Very high-energy γ-ray observations of the Crab nebula and other potential sources with the GRAAL experiment

a Xi :as o-ph/0108270 1 16 Aug 2001 Ve y high-ene gy γ- ay obse a ions o he C ab nebula and o he po en ial sou ces wi h he GRAAL expe imen F.A que os1, J.Balles in2, M.Be enguel3, D.M.Bo que1, E.F.Camacho4, M.Diaz5, H.-J.Gebaue 5, R.En iquez1, R.Plaga5 1Facul ad de Ciencias Fisicas, Uni e sidad Complu ense, E-28040 Mad id, Spain 2CIEMAT-Depa amen o Ene gias Reno ables, Pla a o ma Sola de Alme ia, E-04080 Alme ia, Spain 3Depa amen o de Lenguajes y Compu aci´on, Uni e sidad de Alme ia, 04120 Alme ia, Spain 4Escuela Supe io de Ingenie os, Uni e sidad de Se illa, E-41012 Se illa, Spain 5Max-Planck-Ins i u ¨u Physik, 80805 M¨unchen, Ge many Oc obe 23, 2018 Abs ac The “Gamma Ray As onomy a ALme ia” (GRAAL) expe imen uses 63 helios a - mi o s wi h a o al mi o a ea o ≈2500 m2 om he CESA-1 ield a he “Pla a o ma Sola de Alme ia” (PSA) o collec Che enko ligh om ai showe s. The de ec o is loca ed in a cen al sola owe and de ec s pho on-induced showe s wi h an ene gy h eshold o 250 ±110 GeV and an asymp o ic e ec i e de ec ion a ea o abou 15000 m2. A compa ison be ween he esul s o de ailed Mon e-Ca lo simula ions and da a is p esen ed. Da a se s aken in he pe iod Sep embe 1999 - Sep embe 2000 in he di ec ion o he C ab pulsa , he ac i e galaxy 3C 454.3, he uniden i ied γ- ay sou ce 3EG 1835+35 and a “pseudo sou ce” we e analyzed o high ene gy γ- ay emission. E idence o a γ- ay lux om he C ab pulsa wi h an in eg al lux o 2.2 ±0.4 (s a ) +1.7 −1.3(sys ) × 10−9cm−2sec−1abo e h eshold and a signi icance o 4.5 σin a o al measu ing ime o 7 hou s and 10 minu es on sou ce was ound. No e idence o emission om he o he sou ces was ound. Some di icul ies wi h he use o helios a ields o γ- ay as onomy a e poin ed ou . In pa icula he e ec o ield-o - iew es ic ed o he cen al pa o a de ec ed ai showe on he la e al dis ibu ion and iming p ope ies o Che enko ligh a e discussed. Upon es ic ion he sp ead o he iming on o p o on induced showe s sha ply dec eases and he econs uc ed di ec ion becomes biased owa ds he poin ing di ec ion. This is shown o make e icien γ-had on sepa a ion di icul . 1 1 In oduc ion - aims and plan o he pape Measu ing a mosphe ic Che enko adia ion is p esen ly he mos e ec i e way o de ec cosmic γ- ays wi h p ima y ene gies be ween abou 100 GeV and 1 TeV [1]. In o de o each low ene gy h esholds wi h echniques based on Che enko ligh , la ge mi o collec ion a eas a e needed. GRAAL is an expe imen ha employs he la ge mi o a ea o an exis ing owe sola -powe plan o his pu pose. This pape b ie ly desc ibes he GRAAL de ec o and epo s esul s abou he de ec ion o γ- ays om cosmic sou ces. In addi ion some gene al lessons we lea n abou he helios a - ield app oach o γ- ay as onomy a e epo ed. In sec ion 2 he GRAAL de ec o is desc ibed and compa ed o o he helios a - ield de ec o s o Che enko ligh . Sec ion 3 desc ibes he e en econs uc ion based mainly on he a i al ime o signals a he cen al de ec o . Sec ion 4 ea s he Mon e Ca lo simula ion o he expe imen . Sec ion 5 explains how he da a se used o he analysis o his pape was chosen om he o al se o all aken da a. The da a educ ion p ocedu es—and he undamen al p oblems bese ing i — a e explained in sec ion 6 and he esul s a e p esen ed in sec ion 7. Finally some concluding ema ks a e o e ed in sec ion 8. A mo e de ailed epo abou hese esul s will be a ailable in wo heses[2, 3]. 2 The GRAAL de ec o 2.1 The CESA-1 helios a ield a he PSA CESA-1 is a helios a ield comp ising o 300 s ee able mi o s o he no h o a cen al owe loca ed wi hin he “Pla a o ma Sola de Alme ia”(PSA) a sola he mal-ene gy esea ch cen e ope a ed by he Spanish CIEMAT. The PSA is loca ed in he dese o Tabe nas (37◦.095 N, 2◦.360 W) abou 30 km om he ci y o Alme ia and he sea, a he oo hills o he Sie a-Ne ada moun ains (heigh a.s.l. o 505 m). The 63 helios a s used o GRAAL ha e a mi o a ea o 39.7 m2each and consis o 12 ec angula “ ace s” (sub mi o s) wi h a sphe ical cu a u e ha a e “can ed” (adjus ed ela i e o he o e all ame) o a oughly sphe ical o e all helios a shape. The beam sp ead unc ion o he helios a s has a RMS o abou 0.25◦. Each helios a is indi idually s ee able wi h s epping mo o s ia a cen al PC. Fo he pu pose o GRAAL a con ol p og am was de eloped ha allowed o pe o m he special acking needed o he use o he ield o Che enko as onomy. The helios a ocus he Che enko ligh o ai showe s om he di ec ion o po en ial gamma- ay sou ces o so wa e adjus able “aiming poin s” in he cen al owe (see ig.1). The so-called “con e gen iew”[4]— he poin ing o he helios a s owa ds a poin in he a mosphe e co esponding o an a mosphe ic dep h o 230 g/cm2in he gene al di ec ion o he po en ial sou ce o gamma ays—was always applied. The ela i ely hin glass used o he helios a mi o s (4 mm hickness) —leading o a low 2 o e all hea capaci y—and he p oximi y o he ocean lead o equen dew o ma ion on he mi o s in he win