray data emitted by GR regionfrom few GeVs up to several TeVs.
3. High-energy Neutrino flux from the inner galactic plane �������������������������������������
Figure 2:
The computed neutrino spectra for KRA and KRA γ models over the entire galactic plane (leftpanel) and for the same region considered in [16] (right panel). Green points: spectra of the atmosphericneutrinos obtained with 3 years of IceCube data in those regions. The black stars in the left plot are obtainedin [16] with the standard GALPROP code assuming a uniform diffusion coefficient.
Recently, the IceCube collaboration reported the detection of 37 neutrino events of cosmic ori-gin [21] with a significance of 5.7 σ . The sky distribution of the reconstructed events did not showany significant correlation with the position of known galactic or extragalactic sources. However,the association with a particular source or region of the sky is difficult due to bad angular resolutionof shower-like reconstructed events (representing 29 neutrinos). The 37 events are reported in the4 amma-ray and Neutrino Galactic emissions above the TeV Antonio Marinelli
Figure 3: Left plot:
The cosmic neutrino skymap obtained with 3 years of IceCube data. Red cross: Track-like events; Red dots: Shower-like events. The gray surfaces indicate the estimated angular uncertaintiesfor each of these 37 events. As we can see only 3 of these events can be associated to the region | l | < ◦ and | b | < ◦ . Right plot:
The skymap of neutrino events obtained with KRA γ at 1 TeV (the shape does notchange significantly at larger energies). The same inner region is highlighted in red. skymap of Fig. 3 (left panel).Here, we report the ν spectra in several regions of the Galactic plane computed with the KRA γ setup using the ν µ and ν e emissivities tuned on accelerator and CR data [22, 23]. We also accountfor the neutrino oscillations, which redistribute the composition almost equally among all the threeflavours. As we did for γ -ray emission, we only consider proton and helium CRs/gas since heaviernuclear species give a negligible contribution in the energy range we consider [24].Our prediction for the neutrino spectra integrated over the entire Galactic plane and in theinner region are reported in Fig. 2. The differences between the predictions of the conventionalKRA setup and that of the KRA γ model featuring a spatial dependent diffusion coefficient areconsiderable. In particular, as we can see from the right panel of Fig.2, in the innermost part of theGalactic plane the neutrino flux computed with the KRA γ model becomes significantly larger thanthe conventional KRA model and the expected signal exceeds the atmospheric ν flux measured byIceCube experiment at ∼
20 TeV for both choices of the CR cutoff.At the moment, IceCube results do not allow to infer strong constrains on the KRA γ model. Infact, being the GR region all the time above the IceCube horizon, most of the reconstructed cosmic ν from this region are represented by high-energy shower-like event with a poor angular resolution.In Fig. 4 we report the IceCube astrophysical flux corresponding to the events compatible with thecentral region defined by | l | < ◦ and | b | < ◦ as derived in [25, 26] considering the IceCube datasets of 28 and 37 events [27, 21] showed in Fig. 3.For a spatial analysis of neutrino emission from the GR, the Mediterranean neutrino telescopesare favored considering that the region is almost all the time below the horizon. For this reason, inthe following paragraph we also discuss the expected sensitivity for the future KM3NeT observa-tory and the upper limits set by Antares experiment, through a unblinding analysis.
