198 6Apj. . .306. .587B the Astrophysical Journal, 306:587-598
198 6ApJ. . .306. .587B h -1 hm 15 The AstrophysicalJournal,306:587-598,1986July15 C 1986.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A. ence insiderealtime(amplitudeofthepeakregion~25% formity withPoissonstatisticsbecameinterestingwhenitwas (~12%) withinterseparationi<40s.Theimpliednoncon- found (Bhat,Sapru,andKaul1980)thatthearrivaltimedis- realized thatthehighereventrates,causingthisdeviationfrom hole outbursts(PageandHawking1976;Rees1977),itwas (R.A. «20±04).Inouroriginalpaper(Bhat,Sapru, and relative tobackground;seeFig.10)and,providedsome the expectedexponentialbehavior,exhibitedamarkedprefer- Gulmarg, India,revealedasignificantoverabundanceofevents tribution ofatmosphericCerenkovpulsesrecordedat best explainedintermsofy-raysfromoneormorepoint unknown experimentalbiaseswerenotresponsible,appeared northern hemispherewouldbehelpfulinelucidatingthenature Kaul 1980),wepointedoutthatfurtherexperimentsin the sources presentinthedirectionofGalacticplane expectedly smalleramplitude(~1%),inthetemporaldistribu- detection ofaGulmarg-likeoverabundance(i<37.5s), of of therecordedexcess.Wenotedwithinterest,therefore, the January 20,ahighlysignificant nonrandomcomponentatthe ment atManitoba(Smithet al.1983a)registered,on1981 et al1980),andoftheabsenceeffectamong air tion ofmuonsrecordedinsidetheMontBlanctunnel(Badino laide (ClayandDawson1981). Similarly,theairshowerexperi- expected numberof0.2events minute.Noothersimilar local siderealtimeof1725, comprising a5minutelongburst shower eventsdetectedatthesouthernlatitudestationofAde- 1980) forepisodiccosmicevents,includingprimordialblack of 32particles(meanenergy of ~3x10eV),asagainstan During aground-basedsearch(Bhat,Razdan,andSapru -125 -1 12 153 Cygnus X-3modulationperiodof4.8hr.Itsamplitude,givenbythenumberexcesseventsinphase photomultiplier systematGulmarg,India,revealsaphase-dependentcomponentexhibitingthecharacteristic constant of~2.3yr).ThisintriguingpossibilityisfurtherstrengthenedbyanexaminationtheHaverah average fluxof1.6±0.4x10ycmsabove0.5PeV(1=eV).Takentogetherwiththespec- peak relativetothetotalphase-independentevents,isfoundbe1.8%±0.4%,correspondingadetected reduction intheluminosityofPeVsourcebyafactor~1.5yr(exponentialdecaylawwithtime tral dataforthefollowingyearsfromseveralotherexperiments,thereissuggestionofalong-term fluxes fromCygnusX-3withthoseinthelO^-lOeVenergyregionshowsthatlatteraresignificantly due toy-yinteractionswiththe2.7Kmicrowavebackground,acomparisonofultra-highenergyphoton than 10eVandgreater2xrespectively.AfteraccountingforlossesinthePeVphotonbeam recorded between1981Decemberand1985March,whichdisplayanalogouslong-termbehavioratgreater lower. ThissuggeststhattheTeVphotonsundergoseverecircumstellarabsorptionthroughinteractionswith responsible forgeneratingthePeVfluxorboth. Park phasehistogramsofCygnusX-3fortheperiod1979January-1984DecemberandPlateauRosadata Subject headings:gamma-rays:general—X-rays:binaries optical/infrared photonsorhaveaproductionspectrumwhichdiffersinsomesignificantmannerfromtheone © American Astronomical Society • Provided by the NASA Astrophysics Data System An analysisofatmosphericCerenkovpulsesrecordedduring1976January-1977December,byawide-angle I. INTRODUCTION ESTIMATE FROMGULMARGOFPeVPHOTONFLUX Bhabha AtomicResearchCentre,NuclearLaboratory,Srinagar,India C. L.Bhat,M.Sapru,andH.Razdan CYGNUS X-3ANDITSRELEVANCE Received 1985February19;accepted1986January3 ABSTRACT 587 14 h bursting activitywasnoticed,however,andtheoveralldata conform wellwiththePoissonbehavior(Smithetal.1983h). jected theGulmargdatatoa phasogram analysisandfindthat ed thattheyaregenericallyrelated,thisdisparityinamplitudes essentially disappearswhensinglemultiple-muoneventsfrom which assuchiscomparabletothatfortheGulmargpeak, estingly, matchtheGulmargsiderealpeakinpositionand multimuon events(primaryenergyabove10eV)recordedby by appealingtothereportedtimevariability(Barteltet al. between MinnesotaandGulmargeffectsmaybeunderstood the generalcosmic-raybackgroundareincluded;and,provid- structure. However,theamplitudeofSoudan-1anisotropy, strates thepresenceofburstsamongtheseeventswhich,inter- the Soudan-1protondecaydetectorinMinnesotademon- obtained between1980Octoberand1982Aprilwasfoundto the Minnesotagroupprefersahadronicbeamfortheirresult). sists ofultra-highenergy(UHE)photons(thoughwenote that More recently,anexaminationbyBarteltetal.