Observation of Extended VHE Emission from the Supernova Remnant IC 443 with VERITAS
VERITAS Collaboration, V. A. Acciari, E. Aliu, T. Arlen, T. Aune, M. Bautista, M. Beilicke, W. Benbow, S. M. Bradbury, J. H. Buckley, V. Bugaev, Y. Butt, K. Byrum, A. Cannon, O. Celik, A. Cesarini, Y. C. Chow, L. Ciupik, P. Cogan, P. Colin, W. Cui, M. K. Daniel, R. Dickherber, C. Duke, V. V. Dwarkadas, T. Ergin, S. J. Fegan, J. P. Finley, G. Finnegan, P. Fortin, L. Fortson, A. Furniss, D. Gall, K. Gibbs, G. H. Gillanders, S. Godambe, J. Grube, R. Guenette, G. Gyuk, D. Hanna, E. Hays, J. Holder, D. Horan, C. M. Hui, T. B. Humensky, A. Imran, P. Kaaret, N. Karlsson, M. Kertzman, D. Kieda, J. Kildea, A. Konopelko, H. Krawczynski, F. Krennrich, M. J. Lang, S. LeBohec, G. Maier, A. McCann, M. McCutcheon, J. Millis, P. Moriarty, R. A. Ong, A. N. Otte, D. Pandel, J. S. Perkins, M. Pohl, J. Quinn, K. Ragan, L. C. Reyes, P. T. Reynolds, E. Roache, H. J. Rose, M. Schroedter, G. H. Sembroski, A. W. Smith, D. Steele, S. P. Swordy, M. Theiling, J. A. Toner, L. Valcarcel, A. Varlotta, V. V. Vassiliev, S. Vincent, R. G. Wagner, S. P. Wakely, J. E. Ward, T. C. Weekes, A. Weinstein, T. Weisgarber, D. A. Williams, S. Wissel, M. Wood, B. Zitzer
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Preprint typeset using L A TEX style emulateapj v. 08/22/09
OBSERVATION OF EXTENDED VHE EMISSION FROM THE SUPERNOVA REMNANT IC 443 WITH VERITAS
V. A. A
CCIARI , E. A LIU , T. A RLEN , T. A UNE , M. B AUTISTA , M. B EILICKE , W. B ENBOW , S. M. B RADBURY , J. H. B UCKLEY ,V. B UGAEV , Y. B UTT , K. B YRUM , A. C ANNON , O. C ELIK , A. C
ESARINI , Y. C. C HOW , L. C IUPIK , P. C OGAN , P. C OLIN ,W. C UI , M. K. D ANIEL , R. D
ICKHERBER , C. D UKE , V. V. D WARKADAS , T. E RGIN , S. J. F EGAN , J. P. F
INLEY ,G. F INNEGAN , P. F ORTIN , L. F
ORTSON , A. F URNISS , D. G ALL , K. G IBBS , G. H. G ILLANDERS , S. G ODAMBE ,J. G RUBE , R. G UENETTE , G. G YUK , D. H ANNA , E. H AYS , J. H OLDER , D. H ORAN , C. M. H UI , T. B. H UMENSKY ,A. I
MRAN , P. K AARET , N. K ARLSSON , M. K ERTZMAN , D. K IEDA , J. K ILDEA , A. K ONOPELKO , H. K RAWCZYNSKI ,F. K RENNRICH , M. J. L ANG , S. L E B OHEC , G. M AIER , A. M C C ANN , M. M C C UTCHEON , J. M ILLIS , P. M ORIARTY ,R. A. O NG , A. N. O TTE , D. P ANDEL , J. S. P ERKINS , M. P OHL , J. Q UINN , K. R AGAN , L. C. R EYES , P. T. R EYNOLDS ,E. R OACHE , H. J. R OSE , M. S CHROEDTER , G. H. S EMBROSKI , A. W. S MITH , D. S TEELE , S. P. S WORDY , M. T HEILING ,J. A. T ONER , L. V ALCARCEL , A. V ARLOTTA , V. V. V ASSILIEV , S. V INCENT , R. G. W AGNER , S. P. W AKELY , J. E. W ARD ,T. C. W EEKES , A. W EINSTEIN , T. W EISGARBER , D. A. W ILLIAMS , S. W ISSEL , M. W OOD , B. Z ITZER Draft version October 24, 2018
ABSTRACTWe present evidence that the very-high-energy (VHE, E >
100 GeV) gamma-ray emission coincident withthe supernova remnant IC 443 is extended. IC 443 contains one of the best-studied sites of supernova rem-nant/molecular cloud interaction and the pulsar wind nebula CXOU J061705.3+222127, both of which areimportant targets for VHE observations. VERITAS observed IC 443 for 37.9 hours during 2007 and detectedemission above 300 GeV with an excess of 247 events, resulting in a significance of 8.3 standard deviations ( σ )before trials and 7.5 σ after trials in a point-source search. The emission is centered at 6h16m51s + ′ ′′ (J2000) ± . stat ± . sys , with an intrinsic extension of 0 . ± . stat ± . sys . The VHE spectrumis well fit by a power law ( dN / dE = N × ( E / TeV) - Γ ) with a photon index of 2 . ± . stat ± . sys and anintegral flux above 300 GeV of (4 . ± . stat ± . sys ) × - cm - s - . These results are discussed in thecontext of existing models for gamma-ray production in IC 443. Subject headings: gamma rays: observations — ISM: individual (IC 443 = VER J0616.9+2230 = MAGICJ0616+225) Fred Lawrence Whipple Observatory, Harvard-Smithsonian Center forAstrophysics, Amado, AZ 85645, USA Department of Physics and Astronomy and the Bartol Research Institute,University of Delaware, Newark, DE 19716, USA Department of Physics and Astronomy, University