4. Neutrino expectations for the Mediterranean telescopes considering the KRA γ model The undersea neutrino telescopes located in the Mediterranean Sea have the capabilities to5 amma-ray and Neutrino Galactic emissions above the TeV
Antonio Marinelli ���������������������������������������������������� � ����������������������� Figure 4:
The expected neutrino spectra in the inner Galactic plane region computed for the conventionalKRA and our KRA γ models are reported. We also show experimental constrains from IceCube (662 and 988days of livetime) and ANTARES observations (upper limits obtained with 1500 days of livetime) as well asthe deduced sensitivity for the future Mediterranean KM3NeT[17] considering 4 years ( ∼ observe the inner GP emission with a good sensitivity and angular resolution below few hundredsTeVs. This is mostly due to the possibility of reconstructing up-going ν µ signal coming from thisregion.Here we present the capabilities of ANTARES and future KM3NeT to constrain the emission modelfrom the central region of the GP | l | < ◦ and | b | < ◦ .In the last few months an unblinding analysis for this region was performed with the ANTARESreconstructed ν µ events collected between 2007 and 2013 [28] corresponding to a 1500 days ofexperiment livetime. This analysis covered the energy range between 3 and 300 TeV and did notfind any significant signal excess with respect to the estimated expected background. This turnedin 90% confidence level upper limit reported in Fig. 4. We see from that figure that a naive extrapo-lation of the flux measured by Fermi (accounting for a ν / γ ratio ∼ . γ model, which accounts also for the leptonic and extra-Galactic contributions to diffuse γ -ray fluxmeasured by Fermi (see Fig. 1) is consistent with that limit.Here we also present the extrapolated sensitivity of the future KM3NeT considering the innerGalactic region. The orange line reported in Fig. 4 shows that in 4 years ( ∼ γ model.6 amma-ray and Neutrino Galactic emissions above the TeV Antonio Marinelli
5. Conclusions
In this work we used the already introduced KRA γ model, featuring a radially dependentdiffusion coefficient in our Galaxy, to describe the expected diffuse γ -ray and neutrino emissionsfrom the Galactic ridge region.We first showed that our setup is able to reproduce the H.E.S.S. and Fermi γ -ray spectra for theinner Galactic plane. With this important validation in hand, we computed the expected neutrinospectra for different regions of the Galactic plane. We showed how the neutrino fluxes obtainedwith the KRA- γ is significantly larger than the predictions of the conventional scenarios. We alsocompared our predictions with the experimental constraints obtained from IceCube and ANTARESdata, and discussed the exciting future opportunities provided by the KM3NeT project. The Ice-Cube constrains to the possible neutrino spectra were obtained considering the cosmic events possi-bly related to the inner Galactic region | l | < ◦ and | b | < ◦ . Instead, for ANTARES, we reportedthe upper limits obtained with 2007-2013 data for the same region between 3 and 300 TeV. References [1] H.E.S.S. Collaboration, “Discovery of very-high-energy gamma rays from the Galactic Centre ridge,”
Nature , vol. 439, pp. 695–698, Feb. 2006.[2] H.E.S.S. Collaboration, “Very high energy gamma rays from the direction of Sagittarius A,”
A&A ,vol. 425, pp. L13–L17, Oct. 2004.[3] H.E.S.S. Collaboration, “Very high energy gamma rays from the composite SNR G 0.9+0.1,”
A&A ,vol. 432, pp. L25–L29, Mar. 2005.[4] F. Yusef-Zadeh, J. W. Hewitt, M. Wardle, V. Tatischeff, D. A. Roberts, W. Cotton, H. Uchiyama,M. Nobukawa, T. G. Tsuru, C. Heinke, and M. Royster, “Interacting Cosmic Rays with MolecularClouds: A Bremsstrahlung Origin of Diffuse High-energy Emission from the Inner 2deg × ApJ , vol. 762, p. 33, Jan. 2013.[5] O. Macias and C. Gordon, “Contribution of cosmic rays interacting with molecular clouds to theGalactic Center gamma-ray excess,”