(1985)ofthe experiments atKiel(SamorskiandStamm1983a) and (R.A. ä20±04),theamplitudeofGulmargeffectseems as aperiodicsourceofsuchUHEy-raysbytheairshower lies inthegeneraldirectionofourintensitypeak Haverah Park(LloydEvansetal.1983).Althoughthissource (Bhat 1982;EichlerandVestrand 1984).Wehavenowsub- too largeandgivesforCygnusX-3afluxthatisabout two law extrapolationofthelower energy(~1TeV)spectrum compatible withaphase-dependent emissionfromCygnus orders ofmagnitudehigherthan thatbasedonasimplepower- 1985) andbyassumingthattheprogenitorparticlebeam con- only 1.8%ofthetotalon-source events(definedin§III)are The X-raybinaryCygnusX-3hasrecentlybeenestablished 198 6ApJ. . .306. .587B 12 14 13 2 12 -1 5-1 - m ing anindependentestimateoftheperiodicfluxfromCygnus X-3 basedontheatmosphericCerenkovtechnique,itrefersto these eventstobey-raysleadsadetectedfluxof X-3, andthisisclearlyamorereasonablefigure.Assuming immediate sourceenvirons(atlO^-lOeVwithinfraredand process, bothintheinterstellarspace(at>0.3PeVwith an energydomain(5x10eV)aroundwhichtheapparent background microwavephotons;Gould1983)andalsointhe spectral shapeislikelytobeinfluencedbythey-ycollision 588 (>3 x10eV)ofCygnusX-3madeovertheintervening metrical coneofsemiangle70°.Noattemptwasmadetokeep m), eachviewingtheskyonclear,moonlessnightsinageo- ferences ofman-madeorigin.Detailstheexperimental and, whencomparedwithotherUHEmeasurements Further, ourobservationsbelongtothe2yrperiod1976-1978 optical photons;CawleyandWeekes1984;Apparao1985). with theexperienceofotherworkers(see,forexample,Bunner excellent seeingandnegligibleopticalelectricalinter- variation intheaveragesourceintensity(Ranaetal.1984). period, enableustoexaminethepossibilityofalong-term (resolution time~10fis,amplitudediscriminationlevel>4 the backgroundlightlevelconstant,though,inaccordance 9545B photomultipliers(cathodearea490cm,separation~1 arrangement aregiveninBhat(1982).Briefly,itusedtwoEMI tain site(altitude2743m,geographiclatitude34?1N),having 1.6 ±0.4x10”ycm“s"above0.5PeVfromthissource. tomultipliers atsuitablyreducedextremehightension(EHT). variations intheambientlightlevelbyoperatingtwopho- oscilloscope-camera recordingsystem(deadtime<100msper times thermsnoise)wererecordedbyacathoderay Prompt coincidencesbetweenthetwodetectorchannels event) alongwiththeeventoccurrencetimeprovidedbya /ns, decaytime~10/¿s)withtheexperimentallydetermined fied bycomparingtheirtemporalcharacteristics(risetime~1 temperature-controlled crystalclockhavinganaveragedrift 1966), asufficientdegreeofgainstabilitywasachievedagainst multipliers forlessthan100ns(Fig.1).Simulationexperiments spheric Cerenkovpulses,recordedinthismanner,wereidenti- rate of~1msday(Bhat,Kaul,andYadav1981).Atmo- profile ofanatmosphericCerenkovpulseandcanbereadily have shownthatelectromagneticinterference,including that simultaneously flashingtwoLEDlampsontopofthephoto- system timeresponseforanimpulsivelightpulse,producedby can eventsduetoopticalemissionsfromlightning(typically distinguished duringavisualexaminationoftherecords, as generated byanEHTspark,doesnotmimictherecorded time night andalsoonanight-to-nightbasis.Thesinglechannel regularly monitoredandappropriatelyadjustedtoguard tens of/¿slong;Bhat1982).Theoverallchannelgainswere exponent mitselfvarieswith \¡/ (Greisen1956).Thisleadstoan than 6x10“hr. corresponding toanegligiblechancecoincidencerateof less time toandwerefoundlieintherange~70-100hr \ rates (afteramplitudediscrimination)werealsocheckedfrom against anynoticeabledriftsinthecourseofanobservation detected perunitsolidangle in thezenithdirectioni/^,and I(i¡/) ^cosij/,where7(i/0is the numberofCerenkovevents pulses isfoundtoapproximately followtheanalyticform The importanceofthepresentworkisthat,apartfromyield- The experimentwascarriedoutinGulmargatadarkmoun- The zenithangledependence oftheatmosphericCerenkov © American Astronomical Society • Provided by the NASA Astrophysics Data System II. EXPERIMENTALARRANGEMENT BHAT ETAL. 4 -2 -2 -2 -1 15 15 14 2 day oftheyear,hour,minute,andsecond. for convenienceofviewing.Thedigitsatthetopeachframegiveeventin in (a).Oneofthephotomultiplierchannelsisdisplayedinvertedmode than 100ns.NotetheshotnoiseamplitudeaccompanyingCerenkovpulse generated byflashingapairofLEDlampsonthetopdetectorsforless system displayssimilartemporalfeaturestothoseof(b)asimulationevent effective fieldofviewforourflatdetector,asgivenbyQ= 0.5 and1.6sr. 2n/(m +2).Form&2-10,assuggestedbyvariousnarrow- lator slab,andbothmethodsyieldaminimumdetectableflux puiser, and(2)relativisticcosmicrayspassingthroughascintil- and wide-angleCerenkovexperiments(JelleyGalbraith background lightlevelatGulmarg(~3x10photonscm cm thatfollowsfromconsiderationsoftheaverage sky- value isconsistentwithathresholddensityof~10photons during theexperiment.Thisgivesaneffectivedetectionarea of of 12+3photonscmfortheCerenkovlightpulse.This proton primary.ReferringagaintotheworkofHara,Kamata, results ofHara,Kamata,andTanahashi(1977)Chantier et s srÂ)andtheamplitudediscriminationlevel used photon-initiated shower(Fazio 1974).Weusethisvaluehere- then, forathresholdproton energy of1.4x10eV,wecan cosmic-ray protonofthesame initialenergy.Equivalently, be afactorof~3higherfor ay-rayprimarythanthatfor tion rangeof~80m,theCerenkov photondensityislikelyto responding toathresholdvalueof~1.4x10eVfor a this matchesthecosmic-rayspectrum(Wolfendale1984), cor- spheric Cerenkovlight.ForanassumedQ=0.5sr(White, al. (1981)onthelateraldistributionfunctionofatmo- take aminimumdetectable energy of~5x10eVfora and Tanahashi(1977),wenotethat,withinoureffectivedetec- Porter, andLong1961)anaverageeventrateof~66hr“ ^ 1955; White,Porter,andLong1961),weobtainQbetween eff ~7c (80m)fortheCerenkovlightpool,whenweuse the eff eff Fig. 1.