of California, Los An-geles, CA 90095, USA Santa Cruz Institute for Particle Physics and Department of Physics, Uni-versity of California, Santa Cruz, CA 95064, USA Physics Department, McGill University, Montreal, QC H3A 2T8,Canada Department of Physics, Washington University, St. Louis, MO 63130,USA School of Physics and Astronomy, University of Leeds, Leeds, LS2 9JT,UK Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cam-bridge, MA 02138, USA Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439,USA School of Physics, University College Dublin, Belfield, Dublin 4, Ire-land School of Physics, National University of Ireland, Galway, Ireland Astronomy Department, Adler Planetarium and Astronomy Museum,Chicago, IL 60605, USA Department of Physics and Astronomy, University of Utah, Salt LakeCity, UT 84112, USA Department of Physics, Purdue University, West Lafayette, IN 47907,USA Department of Physics, Grinnell College, Grinnell, IA 50112-1690,USA Department of Astronomy and Astrophysics, University of Chicago,Chicago, IL, 60637 Department of Physics and Astronomy, Barnard College, ColumbiaUniversity, NY 10027, USA N.A.S.A./Goddard Space-Flight Center, Code 661, Greenbelt, MD20771, USA Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS/IN2P3, F- INTRODUCTION
IC 443 (G189.1 +3.0) is one of the most thoroughly stud-ied supernova remnants (SNRs) and remains one of the clear-est examples of an SNR interacting with molecular clouds.IC 443 has a double-shell structure in the optical and radio(Braun & Strom 1986). The northeast shell’s interaction withthe H II region S249 places IC 443 at a distance of 1 . Enrico Fermi Institute, University of Chicago, Chicago, IL 60637, USA Department of Physics and Astronomy, Iowa State University, Ames,IA 50011, USA Department of Physics and Astronomy, University of Iowa, Van AllenHall, Iowa City, IA 52242, USA Department of Physics and Astronomy, DePauw University, Greencas-tle, IN 46135-0037, USA Department of Physics, Pittsburg State University, 1701 South Broad-way, Pittsburg, KS 66762, USA Department of Physics, Anderson University, 1100 East 5th Street, An-derson, IN 46012 Department of Life and Physical Sciences, Galway-Mayo Institute ofTechnology, Dublin Road, Galway, Ireland Kavli Institute for Cosmological Physics, University of Chicago,Chicago, IL 60637, USA Department of Applied Physics and Instrumentation, Cork Institute ofTechnology, Bishopstown, Cork, Ireland * Corresponding author: [email protected] † Currently at N.A.S.A./Goddard Space-Flight Center, Code 661, Green-belt, MD 20771, USA ‡ Currently at Department of Physics, Durham University, Durham, DH13LE, UK § Currently at Laboratoire Leprince-Ringuet, Ecole Polytechnique,CNRS/IN2P3, F-91128 Palaiseau, France
Acciari et al.region to the southwest. The age of IC 443 remains uncer-tain, with various estimates placing it in the range ∼ ∼ M ⊙ (Torres et al. 2003). Dickman et al.(1992) and Lee et al. (2008) estimate that ∼ ⊙ of cloud material have been directly perturbed by the SNRshock.IC 443’s X-ray emission is primarily thermal and peakedtowards the interior of the northeast shell (Petre et al. 1988;Troja et al. 2006), placing IC 443 in the mixed-morphologyclass of SNRs. Observations by Chandra (Olbert et al. 2001;Gaensler et al. 2006) and XMM (Bocchino & Bykov 2001) ofthe southern edge of the shell have resolved a pulsar wind neb-ula (PWN), CXOU J061705.3+222127, that may be the com-pact remnant of IC 443’s progenitor. Although the directionof motion of the neutron star, inferred from the morphologyof the X-ray PWN, does not point back to the center of theshell, asymmetric expansion of the shell and distortion of thePWN tail by turbulence in the ambient medium could be ex-aggerating the apparent misalignment (Gaensler et al. 2006).No pulsations have been detected at any wavelength.The centroid of the unidentified EGRET source 3EGJ0617+2238 is located near the center of the IC 443 shell,but the limited angular resolution of EGRET makes it im-possible to resolve the ∼ ′ extension of the remnant com-plex. The EGRET source has a spectral index of - . ± .
