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markarian 501 ( mkn501 ) is an extragalactic bl - lac type object at z=0.034 .
it is observed in radio , optical and x - ray bands , and can be characterized in flat spectrum radio , highly optically polarized and optically violently variable .
bl - lac type objects are considered to be a kind of agn , which have a jet oriented to our line of the sight .
recently , mkn501 ( @xcite ) , mkn421 ( @xcite ) , and 1es 2344 + 514 ( @xcite ) have been identified as tev gamma ray sources , and they are all bl - lac objects . gamma rays from mkn501 around 100mev to 10gev was not observed by the cgro egret detector(@xcite ) , however , emission in the tev energy region were discovered by the whipple telescope ( @xcite ) and confirmed by the hegra telescopes ( @xcite ) .
detailed study of gamma rays from agn will give us information about the environment surrounding the huge black hole located at the center of the agn , and the high energy phenomena and particle acceleration in the jet .
the time scale of the intensity variations of the tev gamma rays may explain the particle acceleration site is close to the black holes .
tev gamma rays from such extragalactic object interacts with the infrared background photons , and a cutoff of the energy spectrum is expected around 7 - 15tev(@xcite ) .
there is an ambiguity in the prediction of the cutoff energy due to the uncertainty of the number density of infrared photons . in the discovery stage ,
the intensity in the tev energy range was very small , corresponding to 8% of the crab nebula flux , however , from mar 1997 it increased dramatically and variated from 0.3 to 4 crab flux .
it is found that the variation of the intensities are larger in the tev range than x - ray and other ranges(@xcite ) .
these flares are observed by whipple , hegra , cat , tactic and the telescope array prototype ( tap ) ( @xcite ) .
the flares were continuously observed until the end of july 1997 .
here we will report the details of the mkn501 observations by tap .
the telescope array prototype detector @xcite , a seven telescope array is under construction at utah , dugway .
its geographical position is @xmath0 n , @xmath1 w at an altitude of 1600 m above sea level .
the prototype detector works in dual modes : the cherenkov and the air fluorescence mode .
construction started in the summer of 1996 , and currently three telescopes with the respective separation of 120 m are in operation .
the seven telescopes will be arranged in a hexagonal grid with a separation of 70 m to maximize the detection efficiency of tev gamma rays .
each telescope is the alt - azimuth mount with a 6m@xmath2 main dish .
the main dish consists of 19 hexagonal shape segmented mirrors coated with anodized aluminum .
the reflectivity of mirrors are about 90% at wave length of 400 nm . at the focal plane ,
a high resolution imaging camera of 256ch photomultipliers is installed to measure detailed images of cherenkov light from gamma rays and cosmic rays .
the cherenkov light images are used to distinguish the gamma rays from the huge number of background cosmic rays .
the typical cosmic - ray rate is about 1000/min and the gamma - ray rate from the crab nebula is about 0.5/min ( @xcite ) with three telescopes .
therefore rejection of cosmic ray background events using the shape parameter of the cherenkov light is essential to obtain a reasonable s / n ratio in this experiment , and we use the techniques originally developed by the whipple group ( @xcite ) .
the absolute pointing accuracy of the telescopes is typically 1 arcmin which is frequently calibrated by imaging bright stars .
the signals from the 256ch camera are amplified just behind the camera in order to minimize electric noise and to obtain better timing resolution , and are then fed to adc and tdc modules mounted at the telescope base to measure the amplitude and the timing after passing through 10 m of twisted pair cables . the triggering requirement to record
the event is four folds out of 256 tubes .
the threshold of the discriminators are set at 5 photoelectron level .
the single counting rates in each tube are set to 3 - 5khz in each channel .
the threshold energy for detectable gamma rays is 600 gev for vertical showers .
the mkn501 was observed with the raster scan tracking mode . in this mode
, the telescope tracking center scans the sqaure region of @xmath3 in right ascension and declination coordinate centered on the target .
there are several advantages in this method compared with the conventional on - source / off - source tracking mode .
the on - source and off - source sky region can be observed simultaneously .
the systematics of the imaging devices can be reduced significantly . by observing the bright star images ,
the calibration of the telescope absolute direction can be done with the accuracy of @xmath4 .