—ACerenkovlightpulse(a)recordedbythetwo-photomultiplier The Gulmargsystemwascalibratedusing(1)anLEDlamp L I33/22/42/1S2 341/23/06/50-4 20^s Vol. 306 198 6ApJ. . .306. .587B -1 h h - -1 1 1 No. 2,1986 undertaken tosearchforthe4.8hrmodulationofCygnusX-3 after, bearinginmindthatitcanbeerrorbyafactorofupto origin inourdatastream,comprisingatotalof16,794atmo- spheric Cerenkovpulses.Theseeventswererecordedbetween distributed over103days.Figure2agivesthefrequencydis- mean rateof52±14.7hr,or,equivalently12.53.5events by~45% relativetoaPoissoniandistributionwiththesame tribution ofthedaily-averagerates;itisfoundtobebroader 1.5 onaccountofvariousuncertainties. X-3 periodandwhichweshallbeusingasthephasebinwidth per timeintervalof14.4minutes,whichisequalto0.05Cygnus 1976 Januaryand1977Decemberin270hrofobservations for part-runswhenR.A.«20isatazenithangle(b)i¡/<40°and(c)ij/> 40°, component ofCygusX-3origin. ment withexpectationisreasonably good in(b)and(c),theobserveddistribu- are comparedwiththecorrespondingPoissonfitsmeanvalues of (a) related tothedetectioninGulmarg datastreamofaphase-dependent when R.A.»20isati/f<40°arediscussed inthetext,butthisvariationisnot and (c)50.8±14.5hr^Thepossible reasonswhythemeanratehaschanged tion issignificantlywiderin(a),mainly becauseitisacompositeoftwo Poissonian distributionswithappreciably differentratesof(b)64±16.3hr 52 ±14.7hr",{b)6416.3hr'and (c)50.8±14.5hr“.Whiletheagree- This sectionprincipallydealswithaperiodogramanalysis Fig. 2.—Distributionsofaverageeventratesfor(a)allthedailyruns,and © American Astronomical Society • Provided by the NASA Astrophysics Data System LU III. DATAANALYSIS RATE PERHOUR CYG X-3PeVPHOTONFLUX h -1 h o d in ouranalysis.TheexcessivewidthofthehistogramFigure invite attentiontoFigures2band2c,wheretheratedistribu- was lyinginthedetectorfieldofview(Fig.10).Insupport,we have beenhigherby~25%whentheR.A.region~20±04 zenith angleoflessthan40°(Fig.2b)andoutsideit2c). 2a isnotsurprisingifwerecallthattheGulmargeventrates values, 64±16.3hrand50.814.5)isnowfoundto The agreementwiththecorrespondingPoissonfits(mean the courseofdailyobservationsR.A.æ20waslyingata tions havebeenplottedseparatelyforthesituationswhen,in changes intheskytransparencyconditionsaswellsuch by someextraneousfactors,includingunknownday-to-day likely thatthedistributionsobservedinFigure2areinfluenced be markedlybetter.Apartfromthismainreason,itisalso monitor sufficientlycloselyduringthelongoperationperiodof experimental parametersaswemaynothavebeenableto the presentexperiment. recorded perphasebin,butinsteadtoworkwiththedeviation analysis, itispreferablenottousedirectlythenumberofevents one runtoanothernotaffecttheoutcomeofdesiredphase 2 yrdata;thereasonableagreementfoundwithexpected A ofthisnumberfromthecorrespondingdailyaveragevalue. formance inthecourseofagivennightas,forexample, been anynoticeabledrift,onaverage,inthesystemper- ing. Wehavealsoutilizedthisparametertocheckiftherehas normal distribution(standarddeviation,
DEVIATION FROM MEAN (%) M M J S MONTH Fig. 4.—Monthly breakdown of the total observation time for the on- source {ij/ < 40°), intermediate (40° <\J/ < 70°), and oflf-source (^ > 70°) situ- 20 r (b) ations {\¡j is the zenith angle of Cygnus X-3). Poor sky conditions have restricted the on-source observations mainly to the months of May and June. I ^ 10 - 0 z to mention here that the present analysis includes all the data < •10 shown in Figure 2a, including the days contributing to its tail 22 Ó 02 ' 04 06 portions. OBSERVATION HOUR (1ST) The mean event rate on the on-source phase histogram (Fig. 5a) is 15.78 + 0.28 per bin, and this is ~25% higher than the corresponding values for the intermediate case (Fig. 5b) or the two off-source phase histograms (Fig. 5c). We return to a dis- cussion of this difference in § IV but note here that it is to be expected in view of the observation made in § I about the presence of an additional, phase-independent component in the Gulmarg data belonging to R.A. « 20h ± 04h (Bhat, Sapru, and Kaul 1980), but not necessarily related to Cygnus X-3. The on-source phase histogram (Fig. 5a) shows a 4.5 a peak at 0 = 0.6, corresponding to 265 events being recorded in this phase bin against an expected number of 189 Cerenkov pulses. Assuming Poisson statistics, the probability that this over- abundance may occur in any of the 20 phase bins on a random basis works out to be 6.3 x 10-7. The remaining 19 bins give X2/18 = 1.24 (as against x2/19 = 2.5 for the whole distribution) Fig. 3.—(a) Frequency distribution of A, the deviation from the mean run and are in reasonable agreement with a flat distribution value in the number of events recorded in a time interval of 14.4 minutes (i.e., 2 0.05 Cygnus X-3 period). The distribution is found to be in reasonable agree- (P[>X ] = 23% and 0.2% for 18 and 19 degrees of freedom ment with the expected normal curve (standard deviation a & 25%) and reass- respectively). Referring now to the phase histogram for the ures that the parameter is not unduly influenced by the non-Poissonian effects intermediate case (Fig. 5b), an analogous behavior is found responsible for the shape of Fig. 2a. {b) Weighted average deviation for all there. Thus, while all other phase bins conform to a phase- the days with an observation time of 3-10 hr per night is plotted as a function independent distribution (reduced %2 = 0.4), the phase bin at of Indian Standard Time. No perceptible long-term