e . To p e en mic o d op o ma ion, all mi o s we e sp ayed e e y second day wi h a ensid solu ion in he e ening using a specially cons uc ed sp ay ca . This p ocedu e was ound o wo k well a e all mi o s had been cleand wi h sul onic acid om aces o silicon gel—a common con aminan in glass p oduc ion. 2.2 De ec o se up 2.2.1 Seconda y op ics Che enko ligh om ou g oups o helios a s (wi h 13,14,18,18 membe s, espec i ely) is di ec ed on o ou single non-imaging “cone concen a o s” each con aining a single la ge- a ea pho omul iplie ube (PMT). The ligh collec o s ha e he o m o unca ed Wins on cones wi h an opening angle o 10◦. Each cone has a on diame e 1.08 m and a leng h o 2.0 m. The cones a e housed in a special enclosu e ha is as ened o he ou side o he cen al owe a he 70 m le el (see ig.2). Each cone is di ec ed on o a poin on he g ound in he helios a ield and collec s he ligh om all helios a s which a e loca ed wi hin he ellipse p ojec ed by he cone opening angle on he g ound (see ig. 3). A he end o each cone, a six-s age 8 inch hemisphe ical PMT, op imized o ope a ion unde high backg ound ligh le els (EMI 9352KB) is si ua ed. The ubes we e ypically ope a ed wi h abou 1300 - 1600 V a a gain o abou 8000. The signals we e ansmi ed ia AC coupling o one as ampli ie di ec ly adjacen o he PMT and a second one nea he da a acquisi ion elec onics wi hin he owe . These ampli ie s ha e a bandwid h o abou 350 MHz and a gain o abou 15 each. The inal FWHM wid h o Che enko pulses is abou 3.6 ns, and mainly de e mined by pa h leng h di e ences wi hin he PMT. The incoming ligh om an ai showe consis s o a ain o pulses om he di e en helios a s, usually ully sepa a ed by pa hleng h di e ences. The a i al ime and ampli ude o each helios a can hus be de e mined wi h a lash-ADC in a sequen ial mode ( ig.4). 2.2.2 T igge logic Two comple ely independen igge s a e used. Fo he “sequence igge ” a e a disc im- ina ed signal abo e 30 mV a ga e o 40 ns leng h is opened a e a delay o 20 ns. I a u he signal is de ec ed du ing ga e du a ion, ano he ga e o 40 ns is opened wi h a delay o 20 ns. I a hi d signal is de ec ed in his second ga e an e en - igge ga e o 200 ns is opened. I he i s and second cone ha e a coinciden e en - igge he inal e en igge is o med. Fo he “cha ge(q) igge ” a iming-ampli ie in eg a es he signal wi h an exponen ial ime scale o 100 (200) ns o Cone 1+2 (3+4). The in eg a ed signal is ed in o a disc imina o in all ou cones and opens a coincidence ga e o 200 ns du a ion i a p ese h eshold is su passed (see also ig.16, whe e he Mon e Ca lo simula ion o his igge is discussed). 3 Figu e 1: Scheme o he expe imen seen om he side, no h is o he igh . The Che enko ligh o a schema ic ai showe (no o scale wi h espec o he ield) is concen a ed by he helios a s o he CESA-1 ield o a ocus a he cen al owe . A dedica ed pla o m moun ed a he ou side o he owe a he 70 m le el houses ou Wins on cones which ecei e ligh om 13 - 18 helios a s in he ield. The da a-acquisi ion elec onics is loca ed inside he owe . The singles a e o his in eg a ed signal is he “q- a e” ( able 2 - 5). A majo i y coinci- dence equi emen o “3 ou o 4 cones” is equi ed o he inal e en igge . Bo h igge s a e always in a logical OR mode in da a aking. The e en a e o he “sequence igge ” depends sensi i ely on he incoming di ec ion o he showe bu is el- a i ely insensi i e on he le el o nigh -sky backg ound induced backg ound ligh (NSB). The “cha ge igge ” is mo e s ongly in luenced by he NSB, bu igge s on coinciden signals independen o de ailed hypo heses on he a i al- ime s uc u e. 2.2.3 Da a eadou GRAAL achie es a good ime esolu ion because he e exis only ou sho cables ha un exclusi ely wi hin he pla o m enclosu e om he pho omul iplie s o he da a acquisi ion elec onics. We egis e all ou pulse ains in only one Digi al Oscilloscope (Le C oy LC 564A) wi h a bandwid h o 1 GHz and a ime bin o 500 ps. This ensu es ha he FWHM o indi idual pulses o abou 3.6 ns is negligibly inc eased by elec onics e ec s. The digi al scope is ead ou in sequence mode o e a GPIB in e ace in o a PC, eaching a speed o abou 260 “wa e o ms”/sec (i.e. 1000 ime bins o 0.5 ns wid h wi h 1 by e each), which is su icien o a dead ime below 10 % o ou mas e igge a e which always emains below 5 Hz and is ypically abou 2-3 Hz (each igge con aining ou wa e o ms). 4 1 m Figu e 2: Uppe plo : Side iew o he de ec o pla o m a he 70 m le el o he cen al owe . Two o he ou Wins on cones poin ing owa ds hei espec i e helios a sub ields wi h he la ge-a ea PMTs a he ends a e ske ched. The wall o he cen al owe is a he le wi h a manhole o en e he pla o m. Lowe plo : View om abo e, all ou Cones a e shown, he hal ci cle is he wall o he cen al owe . 