227 Acta Polytechnica CTU Proceedings 1(1): 227–230, 2014 227 doi: 10.14311/APP.2014.01.0227 The Galactic Center Region Imaged by VERITAS from 2010–2012 Matthias Beilicke1 for the VERITAS Collaboration2 1Department of Physics and McDonnell Center for the Space Sciences, Washington University, St. Louis, MO, USA 2http://veritas.sao.arizona.edu/ Corresponding author: beilicke@physics.wustl.edu Abstract The galactic center has long been a region of interest for high-energy and very-high-energy observations. Many potential sources of GeV/TeV γ-ray emission are located in this region, e.g. the accretion of matter onto the central black hole, cosmic rays from a nearby shell-type supernova remnant, or the annihilation of dark matter. The galactic center has been detected at MeV/GeV energies by EGRET and recently by Fermi/LAT. At TeV energies, the galactic center was detected at the level of 4 standard deviations with the Whipple 10 m telescope and with one order of magnitude better sensitivity by H.E.S.S. and MAGIC. We present the results from 3 years of VERITAS galactic center observations conducted at large zenith angles. The results are compared to astrophysical models. Keywords: gamma-rays - galactic center - black hole - non-thermal - VERITAS. 1 Introduction The center of our galaxy harbors a 4×106 M� black hole (BH) coinciding with the strong radio source Sgr A*. X-ray/MeV/GeV transients in this region are observed on a regular basis. Various astrophysical sources lo- cated close to the galactic center (GC) may potentially be capable of accelerating particles to multi-TeV ener- gies, such as the supernova remnant Sgr A East or a pulsar wind nebula [1]. Furthermore, super-symmetric neutralinos χ are discussed as potential candidates for dark matter accumulating in the GC region and anni- hilating into γ-rays [2]. The resulting spectrum would have a cut-off near the neutralino mass mχ. Assuming a certain dark matter density profile the expected γ-ray flux along the line-of-sight integral can be calculated as a function of mχ and the annihilation cross section [3] and can in turn be compared to measured upper limits. EGRET detected a MeV/GeV source 3EG J1746- 2851 coincident with the GC position [4] and recently Fermi/LAT resolved several sources in the GC region [5], see Fig. 3. However, uncertainties in the diffuse galactic background models and limited angular reso- lution at MeV/GeV energies make it difficult to study the morphologies of these sources. At GeV/TeV ener- gies a detection from the direction of the GC was first reported in 2001/02 by the CANGAROO II collabora- tion with a steep energy spectrum dN/dE ∝ E−4.6 at the level of 10% of the Crab Nebula flux [6]. Shortly after, evidence at the level of 3.7 standard deviations (s.d.) was reported from the Whipple 10 m collabora- tion [7]. The GC was finally confirmed as a GeV/TeV γ-ray source by the H.E.S.S. collaboration [8] (the posi- tion of the supernova remnant Sgr A East could be ex- cluded as the source of the γ-ray emission). The energy spectrum measured by H.E.S.S. is well described by a power-law dN/dE ∝ E−2.1 with a cut-off at ∼15 TeV. The H.E.S.S. observations revealed a diffuse GeV/TeV γ-ray component (dashed contour lines in Fig. 3) which is aligned along the galactic plane and follows the struc- ture of molecular clouds [9]; the emission is explained by an interaction of local cosmic rays (CRs) with mat- ter of the molecular clouds. The MAGIC collaboration detected the GC (7 s. d.) in 2004/05 observations per- formed at large zenith angles (LZA) [10], followed by a strong (> 10 standard deviations) VERITAS LZA de- tection in 2010 [11]. 2 VERITAS Observations of the Galactic Center GC observations Due to its declination the GC can only be observed by VERITAS at LZA (zenith angles 60 − 66 deg) – strongly decreasing the angular resolu- tion and sensitivity. The use of the displacement pa- rameter [12], between the center of gravity of the image and the shower position, has been used in the VERI- TAS event reconstruction which strongly improved the sensitivity for LZA observations [11]. The performance and energy reconstruction have been confirmed on LZA Crab Nebula data. The column density of the atmo- sphere changes with 1/ cos(z). In a conservative es- 227 http://dx.doi.org/10.14311/APP.2014.01.0227 Matthias Beilicke timate, the systematic error in the energy/flux recon- struction can be expected to scale accordingly. More detailed studies are needed for an accurate estimate; for the GC observations we currently give a conserva- tive value of a systematic error on the LZA flux nor- malization of ∆Φ/Φ ' 0.4. The GC was observed by VERITAS in 2010–2012 for 46 hrs (good quality data, dead-time corrected) with an average energy threshold of Ethr ' 2.5 TeV. GC results The VERITAS sky map of the GC region is shown in Fig. 3. An 18 s.d. excess is detected. No evidence for variability was found in the 3-year data. The energy spectrum is shown in Fig. 1 and is found to be compatible with the spectra measured by Whipple, H.E.S.S., and MAGIC. Since the large LZA effective ar- eas of the VERITAS observations compensate a shorter exposure of low-zenith observations, the statistical er- rors of the E > 2.5 TeV data points are comparable or even smaller than those of the H.E.S.S. measurements. ] -1 s -2 d N /d E [ e rg c m 2 E -1310 -1210 -1110 Energy [eV] 1210 13 10 V E R IT A S ( 2 0 1 0 -2 0 1 2 , p re li m .) galactic center VERITAS (GC) H.E.S.S. (GC) Whipple (GC) MAGIC (GC) Figure 1: VERITAS energy spectrum measured from the direction of the GC (statistical errors