GRIPS is planned to be the next great satellite-born survey mission lead by
Europe; it will look into the cosmos with unprecedented accuracy in several
bands of the EM spectrum (infrared, X-rays, MeV gamma-rays); in particular in
gamma-rays, GRIPS will be able to bridge the so-called MeV gap and to answer
several questions brought forth by GeV-TeV gamma-ray observations. We will
discuss here several connections to GeV-TeV gamma-ray astrophysics, focussing
in particular to show how GRIPS will be crucial in revealing the origin of
cosmic rays.Comment: Nuovo Cimento C; published online: 18 May 201

Greiner, J.

Kanbach, G.

Mannheim, K.

Paravac, A.

Tibolla, O.

English

arXiv

GRIPS is planned to be the next great satellite-born survey mission lead by Europe; it will look into the cosmos with unprecedented accuracy in several bands of the EM spectrum (infrared, X-rays, MeV gamma-rays); in particular in gamma-rays, GRIPS will be able to bridge the so-called MeV gap and to answer several questions brought forth by GeV-TeV gamma-ray observations. We will discuss here several connections to GeV-TeV gamma-ray astrophysics, focussing in

particular to show how GRIPS will be crucial in revealing the origin of cosmic rays

Tibolla, Omar

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2011-10878-9Colloquia: Scineghe2010IL NUOVO CIMENTO Vol. 34 C, N. 3 Maggio-Giugno 2011Highlights from the ARGO-YBJ experimentG. Di Sciascio(∗) for the ARGO-YBJ CollaborationINFN, Sezione di Roma Tor Vergata - Via della Ricerca Scientifica 1, I-00133 Rome, Italy(ricevuto il 25 Febbraio 2011; pubblicato online il 18 Maggio 2011)Summary. — The ARGO-YBJ experiment is a multipurpose detector exploitingthe full coverage approach at very high altitude. The apparatus, in stable datataking since November 2007 with an energy threshold of a few hundreds of GeV anda duty-cycle > 85%, is located at the YangBaJing Cosmic Ray Laboratory (Tibet,P.R. China, 4300 m a.s.l., 606 g/cm2). A number of interesting results are availablein cosmic ray physics and in gamma-ray astronomy after the first 3 years of stabledata taking. In this paper gamma-ray astronomy results are summarized.PACS 96.50.sd – Extensive air showers.PACS 07.85.-m – X- and γ-ray instruments.PACS 97.60.Gb – Pulsars.PACS 98.54.-h – Quasars; active or peculiar galaxies, objects, and systems.1. – IntroductionThe ARGO-YBJ experiment is in stable data taking since November 2007 with a dutycycle > 85% and with excellent performance. The large field of view (∼ 2 sr) and thehigh duty cycle allow a continuous monitoring of the sky in the declination band from−10◦ to 70◦. Several interesting results are available in cosmic ray (CR) physics as wellas in gamma-ray astronomy with the first 3 years of operation.We observed TeV emission from 3 sources with a significance greater than 5 standarddeviations (s.d.): Crab Nebula, Mrk421 and MGRO J1908+06. In particular we observedthe Crab Nebula with a significance of 14 s.d. in ∼ 800 days. A detailed long-termmonitoring of the Mrk421 flaring activity has been carried out in 2008-2009. A clearcorrelation with X-ray data was found. The relation between spectral index and flux hasbeen studied in one single flare (June 2008) [1] and averaging the data over 2 years.Working in scaler mode [2] ARGO-YBJ has performed a search for emission fromGRBs in coincidence with 93 events observed by satellites (16 with known redshift),(∗) E-mail: disciascio@roma2.infn.itc© Società Italiana di Fisica 2728 G. DI SCIASCIO for the ARGO-YBJ COLLABORATIONsetting upper limits on the fluence between 10−5 and 10−3 erg cm−2 in the 1–100 GeVenergy range [3].A medium-scale CR anisotropy has been observed with a significance greater than10 s.d. at proton median energy of about 2 TeV. Two excesses (corresponding to a fluxincrease of ∼ 0.1%) are observed by ARGO-YBJ around the positions α ∼ 120◦, δ ∼ 