trend is evident, suggesting that the detection system has remained fairly stable during observations on 0 = 0.6 again exhibits a 3.2 a excess, the corresponding various nights, (c) The behavior of A as a function of local time, as seen on 1976 Poisson probability for which (274 events observed versus 218 January 3 and 4, two consecutive nights with the longest observation time. No events expected) is 6.3 x 10 “4, with all 20 bins being équi- correlated long-term behavior is seen, suggesting that the detection system was probable. On the contrary, there is no evidence of a similar 4.8 not particularly sensitive to long-term changes in factors like ambient tem- hr modulation among the off-source data. This is evident from perature (expected to follow a similar hourly pattern on two contiguous days). the two phase histograms in Figure 5c, obtained after the runs with 0 > 70° were divided into two subsamples of a compara- during the various observations runs. Cases (a)-(c) have been ble size : no correlated behavior is discerned at any 0 between treated separately and the results, showing average deviation these two periodograms themselves or with those in Figures 5a from mean as a function of 0, are presented in Figure 5 and 5b. Further, all the fluctuations in the off-source phaso- along with the frequency n with which a given phase bin has grams are restricted to within ±2 cr and, with x2/19 = 1.29 and been covered in the course of the observations. It is obvious 1.23 for the two cases, both can be regarded as being consistent that n determines the total number of events observed in a with a phase-uniform distribution (P[ > x2] ~ 20%). given 0 and is also related to the standard deviation a, in units In order to examine the possible effect of a change of ephem- of which the phase histograms have been plotted. It is pertinent eris, we have combined the data belonging to the on-source
© American Astronomical Society • Provided by the NASA Astrophysics Data System 198 6ApJ. . .306. .587B 1/2 10-8 (\l/ <70°)basedontheCygnusX-3phaseparametersdueto procedure describedabove,twocompositephasehistograms and theintermediatecaseshaveobtained,following phase peakat0=0.6(Fig.6a),obtainedonthebasisof the been coveredinthetwocases.Asshouldbeexpectedfor our Bonnet-Bidaud (1981).Theseperiodogramsarecompared in Parsignault etal(1976)andthosefromvanderKlis and comparison oftheplotsconvincinglydemonstratesthatpeakfeaturein(a)and(b)isrelatedtoCygnusX-3. is typically~15/n%.Thephase-averagedmeanrate(a)15.78±10.28perbin,(b)12.810.32and(c)12.580.23bin12.79 ±0.24perbin.A position towhichthepeaksseenin{a)and(b)shiftwhenvanderKlisBonnet-Bidaud(1981)ephemerisisused.Thedottedhistogramat the bottomofeach the weightedaveragedeviationfrommeanrunrateasafunctionofCygnusX-3phase(basedonParsignaultetal.1976elements).Arrows indicatethe phase binto0=0.55(Fig.6b),whent,p,andpvaluesfrom observation epochof1976-1978,essentiallythewhole the panel givesthefrequencynwithwhichagivenhasbeencoveredduringthreecases.isplottedinunitsofstandarddeviationa,for agivenphasebin tion turnsouttobe1.3x10“forFigure6aand10 for the morerecentephemerisofvanderKlisandBonnet-Bidaud Parsignault etalephemeris,isfoundtoshiftbackwardby one Figure 6alongwiththenumberoftimes(n)eachcj)value has Figure 6b. are used.TheprobabilitythatthepeaksseeninFigure 6 at former valuewaschosenbecause ofitsproximitytothe 0 =0.55and0.6canariseduetoarandomstatisticalfluctua- Cygnus X-3periodof4.8hr,while the8hrtrialperiod,beinga similar analysisusingtrialperiods of4.5hrand8hr.The smaller submultipleof24hrthan 4.8hr,wasexpectedtocheck No. 2,1986 0 Fig. 5.—PeriodogramsderivedfromtheGulmargdataforCygnuszenithangleof(a)\¡/<40°,(b)40°\j/70°,and(c)^>70°obtainedby plotting, We havealsosubjectedthecombined datafori¡/<70°toa © American Astronomical Society • Provided by the NASA Astrophysics Data System ’10 20 CYG X-3PeVPHOTONFLUX tc) OFF-SOURCE(4>>70*) independent, sinceinnophasebinistheeventratefound to The resultingdistributionscanwellberegardedasphase- each involvinganobservationperiodcomparableinlength to depart significantlyfromthesamplemeanratebymore than has someunknownconnectionwithEarth’srotationperiod. whether theeffectobservedinFigure6(andFigs.5aand5b) display nounexpectedlylargedeviationsfromauniform dis- that fortheon-sourcesituation(Fig.5a),arefoundtobeessen- 2.5
© American Astronomical Society • Provided by the NASA Astrophysics Data System No. 2, 1986 CYG X-3 PeV PHOTON FLUX 593 tainties, which can affect both the flux estimates and the energy values assigned to them. Nevertheless, there are other factors a as well which merit attention in this regard, and we proceed to consider some of them now. %/) Turning first to energies above 1013 eV, most of the mea- 'ë 1Ô10 surements here involve continually long observation periods (months to years) and they should as such represent long-term average values. The significant dispersion found among these .