I present an updated measurement of the cosmic-ray electron and positron spectrum between 7GeV and 2TeV, based on 7 years of data collected with the Fermi Large Area Telescope (LAT). The LAT is the ﬁrst space-based instrument to directly explore the region above 1TeV. The best ﬁt to the spectrum that we measure is given by a broken power-law, with the break located at ∼ 50GeV. Above 50GeV our data are well described by a single power-law with a spectral index of 3.07 ± 0.02 (stat + syst) ±0.04 (energy scale). An exponential cutoﬀ lower than 1.8TeV is excluded at 95% CL

Manfreda, A.

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DOI 10.1393/ncc/i2017-17112-8Colloquia: SciNeGHE 2016IL NUOVO CIMENTO 40 C (2017) 112A new measurement of cosmic-ray electrons with the Fermi LargeArea TelescopeA. Manfreda(1)(2) for the Fermi LAT Collaboration(1) INFN, Sezione di Pisa - Pisa, Italy(2) Università di Pisa - Pisa, Italyreceived 3 June 2017Summary. — I present an updated measurement of the cosmic-ray electron andpositron spectrum between 7 GeV and 2 TeV, based on 7 years of data collected withthe Fermi Large Area Telescope (LAT). The LAT is the first space-based instrumentto directly explore the region above 1 TeV. The best fit to the spectrum that wemeasure is given by a broken power-law, with the break located at ∼ 50 GeV. Above50 GeV our data are well described by a single power-law with a spectral index of3.07 ± 0.02 (stat + syst) ±0.04 (energy scale). An exponential cutoff lower than1.8 TeV is excluded at 95% CL.1. – IntroductionHigh-energy cosmic-ray electrons and positrons (CREs) constitute a peculiar compo-nent of the cosmic radiation. In fact, unlike protons and heavier nuclei, they rapidlylose energy by synchrotron radiation on galactic magnetic fields and by inverse Comptonscattering on the interstellar radiation field. At ∼ 1TeV, the diffusion distance of CREsis of the order of a few hundred parsecs, much shorter than the radius of our Galaxy [1];thus, measuring the CRE spectrum up to a few TeV can provide evidence of local CREsources from astrophysical origin (supernova remnants and pulsar wind nebulae [2-7]) orfrom exotic origin (dark matter [8-10]) and constrain theoretical models of propagationof CRs in the nearby or local galactic space.Here I present a measurement of the CRE spectrum from 7 GeV to 2 TeV based onthe data collected by the Large Area Telscope (LAT) on the Fermi mission betweenAugust 4, 2008 and June 24, 2015. A publication describing the work in full detail wasin preparation at the time this document was being written and is now published in [11].The LAT [12] is a pair conversion telescope, designed for measuring γ rays in an en-ergy range from a few tens of MeV to a few hundreds of GeV. It is composed of threemain subsystems: a silicon tracker (TKR), for measuring the direction of incident par-ticles; a CsI(Tl) calorimeter (CAL), located below the TKR, for energy measurement; aCreative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0) 12 A. MANFREDA for the Fermi LAT COLLABORATIONsegmented anti-coincidence detector (ACD) for charged cosmic-ray background rejection.Since electromagnetic cascades are common to both electron and photon interactions inmatter, the LAT is also by its nature a detector for electrons and positrons. Measure-ments of the inclusive CRE spectrum up to 1 TeV have been published by the LATCollaboration in 2009 [13] and 2010 [14], based on six months and one year of data,respectively.Recently, the Fermi LAT Collaboration has released a radical revision of the entireevent analysis, going under the name of Pass 8 [15]. Pass 8 came with improvementsin all the main areas of the analysis, including Monte Carlo simulation of the detector,track and shower reconstruction, background rejection and control of the systematic un-certainties. Thanks to Pass 8, as well as to the increased statistics, we were able to extendthe upper energy limit of the measurement to 2 TeV. We also seized the opportunity torefine our estimate of the systematic uncertainties and to further investigate the effectsof the geomagnetic field on the low-energy end of the spectrum, as described in detail insect. 