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PoS(ICRC2015)795 http://pos.sissa.it/ ∗ ) to reveal a source on the scale of the Milagro de- ◦ [email protected] Speaker. tection. In this talk, we describe twovery approaches high being energy developed to gamma-ray search emission for surrounding angularly the extended Geminga . Geminga was first detected asservatory a gamma-ray and point the source COS-B by X-rayGeminga the satellite as SAS-2 observatory. a gamma-ray heavily satellite Subsequent obscured ob- observationsphase radio-quiet (300,000 have pulsar year) associated identified with remnant. a Thetected nearby Geminga by (250 pulsar the is pc) Large the late second Area Sedov brightest Telescopebeen source aboard frequently de- the advanced Fermi as gamma-ray a satellite sourceby (Fermi-LAT) of PAMELA, and Fermi-LAT, and the has AMS-2. anomalous It excess is ofObservations surrounded cosmic above by ray 10 a compact TeV reported X-ray by pulsardiffuse the wind gamma-ray water . halo Cherenkov around observatory Geminga extending MilagroVERITAS over IACT have several observatory also square has revealed degrees. performed a Since observationsregion. 2007 of the Geminga However, the and the standard surrounding methodssensitivity halo to of angularly source extended sources detection (>0.5 in VERITAS data have insufficient ∗ Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. c