-ii- values cannot, therefore, be attributed to Cygnus X-3 variabil- 10 ity on short time scales (days to weeks), for which there is now convincing evidence at TeV energies (see, for example, Cawley et al 1985a) and possibly at still lower energies, as is suggested 12 1Ô Weekes and Htlmken (1977) by the balloon observations of Galper et al (1977) and also by 0 Lamb etal (1982) a comparison of SAS 2 (Lamb et al 1977) and COS B (Hermsen et al 1983) satellite data on Cygnus X-3. On the 13- ▼ Cawley et al( 1985a) 10 0 Dowthwaite etal(1983) other hand, the UHE measurements refer to different observa- □ Neshpor etal (1979) tion epochs, and it is therefore worth considering if they reflect V Danaher etal (1981) LU 4/ a long-term trend in Cygnus X-3 emission characteristics _ A Mukanov etal (1979) (Rana et al 1984). But before looking into this possibility, we ^ 10H . $ Morello etal (1983) should take note of the recently reported lower bound of 11.6 S Í? Tonwar etal (1985) kpc on Cygnus X-3 distance (Dickey 1983) and the attendant Alexeenko etal (1985) f attenuation of UHE photons through pair production inter- ? 1015 ? Bhat etal (1985) LU 5? Baltrusaitis etal(1985a) actions with the 2.7 K microwave background radiation (/) Fomin etal (1985) (MWB) in the interstellar medium (see, for example, Gould X .-16 Li Kifune etal(1985) 1983). This leads to an upward revision of the Gulmarg flux for Q. 101f 1 Watson (1985) 1976-1978 to 2.9 ± 0.7 x 10~12y cm-2 s-1 at greater than 0.5 i Lloyd-Evans etal (1983) PeV. This is compared in Figure 8 with the results of other PeV Samorski & Stamm(1983a) measurements, which have also been similarly corrected for This work attenuation by the MWB field. We have not plotted here the ■ I ■ i 1 i i Fly’s Eye estimate for 1983, since it is derived from only 25 hr r\13 r\15 06 10" 1012 1013 1014 10D 10 of observations and may not represent a long-term average value. Again, upper limits cannot provide any definite leads in PHOTON ENERGY E^leV) the present context and are not being considered ; however, in Fig. 7.—Phase-averaged photon flux estimate for Cygnus X-3 as measured the case of the Gulmarg limit for 1984 epoch, we would like to -13 -2 1 by several ground-based 1,1experiments during the period 1976-1985. A plausible mention that this smaller value (1.7 x 10 y cm s' ) is integral spectrum ; Ey is the photon energy) through the Kiel point at quite consistent with other contemporaneous measurements 2 x 1015 eV suggests apparent disagreement with a single power-law spectral fit, a feature which can be accentuated when it is recalled that the UHE data and in that sense strengthens our confidence in the Gulmarg points need to be corrected for absorption by the 2.7 K background in the results obtained by the same experiment for the earlier period interstellar medium. Note the compatibility of the Gulmarg upper limit for the (1976-1978). 1984 epoch with other more contemporary measurements. An inspection of Figure 8 reveals that, in general, the MWB- 13 corrected spectral data at Ey > 10 eV are fitted reasonably well by a series of lines, each having a slope of —1.1 and (j) = 0.575-0.625. The Fly’s Eye group (Baltrusaitis et al 1985a) representing a different observation epoch. (It is interesting to have seen a 3.5 © American Astronomical Society • Provided by the NASA Astrophysics Data System r"PQ ooLO 594 BHAT ET AL. Vol. 306 -10 TABLE 1 'O 10 Possible Cygnus X-3 Photon Signal from Haverah Park ft 1976-1978 1976-1979 Year Noise Phase Bins Number of 1 c*x\n of Rejection 0a Excess 1979-1982 b \ Observation Number iV0 (1 12 JB 15 10* 10 10,j 10” 10 PHOTON ENERGY E^teV) Fig. 8.—Integral photon spectrum of Cygnus X-3 (>1012 eV) replotted after correcting for interactions with the cosmic microwave background for an assumed source distance of 11.6 kpc. The fitted lines have a slope of —1.1 and are representative of different mean observation epochs. There is seen to be some support for the view that the source luminosity around 1 PeV has been undergoing a long-term reduction, at least from 1976, for which the earliest observations in this energy region are available. The average photon flux (1976-1984) above 1 PeV (derived from the measurements above 1013 eV) is shown in the figure by a bracketed star. When compared with a similarly averaged value for the TeV region (to remove short-term jitter to which the air-Cerenkov observations are particularly prone because of short observation runs) on the basis of a ~ 1-1 spectrum, the latter (shown by a bracketed filled circle) is found to be anomalously lower by a factor of ~20, possibly due to significant attenuation in the infrared/optical field of the stellar companion of the Cygnus X-3 binary system (see text for additional details). yr if only post-1979 observations (i.e., all except Gulmarg and Kiel) are considered. With the recent publication of Haverah Park observations of Cygnus X-3 (Lambert et al. 1985), it is now possible to show that the above result is not unduly biased by ambiguities that may be inherent in a comparison of observations from different 1976 77 78 79 80 81 82 83 84 1985 experiments. These Haverah Park measurements hold for energies above 1 PeV and have been presented as a series of OBSERVATION EPOCH (YEAR) phase-histograms (bin size A0 = 0.025) on a yearwise basis Fig. 9.