2.2.4 Calib a ion The ime and ampli ude calib a ion o ou se up is pe o med using blue LEDs (Nichia NSPB 500, maximal ou pu a 470 nm) wi h a calib a o module ha is as ened a he window o he Wins on cones. The amoun o ligh emi ed by he indi idual LEDs is de e mined wi h a Quan acon RCA C31000 (a pho omuliplie yielding a well sepa a ed single pho oelec on (p.e.) peak) ha was p e iously calib a ed by de e mining i s single p.e. peak and luc ua ion beha iou . The LED ope a ing ol age is adjus ed so ha one LED pulse co esponds o abou 100 p.e. These LED pulses a e egula ly used in each un o e i y ime and ampli ude calib a ion. In addi ion a LED module wi h highe o al ligh ou pu shines on o he helios a ield. When he helios a s a e b ough in o a “back e lec ion” posi ion, he e lec ed LED pulses a e used o e i y he geome y and check he 5 64 m Cone 2 Cone 1 Cone 3 Cone 4 Figu e 3: Scheme o he de ec ion geome y seen om abo e, no h is o he op o he page. The small ci cle is he owe , he iled double squa e symbolize he helios a s o CESA-1 in he 2nd o 7 h ow om he owe . The ligh om one o he ou g oups o helios a s used in GRAAL - indica ed by he ellipses - is concen a ed in o one o he ou cones. The cone numbe ing indica ed is used h oughou he ex . mi o quali y. The iming and gain p ope ies o he elec onics chain we e calib a ed on-line wi h a PC- con olled Phillips Scien i ic PS7120 cha ge injec ion module. Cha ge pulses wi h p ope ies simila o PMT pulses and di e en ampli udes we e injec ed di ec ly a e he PMT in each un. 2.2.5 Remo e ope a ion All ope a ions (like opening o he doo , high- ol age con ol e c.) a he cen al ecei e and he acking o he helios a ield a e unde emo e con ol ia he in e ne . Va ious en i onmen al pa ame e s like humidi y, ambien ligh , wind speed, a es e c. a e checked by he da a-acquisi ion compu e . Unde condi ions ha indica e some mal unc ion, a physicis on shi is phoned by he PC and can check all pa ame e s and images o web came as, emo ely. Fo he ope a ion o he helios a ield and eme gencies only he egula nigh -ope a o o he PSA is on-si e in all obse a ion nigh s. 2.3 Di e ences o he basic “CELESTE” e sus “GRAAL” cen al- ecei e app oach A e ea ly es s o he use o helios a ields o γ- ay as onomy [5] he basic idea o T¨ume [6, 7] o image he Che enko ligh o one helios a o a single pho omul iplie has been wo ked ou in echnical de ail in he p oposal o he CELESTE expe imen [4]. I was 6 0 1000800600400200 Time (ns) 0 1000800600400200 Time (ns) Signal (mV) Signal (mV)Signal (mV)Signal (mV) 0 1000800600400200 Time (ns) 0 1000800600400200 Time (ns) 400 1000 500 40 20 0 0 200 400 0 0 200 Cone1 Cone2 Cone3 Cone4 Figu e 4: The signal heigh in mV a e ampli ica ion eco ded in all ou cones om one ypical ai showe is displayed as a unc ion o ime. The igge occu s a 500 ns. The y-gain depends on ampli ude, a 100 mV one mV co esponds o ypically 0.25 pho o elec ons. Each peak co esponds o he Che enko -ligh lash o he showe e lec ed by a di e en helios a . The dis ibu ion o ligh in ensi y on he g ound wi hin he ield o iew o he cones is e y une en, no e he di e en y-scales. hen p o en echnically a he Themis helios a a ay [8]. Two o he helios a - ield expe i- men s “STACEE”[9, 10] and “Sola 2”[11] ollow his basic design. Recen ly CELESTE[12] and STACEE[13] epo ed he de ec ion o VHE γ ays. The majo di e ences be ween his well documen ed me hod o de ec ai showe s wi h helios a a ays and he “non-imaging” p inciple o GRAAL—which collec s he ligh om 13-18 helios a s in a Wins on cone on o asingle la ge-a ea pho omul iplie —a e desc ibed in he ollowing. •The mos impo an d awback o he non-imaging app oach o GRAAL is ha he nigh - sky backg ound is highe oughly by he numbe o helios a s iewed by one cone. This esul s in a ypical expec ed backg ound o 8-10 p.e./ns in GRAAL, compa ed o 0.7 p.e./ns in CELESTE. The ha dwa e ene gy h eshold o he de ec ion o γ- ays in p inciple achie - able wi h he same mi o a ea used is abou 4 imes highe in GRAAL. Fo pulses a abo e h eshold he pe o mance o he wo app oaches is no expec ed o be e y di e en be- cause a simila amoun o Che enko ligh is ga he ed by GRAAL and CELESTE. •The ad an age o he non-imaging app oach is i s g ea e simplici y leading o sa ings by abou a ac o 5-10 in ha dwa e cos s. The p esence o only ou da a-acquisi ion channels makes au oma iza ion and emo e con ol mo e easible, leading o compa able sa ings in ope a ion cos s. In i s p esen con igu a ion GRAAL no mally uns unde emo e con ol wi h only a PSA ope a o (who is p esen o main enance o he acili ies independen ly o GRAAL) on-si e. The small numbe o channels allows o use lash-ADCs wi h a ime 7 esolu ion o 0.5 ns/bin, highe han any o he Che enko expe imen . •In CELESTE he angula ield-o - iew in he sky o each PMT is designed o be cons an a 10 m ad ( ull angle). In GRAAL his is impossible because he con ibu ing helios a s’ dis ance om he collec ing cone a ies. This ield-o - iew he e o e a ies be ween 6.5 and 12.1 m ad. I is no easy o de e mine he “op imum” alue o he ield o iew since i depends on se e al di e se ac o s. The o al accep ance has o be de i ed om de ailed Mon e Ca lo simula ions e en in case o a ixed accep ance. The e o e his di e ence seems o li le impo