40◦and α ∼ 60◦, δ ∼ −5◦ [4], in agreement with a similar detection reported by the MilagroCollaboration [5]. The origin of this medium-scale anisotropy is puzzling. In fact, theseregions have been interpreted as excesses of hadronic CRs, but TeV CRs are expectedto be randomized by the magnetic fields. Understanding these anisotropies should be ahigh priority as they are probably due to a nearby source of CRs, as suggested by someauthors (see, e.g., [6]).With 2 years of data we carried out a 2D measurement of the CR large-scale anisotropyto investigate detailed structural information beyond the simple Right Ascension profiles.The p-air cross-section has been measured in the range 1–100 TeV and the corre-sponding p-p cross-section inferred [7].The light-component (p + He) spectrum of primary CRs has been measured in therange 5–250 TeV. The preliminary results are in good agreement with the CREAMballoon data. For the first time direct and ground-based measurements overlap for awide energy range thus making possible the cross-calibration of the different experimentaltechniques.A measurement of the p/p ratio at few-TeV energies has been performed settingtwo upper limits at the 90% confidence level: 5% at 1.4 TeV and 6% at 5 TeV [8]. Inthe few-TeV range these results are the lowest available, useful to constrain models forantiproton production in antimatter domains.The main results after about 3 years of stable operation are summarized in [9]. Inthis paper gamma-ray astronomy results are summarized.2. – The ARGO-YBJ experimentThe ARGO-YBJ experiment, located at the YangBaJing Cosmic Ray Laboratory(Tibet, P.R. China, 4300 m a.s.l., 606 g/cm2), is currently the only air shower arrayexploiting the full coverage approach at high altitude, with the aim of studying thecosmic radiation at an energy threshold of a few hundred GeV.The detector is composed of a central carpet large ∼ 74 × 78 m2, made of a singlelayer of Resistive Plate Chambers (RPCs) with ∼ 93% of active area, enclosed by a guardring partially (∼ 20%) instrumented up to ∼ 100 × 110 m2. The apparatus has modularstructure, the basic data acquisition element being a cluster (5.7 × 7.6 m2), made of 12RPCs (2.8×1.25 m2 each). Each chamber is read by 80 external strips of 6.75×61.8 cm2(the spatial pixels), logically organized in 10 independent pads of 55.6 × 61.8 cm2 whichrepresent the time pixels of the detector. The read-out of 18360 pads and 146880 stripsare the experimental output of the detector [10]. The RPCs are operated in streamermode by using a gas mixture (Ar 15%, Isobutane 10%, TetraFluoroEthane 75%) forhigh-altitude operation. The high voltage set at 7.2 kV ensures an overall efficiency ofabout 96% [11]. The central carpet contains 130 clusters (hereafter, ARGO-130) and thefull detector is composed of 153 clusters for a total active surface of ∼ 6700 m2. A simple,yet powerful, electronic logic has been implemented to build an inclusive trigger. Thislogic is based on a time correlation between the pad signals depending on their relativedistance. In this way, all the shower events giving a number of fired pads Npad ≥ Ntrig inHIGHLIGHTS FROM THE ARGO-YBJ EXPERIMENT 29multiplicity10 210 310)° (αΔ-1.8-1.6-1.4-1.2-1-0.8-0.6-0.4-0.200.2DATA1.25 1.76 2.30 3.55 6.05 12.2 26.0[TeV]/Z50EMCMC best fit 10%±MC best fit  Fig. 1. – (Color online) Measured westward displacement of the Moon shadow as a function ofparticle multiplicity (red filled circles), i.e. the number of fired strips. The data are comparedto Monte Carlo simulation (blue filled squares). The upper scale refers to the median energy ofrigidity (TeV/Z) in each multiplicity bin.the central carpet in a time window of 420 ns generate the trigger. This can work withhigh efficiency down to Ntrig = 20, keeping the rate of random coincidences negligible.The whole system, in smooth data taking since July 2006 firstly with