06 over the band 100 MeV - 30 GeV and an integralflux above 100 MeV of 5 × - cm - s - (Hartman et al.1999). AGILE (AGL J0617+2236) and the Fermi Gamma-ray Space Telescope (0FGL J0617.4+2234) have recently re-ported gamma-ray detections with position and flux consis-tent with EGRET (Pittori et al. 2009; Abdo et al. 2009). IC443 was detected in very-high-energy (VHE, E >
100 GeV)gamma rays by MAGIC (Albert et al. 2007) and VERITAS(Humensky et al. 2007). MAGIC J0616+225 is a point-likesource coincident with the densest part of the molecularcloud and the OH maser emission observed therein. It hasa power-law spectrum described by (1 . ± . stat ± . sys ) × - ( E / . - . ± . stat ± . sys TeV - cm - s - in the range0 . . OBSERVATIONS AND ANALYSIS
VERITAS (Holder et al. 2006) consists of four 12-m imag-ing atmospheric Cherenkov telescopes located at an altitudeof 1268 m a.s.l. at the Fred Lawrence Whipple Observatoryin southern Arizona, USA (31° 40’ 30” N, 110° 57’ 07” W).Each telescope is equipped with a 499-pixel camera of 3 . σ in less than 50 hours at 20° zenithangle.VERITAS observed IC 443 during two epochs in 2007.Data set 1 was acquired with three telescopes during thecommissioning phase in February and March of 2007. The PWN location of 6h17m5 . + ′ ′′ (J2000) was trackedin wobble mode, in which the source is displaced by 0 . . + ′ ′′ (J2000). Application of stan-dard quality-selection cuts on weather conditions and hard-ware and rate stability resulted in 16.8 hours live time in dataset 1 and 20.9 hours in data set 2, with mean zenith anglesof 20° and 17°, respectively. Data set 2 is used for the spec-tral analysis. In this data set, 0.8 hours were taken with anon-standard array trigger configuration, and these runs areexcluded from the spectral analysis.The data are analyzed following standard procedures de-scribed in Cogan et al. (2007) and Daniel et al. (2007). Show-ers are reconstructed for events in which at least two telescopeimages passed pre-selection criteria: number of pixels in thecamera image >
4, number of photoelectrons in the image >
75, and distance of the image centroid from the cameracenter < of 300 GeV.The event-selection cuts are optimized for weak ( ∼ mean-scaled width (selecting events within the range0.05-1.24) and mean-scaled length (selecting events withinthe range 0.05-1.40) parameters, as described in Konopelko(1995). The ring-background model (Aharonian et al. 2005)is used to study the morphology and the reflected-regionmodel (Aharonian et al. 2001) is used to determine the spec-trum. Regions of radius 0.4° around the source location and0.3° around two bright stars are excluded from the back-ground estimation. All results presented here have been veri-fied by an independent calibration and analysis chain; in par-ticular, the morphology was cross-checked using a templatebackground model (Rowell 2003) because it is sensitive toa different combination of systematic effects than the ring-background model; this is especially relevant for fields thatcontain bright stars.One systematic issue particular to this data set is the pres-ence of two bright stars in the IC 443 field: Eta Gem (V bandmagnitude 3.31), located 0.53° from the PWN; and Mu Gem(V band magnitude 2.87), located 1.37° from the PWN. Thehigh flux of optical photons from the stars requires that sev-eral photomultiplier tubes (PMTs) in each camera be turnedoff during the observations and produces higher noise levelsin the signals of nearby PMTs. These effects reduce the ex-posure in the vicinity of the stars and degrade the angular res-olution. A high telescope-multiplicity requirement, describedbelow, was chosen for studying