our telescopes will be described in more detail elsewhere .
this observation of mkn501 was carried out from the end of march to the end of july , 1997 .
we observed a total number of 47 nights for a total observation time of 105.4 hrs .
we have observed 3,400,000 events in the f.o.v . of @xmath5 around mkn501 , which are mainly cosmic - ray protons and helium nuclei . in the analysis
, we have limited the zenith angle to the range of @xmath6 to @xmath7 in order to reduce the systematic errors in the energy and the aperture estimates .
this leaves 2,160,000 events and 64.0 hrs of live time , with an average event rate of 9.4hz with three telescopes . among these events
, we selected gamma ray candidates using the shape parameters of cherenkov images and their directionality .
we can determine the arrival direction of gamma rays and the cosmic rays with an accuracy of @xmath8 ( the elliptical errors ) in each event .
therefore , 97% of the background events around the target , say within @xmath9 , can be rejected .
the cosmic rays show larger images than gamma rays , and they are rejected with 95% efficiency through the image selection .
therefore , typically 99.95 % of the background events around the target are rejected with the image and the arrival direction using the present analysis method .
35 % of the gamma rays remain through this process . therefore , even if the original @xmath10 is bad as @xmath11 , we can obtain @xmath10 of order @xmath12 .
at first , the clean cherenkov images are obtained by removing the background photons with the timing information .
the signal timings of each photomultipliers are recorded with an accuracy of 1nsec .
the cherenkov light signals from gamma rays and cosmic rays are concentrated within a 10 nsec interval , however noise including the photons from star light and the air glow are randomly distributed in time . therefore we require a timing alignment with a software gate set at 40 nsec .
then we require the clustering of hit tubes , geometry .
after these selections , the chance coincidence events due to the random coincidence are completely rejected and the pure cherenkov events induced by gamma rays and cosmic rays remain .
then , the image parameters are calculated : signal size size , the centroide position of images ( x , y ) , width , length and conc .
the events with images located near the camera boundary are cut by requiring the condition @xmath13 , because the images near the camera boundary are distorted due to the boundary .
the gamma rays show the compact images compared with the hadronic showers , so we then selected the events which has a smaller width and length region similar to what is predicted by a monte carlo simulation .
we select events with the following conditions , @xmath14 , and @xmath15 , where @xmath16 and @xmath17 correspond to 30% and 50% of the points obtained by integrating the width distribution from off - source events ( hadron events ) .
the typical values of @xmath16 and @xmath17 are @xmath18 and @xmath19 , respectively , and they naturally contain the zenith angle dependencies and the weather conditions , and we could obtain the constant fraction of data as gamma ray candidates .
the size of 400 corresponds to the the average value of size .
the second terms ( 0.020 and 0.023 ) represent the size dependence of the images as predicted by the monte carlo simulation .
the parameter @xmath20 corresponds to the light concentration , as gamma ray showers have a higher light concentration than hadronic showers .
after these selections , 8 - 10% of events remain . finally , we select for gamma ray showers using the directional information obtained by the asymmetry of shower images .
we select for events which develop from neighbouring of the target source to outer direction .
finally , 97% of the hadronic showers are rejected and 30 - 40% of gamma rays are selected . after these cuts we obtained the excess of @xmath21 from the direction of mkn501 .
the monthly excesses in the alpha distribution are shown in figure 1 .
the absolute gain calibration is carried out by the measurement of a single photoelectron .
we found that a single photoelectron corresponds to 4 adc counts .
the relative gain of each channel of 256ch is adjusted to within 5% accuracy using the led pulsar .
we confirmed that the @xmath22 distribution observed is consistent with the simulated showers assuming the cosmic ray energy spectra in each composition .
the uncertainty of the absolute gain or the @xmath22 and the energy relation is estimated as to be 20% . with a monte carlo simulation
, we can determine the effective area for gamma rays and hadrons , @xmath23 and @xmath24 .
we also obtain experimentally the number of the excess events @xmath25 , and number of background events @xmath26 . in order to minimize the systematic errors
, we took the ratio @xmath27 .
the denominator @xmath28 has a weak dependence on size with our imaging selection of approximately @xmath29 .
the energy and the @xmath22 relations for gamma rays and hadrons are determined by our monte carlo simulation to be @xmath30 tev and @xmath31 tev , respectively .
then we can obtain the gamma ray energy spectrum from @xmath32 .
the derived energy spectrum is shown in the figure 2 .