The 34th International Cosmic Ray Conference, 30 July- 6 August, 2015 The Hague, The Netherlands University of Utah E-mail: Andy Flinders for the VERITAS collaboration VERITAS Observations of the Geminga Supernova Remnant PoS(ICRC2015)795 at 1 TeV. ◦ 57’W, 1.3km ◦ -ray sources. Additional 40’N, 110 γ ◦ ]. 9 , 8 ]. The Geminga supernova itself has 2 Andy Flinders for the VERITAS collaboration ]. These images are then used to determine 7 ]. The use of established point source analysis 6 -rays (as well as high energy cosmic rays) induce 2 γ ]. The Milagro water Cherenkov, observatory which has sen- ] supernova remnant. Due to its proximity to Earth, it may 5 1 ]. A later search for pulsed VHE emission in the VERITAS data 4 [ . The charged component of the shower produces Cherenkov radiation 1 − s 2 − ]) surrounding our . Supernova remnants contribute energy to the inter- 3 showers -rays from 85 GeV to > 35 TeV with an energy resolution of 15-25%. VERITAS γ photons cm ) PWN surrounding the pulsar location [ 12 ◦ − 10 The ring background model (RBM) and reflected-region model are widely used to estimate The Very Energetic Radiation Imaging Telescope Array System (VERITAS) is located at the Geminga is a nearby (250pc) [ When incident on the Earth’s atmosphere, ∗ 2 information regarding the VERITAS data analysis can be found in [ 3. Analysis strategy stereoscopic parameters of individual showers,position such on the as ground. the direction CosmicITAS data. on ray the showers Selection serve sky cuts as are andnumber the applied core of primary to background impact source the cosmic of combined ray background events image while in parameters optimizing VER- to sensitivity drastically to reduce the Fred Lawrence Whipple Observatory (FLWO) in southern Arizona (31 also yielded no significant detection [ sitivity in the range 1 to(about 4 100 TeV, has detected verytechniques high is energy emission not from suitable a fordevelopment highly of analyzing extended techniques data capable from of potentially analyzingmay highly highly detect extended extended extended source sources. emission data from it Withalso the is be the Geminga possible useful PWN that in as we analyzing seen other by potentially Milagro. extended sources. Such techniques will 2. VERITAS stellar medium primarily through theby shock the front spin-down of expanding luminosityemission ejecta of e.g. and the the the neutron Crab pulsar pulsar wind .Cherenkov wind driven telescope nebula array Pulsar (PWN) has and driven observed the windsVERITAS the Vela data PWN. Geminga commonly The pulsar using exhibit VERITAS location standard imaging VHE since air pointproduced 2007. source a Analysis techniques 99% of has the confidence yielded< level no limit significant for detection, a and steady/point source at energies above 300 GeV of particle cascades, or which is observed by the individual VERITAScharacterized telescope using cameras. Hillas The parameters resulting as camera described images in are [ is able to detectobservation a and has point an source angular resolution with (68% 1% containment radius) the of flux better than of 0.1 the within about 25 hours of VERITAS Observations of the Geminga Supernova Remnant 1. Introduction play a significant rolehas in been the put nature forth as ofexcess the our measured explanation local by for PAMELA, interstellar several Fermi-LAT, astrophysicalalso environment. and phenomena been AMS-2 including considered The [ the as Geminga as the pulsar the source local of bubble the [ low density region of the (known a.s.l.). The array consists ofand is four sensitive 12-meter to Imaging Atmospheric Cherenkov Telescopes (IACTs) PoS(ICRC2015)795 OFF -rays γ or data ON ]. The MLM 12 -LAT data [ Fermi means that the contribution of the back- Andy Flinders for the VERITAS collaboration α gives a visual representation of these tech- 1 ]. This background emission typically consists -ray emission and has a similar background rate. 10 3 γ ]. This approach allows a source filling the entire field 11 observation. This method has been successfully used by . A high value of ]. The second technique is a 3D maximum likelihood method α OFF 11 method) which selects a second field of view external to the source and observation, including its elevation angle, azimuthal angle, time of year, OFF ON / ON A search algorithm has been developed to help identify which fields of view are suitable back- Our first approach to extended source analysis is the matched run method. This method at- and cosmic-rays detected, as well as their energy distributions. When searching for a background (MLM) similar to the 2D likelihood analysis used in studies of of view of the instrumentobservation to would be be analyzed. taken immediately To before obtain orsame a after path the well in