morphology in order to makethe direction reconstruction more robust against these effects.In order to verify that the presence of a bright star in the fielddoes not produce a false excess, a 4 . RESULTS Peak of the differential counting rate for a Crab Nebula-like spectrum. bservations of IC 443 with VERITAS 3Here an updated, standard point-source analysis of thecombined data set (1 & 2) is presented, in which a maxi-mum significance of 8.3 standard deviations ( σ ) before tri-als (7.5 σ post trials, accounting for a blind search overthe region enclosed by the shell of IC 443) is found at06h16m49s+22°28 ′ ′′ (J2000) and a significance of 6.8 σ at the location of MAGIC J0616+225. The significance iscalculated according to equation 17 of Li & Ma (1983). Ta-ble 1 summarizes the results of this analysis at the locationof maximum significance, listing the counts falling within thepoint-source integration radius of 0.112° ( on ), the counts in-tegrated in a background ring spanning radii 0.6-0.8° ( o f f ),the ratio of on exposure to o f f exposure ( α ), and the resultingnumber of excess counts and significance. Morphology
Figure 1 shows the significance map for the IC 443 field. Tostudy the source morphology, an integration radius of 0.112°is used and the best-reconstructed gamma rays are selectedby requiring that an event must have all 3 images and all4 images surviving the pre-selection cuts in data sets 1 and2, respectively. This requirement increases the analysis en-ergy threshold by ∼
15% and reduces the excess by ∼ of the instrument, to anacceptance-corrected uncorrelated excess map with a bin sizeof 0.05°. The PSF has a 68% containment radius of 0.11°.The centroid is located at 06h16m51s+22°30 ′ ′′ (J2000) ± . stat ± . sys , consistent with the MAGIC position. Theextension derived in this fashion is 0 . ± . stat ± . sys .The difference in extension between this work and the point-like source detected by Albert et al. (2007) can be explainedby the difference in angular resolution and sensitivity betweenVERITAS and MAGIC, the latter of which has an angularresolution of ∼ .
15° for 68% containment and a sensitivityabove 200 GeV of 2% of the Crab Nebula flux in 50 hours(Albert et al. 2008).
Spectrum
The threshold for the spectral analysis is 300 GeV; theenergy resolution is less than ∼
20% at 1 TeV. The pho-ton spectrum, integrated within a radius of 0.235°, is shownin Figure 2. The photon spectrum is well fit ( χ / ndf =3 . /
3) by a power law dN / dE = N × ( E / TeV) - Γ in the range0 . . . ± . stat ± . sys ) × - TeV - cm - s - and an index of 2 . ± . stat ± . sys . The integral flux above 300 GeV is (4 . ± . stat ± . sys ) × - cm - s - (3.2% of the Crab Nebulaflux), consistent within errors with the spectrum reported byMAGIC (Albert et al. 2007). DISCUSSION AND CONCLUSIONS
Figure 3 shows the inner 0 .
8° of the excess map and placesthe VHE emission in a multi-wavelength context. The TeVcentroid is ∼ .
15° from the position of the PWN and ∼ . The PSF is characterized as a sum of two, two-dimensional Gaussiansdescribing a narrow core and a broader tail. It is determined from data takenon the Crab Nebula, which is a point source at these energies (Albert et al.2008). remnant, east of the PWN (Hewitt et al. 2006). The VHEemission overlaps the foreground molecular cloud. While theFermi source 0FGL J0617.4+2234 is displaced from the cen-troid of the VHE emission by ∼ ∼ . . . × erg s - . The spin-down luminosity of the pul-sar has been estimated as ∼ erg s - (Olbert et al. 2001;Bocchino & Bykov 2001) and 5 × erg s - (Gaensler et al.2006). If the PWN association is correct, the VHE luminosityin the 0 . . ∼ .