ance. •Because he non-imaging app oach o GRAAL equi es ha g oups o di ec ly adjacen helios a s in he ields a e chosen, i s con igu a ion is mo e compac . In GRAAL 63 he- lios a s ha co e an a ea o abou 160 ×80 m2a e used, whe eas CELESTE p esen ly uses 40 helios a s ha co e an a ea o 240 ×200 m2, i.e. he sampling densi y is abou a ac o 5 lowe . F om he Mon e-Ca lo simula ions i seems ha wi h a es ic ed ield o iew he i egula s uc u e o he ligh pool in had onic showe s ends o be mo e p o- nounced a la ge dis ance scales, so a mo e ex ended a ay ends o be ad an ageous o a possible γ-had on sepa a ion. •In he non-imaging app oach i is impossible o a oid a empo al o e lap o he signal om ce ain helios a s depending on he poin ing di ec ion. This educes he numbe o imes/ampli udes usable in he econs uc ion by abou 20%. When he inciden di ec ion lies no hwa d ( his is he case o he sou ce 3EG 1835+35 a he loca ion o GRAAL), he o e lap becomes s onge leading o a subs an ial dec ease in he quali y o econs uc ion. On he posi i e side, calib a ion is easie when signals om se e al helios a s a e measu ed in he same PMT. 3 E en econs uc ion 3.1 So wa e- igge h eshold The nigh -sky backg ound (NSB) RMS luc ua ion was es ima ed om a po ion o he lash-ADC eco ded aces ha do no con ain Che enko signals, o each e en and cone indi idually. The a i al ime o all de ec ed signals wi h an ampli ude exceeding n × σNSB was de e mined om he eco ded ull pulse shape in he ela ed lash-ADC. The pa ame e n was chosen o be ypically be ween 5 and 7. These a i al imes we e co - ec ed o pa h leng h di e ences in cables and wi hin PMTs wi h he online-calib a ion (sec ion 2.2.4) and we e hen used o econs uc he iming showe on o he indi idual e en s. A i al imes close o each o he han 6 ns we e excluded o a oid any bias om o e lapping pulses. Signals ha sa u a ed any channel we e also excluded om u he analysis. Only he NREMAIN emaining signals we e used in he u he analysis. Be o e u he analysis a so wa e- igge h eshold was applied. In o de o allow a mean- ing ul econs uc ion o showe pa ame e s NREMAIN ≥5 was equi ed. n was chosen a 8 Zeni h (deg) Numbe o e en s Azimu h (deg) Zeni h (deg) Numbe o e en s Azimu h (deg) Figu e 5: P ojec ions o he numbe o showe s as a unc ion o showe di ec ions as econ- s uc ed om he iming da a. Shown is de ia ion o he econs uc ed di ec ion om he poin ing di ec ion on he ele a ion-axis (le wo panels a. and c.) and azimu h-axis ( igh wo panels b. and d.). The o igin hen co esponds o he poin ing di ec ion as de e mined by he o ien a ion o he helios a s. Two componen s a e appa en : a peak nea he o igin, and a “ la backg ound” co esponding o e en s mis econs uc ed in di ec ion (see ex ). The da a sample comp ises o 32 hou s o ON-sou ce ime on he C ab pulsa (uppe panels a. and b.) and an equal amoun o OFF-sou ce ime (lowe panels c. and d.) aken unde a iable wea he condi ions in he season 1999/2000. The “Gaussian plus linea unc ion” i is pe o med o each subsample. I is seen ha he Gaussian - co espondig o success ully di ec ion econs uc ed e en s - is always cen ed wi hin <0.05◦. a alue as low as possible, be o e a la ge numbe o NSB induced “ ake” signals we e ound o en e he sample. The lowes possible alue o n was ound o depend on he sou ce posi ion somewha , due o he a ying empo al o e lap o signals in he ace. The inal choice was: n =5(7,9,7) o he C ab (3C454, 3EG+1835, pseudo sou ce) sample. This so wa e h eshold also equalized he e ec o he NSB on he econs uc ion. A highe le el o NSB σNSB leads o a co espondingly highe so wa e h eshold. This is expec ed o co ec o he e ec o a lowe ha dwa e igge h eshold and dec eased econs uc ion e iciency wi h highe NSB o i s o de . The choice o n in he analysis o MC da a was di e en and is explained in sec ion 4.3. 9 0 50 100 150 200 250 300 0 10 20 30 40 50 60 70 Numbe o peaks Numbe o showe s Figu e 8: Numbe o showe s wi h a gi en numbe o peaks iden i ied in all ou eco ded iming aces. The ull (dashed) line is o MC γs (p o ons), and he do ed line o ex- pe imen al da a aken unde simila inciden angles. The o al numbe o MC showe s was no malized o he expe imen al da a o compa ison. hese ails ha inc ease he mean o he expe imen al lsq2 dis ibu ion. 4.4.3 To al-cha ge spec um Fig.12 displays he “ o al cha ge” spec um bo h in da a and Mon e Ca lo. The “ o al cha ge” is de e mined by in eg a ing he a ea unde all peaks de ec ed in he lash-ADC aces adding all ou cones in one e en . Fa abo e h eshold he expe imen al spec um ollows a powe law wi h a di e en ial index o abou -1.6—which is much la ge han ha o he p ima y spec um o -2.7. The eason o his is a e y la ge sca e in he co ela ion o o al cha ge and ene gy. The Mon e Ca lo simula ed spec um looks quali a i ely simila o he expe imen al da a bu ollows a sligh ly s eepe index o abou -1.9. One eason o his is ha a abo e he h eshold he cu o in simula ed p o on ene gy a 10 TeV is al eady expec ed o ha e a s eepening e ec on he MC