3. For all these reasons, the work presented here should be regarded not merely asan update of our previous measurements, but as an effectively new one.2. – Event selectionThe LAT is equipped with an on-board filter, designed to reject charged particlesand reduce the data flow to a level compatible with the available telemetry bandwidth.However, the filter accepts by default all the events that deposit at least 20 GeV in thecalorimeter, a condition which is typically satisfied by CREs above 40 GeV. Below thatenergy, it is possible to extend the measurement by using an unbiased sample of all theinstrument triggers, pre-scaled by a factor 250, which is provided on the LAT mostlyfor diagnostic purposes. Thus, two independent event selections have been developed:Low Energy (LE), from 7 GeV to 70 GeV, using events from the unbiased trigger sample;High Energy (HE), above 42 GeV, using events from the standard on-board filter. Theregion of overlap is useful for cross-checking the two analyses.We select particles with a successfully reconstructed track, a direction within 60◦ fromthe instrument boresight and which cross at least 8 radiation length of material in theCAL. We also impose minimal cuts on the quality of energy and direction measurement,in order to get rid of poorly reconstructed events. We exploit the dependence of ionizationon Z2 (where Z is the atomic number of the bullet) to easily tag and remove particleswith Z > 1 from the dataset, cutting on the amount of ionization produced in the TKRand in the ACD.Discrimination between protons and electrons is based on the different topology ofhadronic and leptonic events in the detector. For the task, we made use of machine-learning algorithms, namely Boosted Decision Trees (BDTs), which are able to combinethe information carried by several different individual observables into a single variable,PCRE, which can then be used to select signal events. BDTs were trained on simulatedMonte Carlo (MC) samples of electrons and protons interacting in the LAT. We traineddifferent BDTs in different energy ranges to account for the changes in event topologyin the LAT; 8 BDTs were trained for the HE analysis, and one for the LE analysis.As shown in fig. 1, we estimate the residual background contamination by fitting, ineach energy bin, the distribution of PCRE in data with the sum of the MC templatesfor electrons and protons, letting their normalization float as free parameters for the fit.We use the renormalized MC proton template to estimate the number of backgroundevents surviving the selection, which are then subtracted from data counts to obtain theA NEW MEASUREMENT OF COSMIC-RAY ELECTRONS WITH THE Fermi LAT 3CREP-4 -3.5 -3 -2.5 -2 -1.5 -1 -0.5 0Counts01002003004005006007008009001000DataMC totalMC electronMC protonFig. 1. – Distribution of the CRE estimator PCRE for events between 866 and 1000 GeV. TheMC prediction (green) is the sum of the signal (red) and background (blue) templates scaled bythe normalization factors given by the template fit to flight-data (black). The selection cut forthis energy bin is indicated by the black vertical line.number of signal CRE events.The acceptance and the estimated residual background contamination for the HE andLE selection are shown in fig. 23. – LE orbital selectionsBelow ∼ 20GeV, the spectrum of CREs observed by the LAT is strongly influenced bythe shielding effect of the magnetic field of the Earth. At a given geomagnetic position,only Galactic charged particles above a certain rigidity (which depends on their directionrespect to the local zenith) can reach the detector. The orbit of Fermi spans an intervalof vertical rigidity cut-off values from ∼ 6GeV to ∼ 14GeV; in order to meaure theCRE spectrum in this energy range we selected only data collected in orbital regions inwhich the entire bin was above the local rigidity cutoff. For the selection, we use theMcIlwain L parameter [16] to conveniently parametrize the dependence of the rigidityE [GeV]10 210 310 sr]2Acceptance [m00.511.522.53Residual contamination00.10.20.30.40.50.6LE acceptance (x 250)HE acceptanceLE contaminationHE contaminationFig. 