—{a) Cygnus X-3 photon flux above 1 PeV (after correcting for the between 1979 and 1984. Before they can be used for the present 2.7 K attenuation) vs. the observation epochs of several experiments. A long- study, however, it is logical that, because of the presence of term reduction in the UHE y-ray emission is suggested, in approximate agree- noise in these periodograms, we should first decide about what ment with an exponential law with a time constant T » 2.4 yr (equal weight being assumed for all the data), (b [inset]) Excess events recorded at Haverah minimum value of a positive excess to associate with Cygnus Park on a yearwise basis between 1979 and 1984 in the Cygnus X-3 phase X-3. Further, in order to accomodate the possibility of the intervals (¡) = 0.2-0.3 and 0 = 0.525-0.7. Positive excesses beyond 1 a noise suggested phase wandering of the signal with time (Watson level N0 (Table 1) alone are regarded as y-rays from the source in the present 1985), the criterion followed here searches the rather extended analysis. As in (a), a decreasing trend is again evident, with T = 5.4 yr (all data) and 2.2 yr (when 1984 signal is excluded). This independent support for a phase intervals between cj) = 0.2-0.3 and (j) = 0.525-0.70 (total long-term trend in the PeV source luminosity is important, for various uncer- of 11 bins) for positive excesses N0 or greater, where the noise- tainties possibly involved in comparing multistation observations are thus rejection level N0 is obtained by studying the frequency dis- automatically removed. © American Astronomical Society • Provided by the NASA Astrophysics Data System 198 6ApJ. . .306. .587B -1 45 37-1 14 quite probable. No. 2,1986 ing valueofN,isplottedinFigure9bagainsttheyear by addingallthepositiveexcessesgreaterthancorrespond- for theremaining5yrconformbettertoanexponentialdecay nential least-squaresfittoalltheobservationyearsyields observation. Againadecreasingtrendisevident,andanexpo- improves remarkably(from~50%to>96%),andthedata with thevalueinferredearlierfromFigure9a.Wewouldliketo law withTä2.2yr.Thisisinsurprisinglygoodagreement 1985) andsuggeststhatthepresentnoise-rejectioncriterionis point outthatwehavedrawnguidancefromotherexperiments and haveusedsomeotherreasonablecriteriaalsotopickup cally allthewayfrom1979to1983,but1984standsanom- the sameresult,viz.,signalstrengthdecreasessystemati- the CygnusX-3signalinHaverathParkdata,alwayswith T ä5.4yr.Ifweexclude1984,thesignificanceoffit 0 alously higher. been undergoingalong-termreduction,atleastfrom1976 persuasive toacceptthattheCygnusX-3UHEphotonfluxhas trend andsuggestsafluxdecreaseattheseenergiesbyfactor and 1985March,conformsremarkablywellwiththeabove three contiguousobservationperiodsbetween1981December as quotedbythePlateauRosagroup(Morelloetal.1985)for of ~2yr. patible withbothFigures9aand9b,itfollowsthattheCygnus energy rangeareavailable.TakingT=2.3yr,whichiscom- X-3 fluxmayhavedecreasedbyafactorof~80overthe onward, theepochforwhichearliestobservationsinthis can varyoverawiderangeissupportedbytherecentdetection intervening period(10yr).ThattheluminosityofUHEsources would beunrealisticonenergyconsiderationstoexpectthis decrease suggestedbytheaboveanalysis.Ifreallytrue,it before present.Amorepuzzlingfeatureisthelargerateofflux intense sources)activeintheGalaxybetween10andyr X-3-like PeVsources(or,moreprobably,onlyseveral, Toyoda etal.1965)intermsoftherebeing~200Cygnus extragalactic sourceLMCX-4(ProtheroeandClay1985)in of VelaX-l(Protheroe,Clay,andGerhardy1984)the trend tohavestartedtoofarbackorcontinuemuchfurther the PeVregion.Itisalsoinlinewithinterpretationby X-3 hasbeengoingthroughthesehighandlowemissionstates natives, themostappealinginourviewisonewhereCygnus in time.Whilethisplausibleconstraintthrowsupseveralalter- of muon-poorshowerfromhighGalacticlatitudes(|h|>20°; Bhat, Kifune,andWolfendale(1985)oftheChacaltayaexcess cated experimentswillprovideanunambiguousanswerand, if the past10yrorso.Undoubtedly,futureobservationsbydedi- regularly, sothatwemayjustbewitnessingonesuchcycle in confirmed, willtelluswhenCygnusX-3gotoahigh state again. Theresultitselfhasimportantimplicationsfor the nature ofthesource(pulsar?)involvedandprocess(es) X-3-like systems(VestrandandEichler1982; Ves- responsible forgeneratingUHEphotonsinthecaseofCygnus by paralleltheoreticaleffort. trand 1984;HillasPorter 1984;Chanmugamand Gulmarg fluxestimateleads toaCygnusX-3luminosityof Brecher 1985).Thisunderstandably needstobeaddressedtoo, 6 x10ergssabove5 10 eV,assumingisotropicemis- sion andasourcedistanceof 11.6 kpc. 13 The CygnusX-3signalNobtainedforeachyear(Table1) r Similarly, theCygnusX-3fluxatgreaterthan3x10eV, Putting alltheevidencetogether,itnowseemssufficiently It isofinteresttoknowthat forthe1976-1978epoch, © American Astronomical Society • Provided by the NASA Astrophysics Data System CYG X-3PeVPHOTONFLUX 1A -132 12 12 1 1 found toliewellbelowessentiallyallthefittedlinesin et al.1983;Cawley1985a)andbasedonapower-law recall thattheyinvolverelativelysmallerobservationperiods figure. Morequantitatively,itisseenfromFigure8thatthe drawn. InordertoseekanextensionoftheCygnusX-3pro- and arethereforesusceptibletotheCygnusX-3variabilityona linear extrapolationofthe~E~spectrallinepassing spectrum withanintegralexponentyj=—1.1.Thisvalueis duction spectrumtotheseenergies,itisthereforemoreuseful