spec um. 4.5 E ec s o he small ield-o - iew on econs uc ed showe p ope ies To gain he ad an age o using many la ge mi o s wi h only one cen al de ec o helios a s need o ha e a ocal leng h abou a ac o 20 - 30 la ge han hose o he elescopes used o he imaging o VHE γ- ay showe s. Fo space easons in he cen al owe he ligh de ec o a he ocus canno be scaled up by such eno mous ac o s. Mo eo e he cons uc ion o an imaging came a o each helios a would be p ohibi i ely expensi e. These wo ac o s o ce a c ucial comp omise in Che enko de ec o s using helios a ields: he ield-o - iew has o be chosen abou one o wo o de s o magni ude smalle in solid-angle han in adi ional 16 0 50 100 150 200 250 0 10 20 30 40 50 60 70 Numbe o iden i ied peaks Numbe o showe s Figu e 9: Numbe o showe s wi h a gi en numbe o peaks ha we e a ached o indi idual helios a s and we e used in he inal de e mina ion o he showe di ec ion. The ull (dashed) line is o MC γs (p o ons), and he do ed line o expe imen al da a aken unde simila inciden angles. The o al numbe o MC showe s was no malized o he expe imen al da a o compa ison. Che enko elescopes. Ou Mon e Ca lo simula ions show ha abou 60% o he Che enko ligh o showe s induced by gamma- ays wi h small ene gies (100 GeV) is collec ed in he GRAAL se up, a numbe ha is accep able when aking in o accoun he la ge mi o a ea. Ne e heless, we ind ha he angula es ic ion has se e al disad an ageous e ec s. 4.5.1 S uc u e o iming on o p o on induced showe s I is well known ha he a i al imes in p o on-induced showe s ha e a much wide sca e a ound he mean a i al ime han in gamma-induced showe s due o hei mo e i egula de elopmen in he a mosphe e[19]. The expe imen al de e mina ion o his sca e has been p oposed o be an e icien me hod o gamma/had on sepa a ion[20]. Fig.13b. shows he s uc u e o he showe on o a ypical gamma, ig.14b. o a ypical p o on showe om he Mon e Ca lo simula ion wi hou simula ion o he de ec o . The la ge sca e o he p o on showe is e iden . In panel c. o hese igu es he showe on is shown wi h a es ic ion on he inciden angle o he pho on. Only pho ons wi h an inciden angle di e en by less han 0.3◦ om he di ec ion poin ing owa ds he showe maximum om a posi ion on g ound we e e ained. This es ic ion has a e y simila e ec o he small ield o iew dicussed abo e. Upon angula es ic ion he showe on na ows bo h o p o ons and gammas, bu he e ec is s onge o he p o ons. Panels a. o hese igu es demons a e he cause o his beha iou . The es ic ed ield o iew mainly p e en s he 17 0 50 100 150 200 250 10 -2 10 -1 1 10 lsq 2 Numbe o showe s Figu e 10: The dis ibu ion o lsq2 . The ull (dashed) line is o MC γs (p o ons), and he do ed line o expe imen al da a aken unde simila inciden angles. The o al numbe o MC showe s was no malized o he expe imen al da a o compa ison. de ec ion o Che enko pho ons emi ed a below he maximum a abou 11 km heigh . De ia ions om he ideal sphe ical iming- on a e mainly due o he deeply pene a ing pa o he showe . P o ons a e mo e pene a ing and a e he e o e mo e a ec ed by he angula es ic ion. The o al e ec is ha wi h a small ield o iew, p o ons and gammas ha e i ually iden ical, nea ly sphe ical showe on s, wi h e y li le sca e , displayed and compa ed wi h expe imen al da a in ig.11. This makes e icien gamma/had on sepa a ion wi h iming me hods in helios a ields all bu impossible (see igs.11,15). 4.5.2 Recons uc ed di ec ion o p o on induced showe s Ano he impo an me hod o disc imina e gamma- and p o on induced showe s is o ex- clude all showe s ha do no a i e om he sou ce di ec ion wi hin he angula esolu ion as de e mined wi h a i o he iming on . Wi h a es ic ed ield o iew he e is a bias o he showe di ec ion econs uc ed om iming in o ma ion owa ds he sou ce di ec ion (see ig.15). The ield-o - iew “selec s” a pa o he showe which lies owa ds he showe maximum o a showe a i ing om he sou ce di ec ion. The iming- i hen inds he di ec ion o his subpa o he showe , which is biased owa ds he sou ce di ec ion. In Mon e-Ca lo simula ions o p o on induced showe s we ound ha he mean di e ence be- ween ue showe di ec ion and econs uc ed showe di ec ion is 0.71◦±0.002◦(s a is ical e o ), whe eas he mean di e ence be ween sou ce di ec ion and econs uc ed showe di- ec ion is only 0.44◦±0.002◦(s a is ical e o ). This bias dec eases he ac ion o p o on showe s which can be excluded due o hei angula dis ance o he sou ce di ec ion. 18 0 500 1000 1500 2000 2500 3000 x 10 3 -20 -15 -10 -5 0 5 10 15 20 Time de ia ion (ns) Numbe o peaks Figu e 11: The de ia ion o measu ed a i al imes om he inal i ed sphe ical showe on o MC γs ( ull), p o ons (dashed) and expe imen al da a (do ed). The isible sha p educ ion o e en s wi h a ime de ia ion somewha smalle han 5 ns is due o he ac ha he econs uc ion p og am allows he exclusion o 3-5 peaks wi h a de ia ion om he showe on la ge han 5 ns (see ex ) om he inal i . 