data matched observation elevation field is and taken, oftime azimuth. following view, compared the ideally to This the an is RBM or undesirable thea as RRM. different it Another field way requires of to view twice obtain that a as has good much no background observation evidence match is of to find ground estimation candidates.match There for are a many given factorshardware to configuration, consider night when sky backgroundsion identifying level, and a weather, possible etc. of thesource, Furthermore, source the the primary can difference right also being have ascen- whetherfactors a it effect is significant on the effect or background on off rate the ofThe of the background elevation galactic IACTs rate angle plane. to for at While some which a all degree, of an several these observation of is them made are has very a significant. drastic effect on the number of of cosmic ray hadrons andare distributed which throughout have the passed entire any fieldregion selection of to cuts view. estimate The applied the RBM to level method of the uses contaminationobservations data in a to and the ring region be around of the taken interest. source from offset The reflected-region the from requires camera the center source, as then the source assigns region. background Figure regions equally offset tempts to match a givenestimation. data The observation with second a field second of field view of needs view to to have a be similar used background for rate background to the VERITAS Observations of the Geminga Supernova Remnant the level of background emission in IACT data [ niques. The ratio of thethe area background region on is the known sky as designated as the source region to the area designated as presented below includes an added dimensionto based increase on sensitivity a to gamma/hadron extended discriminating sources. parameter 4. Matched Run Method observation and is often calledprevious the IACTs such as the Whipple 10m [ ground estimate’s uncertainty to the final uncertaintyBecause the in RBM the and significance RRM of require an apossible observation dedicated is to background larger. obtain region within suitable the values field ofFurthermore, of alpha view for it for is significantly significantly not extended extended sources sourcessource using photons the these leaking true methods. into morphology the isthe background source rarely regions under known, and investigation. VERITAS leading the is obvious currently to have implementing drawback been two of successfully analysis employed self techniques in which subtracting other VHEpresented experiments to by answer the extended observational sources. challenges known as The the first of theseuses it techniques for is background estimation the [ Matched Run Method (also PoS(ICRC2015)795 data ON observations and reduce the α of 1, meaning an OFF α -ray emission from standard γ (b) Reflected Region Method run results in an observations to be taken in the future. ON Andy Flinders for the VERITAS collaboration field of view for a RBM or reflected region OFF field of view can also decrease 4 OFF OFF observations or finding more suitable matched runs. OFF and reduce the statistical uncertainty of the background but comes α (a) Ring Background Method Once a suitable background observation has been found, there are several ways to analyze The matched run method is under development and has been used in several test cases. Testing could decrease the value of at the cost of either taking additional Another way to analyzeanalysis. the Using pair a is ring (as to in use the RBM) the in the the matched pair. One methoddisadvantage is is to that use only one using data one run background and run one per background run directly. The main Figure 1: The standard background estimationthe techniques, Reflected the Region Ring Method Background (right). Method (left) and match for a given data run,matched elevation angle in is the the current first parameter algorithmfected considered. is by The night the second right sky parameter ascension background. andof declination The year, of and night the elevation observation sky angle as background and well azimuth levelangle as angle is and weather of conditions, af- night the time sky observation. background Closelyeffective levels matching in between both elevation the finding source suitable and backgroundthat background regions observations we for has also been several require tests theand thus matched prefer far. run to It to use observations is havesources recorded the important continue, as same to other closely hardware note restrictions together configuration may inAt of time be this the as required time telescopes, possible. and no implemented Asobservations adjustments more into tests are closely, the on as being other search careful made matching algorithm. of to the the runs noise has been levels sufficient. of the pixels in order match the equal statistical error in the background and signal uncertainty. Using multiple VERITAS Observations of the Geminga Supernova Remnant statistical uncertainty of the background estimation.be Successful advantageous development for of this several technique reasons will including the ability to analyze previously recorded for extended sources, as well as not requiring further analysis techniques, as well as pointsources source testing, is has underway. gone well. Testing on moderately extended of the matched run method on fields of view lacking evidence of PoS(ICRC2015)795 ]. 