2. – Acceptance and residual background contamination for the LE and HE selections. Thedisplayed LE acceptance is multiplied by 250 (as if there were no prescale factor due to theon-board filter). For the acceptance, the energy is the true energy, while it is the measuredenergy for the contamination.4 A. MANFREDA for the Fermi LAT COLLABORATIONMcIlwain L1 1.1 1.2 1.3 1.4 1.5 1.6 1.7Livetime fraction00.20.40.60.811.27.0 GeV7.8 GeV8.7 GeV9.7 GeV10.8 GeV12.0 GeV13.3 GeV14.8 GeV16.4 GeVFig. 3. – Normalized integral distribution of the instrument live time in different McIlwain Lbins for the data sample. Grey lines show the McIlwain L cut in each energy bin (with numbersindicating the lower edge of the bin).cut-off on geographic coordinates. The corresponding fraction of live time spent by theLAT in the selected regions is shown in fig. 3.The cutoff is smooth and even above the nominal rigidity cutoff there is still a fractionof electrons and positrons which are prevented from reaching the detector by the magneticshadow of the Earth. The effect is enhanced by a combination of the wide angularaperture of the LAT and its periodic rocking motion with respect to the local zenith,causing the Earth to be often very close to the edge of the instrument’s field of view.In order to estimate this fraction of undetected CREs and to correct the observedflux accordingly, we developed a realistic simulation of the trajectories of electrons andpositrons in the orbital environment of the LAT. We use the particle trajectory tracingcode developed by Smart and Shea [16] and the 2010 model of the Earth’s magnetic fieldfrom the International Geomagnetic Reference Field (IGRF) [17], as we did in [18]. For agiven McIlwain L selection, we use the output of the simulation to estimate the fraction oftrajectories blocked by the magnetic shield of the Earth, corresponding in the real worldto the fraction of unobserved CREs. The correction factors found are shown in fig. 4.E [GeV]8 9 10 20 30 40 50 60 70Correction factor11.051.11.151.21.251.31.351.4Aug 2008 - Aug 2009nominal cutoffnominal cutoff + 30%Sep 2009 - Jun 2015nominal cutoffnominal cutoff + 30%Fig. 4. – LE correction factors for the first year of the mission (triangles) and for the followingyears (squares). Full markers correspond to the nominal energy cutoff, while the empty markerscorrespond to a cutoff 30% higher.A NEW MEASUREMENT OF COSMIC-RAY ELECTRONS WITH THE Fermi LAT 5E [GeV]10 210 310]2 GeV-1 sr-1 s-2 Intensity [m×3E80100120140160180200220240 Fermi LE (2010)Fermi HE (2010)Fermi Pass 8 - LE selectionFermi Pass 8 - HE selectionenergy reconstruction uncertainty onlyFig. 5. – CRE spectrum between 7 GeV and 2 TeV measured by the LAT and the previousLAT measurement [14]. All error bars represent the quadratic sum of statistical and systematicuncertainties (except the one on the energy measurement). The area between the dashed linescorresponds to the uncertainty band due to the LAT energy reconstruction uncertainty only.A further 2% systematic uncertainty on the energy scale is not indicated in the plot.We note that this effect was not explicitly accounted for in our previous measurements,though it was mitigated by the fact that the rocking angle of the instrument was smallerduring the first year of the mission (it has been increased from 35◦ to 50◦ since then).E [GeV]10 210 310]2 GeV-1 sr-1 s-2 Intensity [m×3E050100150200250H.E.S.S. (2008)H.E.S.S. (2009)AMS-02 (2014)Fermi Pass 8 - LE selectionFermi Pass 8 - HE selectionenergy reconstruction uncertainty onlyFig. 