. the differential energy spectrum can be well fit with the power law spectrum of index -2.5 up to 5 tev .
the spectrum becomes steeper above 5tev which may suggest a cutoff of the energy spectrum .
however , it is possible that statistical fluctuations may make this effect .
we need more statistics to obtain conclusive results .
the saturation of the photomultipliers , the amplifiers , and the adc is considered to occur at higher energy , above 30 tev .
we could minimize this saturation effect by the above ratio method .
the observation of the mkn501 was carried out for 47 nights from the end of march to the end of july .
the gamma ray event rate is plotted as a function of mjd in figure 3 . for reference ,
the gamma ray rate from the crab nebula measured by our detectors is shown by a horizontal line .
the event rate was highly variable day by day , and the maximum event rate was about 4 crab and the minimum rate was @xmath34 crab .
the time scale of the intensity change was about a few days .
we searched for a short time variation of the intensity but we could not find clear evidence of any short time variation in our data set .
we can see high states and low states clearly in our data set .
this feature ( the time scale and the intensity change ) in april and in july appear to be similar , showing `` u '' shapes , and the interval of the two high states are 14 days and 12 days .
may and june data each show only one high state `` @xmath35 '' shapes . in order to examine the periodicity in the data set , the data shifted by 25.5 days are superposed in the bottom pannel of figure 3 .
we can see good coincidence of the high states .
this test suggests a @xmath36 day periodicity .
the periodicity of the gamma ray intensities was examined by the rayleigh test .
we calculated the powers ( amplitude when fitting sinusoidal function ) in each test period from 5 days to 45 days . in order to evaluate the significance in each test period ,
we have generated @xmath37 data set artificially by shuffling the relation of ( @xmath38 , mjd ) , where @xmath38 is 47 flux points in figure 3 .
( these data set also has the same gaps in the observation time . )
they are analyzed in the same way as the real data set . by comparing the amplitude of the real data set with the amplitude distribution of the generated @xmath37 data sets , we can estimate the probability of the appearance of observed amplitude in each test period . with this method
we could remove the spurious effect ( the gaps in the observations making the false periodicities ) , and succeed to obtain the chance probability .
figure 4 shows the obtained chance probability as a function of the test period .
we can see prominent peak at 12.7 days with the chance probability of less than @xmath39 .
this period corresponds to a half of the period obtained in the simple test mentioned above .
we obtained the differential energy spectrum of the gamma rays from mkn501 .
it shows a possible cutoff feature above 5 - 7 tev .
this cutoff is predicted to be caused by the interaction with the infrared photons or the the limit of the electron acceleration in the jet .
the cutoff energy of the gamma rays due to the infrared photon interaction from the distance of the mkn501 , z=0.034 is estimated 7 tev to 15 tev by @xcite .    in the time variation of the gamma ray intensities
, we found a periodicity , which may be a quasi periodic oscillation . the observed periodicity corresponds to 12.7 days or 25.5 days .
the relationship of this periodicity with high energy phenomena around the massive black holes could be influenced by factors including the precession of the jet , and the rotation of the black hole(s ) .
r. protheroe ( @xcite ) suggested the interaction of the shock wave and the helical structure of the jet may cause this observed type of periodicity .
authors would like to thanks to j.como , a.larsen , r.smith , f.misak , l.sutton , d.gardener , s.davis , c.davis , j.parry and b.larsen for their contribution to our experiment in usa .
this project is supported by the grant aid ( # 07247102 ) and aid ( # 08041096 ) by monbushou ( the ministry of education and science ) and also partly supported by the science research promotion fund of japan private school promotion foundation .
we will report the observations of tev gamma ray flares from markarian 501 using telescope array prototype . 
 the observation were carried out continuously from the end of march to the end of july in 1997 . 
 the energy spectrum , and the time variation of the gamma ray intensities are shown . 
 the intensity has been changed by the order of magnitude in this period and the possible quasi periodic oscillation of 12.7days were discovered .