4.5.3 Ene gy esolu ion The es ic ion in he ield o iew dec eases he ene gy esolu ion p og essi ely o la ge showe s, because he ac ion o he showe image seen canno be in e ed. We ind ha nea ou ene gy h eshold o gamma ays he esolu ion de i ed om choosing he o al cha ge eco ded in all 4 cones as simple p ima y-ene gy es ima o is abou 110% and wo sens apidly o highe ene gies. 5 Da a selec ion 5.1 De ec o condi ion Only nigh s in which all ou de ec o channels and he helios a s in he ield we e unc- ioning no mally acco ding o he eco ded moni o iles we e chosen o u he analysis. 5.2 Me eo ological selec ion I was ound ha he econs uc ion quali y depends on he a mosphe ic ansmission. Fo example in nigh s which we e isibly hazy wi h a high ela i e humidi y abo e 80% (a ela i ely equen nigh ly wea he condi ion a he PSA), he o al igge a e was low, he a io o well econs uc ed e en s o e en s wi h a mis econs uc ed angula di ec ion (called “PT” below) was educed by up o a ac o 2 and he lsq2 o he i o he iming 19 To al cha ge (mV) 10 -3 10 -2 10 -1 104105 Figu e 12: The numbe o showe s as a unc ion o “ o al in eg a ed cha ge” in all 4 Cones in one showe . The dashed line a e expe imen al da a, he ull line is om he MC simula ion. The cu es we e no malized o he same numbe o showe s. The x-axis is in uni s o summed lash-ADC ampli udes in mV. on signi ican ly inc eased. This is p obably he esul o selec i e abso p ion, by which Che enko ligh om he deeply pene a ing pa o he ai showe , wi h inc eased empo al luc ua ions, domina es he eco ded signal. As γ-induced showe s de elop mainly in he uppe a mosphe e a selec ion o da a wi hou selec i e abso p ion is impo an . Besides a ela i e humidi y below 70% and gene ally clea skies we equi ed he ollowing c i e ia om he econs uc ed da a o a gi en nigh . The pa ame e limi s o each indi idual poin ing di ec ion we e chosen such ha a se o “good” nigh s —de ined as showing ai ly cons an pa ame e alues—was e ained. The pa ame e limi s hus sligh ly a ied o di e en poin ing di ec ions. Fi s a cu o exclude uns able wea he condi ions was applied. Fo his he ac ion o e en s wi h a econs uc ed angle a om he sou ce di ec ion was chosen. Condi ion 1: 0.95 <RO <1.05 RO = (Numbe o e en s wi h econs uc ed di ec ion >3◦ om poin ing di ec ion ON sou ce) / (Numbe o e en s wi h econs uc ed di ec ion >3◦ om poin ing di ec ion OFF sou ce) The o he wo un-cu c i e ia a e mean o exclude nigh s wi h low a mosphe ic ansmis- sion. Condi ion 2: Ra e a e all so wa e cu s in OFF sou ce di ec ion >50/min Condi ion 3: PT >0.8 PT = (Numbe o e en s wi h econs uc ed di ec ions <1◦ om poin ing di ec ion OFF sou ce)/ (Numbe o e en s wi h econs uc ed di ec ions >3◦ om poin ing di ec ion OFF 20 0 250 500 750 1000 1250 1500 1750 2000 2250 0 10000 20000 Heigh (m) Numbe o C-pho ons 0 5 10 15 20 25 30 0 100 200 102103104 Co e dis ance (m) A i al ime (ns) 0 5 10 15 20 25 30 0 100 200 102103104 Co e dis ance (m) A i al ime (ns) Figu e 13: Time s uc u e o a ypical gamma- ay ini ia ed showe . b. The a i al ime as a unc ion dis ance om he co e in me e s o a ypical gamma showe . The shading is p opo ional o he Che enko -pho on densi y. c. Same as b. bu only hose pho ons wi h an a i al di ec ion wi hin 0.3◦ om he di ec ion owa ds he showe maximum om a posi ion on he g ound a e displayed o he same showe s. a. Numbe o Che enko - pho on emi ing elec ons in he showe as a unc ion o heigh a.s.l. sou ce) These “me eo ological cu s” a e se e e unde he wea he condi ions a he PSA. In he da a sample on C ab in Feb ua y/Ma ch 2000 only 22% o all da a aken on he C ab pulsa passed all cu s. 6 Da a educ ion— he p oblem o di e en condi ions in he sou ce and OFF-sou ce egion A undamen al p oblem o all Che enko expe imen s—specially o hose a emp ing o de ec an excess due o gamma- ays in he o al a e—is he ac ha he nigh -sky back- g ound be ween ON- and OFF-sou ce di e s in gene al. This can in luence he coun ing a e and analysis e iciency in a ious ways. This p oblem is mos c i ical o he helios a - a ay based expe imen s because hey aim o wo k wi h a igge h eshold nea o andom luc ua ions o he nigh -sky backg ound in a single channel. We discuss he obse ed di - e ences in he ON- and OFF-sou ce egion in sec ion 6.1. Sec ions 6.2 and 6.3 discuss he e ec o a di e ence in he ON- and OFF-sou ce in ensi y o he NSB on he o al a e and he econs uc ion, espec i ely. Sec ion 6.4 desc ibes he me hod we inally chose o calcula e an excess o e en s in he ON-sou ce di ec ion. 6.1 De ailed compa ison o condi ions ON and OFF sou ce Fo he coun ing condi ions chosen in he 1999/2000 season, he “q- igge ” (see sec ion 2.2.2) leads o andom e en igge s due o nigh -sky noise. Because his andom a e is 21 0 200 400 600 800 1000 1200 1400 0 10000 20000 Heigh (m) Numbe o C-pho ons 0 5 10 15 20 25 30 0 100 200 10 102103 Co e dis ance (m) A i al ime (ns) 0 5 10 15 20 25 30 0 100 200 1 10 102 Co e dis ance (m) A i al ime (ns) Figu e 14: Time s uc u e o a ypical p o on ini ia ed showe . The panels show he same quan i ies as in he p e ious ig.13. No e ha he p o on