14 (5.1) ] and ]. The 12 2 -ray showers γ -LAT [ Fermi -ray photons. The re- γ -LAT analysis this is sufficient ] as compared to 7 -ray events is expected to peak at γ Fermi resulting in images with much larger > 2 events can be quite similar leading to a i i 1 w w Andy Flinders for the VERITAS collaboration < 1 n -ray detectors, such as the = ∑ i γ  /hadron discriminating parameter known as mean 5 -ray-like γ 1 n γ  -ray and cosmic ray MSW distributions. The distinct = -ray. Observed (or reconstructed) energy is the energy of the photon γ γ -ray and γ MSW -LAT background from cosmic ray events is small compared to the that the MSW distribution for -ray sources. These events are the main source of background in VERITAS data. γ bins. Each IRF component is also computed in narrow logarithmic bins 5.1 Fermi ◦ 0.025 ], rely on spatial information alone. For the -ray and background cosmic ray components of our data are distributed in MSW. × γ 13 ◦ in steps of 0.05. The final source model for a given spatial bin is then computed is the true energy of an observed 3 ). Of the standard set of selection cuts used to reduce the level of background in events typically consist of cosmic ray electrons and/or hadrons which survive the standard analysis cuts 2 -ray-like γ This is due to the largertrue transverse energy momentum of the secondary pions produced in hadronic cosmic ray showers. The source models are based on the instrument response functions (IRFs) relevant to the VER- The maximum likelihood method (MLM) works by modeling the expected distribution of the MSW is determined as an average of the Hillas width parameters from each telescope image 1 2 3 1. Cosmic ray events typically produce broader showers sulting contribution to the contribution from background galactic diffuse photons.photons For from IACT detectors, point the sources localization is ofextended sufficient the sources to implement the a distribution purely of spatial MLM. However, for highly It can be seen from Eq. calculated widths. This leads to a(see peak Figure at higher values of MSW [ chosen to optimize sensitivity to as calculated by the VERITAS analysis. VERITAS data, a cut on MSW is amongevents the with most MSW>1.1 efficient. are For a cut. typicalto However, VERITAS source in provide analysis, the ample MLM separation analysis between the MSW the range is extended to 1.3 ITAS detector. The IRFs are combined based on the formulation of Mattox (1996) Eq. 2 [ reduced sensitivity of the analysis. Toanalysis overcome this, includes the a MLM third in dimension development based for on VERITAS data a Details regarding the VERITAS MLM, including informationground on spatial the model likelihood determination, construction, back- and characterization of the various IRFs can be found in [ difference in the shape ofability the to distinguish MSW source distribution emission for from these background as two opposed components to improves using the a MLM’s spatial only model. as the instrument also includesevents by an over anticoincidence 99.97% detector leading to for the reducing dominant background triggers cosmic being the ray result of scaled width (MSW). This third dimensionof is how incorporated both into the the likelihood by including models ∼ actual construction of the source modelmodels in is 0.025 made more tractable by computing the expected spatial of true energy data across a set ofnent observables. of This the data. includes Likelihood modeling analyses both used the in source other and background compo- normalized by the expectedimage width intensity derived and from shower core simulated impact air-showers distance from with the the telescope. same integrated VERITAS Observations of the Geminga Supernova Remnant 5. Maximum Likelihood Method ctools for CTA [ PoS(ICRC2015)795 300 200 100 0 900 800 700 600 500 400 3500 3000 2500 2000 1500 1000 500 0 2.5 2.5 2 2 -ray air-shower from known and γ ◦ Mean Scaled Width Mean Scaled Width 1.5 1.5 and the changing sensi- ◦ 1 1 -ray MSW distribution γ (b) 0.5 0.5 (d) Background MSW distribution Andy Flinders for the VERITAS collaboration 0 0 1 1 1 1 2 0 2 0