only). Also shown are bow ties representing the spectra measured by Whipple [7], H.E.S.S. [8], and MAGIC [10]. Diffuse flux limit and dark-matter annihilation OFF-source observations were performed in a field lo- cated in the vicinity of the GC region (similar zenith angles and sky brightness) without a known TeV γ-ray source. These observations are used to study the back- ground acceptance throughout the field of view and al- low the estimate of a diffuse γ-ray component surround- ing the position of the GC. An upper limit of the diffuse γ-ray flux can in turn be compared with line-of-sight integrals along the density profile ∫ ρ2dl, in order to constrain the annihilation cross section for a particular dark matter model, dark matter particle mass and den- sity profile ρ(r). Due to its likely astrophysical origin the excess at the GC itself, as well as a region along the galactic plane, will be excluded from this analysis (work in progress). Hadronic models Hadronic acceleration models [13, 14] involve: (i) hadrons being accelerated in the BH vicinity (few tens of Schwarzschild radii). (ii) The accel- erated protons diffuse out into the interstellar medium where they (iii) produce neutral pions which decay into GeV/TeV γ-rays. Linden et al. (2012) discuss the sur- rounding gas as proton target defining the morphology of the TeV γ-ray emission [15]. Changes in γ-ray flux in those models can be caused by changing conditions in the BH vicinity (e.g. accretion). The time scales of flux variations are ∼104 yr at MeV/GeV energies (old flares) and ∼10 yr at E > 10 TeV (’new’ flares caused by recently injected high-energy particles) [13]. Constrain- ing the E > 10 TeV spectral variability would serve as an important test for this class of models. ] -1 s -2 d N /d E [ e rg c m 2 E -1310 -1210 -1110 -1010 Energy [eV] 8 10 1110 1410 VERITAS (preliminary) galactic center VERITAS (GC) Fermi (Chernyakova et al., 2011) Ballantyne et al. (2011) [hdr] Chernyakova et al. (2011) [hdr] Linden et al. (2012) [hdr] Atoyan et al. (2004) [lep] Figure 2: VERITAS energy spectrum compared to hadronic [13, 14, 15] and leptonic [16] emission models discussed for the GC source. The Fermi/LAT bow tie is taken from [13]. Leptonic models Atoyan et al. (2004) [16] discuss a BH plerion model in which a termination shock of a leptonic wind accelerates leptons to relativistic energies which in turn produce TeV γ-rays via inverse Comp- ton scattering. The flux variability time scale in this model is on the order of Tvar ∼100 yr. The hadronic and the leptonic models are shown together with the VER- ITAS/Fermi data in Fig. 2. The leptonic model clearly fails in explaining the flux in the MeV/GeV regime. However, this emission may well originate from a spa- tially different region or mechanism other than the TeV γ-ray emission. The hadronic models can explain the SED by the superposition of different flare stages. Fu- ture Fermi/VERITAS flux correlation studies, as well as the measurement of the TeV energy cut-off and lim- its on the E > 10 TeV variability will serve as crucial inputs for the modeling. 3 Summary and Conclusion VERITAS is capable of detecting the GC within 3 hrs in observations conducted at zenith angles greater than 228 The Galactic Center Region Imaged by VERITAS from 2010–2012 Figure 3: VERITAS sky map of the GC region (smoothed excess significances, ring background, scale truncated). The black contour lines indicate the GC and the supernova remnant G 0.9+0.1 as seen by H.E.S.S. [8]. The gray dashed lines indicate the H.E.S.S. diffuse emission along the galactic plane and from HESS J1745-303 [9]. The position of HESS J1741-302 is indicated, as well (circle); the flux/spectrum of this source make it very unlikely to be detected in VERITAS LZA observations. The solid circles (cyan color) indicate the positions of the MeV/GeV sources taken from the second Fermi/LAT catalog [5]. 60 deg. The measured energy spectrum is found to be in agreement with earlier measurements by H.E.S.S., MAGIC, and Whipple. Future observations to measure the cut-off energy in the spectrum and to determine limits on the flux variability at the highest energies will place constraints on emission models. The recently dis- covered giant molecular cloud heading towards the im- mediate vicinity of the GC BH [17] represents further motivation for future TeV γ-ray monitoring of this re- gion. An upper limit on diffuse γ-ray emission and, in consequence, a limit on the photon flux initiated by the annihilation of dark matter particles is work in progress. Acknowledgement This research is supported by grants from the U.S. Department of Energy Office of Science, the U.S. Na- tional Science Foundation and the Smithsonian Insti- tution, by NSERC in Canada, by Science Foundation Ireland (SFI 10/RFP/AST2748) and by STFC in the U.K. We acknowledge the excellent work of the tech- nical support staff at the Fred Lawrence Whipple Ob- servatory and at the collaborating institutions in the construction and operation of the instrument. References [1] Q.D. Wang, F.J. Lu, E.V. Got- thelf, et al., MNRAS 367, 937 (2006). doi:10.1111/j.1365-2966.2006.09998.x [2] G. Jungman, M. Kamionkowski, & K. Griest, PhR 267, 195 (1996). [3] L. Bergström, P. Ullio, & J. Buckley, APh 9, 137 (1998). [4] R.C. Hartman, D.L. Bertsch, S.D. Bloom, et al., ApJS 123, 79 (1999). [5] A.A. Abdo, et al., ApJS 188, 405 (2010). doi:10.1088/0067-0049/188/2/405 [6] K. Tsuchiya, R. Enomoto, L.T. Ksenofontov, et al., ApJ 606, L115 (2004). doi:10.1086/421292 [7] K. Kosack, H.M. 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