ARGO-130, isin stable data taking with the full apparatus of 153 clusters since November 2007 withthe trigger condition Ntrig = 20 and a duty cycle ≥ 85%. The trigger rate is ∼ 3.6 kHzwith a dead time of 4%.2.1. Detector performance. – The performance of the detector and the operationstability are continuously monitored by observing the Moon shadow, i.e. the deficit ofCR detected in its direction. Indeed, the size of the deficit allows the measurement of theangular resolution and its position allows the evaluation of the absolute pointing accuracyof the detector. In addition, positively charged particles are deflected towards East dueto the geomagnetic field by an angle ∼ 1.6◦Z/E (TeV). Therefore, the observation of thedisplacement of the Moon provides a direct calibration of the relation between showersize and primary energy (fig. 1).ARGO-YBJ observes the Moon shadow with a sensitivity of ∼ 9 s.d. per month forevents with a multiplicity Npad ≥ 40 and zenith angle θ < 50◦, corresponding to aproton median energy Ep ∼ 1.8 TeV. With all data from July 2006 to December 2009(about 3200 hours on-source in total) we observed the CR Moon shadowing effect with asignificance of about 55 s.d. [9]. The measured angular resolution is better than 0.5◦ forCR-induced showers with energies E > 5 TeV (fig. 2) and the overall absolute pointingaccuracy is ∼ 0.1◦. The angular resolution stability is at a level of 10% in the periodNovember 2007-March 2010 [9]. The energy scale error of ARGO-YBJ is estimated tobe less than 18% in the energy range 1–30 TeV/Z.3. – The Crab NebulaWe observed the Crab signal with a significance of 14 s.d. in about 800 days at amedian energy of ∼ 2 TeV, without any event selection or gamma/hadron discriminationalgorithm. In almost 3 years of data taking the observed flux was consistent with asteady emission, and the obtained differential energy spectrum in the 0.3–30 TeV range isdN/dE = (3.0±0.3)·10−11 ·(E/1TeV)−2.74±0.14 cm−2 s−1 TeV−1 in agreement with other30 G. DI SCIASCIO for the ARGO-YBJ COLLABORATIONmultiplicity210 310) °(σang. res. 00.20.40.60.811.21.41.61.82DATAMCFig. 2. – Measured angular resolution of the ARGO-YBJ detector compared to expectationsfrom MC simulation as a function of the multiplicity. The used multiplicity bins are shown bythe horizontal errors.measurements. The distribution of the daily excess significances in the period November2007-October 2010 can be fitted with a Gaussian function with mean value 0.31 ± 0.03and r.m.s. = 0.99±0.02 (χ2red = 15/10), showing that the detector is performing correctlyand that the Crab flux is stationary on one-day scale over the investigated period.4. – The blazar Markarian 421Mrk421 is one of the brightest blazars and the closest to us (z = 0.031), characterizedby a strong broadband flaring activity with a variability time scale ranging fromminutes to months. A correlation of VHE gamma-rays with X-rays has been observedduring single flaring episodes by different detectors and can be interpreted in termsof the Synchrotron Self-Compton (SSC) model. The Mrk421 high variability makesthe long-term multiwavelength observation very important to constrain the emissionmechanisms models.Mrk421 was the first source detected by the ARGO-YBJ experiment in July 2006 whenthe detector started recording data with only the central carpet and was in commissioningphase. ARGO-YBJ has monitored Mrk421 for more than 2 years, studying the correlationof the TeV flux with X-ray data. We observed this source with a total significance of about12 s.d., averaging over quiet and active periods. Indeed, as is well known, this AGN ischaracterized by a strong flaring activity both in X-rays and in TeV γ-rays. ARGO-YBJdetected different TeV flares in correlation with X-ray observations, as can be seen fromfig. 