− −

1.5 1.5 1.5 0.5 0.5 1.5 0.5 0.5

10 true 10

rec − − − − [TeV]) (Energy log [TeV]) (Energy log

6

] [sec Events ] [sec Events

-1 -1 6 6 − − 10 10 0.04 0.02 0 18 16 14 12 10 8 6 4 2 0 0.12 0.1 0.08 0.06 × × 2 2 1.5 1.5 X-offset [Deg] X-offset [Deg] 1 1 0.5 0.5 0 0 0.5 0.5 − − 1 1 − − ]. This model is asymmetric due to the observation offset of 1 (c) Background spatial model 6 -ray sources. Additionally, the spatial background distribution uses events with MSW < (a) Extended source spatial model γ 1.5 1.5 − − 2 2 elevation, due south, with four telescopes participating in the shower reconstruction. − 1 1 − 1 1 2 0 2 2 0 2 − − − −

0.5 1.5 1.5 0.5 0.5 0.5 1.5 1.5

◦

− − − − Y-offset [Deg] Y-offset Y-offset [Deg] Y-offset Figure 2: (a) Extended source spatialscribed model in as [ derived from the Milagrotivity C3 of source the parameters instrument de- over thesimulations.(c) field Background of view. spatial (b) model MSW and distributionTAS data. derived (d) from Both background background distributions MSW are model derivedpotential from derived events greater from than VERI- 0.4 1.3 reflecting the range currently used70 in the MLM analysis. Distributions were derived assuming VERITAS Observations of the Geminga Supernova Remnant PoS(ICRC2015)795 (5.2) ). Spatial S 0 . Integrating k ). Each IRF is j dE , ) i s ~ | 0 E the observed energy. ( S E upper , 0 lower k , indexed by , 0 k E E ), the instrument point spread E’ Z B ) is assumed to be a Gaussian B dE ), and source spectrum ( ) 0 k R E , E ( n Andy Flinders for the VERITAS collaboration , m R max min E 7 E Z ) yields an estimate of the total number of observed ) -ray source with a rich discussion surrounding it in -ray emission which may emanate from this region. 0 k i,j γ γ E ( n ), energy dispersion ( , m A A ) j , i r ~ ( represents the true energy of an event, and n ) is summed at the position of interest (indexed by , n m ) at 35 TeV. The resulting source model was then derived for the E’ , 1 P , n m j − , , ). The extended source uses as input the extension and spectrum as i s m 2 2 r ~ B − n , m cm , ∑ k 1 − ) = s ] for the Milagro C3 source. The morphology ( ~ | j and the spectrum is modeled as a powerlaw with spectral index of -2.6 and flux of (TeV 6 , i ◦ ), effective collection area ( r ~ 17 ( P − src =1.3 S 10 σ This research is supported by grants from the U.S. Department of Energy Office of Science, To demonstrate how the inclusion of the MSW parameter aids in the detection of extended The presented extrapolation of the Milagro C3 source to IACT energies should only be consid- The Geminga pulsar is an interesting × the U.S. National Science Foundation andWe the acknowledge Smithsonian the Institution, excellent and work by of NSERC the in technical Canada. support staff at the Fred Lawrence Whipple parameterized for a given setcamera of noise observing level. conditions in The zeniththe IRFs angle, computed are azimuth, source also source model computed offset, over in and every bin narrow ( bins of These techniques are both progressing well anddata should in be the ready near to future. analyze the VERITAS Geminga 7. Acknowledgments sources, a comparison of the models ofbackground an (see extended source Figure were compared to those of the expected photons in a given dataset. connection with our stellarextended neighborhood. PWN would A contribute VERITAS significantly detectionthe to of local our interstellar either understanding medium. a of Two analysis point the techniqueswill origin source are aid and being or implemented in evolution an by of the VERITAS which study of spatially extended ered a test of the MLM.event In position reality, reconstruction due of to the theenergies Milagro higher may energy detector, have threshold any a and very emission larger different potentially morphology uncertainty seen and in at spectrum. the VERITAS 6. Conclusions where the components consist of thefunction intrinsic ( source morphology ( coordinates are given by To account for the smearing of eventsbin due in to the our map point (indexed spread by function, the contriubtion from every VERITAS energy range 0.5 -the 1 derived TeV. When extended compared source to modelsignificant the separation is expected between very background the two spatial similar. models. model, However, in the MSW dimension there is a reported in [ 37.7 with VERITAS Observations of the Geminga Supernova Remnant according to PoS(ICRC2015)795 ]. , A&A , Proceedings ]. , ]. The , -ray astronomy (2015). arXiv:0904.1018 γ (June, 2015) 23, Proceedings of the 19th , 218 (Apr., 1996) 396. ]. ApJS ]. , 461 astro-ph/0309063 arXiv:1307.5560 ]. ]. ApJ Proceedings of the 30th International , , Andy Flinders for the VERITAS collaboration ]. Proceedings of the 30th International Cosmic , (Aug., 2009) L127–L131, [ A Novel Method for Detecting Extended Sources Search for TeV Emission from Geminga by (July, 2013) [ 8 arXiv:0709.4233 700 arXiv:1412.4734 The trigonometric parallax of the Geminga (Dec., 2003) 909–917, [ arXiv:0709.4006 arXiv:0907.5237 (Aug., 1985) 445–448. (Apr., 2007) 225–230. 599 Background modelling in very-high-energy 308 astro-ph/0610959 (Feb., 2015) 61, [ (2008) 1385–1388, [ Search for High-Energy Gamma Rays from an X-Ray-selected Blazar The Geminga supernova as a possible cause of the Local Interstellar (July, 2009) [ 3 ]. 800 Towards a common analysis framework for gamma-ray astronomy (Feb., 1993) 706. The Astrophysical Journal The Likelihood Analysis of EGRET Data (2008) 1325–1328, [ Milagro Observations of Multi-TeV Emission from Galactic Sources in the Fermi , 3 361 Proceedings of the 34th International Cosmic Ray Conference Fermi Large Area Telescope Third Source Catalog The VERITAS standard data analysis , A Search for Pulsations from Geminga above 100 GeV with VERITAS Cerenkov light images of EAS produced by primary gamma ArXiv e-prints VEGAS, the VERITAS Gamma-ray Analysis Suite , The Astrophysical Journal Nature , , (May, 2007) 1219–1229, [ arXiv:1501.2003 [ 466 of the 33rd International Cosmic Ray Conference Ray Conference Sample Astrophysical Journal with VERITAS Bright Source List Astrophysics and Space Science Bubble International Cosmic Ray Conference Cosmic Ray Conference VERITAS [5] E. Aliu et al., [9] M. K. Daniel, [3] N. Gehrels and W. Chen, [6] A. A. Abdo et al., [7] A. M. Hillas, [8] P. Cogan, [1] J. Faherty, F. M. Walter, and J. Anderson, [2] J. R. Mattox et al., [4] G. Finnegan and for the VERITAS Collaboration, [10] D. Berge, S. Funk, and J. Hinton, [12] F. Acero et al., [13] J. Knödlseder et al., [11] I. de la Calle Pérez et al., [14] J. V. Cardenzana for the VERITAS Collaboration, VERITAS Observations of the Geminga Supernova Remnant Observatory and at the collaboratingment. institutions The VERITAS in Collaboration the is grateful constructionleadership to in and Trevor the Weekes operation field for of of his VHE seminal the gamma-ray contributions instru- astrophysics, and which made this studyReferences possible.