The Large Area Telescope (LAT) on board the Fermi satellite is exploring the gamma-ray sky in the energy range from 20MeV to > 300GeV. Since the start of the science phase of the mission the LAT has detected high-energy gamma rays from the Moon. This emission is produced by interactions of cosmicrays nuclei with the lunar surface and depends on the level of solar activity. Moon was detected by EGRET on CGRO with low statistics, but Fermi is the only gammaray mission capable of detecting the Moon over the full 24th solar cycle. Here we report the detection of gamma-ray emission from the Moon during the first 18 months of observation showing the status of the analysis and interpretation

Brigida, M.

English

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2011-10890-1Colloquia: Scineghe2010IL NUOVO CIMENTO Vol. 34 C, N. 3 Maggio-Giugno 2011Gamma-ray emission from the Moon as observed by FermiM. Brigida for the Fermi CollaborationUniversita` di Bari - Via Amendola 167, 70126 Bari, Italy(ricevuto il 25 Febbraio 2011; pubblicato online l’1 Giugno 2011)Summary. — The Large Area Telescope (LAT) on board the Fermi satellite isexploring the gamma-ray sky in the energy range from 20MeV to > 300GeV. Sincethe start of the science phase of the mission the LAT has detected high-energygamma rays from the Moon. This emission is produced by interactions of cosmic-rays nuclei with the lunar surface and depends on the level of solar activity. Moonwas detected by EGRET on CGRO with low statistics, but Fermi is the only gamma-ray mission capable of detecting the Moon over the full 24th solar cycle. Here wereport the detection of gamma-ray emission from the Moon during the first 18months of observation showing the status of the analysis and interpretation.PACS 07.85.-m – X- and γ-ray instruments.PACS 96.20.-n – Moon.PACS 96.60.Tf – Solar electromagnetic emission.1. – IntroductionFermi was successfully launched on 2008 June 11 onto a low Earth circular orbitat an altitude of 565 km, an inclination of 28.5◦ and an orbital period of 96min long.The observatory consists of the Large Area Telescope (LAT) and the Gamma-ray BurstMonitor (GBM). The LAT [1] is a pair-production telescope with a large effective areaand field of view (2.4 sr), sensitive to gamma rays between 20MeV and 300GeV. Aftera commissioning phase devoted to the instrument fine tuning and calibrations, the LATbegan its normal science operations on 2008 August 11. Since then, several scientificresults have been obtained with these early data.In this paper we report the detection and mesurements of gamma-ray emission fromthe Moon, as observed by Fermi -LAT. The Moon emission results from the interactionof cosmic ray nucleons with its surface [2, 3]: the main processes involved are the pro-duction and decay of neutral pions and kaons by ions. Cosmic ray interaction with thelunar surface is well established and gamma-ray spectrum computed in [2]. More recentcalculations have been performed taking into account the Lunar Prospector observations,using a detailed description of the regolite lunar surface [4] and GEANT4 [5] taking intoaccount alla the interactions with the specific composition of the lunar rock [6].c© Societa` Italiana di Fisica 289290 M. BRIGIDA for the FERMI COLLABORATIONEarly analysis of EGRET observations of the Moon yielded the integral flux of F (E >100MeV) = (4.7±0.7)×10−7 ph cm−2 s−1 [3]. A later reanalysis confirmed the detectionand yielded a flux F (E > 100MeV) = (5.55±0.65)×10−7 ph cm−2 s−1 averaged over theentire mission duration [7], about 24% below the expected value [2]. Moreover the samework indicates that the lunar gamma-ray spectrum should exhibit a narrow 67.5MeVline due to neutral pions decaying at rest and a steep spectrum with a cut-off at about3–4GeV.These calculations indicate that the kinematics of the collisions of cosmic rays hittingthe lunar surface produce a secondary particle cascade that develops deep in the rock.A small fraction of the secondary low energy pions are directed toward to lunar surfaceand decay to produce a soft-ray spectrum. Finally since the high energy rays can beproduced by cosmic rays hitting the lunar surface with almost tangential traiectories,the limb of the disk surface should give the larger contribution to the emission.Although similar physical processes are involved, the γ-ray spectra of the Earth, theMoon, and the Sun are very different. The Moon is so far the only observed γ-ray–emit-ting body with the solid surface. For the Sun the γ-ray emission from the disk, due tothe interactions of cosmic ray nuclei with the solar atmosphere [8], is accompanied byextended and brighter γ-ray emission due to the inverse Compton scattering of Galacticcosmic ray electrons off solar photons [4, 9, 10].Calculations of interactions of cosmic rays with the lunar surface are fairly straight-forward and involve a well-measured spectrum and composition of cosmic rays near theEarth and composition of the Moon rock. A similar emission mechanism should be de-tected from any other solid object in the Solar System. Therefore planets should emitrays produced from pion decays coming from the hadronic interactions by cosmic rayshitting the surface of these bodies. We report report here the updated observations ofthe lunar-ray emission, previously presented, and the status of the search.2. – Data