04% of the spin-downluminosity, well within the ∼ . τ of a synchrotron-emitting electronwith energy E is τ ( E ) ∼ . × ( B /µ G) - ( E / - kyr(Gaisser et al. 1998). Assuming the TeV photons are pro-duced by ∼ ∼ µ G(comparable to typical interstellar fields), synchrotron coolingis only just beginning to become important even if the age ofIC 443 is as high as ∼
30 kyr. Note that in the nebula-poweredscenario presented by Bartko & Bednarek (2008), the GeVemission is assumed to be unresolved pulsar emission. Thedisplacement of 0FGL J0617.4+2234 from the PWN locationargues against that scenario.Alternatively, Figure 3 can be interpreted within a scenarioof hadronic cosmic-ray acceleration and subsequent interac-tion with the molecular cloud, which would provide a highdensity of target material for the production of VHE gammarays. The correlation of the VHE emission with the molec-ular cloud is natural within this scenario. The low velocityof the SNR shock implied by the presence of maser emis-sion (Hewitt et al. 2006) indicates that the shock is in its ra-diative phase in this region and cosmic-ray acceleration ismost likely now inefficient. Thus, the cosmic rays acceler-ated during the shock’s earlier propagation have diffused intothe remnant and the upstream region, including the molec-ular cloud. The steep VHE spectrum can be explained ei-ther as a low maximum energy to which particles were ac-celerated prior to the shock hitting the cloud, or an energy-dependent rate of diffusion of cosmic rays out of the cloud(Aharonian & Atoyan 1996). Zhang & Fang (2008) modelthe remnant as evolving partially within and partially outsidethe cloud, and find that a hadronic interpretation can explainthe VHE emission. However, Torres et al. (2008) point outthat in the Zhang & Fang (2008) model, the GeV and TeVemission ought to be spatially coincident; it is not yet clearwhether they are. Torres et al. (2008) present an alternativepicture in which cosmic rays escape from the SNR shock anddiffuse toward the cloud, similar to what may be occurringin the Galactic Ridge (Aharonian et al. 2006b) and the regionsurrounding the SNR W 28 (Aharonian et al. 2008). This pic-ture can naturally accommodate spatially separated GeV and Acciari et al.TeV emission by positing clouds at different distances fromthe SNR shock. Rodriguez Marrero et al. (2009) extend thiswork to make predictions for the Fermi Gamma-ray SpaceTelescope (Fermi) and find that Fermi should see a movementof the centroid towards the TeV position as the photon en-ergy increases, along with a gradual spectral softening. Thecombination of GeV and TeV observations may also provideadditional constraints on the diffusion coefficient and the dis-tance between the SNR and the molecular cloud.Further GeV and TeV gamma-ray observations will helpto clarify the nature of the particle acceleration associatedwith IC 443. The ∼ . ± . stat ± . sys and a flux of (4 . ± . stat ± . sys ) × - cm - s - above300 GeV. The location and flux are consistent with MAGICJ0616+225. This deep VERITAS observation reveals thatthe VHE emission is extended (0 . ± . stat ± . sys ),with its brightest region coincident with the dense cloud ma-terial and maser emission. The emission is offset from thenearby PWN CXOU J061705.3+222127, a viable source forthe VHE emission. If further VHE observations reveal anenergy-dependent morphology, it may become possible to dis-tinguish cleanly between scenarios related to the PWN and tohadronic cosmic rays interacting with the molecular cloud.This research is supported by grants from the US Depart-ment of Energy, the US National Science Foundation, and theSmithsonian Institution, by NSERC in Canada, by ScienceFoundation Ireland, and by STFC in the UK. We acknowledgethe excellent work of the technical support staff at the FLWOand the collaborating institutions in the construction and op-eration of the instrument. Some of the simulations used inthis work have been performed on the Joint Fermilab - KICPSupercomputing Cluster. Facilities:
FLWO:VERITAS.
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VERITAS
OBSERVATIONS OF
IC 443. on o f f α excess significance( σ )data set 1 393 5152 0.0610 78.7 4.1data set 2 609 7331 0.0601 168.1 7.3combined 1002 12483 0.0604 247.5 8.3 Acciari et al. ) − s − * F l u x ( T e V c m . E −13 −12 −11 −10 VERITASMAGICVERITAS Crab
Energy (TeV) R es i du a l s −1−0.500.51 F IG . 2.— Spectrum of IC 443, scaled by E . Red points are the VERITASspectrum (red tick marks along top indicate bin edges). The red line is apower-law fit (see text), with residuals to the fit plotted in the lower box. TheMAGIC spectrum is indicated by the gray band and extends down to 90 GeV.VERITAS error bars and MAGIC error band reflect statistical errors only. TheCrab spectrum (V. Acciari et al. 2009, in preparation) is shown as a dashedline for comparison. bservations of IC 443 with VERITAS 7 D ec li n a t i on ( D e g ) E xcess PSF F IG . 3.— Inner 0 ..