short timescale,afeaturetowhichattentionhasalreadybeen through thefluxof~3x10ycmsatgreaterthan1 time-averaged fluxabove10eVliesafactorof~20below We haveshowninFigure8onesuchvaluederivedfrom spaced measurementsinthelO^-lOeVregionandcompare to firstobtainalong-termaveragedfluxfromvarioustime- recently, byApparao(1985),isthattheTeVy-raysbecome also beenconsideredbyCawleyandWeekes(1984)and,more UHE spectralpointsinFigure8.Onepossibilitywhichhas that valuewiththeMWB-correctedestimatesofPeVflux. helium star(vanderHeuvelanddeLoore1973;Ghoshetal. significantly attenuatedintheopticalandinfraredphotonfield et al.1979;Danaher1981;Lamb1982;Dowthwaite several Cerenkovobservationsspanningtheperiod~1972- of theCygnusX-3stellarcompanion,whichisbelievedtobea PeV, obtainedbyaveragingtheindividualMWB-corrected into twogroups,coveringperiodsof1972-1978and1978- lation ofthesourcespectrumform~downtoTeV also onthespectraldetailsoftheseoptical/infraredphotons. tively onthespatialextentofabsorbingradiationfieldas lower limitresultswhentheestimatequotedinVladmirsky, no long-termdecreaseatallor,best,byafactorofless than flux valuesabove1TeVareconsistentwiththerebeingeither lower energies.Bydividingtheavailableair-Cerenkovdata the rangeabove10eV,largelong-termfluxreduction energies cannotberuledoutonthisbasisatpresent. be large,upto~400,sothatthepossibilityofalinearextrapo- Apparao (1985)suggestthatthecorrectionfactorscanindeed accurately. Thecalculationsmadeinthemeanwhileby required inthefuturetodeterminethesequantitiessufficiently Extensive infraredobservationsaroundCygnusX-3are vary fromtimetotime,itisnotimmediatelyobviouswhy it ing efficiencyofthecircumstellarinfraredphotonfield may 2.8 overthewholeperiod,correspondingtoT>11.5yr (the apparent inthePeVregionshouldalsobereflectedat mate forreasonsthatarenotobviousatpresent,itseems the deductionthatfollowsfromFigure9isagrossoveresti- should bearanout-of-phaserelationshipwiththedecreasing Stepanian, andFomin1973isincluded).Whereastheabsorb- Cygnus X-3emissiononalong-termbasis.Therefore,unless 1984 (Vladimirskyetal.1975;Mukanov1919;Neshpor Cygnus X-3bythesamemechanismasUHEphotons, necessary toconcedethatTeVy-raysarenotproduced in 1981). Themagnitudeofthecorrectionrequireddependssensi- however, VestrandandEichler 1982;EichlerandVestrand thereby possiblyconstrainingtheHillas(1984)model (see, in theCygnusX-3vicinityisindeed significant,weshouldthen photons canbewrongandshould bearrivedatindependently. above tocorrectforthecircumstellar absorptionoftheTeV 1984, inordertosmoothanyshort-termjitter,therespective 1984). Insuchacase,thecorrection factorof~20predicted 12 Turning nowtothemeasurementsinTeVrange,we However, iftheproductionspectrumissamethroughout If itturnsoutthattheabsorption oflO^-lOeVphotons 595 r"PQ ooLO BHAT ET AL. Vol. 306 ooo ; expect these y-rays to be regenerated as X-rays and MeV (Dowthwaite et al. 1984; Cawley et al. 1985h) and PeV ^ photons by synchrotron and Compton processes. It will be (Baltrusaitis et al. 1985h) energy regions. In addition, the rele- è useful to study the attendant consequences, including whether vant celestial region has within it two “Kiel sources” with S the extension of the X-ray peak to the phase interval 0 = 0.6- R.A. ä 18h and æ23.5h (Stamm and Samorski 1983) and AM ^ 0.7 in the Cygnus X-3 light curve and its resulting asym- Her (R.A. æ 18h), which is a Cygnus X-3-like X-ray system metrical shape can be related to the attenuation of the phase with a binary period of 3.1 hr. We also note that several peak at 0 = 0.6-0.7 in the lO^-lO12 eV. It is pertinent to point hadron-poor and muon-poor showers (presumably due to out here that Weekes et al. (1981) have reported a correlated y-ray primaries of energy >6.5 x 1014 eV) have been recorded change in the Cygnus X-3 light curves in the X-ray and the at Tien-Shan with R.A. æ 16h and æl8h and declination of TeV regions, where a reduction in the relative X-ray flux at ~40° ± 10° (Stamenov et al. 1983; S. Nikolsky, private 0 = 0.6-0.8 was seen to be accompanied by a significant communication). However, a multiple y-ray source origin for increase in the peak at 0 = 0.6-0.7 in the very high the peak in Figure 10 needs to be reconciled with the results of energy y-ray flux, possibly imlying that the latter escaped cosmic-ray air shower arrays, and here its disproportionately severe attenuation and subsequent transformation into X-rays large amplitude poses problems. A plausible way of resolving due to some causative changes in the optical emission charac- or reducing this disparity (apart from invoking time variability teristics of the Cygnus X-3 companion star. for the effect) is to hypothesize that the y-ray initiated cascade Finally, we discuss why, during the on-source period (y-EAS) develops faster in the atmosphere than does a hadron- (0 < 40°), the phase-averaged event rate (Fig. 5a) is higher than produced shower (p-EAS). Since the bulk of the Cerenkov light those found for the intermediate and the off-source periodo- emanates at higher altitudes, from close to the shower grams (Figs. 5b and 5c). The reason for this is apparent from maximum, and it can traverse the atmosphere without signifi- Figure 