3 where the ARGO-YBJ cumulative events per day is compared with the cumulativeevents per second of the RXTE/ASM and Swift satellites. To get simultaneous data,556 days have been selected for this analysis. The X-ray/TeV correlation is quite evidentover more than 2 years. The steepness of the curve gives the flux variation, therefore weobserved an active period at the beginning of 2008 followed by a quiet phase.Cross-correlation functions were measured to quantify the degree of correlation andthe phase difference (time lag) between the variations in X-ray and TeV bands, usingthe Discrete Correlation Function (DCF) [12]. The DCF gives the linear correlation co-efficient for two light curves as a function of the time lag between them. For X-rays weconsider two different bands (2–12 and 9–15 keV). The distributions peak around zeroand the correlation coefficients around zero are ∼ 0.78. The peak positions, evaluated byfitting the distributions with a Gaussian function around the maximum, are −0.14+0.86−0.85HIGHLIGHTS FROM THE ARGO-YBJ EXPERIMENT 31Fig. 3. – (Color online) Cumulative light curve from Mrk421 measured by ARGO-YBJ (redcurve) compared with RXTE/ASM (black curve) and Swfit (blue curve) X-ray data.and −0.94+1.05−1.07 days for RXTE/ASM and Swift data, respectively. Therefore, no signif-icant time lag is found between X-ray data and TeV ARGO-YBJ observations.In order to study the Spectral Energy Distribution (SED) of Mrk421 at different fluxlevels, both X-ray and TeV data have been divided into 4 bands based on the RXTE/ASMcounting rate: 0–2, 2–3, 3–5, > 5 cm−2 s−1. For each band a SED with the average flux iscalculated for different X-ray and γ-ray energies. The RXTE/ASM detector is monitoringMrk421 in three energy bands: 1.5–3, 3–5 and 5–12 keV. By assuming a power law X-rayenergy spectrum we calculated the spectral indices in the 4 flux bands: −2.02 ± 0.08,−2.05±0.03, −2.15±0.03 and −2.43±0.04. A correlation is clearly recognized in the sensethat higher fluxes are accompanied by harder energy spectra. This result, obtained byaveraging the Mrk421 emission over about 2 years, extends similar conclusions obtainedby averaging observations over much shorter periods [13].With this information we studied the relation between TeV spectral index and flux.The TeV flux measured by ARGO-YBJ in the different bands ranges from 0.9 to about7 Crab units. As shown in fig. 4 the TeV spectral index hardens with increasing fluxin agreement with the results obtained by the Whipple Collaboration in 2002 (continu-ous line) [14]. This conclusion generalizes our result obtained with the analysis of theJune 2008 flare [1]. We investigated also the correlation between RXTE/ASM X-ray Flux >1 TeV [Crab]1 2 3 4 5 6 7 Differential spectral index-2.8-2.6-2.4-2.2-2-1.8-1.6Fig. 4. – Mrk421 spectral index vs. flux above 1 TeV in Crab units. The solid line is the functionobtained in [14].32 G. DI SCIASCIO for the ARGO-YBJ COLLABORATIONFig. 5. – ARGO-YBJ > 1 TeV flux vs. RXTE/ASM X-ray flux. Solid line: quadratic best fit(χ2/d.o.f. = 1.9/2). Dotted line: linear best fit (χ2/d.o.f. = 7.7/2).and ARGO-YBJ TeV fluxes (fig. 5). A positive correlation is clearly observed and theX-ray/TeV relation seems to be quadratic rather than linear, in agreement with theresults of [15].5. – MGRO J1908+06The gamma-ray source MGRO J1908+06 was discovered by the MILAGRO [16] ata median energy of ∼ 20 TeV and recently associated with the Fermi pulsar 0FGLJ1907.5+0602 [17]. The data were consistent both with a point source and with anextended source of diameter < 2.6◦. HESS confirmed the discovery with the detec-tion of the extended source HESS J1908+063 [18] at energies above 300 GeV, position-ally consistent with the MILAGRO source. The extension of the source was extimatedσext = 0.34◦+0.04−0.03 . The Milagro and HESS energy spectra are in disagreement at a levelof 2–3 s.d., the Milagro result being about a factor 3 higher at 10 TeV [19].Fig. 6. – (Color