6. – CRE spectrum between 7 GeV and 2 TeV measured by the LAT along with other recentmeasurements by AMS-02 [20] and H.E.S.S. [21,22]. All error bars represent the quadratic sumof statistical and systematic uncertainties (except the one on the energy measurement). Thearea between the dashed lines corresponds to the uncertainty band due to the LAT energymeasurement uncertainty only. A further 2% systematic uncertainty on the energy scale is notindicated in the plot.6 A. MANFREDA for the Fermi LAT COLLABORATION4. – ResultsHere I briefly report the most relevant results of this work. A longer discussion is givenin [11], including a detailed description of the techniques used to estimate the systematicuncertainties and how they were accounted for when fitting the data. We note that,at the end of the selection, the data sample contains more than 107 events, nearly 104of which above 1 TeV; consequently, the analysis is limited by systematic uncertaintiesacross the whole energy range.Figure 5 shows the spectra we measured with the LE and HE analysis. We note thatthe two spectra match very well over the overlapping range 42 < E < 70GeV. Below100 GeV, the new LAT measurement differs from the previous one by 10–30%. A largepart of this difference below 30 GeV is due to the lack of correction in the previous analysisfor the loss of CREs above the geomagnetic energy cutoff. After applying this correction,the remaining difference is 10–15% and is due to imperfections in the simulation that wasused in the previous analysis (remnants of electronic signals from out-of-time particleswere not simulated [19]).Figure 5 suggests the presence of a break in the spectrum. Indeed, a broken power-lawfit to our data is preferred over a single power-law at the 4σ level. Fitting with a brokenpower-law yields a break energy of 53± 8GeV and the spectral indices below and abovethe break are 3.21 ± 0.02 and 3.07 ± 0.02, respectively.Figure 6 shows a comparison of the spectrum with other recent measurements fromAMS-02 [20] and H.E.S.S. [21,22]. As can be seen, the LAT CRE spectrum is consistentlyabove the AMS-02 one for energies larger than ∼ 70GeV. The difference in spectralindices above 30.2 GeV, limit above which AMS-02 reports a single power-law spectrum,is at the level of 1.7σ. This could indicate that systematic uncertainties on the energymeasurement in one or both of the results are slightly larger than estimated.At higher energies, H.E.S.S. reported in [21] that its data were well reproduced by anexponentially cutoff power-law with a cutoff at 2.1 ± 0.3TeV. The LAT CRE spectrumabove 50 GeV, as indicated by the previous broken power-law fits, is compatible with asingle power-law with a spectral index of 3.07±0.02 (stat+syst) ±0.04 (energy measure-ment). Fitting the count spectrum above 50 GeV with an exponentially cutoff power-lawE−γe−E/Ec does not yield statistically significant evidence for a cutoff and we excludeEc < 1.8TeV at 95% CL.REFERENCES[1] Nishimura J., Fujii M. and Taira T., in Proceedings of the 16th International CosmicRay Conference, Vol. 1 (Tokyo Univ. Inst. Cosm. Ray Res.) 1980, p. 488.[2] Shen C. S., Astrophys. J., 162 (1970) L181.[3] Nishimura J. et al., Astrophys. J., 238 (1980) 394.[4] Nishimura J., Kobayashi T., Komori Y. and Yoshida K., Adv. Space Res., 19 (1997)767.[5] Aharonian F. A., Atoyan A. M. and Voelk H. J., Astron. Astrophys., 294 (1995) L41.[6] Kobayashi T., Komori Y., Yoshida K. and Nishimura J., Astrophys. J., 601 (2004)340.[7] Blasi P., Phys. Rev. Lett., 103 (2009) 051104.[8] Cholis I., Goodenough L., Hooper D., Simet M. and Weiner N., Phys. Rev. D, 80(2009) 123511.[9] Cirelli M., Kadastik M., Raidal M. and Strumia A., Nucl. Phys. B, 813 (2009) 1;873 (2013) 530.A NEW MEASUREMENT OF COSMIC-RAY ELECTRONS WITH THE Fermi LAT 7[10] Bergstrom L., Edsjo J. and Zaharijas G., Phys. Rev. Lett., 103 (2009) 031103.[11] Fermi LAT Collaboration (Abdollahi et al.), Phys. Rev. D, 95 (2017) 082007.[12] Fermi LAT Collaboration (Atwood W. et al.), Astrophys. J., 697 (2009) 1071.[13] Fermi Lat Collaboration (Abdo A. et al.), Phys. Rev. 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A new measurement of cosmic-ray electrons with the Fermi Large Area Telescope

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A new measurement of cosmic-ray electrons with the Fermi Large Area Telescope

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