10, where we find that the Cerenkov pulse rates exhibit cant attenuation, this possibility would make the Cerenkov a broad peak at R.A. ä 20h. The most striking thing about the technique intrinsically superior for detecting y-EAS to the figure is the large peak amplitude, which is ~25% higher than particle-array one (Bhat and Razdan 1985), which samples the the average rate for the non-peak region. We have already EAS secondaries only at the ground level Und thus would have referred in § I to the correspondence of this Gulmarg result a reduced efficiency for detecting y-ray primaries. It is worth with the time-dependent observations at Mont Blanc (Badino remarking here that the upper limits obtained recently in the et al. 1980), Manitoba (Smith et al. 1983a, b), and Minnesota course of the Fly’s Eye survey for emission from steady y-ray (Bartelt et al. 1985). sources (Baltrusaitis et al. 1985h) exhibits a sidereal distribu- A closer examination of Figure 10 shows that the peak struc- tion which also show a broad peak between R.A. » 15h ture observed there is compatible with at least two point- andæ 18h for the declination range of 180-62°. spread functions (nominal form ~ cos4 0) centered at ~16h The above discussion does not on its own rule out the possi- and ~20h. Taken at its face value, this would suggest that two bility of some systematic instrumental effects leading to or or more y-ray sources are contributing to the apparent excess. augmenting the peak in Figure 10, and these effects must also It is remarkable that both these positions are now known to be be considered. One possibility here is the increase in the back- occupied by PeV y-ray sources: Cygnus X-3 and Her X-l, the ground light level when the Galactic arm feature at R.A. ä 20h latter source having been detected very recently in both TeV transits at Gulmarg. Though it leads to an increase in the photomultiplier anode current (DC component) by a factor of nearly 2, it decreases rather than increases the photomultiplier gain, so that the effect of the appearance of the Galactic arm within the system field should be in opposite sense to that suggested by Figure 10. Similarly, the use of a rather high amplitude discrimination level ( > 4 rms noise) rules out a sig- nificant increase in the accidental coincidence rate due to the increase of shot noise. In any case, if the effect were indeed related to the overhead appearance of the Galactic arm, we should have expected it to be duplicated with a somewhat reduced amplitude, when the similar feature of R.A. æ 06h sweeps through the system field of view. Figure 10 and the off-source phasograms in Figure 5c (the phase-independent rate here would be higher) demonstrate that such is not the case. Moreover, the peak in Figure 10 is too wide and, as already mentioned, is consistent with contributions from at Fig. 10.—Histogram shows Cerenkov event rates averaged for the period least two point (or line) sources at R.A. æ 16h and ^20h and 1976 January-1977 December and plotted as a function of R.A. A broad peak not just one at R.A. « 20h. is seen in the R.A. range 15h-02h. Point-spread functions (PSFs) of the form ~ cos4 ij/ have been fitted to the distribution to illustrate that the peak struc- Another proposal that merits consideration in the present ture is more compatible with contributions from at least two point sources at context is that better observing conditions (e.g., sky R.A. æ 16h and 20h. Dashed line represents the sum of two PSF contributions transparency) were encountered at Gulmarg during the plus a background rate (dot-dash line) of 47.7 hr - A plausible way of reconcil- summer months of 1976-1978 than during the winter, and the ing the disproportionately large peak amplitude with particle detector array resulting systematic variation in the event rates masquerades measurements is discussed in the text, as are various probable technical reasons which can lead to a spurious sidereal effect. This peak is flattened in here as a sidereal effect. While it does not seem as probable in the present phasogram analysis and leads to a 25% increase in the phase- our view, we cannot completely rule it out at the present stage. averaged mean rate in Figure. 5a compared with the corresponding values in It is clear that more experimentation with optical detectors Figs. 5b and 5c. provided with light compensation circuits and sky transpar- © American Astronomical Society • Provided by the NASA Astrophysics Data System 198 6ApJ. . .306. .587B -12 - No. 2,1986 ency monitorswillhelpcheckwhetherFigure10isanartifact X-3 phasepeakseenintheon-sourcehistogram(Fig. phase histogramanalysisandisnotthecauseofCygnus peak featureinFigure10getseffectivelyflattenedduringthe however, itisimportanttorealizethat,whateveritsorigin,the of somesystematicobservationalbias.Forthepresent, for theintermediatecasewhichphase-averagedevent phase peak,withexpectedlysmalleramplitude,inFigure5b, rate perbinisthesameasthatforoff-sourcecase. characteristic modulationperiodof4.8hr,dutycycle~5%, cant (4.5a)phase-dependentsignal(cj)=0.55;vanderKlisand between 1976and1978,haverevealedthepresenceofasignifi- 5a). 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