emi s a much smalle ac ion o ligh wi hin he es ic ed ield-o - iew because o i s la ge angula ex ension. e y sensi i e o he nigh -sky backg ound, sligh ly highe NSB le els in OFF (as obse ed o all po en ial sou ces see ables (2 - 5)) p oduce a highe e en - igge a e in OFF (see en y “ aw e en s” in ables 2-4) The andom a e was calcula ed om he eco ded single a es. A discussion o he o al a e a e a co ec ion o he andom igge and o he small e ec s is gi en below in sec ion 6.2. F om es da a i was shown ha he econs uc ion o he iming showe on always ails o andom- igge e en s, so ha he e en numbe “a e econs uc ion” (“ ec. e en s” in ables 2 - 4) is expec ed o be ee om nigh -sky backg ound induced andom igge s. All 4 sou ces discussed in his pape show a sligh ly highe NSB in he OFF-sou ce egion. This e ec is mos p onounced o he sou ce 3C 454, whe e— om he da a epo ed in able 3— he cu en (q- a e) is 13 (25)% highe in OFF han in ON. Howe e , he measu ed RMS luc ua ion is only 0.4% highe in OFF han in ON and his di e ence is smalle o he o he sou ces (0.04% o he C ab pulsa ). By measu ing he andom noise in comple e da kness, we de e mined a cons an nigh -sky backg ound independen noise le el wi h a RMS o 0.8658. Sub ac ing his cons an noise quad a ically om he o al noise we ge he con ibu ion om he NSB alone (numbe in b acke s in hi d column o able 2 - 5). Fo he sou ce wi h he la ges di e ence in noise le el, he NSB-induced componen di e s in ON- and OFF-sou ce posi ion by abou 2.5%, so ha he di e ence in b igh ness a he wo posi ions can be es ima ed o be abou 5%. An e ec ha is e y di icul o emo e is a sligh expec ed educ ion in he igge h esh- old due o a highe NSB. Due o luc ua ions, smalle e en s can c oss he igge h eshold. The opposi e e ec — ha la ge e en s a e dec eased in ampli ude due o luc ua ions and ail o c oss he h eshold—happens less o en due o a CR spec um ha s eeply alls wi h ampli ude. This e ec was s udied by calcula ing he o al mean cha ge o all ON e sus all OFF sou ce e en s ( see ables 2 - 5 en y “mean q”). I small e en s a e p e e ed, he o al 22 Showe di ec ion o mo ion Res ic ed ield o iew Res ic ed ield o iew sub Gamma induced showe P o on induced showe Figu e 15: Ske ch o illus a e he e ec o a small- ield o iew - necessi a ed by he helios a - ield app oach - on he de e mina ion o he iming s uc u e. A gamma- ay in- duced showe is symbolized in he le pa o he igu e and a p o on induced one wi h a sligh ly di e en incidence di ec ion on he igh . The p o on showe is spa ially mo e ex ended and symbolized as a collec ion o small sub showe s. The es ic ed ield o iew “p ojec s” ou sub showe s in he cen al pa o he showe ou o he mo e ex ended p o- on showe . O he mo e pene a ing and la e ally ex ended sub showe s - ha inc ease he luc ua ion in he iming on - do no con ibu e o he ligh de ec ed wi hin he es ic ed ield o iew. One sub showe wi h an incidence di ec ion biased owa ds he poin ing di ec- ion (symbolized by he label “sub”) is p e e en ially de ec ed and hus biases econs uc ed di ec ions owa ds he poin ing di ec ion. mean cha ge should be smalle by a ce ain ac o bwi h inc eased NSB. The o al a e should be inc eased by a ac o o oughly 1.4 b o ou se up. I can be seen om he esul s in he ables (2 - 4) ha wi hin he s a is ical e o o ypically somewha less han a pe cen he o al mean cha ge is he same o all sou ces. Howe e , wi hin his e o a signi ican educ ion o he h eshold - one which would p oduce a educ ion in ON-OFF a es o he same o de o magni ude as an expec ed signal om he c ab nebula - canno be excluded in his way. 6.2 E ec o NSB di e ences on o al igge a e - Mon e-Ca lo simu- la ion The e ec o he NSB di e ences on he o al igge a e was simula ed by aising he amoun o andom noise by 5% o e i s usual alue. The de ec o Mon e Ca lo models he elec onic pulse shaping and he esponse o he disc imina o in de ail (see ig.16), and so he e ec i e change in h eshold, due o he inc eased noise le el could be deduced o be 23 Table 2: Cu en (mean o 4 Cones), q- a e: single igge a e o cha ge in eg a ing channel (mean o Cone 1+2), σNSB: RMS luc ua ion o he measu ed NSB (in lash- ADC uni s) in he i s 100 channels (be o e signal), numbe in b acke s is NSB induced backg ound alone, log(mean q): base-10 loga i hm o mean ne -cha ge (in lash-ADC uni s) o all e en s in sample, aw e en s: all ha dwa e- igge ed e en s which aces we e eco ded, ec. e en s: numbe o e en s a e angula econs uc ion and so wa e igge , cen . e en s: no malized numbe o e en s in cen al angula egion (wi hin 0.7 deg ees o poin ing di ec ion), calcula ed as explained in sec ion 6.4. Rows a e o he samples wi h poin ing owa ds he C ab pulsa (“ON”) and on a sky posi ion (“OFF”) wi h a igh ascension 2.625 deg ees la ge han in he ON di ec ion. The o al da a- aking ime ON was 430 minu es wi h an equal amoun o OFF ime. cu en [µA] q- a e[kHz] σNSB [ADC uni s] log(mean q) ON 19.0 ±0.4 1.35 0.9493 (0.3893) 2.940 ±0.004 OFF 19.3 ±0.3 1.49 0.9497 (0.3902) 2.937 ±0.004 EXCESS -0.3 -0.14 -0.0004 (-0.0009) 0.003 ±0.006 aw e en s ec. e en s cen . e en s ON 68702 33384 9415 OFF 75198 33056 8678 EXCESS -6496 ±379 328 ±258 737 ±165 abou 6±2%. Ex apola ing, we deduce an expec ed spu ious excess a he OFF sou ce posi ion o he sou ce wi h he la ges di e ence in ON and OFF noise (3C454, see able 3) o abou 1%, his co esponds o abou 1.4 σs a in his case. As he di e ence in he noise le els be ween ON and OFF is smalle in he case o he o he 3 sou ces discussed in his pape , his e ec does no ye con ibu e signi ican ly. Howe e , i is clea ha a e y ca e ul co ec ion o i becomes necessa y when he a ailable s a is ics g ows. 6.3 E ec o NSB di e ences on econs uc ion - So wa e padding Finally a di e ence in NSB leads o a sligh ly di e en noise le els in ON and OFF da a. Fo example o he C ab da a he RMS noise in he ON-sou ce da a was ound o be abou 0.5% smalle han in he OFF-sou ce da a. The e ec o his di e ence on he econs uc ion p ocedu e was s udied by a i icially adding noise a he so wa e le el (“so wa e padding”). Fig.17 demons a es ha he ac ion o e en s nea he sou ce di ec ion (“PT” o sec ion 5.2) dec eases wi h inc easing NSB, bu ha he e ec is impo an only a ela i ely la ge inc eases on he o de o a ew pe cen . F om he esul s shown in igu e 17 i was de i ed ha an inc ease o RMS noise by 1% dec eases he o e all econs uc ion e iciency by abou 0.4% and he peak o ail a io PT (sec ion 5.2) by 0.8%. This e ec emains small 24 Table 3: En ies as in able 2 o he samples wi h poin ing owa ds he adio sou ce 3C 454.3 (“ON”) and on a sky posi ion (“OFF”) wi h a igh ascension 2.625 deg ees la ge han in he ON di ec ion. The o al da a- aking ime ON was 550 minu es wi h an equal amoun o OFF ime. cu en [µA] q- a e[kHz] σNSB [ADC uni s] log(mean q) ON 17.7 ±0.4 3.1 0.9505 (0.3922) 3.119 ±0.003 OFF 20.3 ±0.3 4.1 0.9540 (0.4006) 3.113 ±0.003 EXCESS -2.6 -1.0 -0.0035 (-0.0084) 0.006 ±0.004 aw e en s ec. e en s cen . e en s ON 42516 30570 7525 OFF 44949 30889 7625 EXCESS -2433 ±296 -319 ±248 54 ±141 o he obse ed ac ional di e ences o he RMS NSB-noise in ON and OFF (on he o de a ew en hs o a pe cen a maximum (see able 2- 5)), and was neglec ed in he p esen analysis. I mus be no ed ha so wa e padding is no a pe ec simula ion o he eal condi ions, because he in luence o he NSB on he ha dwa e- igge condi ion—which can in p inciple in luence showe p ope ies and econs uc ion—is no simula ed. 6.4 Calcula ion o he excess To a oid he p oblem men ioned in sec ion 6.2 we chose a me hod ha no malizes any excess o he a io o ON- and OFF-sou ce e en s o he inal esul s epo ed in he nex sec ion. The no malized excess EXCESSnwas calcula ed acco ding o he ollowing equa ion: EXCESSn= ONin −OFFin ON OFFou (3) He e (ON,OFF)in s ands o he numbe o e en s wi hin 0.7◦ om he sou ce and OFF- sou ce di ec ion, espec i ely whe eas (ON,OFF)ou s ands o he numbe o e en s wi h di- ec ions de ia ing mo e han 2◦ om he sou ce di ec ion. The s a is ical e o o EXCESSn, ERRnwas calcula ed acco ding o: ERRn= ONin + OFFin ×ON OFF2 ou +  (1 + ON OFF ou )ONou ×OFF2 in OFF2 ou    0.5 (4) 7 Resul s 25 Angula dis ance (deg) Numbe o showe s Angula dis ance (deg) Numbe o showe s (on-o ) Figu e 18: The uppe plo (a.) shows he numbe o e en s as a unc ion o angula dis ance o econs uc ed di ec ion om sou ce di ec ion o ON-sou ce e en s ( ull line) and OFF- sou ce e en s (dashed line). No no maliza ion o any kind was applied o his plo . The lowe plo (b.) shows he di e ence ON - OFF, no malized o he numbe o e en s in he ou e angula egion, acco ding o eq.3 Da a om he C ab pulsa aken unde good me eo ological condi ions acco ding o he cu s discussed in sec ion 5.2 we e used. The s a is ical e o s o he indi idual bins a e shown. Re e ences [1] F.K enn ich, TeV Gamma-Ray As onomy in he new Millennium, P oc. 7 h Taipei As ophysics Wo kshop, ASP Con e ence Se ies, Vol. XX, 2001, de. C.M. Ko,as o- ph/0101120. [2] D.Bo que, PhD hesis, o be published, Uni e sidad Complu ense, Mad id, (2001). [3] M.Diaz, PhD hesis, o be published, Uni e si ¨a Heidelbe g, (2002). [4] D. Dumo a e al. (CELESTE coll.), Che enko Low Ene gy Sampling & Timing Expe imen - CELESTE expe imen al p oposal, h p://wwwcenbg.in2p3. /As opa icule (1996). [5] S.Danahe , D.J.Fegan, N.A.Po e , T.C. Weekes, T.Cole, Possible applica ions o la ge sola a ays in as onomy and as ophysics, Sola Physics 28 (1982) 335-343. [6] O.T. T¨ume ,T.J. O´Neill, A.D.Zych, R.S.Whi e, A la ge a ea VHE de ec o wi h low-ene gy h eshold, P oc. 21s ICRC, Adelaide, 4 (1990) 238-241. 32 Angula dis ance (deg) Numbe o showe s Angula dis ance (deg) Numbe o showe s (on-o ) Figu e 19: Good-wea he da a o he po en ial sou ce 3C 454.3 plo ed as in ig. 18 [7] O.T. T¨ume , A.D. Ke ick, T.J. O´Neill, R.S.Whi e, A.D.Zych, Sola One Gamma Ray obse a o y o in e media e high ene gies o 10 - 500 GeV, P oc. 22nd ICRC, Dublin 2 (1991) 634-637. [8] B.Giebels e al. 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