Phys. Rev. D , vol. 89, p. 063515, Mar. 2014.[6] D. Gaggero, A. Urbano, M. Valli, and P. Ullio, “Gamma-ray sky points to radial gradients incosmic-ray transport,”
Phys. Rev. D , vol. 91, p. 083012, Apr. 2015.[7] “The dragon code for cosmic-ray transport and diffuse emission production.” .[8] M. Ackermann et al. , “Fermi-LAT Observations of the Diffuse Gamma-Ray Emission: Implicationsfor Cosmic Rays and the Interstellar Medium,”
Astrophys.J. , vol. 750, p. 3, 2012.[9] D. Gaggero, D. Grasso, A. Marinelli, A. Urbano, and M. Valli, “The gamma-ray and neutrino sky: aconsistent picture of Fermi-LAT, H.E.S.S., Milagro, and IceCube results,”
ArXiv e-prints 1504.00227 ,Apr. 2015.[10] A. A. Abdo et al. , “A Measurement of the Spatial Distribution of Diffuse TeV Gamma-Ray Emissionfrom the Galactic Plane with Milagro,”
The Astrophysical Journal , vol. 688, pp. 1078–1083, Dec.2008.[11] “ D .Gaggero, D. Grasso, A. Marinelli, A. Urbano and M. Valli, talk given by D. Grasso at thisconference, Id: 345.” amma-ray and Neutrino Galactic emissions above the TeV Antonio Marinelli[12] C. Evoli, D. Grasso, and L. Maccione, “Diffuse Neutrino and Gamma-ray Emissions of the Galaxyabove the TeV,”
JCAP , vol. 0706, p. 003, 2007.[13] M. Aartsen et al. , “Observation of High-Energy Astrophysical Neutrinos in Three Years of IceCubeData,”
Phys.Rev.Lett. , vol. 113, p. 101101, 2014.[14] M. Aartsen et al. , “Atmospheric and Astrophysical Neutrinos above 1 TeV Interacting in IceCube,”
Phys.Rev. , vol. D91, p. 022001, 2015.[15] IceCube Collaboration, M. G. Aartsen, K. Abraham, M. Ackermann, J. Adams, J. A. Aguilar,M. Ahlers, M. Ahrens, D. Altmann, T. Anderson, and et al., “A combined maximum-likelihoodanalysis of the high-energy astrophysical neutrino flux measured with IceCube,”
ArXiv e-prints1507.03991 , July 2015.[16] M. Ahlers, Y. Bai, V. Barger, and R. Lu, “Galactic TeV-PeV Neutrinos,”
ArXiv e-prints 1505.03156 ,May 2015.[17] “ P. Piattelli et al., for the KM3NeT Collaboration, talk at this conference, Id: 1014.”[18] T. Kamae, N. Karlsson, T. Mizuno, T. Abe, and T. Koi, “Parameterization of γ , e + / − , and NeutrinoSpectra Produced by p-p Interaction in Astronomical Environments,” The Astrophysical Journal ,vol. 647, pp. 692–708, Aug. 2006.[19] “The galprop code for cosmic-ray transport and diffuse emission production.” http://galprop.stanford.edu/ .[20] K. Ferrière, W. Gillard, and P. Jean, “Spatial distribution of interstellar gas in the innermost 3 kpc ofour galaxy,”
A&A , vol. 467, pp. 611–627, May 2007.[21] M. G. Aartsen, M. Ackermann, J. Adams, J. A. Aguilar, M. Ahlers, M. Ahrens, D. Altmann,T. Anderson, C. Arguelles, T. C. Arlen, and et al., “Observation of High-Energy AstrophysicalNeutrinos in Three Years of IceCube Data,”
Physical Review Letters , vol. 113, p. 101101, Sept. 2014.[22] T. Kamae, N. Karlsson, T. Mizuno, T. Abe, and T. Koi, “Parameterization of gamma, e + / − , andNeutrino Spectra Produced by p-p Interaction in Astronomical Environments,” ApJ , vol. 647,pp. 692–708, Aug. 2006.[23] S. R. Kelner, F. A. Aharonian, and V. V. Bugayov, “Energy spectra of gamma rays, electrons, andneutrinos produced at proton-proton interactions in the very high energy regime,”
Phys. Rev. D ,vol. 74, p. 034018, Aug. 2006.[24] M. Kachelrieß and S. Ostapchenko, “Neutrino yield from Galactic cosmic rays,”
Phys. Rev. D ,vol. 90, p. 083002, Oct. 2014.[25] A. Neronov, D. Semikoz, and C. Tchernin, “PeV neutrinos from interactions of cosmic rays with theinterstellar medium in the Galaxy,”
Phys. Rev. D , vol. 89, p. 103002, May 2014.[26] M. Spurio, “Constraints to a Galactic component of the Ice Cube cosmic neutrino flux fromANTARES,”
Phys. Rev. D , vol. 90, p. 103004, Nov. 2014.[27] IceCube Collaboration, “Evidence for High-Energy Extraterrestrial Neutrinos at the IceCubeDetector,”
Science , vol. 342, p. 1, Nov. 2013.[28] “ L. Fusco talk at this conference, ID306.”[29] V. Cavasinni, D. Grasso, and L. Maccione, “TeV Neutrinos from SuperNova Remnants embedded inGiant Molecular Clouds,”
Astropart. Phys. , vol. 26, pp. 41–49, 2006., vol. 26, pp. 41–49, 2006.