online) Differential flux from MGRO J1908+06 measured by ARGO-YBJ (redline) compared with MILAGRO and HESS spectra. The shaded band represents 1 s.d. error.The black dashed line represents the Crab Nebula flux.HIGHLIGHTS FROM THE ARGO-YBJ EXPERIMENT 33ARGO-YBJ observed a TeV emission from MGRO J1908+06 with a significance of6 s.d. in about 730 days. At the ARGO-YBJ site, MGRO J1908+06 culminates at therelatively low zenith angle of 24◦ and is visible for 5.4 hours per day with a zenith angleless than 45◦. By assuming a 2D Gaussian source shape with r.m.s = σext, independentof the gamma-ray energy, we fitted the event distribution as a function of the distancefrom the center (set to the Fermi pulsar position) and, taking into account the PSF, wefound σext = 0.48◦+0.26−0.28 , a value larger but consistent with the HESS measurement.The best-fit power law spectrum is: dN/dE = 3.6+0.7−0.8 × 10−13 (E/6TeV)−2.20+0.34−0.29photons cm−2 s−1 TeV−1 (the errors on the parameters are statistical) (fig. 6). The sys-tematic errors are mainly related to the background evaluation and to the uncertaintyin the absolute energy scale. According to our estimate, they globally affect the quotedfluxes for ≤ 30%. Given the flat slope, above a few TeV the MGROJ1908+06 flux be-comes larger than the Crab Nebula one. A significant disagreement appears between theARGO-YBJ and HESS spectra. Concerning ARGO-YBJ and MILAGRO, consideringthe large errors in the flux measurements (the error on the MILAGRO flux is ∼ 30% at10 TeV and larger at lower energies), the apparent disagreement could be likely due tostatistical fluctuations. In the limit of the statistical accuracy of this result, our datasupports the MILAGRO measurement of a flux significantly larger than that measuredby HESS. One possible cause of this discrepancy is that ARGO-YBJ and MILAGROintegrate the signal over a solid angle larger than the HESS one, and likely detect moreof the diffuse lateral tail of the extended source. A further interesting possibility is thevariability of the source, since the HESS data consist of 27 hours of observations in2005-2007, before our measurements.REFERENCES[1] Aielli G. et al., Astrophys. J., 714 (2010) L208.[2] Aielli G. et al., Astrophys. Phys., 30 (2008) 85.[3] Aielli G. et al., Astrophys. J., 699 (2009) 1281.[4] Vernetto S. et al., in Proceedings of the 31st ICRC, edited by Giller M. and SzabelskiJ. (University of Lodz, Lodz) 2009, 0398, arXiv:0907.4615 preprint (2009).[5] Abdo A. A. et al., Phys. Rev. Lett., 101 (2008) 221101.[6] Malkov M. A. et al., Astrophys. J., 721 (2010) 750 and references therein.[7] Aielli G. et al., Phys. Rev. D, 80 (2009) 092004.[8] Di Sciascio G. and Iuppa R., in Proceedings of the CRIS 2010, edited by Caruso R.,Insolia A., Maccarone M. C. and Pirronello V. (Elsevier, Amsterdam) in press,arXiv:1011.2637 preprint (2010).[9] Di Sciascio G., in Proceedings of the Frontier Objects in Astrophysics and ParticlePhysics Workshop, edited by Mannocchi G. and Giovannelli F. (SIF, Bologna) inpress, arXiv:1010.4401 preprint (2010).[10] Aielli G. et al., Nucl. Instrum. Methods A, 562 (2006) 92.[11] Aielli G. et al., Nucl. Instrum. Methods A, 608 (2009) 246.[12] Edelson R. R. and Krolik J. H., Astrophys. J., 333 (1988) 646.[13] Rebillot P. F. et al., Astrophys. J., 641 (2006) 740.[14] Krennrich F. et al., Astrophys. J., 575 (2002) L9.[15] Fossati G. et al., Astrophys. J., 677 (2008) 906.[16] Abdo A. A. et al., Astrophys. J., 664 (2007) L91.[17] Abdo A. A. et al., Astrophys. J., 700 (2009) L127.[18] Aharonian F. et al., Astron. Astrophys., 499 (2009) 723.[19] Smith A. J. et al., in Proceedings of the Fermi Symposium 2009, eConf ProceedingsC0911022, arXiv:1001.3695 preprint (2010).

GRIPS and its strong connections to the GeV and TeV bands

Abstract

Similar works

Full text

Available Versions

Scientific Open-access Literature Archive and Repository