selectionThe data sample used was includes the scientific data collected since 2008 August 4to 2010 February 4. We use for this analysis the “Diffuse” class [1], corrisponding to theevents with the highest probability candidates as photons.As the Moon is a moving source, we developed a code in order to perform the analysisof the data in a source-centred system: the events were mapped onto a celestial coordinatesystem centred on Moon instantaneous position. Coordinates were computed using JPLlibraries [11] taking into account parallax corrections. In order to have a better sensitivityto the Moon emission, other sources of background have been reduced with the followingselections:– Zenith angle < 105◦ in order to exclude photons from the Earth’s limb;– the Sun or the Moon should be at least 30◦ under or above the galactic planein order to reduce the diffuse components and avoid the brightest sources on thegalactic plane;– the angular separation between Moon and Sun should be more than 20◦, in orderto remove the Sun emission component;– we remove also any time interval occurring when any bright object is within 5◦from the Moon;During the whole period the Sun was at the minimum of its activity.GAMMA-RAY EMISSION FROM THE MOON AS OBSERVED BY FERMI 291Fig. 1. – Count maps of events with angular separation from the Moon and E > 100MeV, asa function of the right ascension and declination. Offset in degrees with respect to the Moonposition. The dashed lines show the fake Moon count map distributions.3. – Analysis method and resultsIn our analysis, the main sources of background are the galactic and extragalacticemission in the source centered frame. Moreover to evaluate the background in thisrelative coordinate frame, we consider a fake source following the same Moon path inthe sky but at least 30◦ displaced from the real source. We evaluate in this way thebackground in the relative coordinates and comparing the two data sets.Figure 1 shows count maps of photons above 100MeV and within 20◦ from the Moonposition projected onto right ascension and declination. The coordinates shown are offsetof celestial coordinates in degrees relative to the Moon position. Counts from the fakesources are superimposed as a dashed line and show the background in our analysis. Theplot are an update of previous observation reported with a reduced statistics [12].Different methods can be used to compute the flux from a source, mainly based on themaximum-likelihood analysis. The Fermi standard method for spectra evaluation andsource flux computation is the GtLike tool, consisting in a binned or un-binned likelihoodanalysis of LAT data [1]. A preliminary analysis is performed by fitting the fake sourceFig. 2. – Moon spectrum for E > 100MeV. Superimposed the best fit.292 M. BRIGIDA for the FERMI COLLABORATIONdata in order to obtain a model for the background events. Then, we fit the sourcedata sample with the proper free function summed to the fixed background model. Theanalysis for the Moon data indicates a best fit with a power law with exponential cut-off.As a result of the fit we obtain a flux as F (> 100MeV) = (1.21 ± 0.02 ± 0.20) ×10−7 ph cm−2 s−1. Figure 2 shows the Moon spectra obtained with GtLike tool.The computed errors include the statistical uncertainties and the estimation of theoverall systematical error of about 20%.4. – ConclusionsIn this paper we demonstrate the observing capabilities of Fermi-LAT by presentingdetection of the Moon over the first 18 months of the Mission. We also report theexstimation of flux from the Moon, in comparison with the previous observations andthe theoretical evaluation. These results indicate that Fermi data analysis will providefundamental information about the Moon emission and the modulation of cosmic rayfluxes during the solar cycle.∗ ∗ ∗The Fermi-LAT Collaboration acknowledges support from a number of agencies andinstitutes for both development and the operation of the LAT as well as scientificdata analysis. These include NASA and DOE in the United States, CEA/Irfu andIN2P3/CNRS in France, ASI and INFN in Italy, MEXT, KEK, and JAXA in Japan,and the K. A. Wallenberg Foundation, the Swedish Research Council and the NationalSpace Board in Sweden. Additional support from INAF in Italy for science analysisduring the operations phase is also gratefully acknowledged.REFERENCES[1] Atwood W. B. et al., Astrophys. J., 697 (2009) 1071.[2] Morris D. J., J. Geophys. Res. A, 89 (1984) 1068.[3] Thompson D. J. et al., J. Geophys. Res. A, 102 (1997) 740.[4] Moskalenko I. V. et al., Astrophys. J., 652 (2006) L65.[5] Agostinelli S. et al. (GEANT4 Collaboration), Nucl. Instrum. Methods A, 506(2003) 250.[6] Moskalenko I. V. and Porter T. A., ICRC, 2 (2008) 759.[7] Orlando E. and Strong A. W., Astron. Astrophys., 480 (2008) 847.[8] Seckel D. et al., Astrophys. J., 382 (1991) 652.[9] Moskalenko I. V. and Porter T. A., Astrophys. J., 670 (2007) 1467.[10] Orlando E., these proceedings.[11] http://iau-comm4.jpl.nasa.gov/access2ephs.html.[12] Giglietto N. for LAT Collaboration, Proceedings of the 31st ICRC Lodz, 2009.

Gamma-ray emission from the Moon as observed by Fermi

http://eprints.bice.rm.cnr.it/17286/1/ncc9933.pdf

Gamma-ray emission from the Moon as observed by Fermi

Abstract

Similar works

Full text

Available Versions

Scientific Open-access Literature Archive and Repository