Astroparticle Physics 32 (2009) 120–128

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Astroparticle Physics

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Search for TeV gamma-rays from Geminga pulsar

B.B. Singh *, V.R. Chitnis, D. Bose 1, M.A. Rahman, S.S. Upadhya, K.S. Gothe, B.K. Nagesh, P.N. Purohit,Shobha K. Rao, Kamesh K. Rao, S.K. Sharma, P.V. Sudersan, B.L. Venkateshmurthy,P.R. Vishwanath 2, B.S. AcharyaTata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400 005, India

a r t i c l e i n f o

Article history:Received 9 January 2009Received in revised form 7 May 2009Accepted 28 June 2009Available online 4 July 2009

Keywords:TeV c-raysGeminga pulsarAtmospheric Cherenkov TechniquePACT

0927-6505/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.astropartphys.2009.06.007

* Corresponding author. Tel./fax: +91 7578252113.E-mail address: bbsingh@tifr.res.in (B.B. Singh).

1 Now at University of Madrid.2 Now at Indian Institute of Astrophysics, Bengaluru

a b s t r a c t

We present the results of observation of the Geminga pulsar carried out in the TeV energy band duringthe 6 year period spanning 2000–2006 using the Pachmarhi Array of Cherenkov Telescopes (PACT). A longstretch of data, new computer codes and the ‘‘Tempo” package have been used in the present analysis. Wehave searched for evidence of pulsed emission of c-rays from the Geminga pulsar using the post-glitchpulsar elements obtained by Jackson and Halpern from X-ray/c-ray satellite data. We do not see any sig-nificant evidence for pulsed emission from the Geminga pulsar at a threshold energy of 825 GeV. In thispaper we present our results on the light curve in the TeV energy band, set an upper limit on the timeaveraged flux of c-rays, and compare our results with other ground based observations.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

The satellite based detectors, SAS-II, COS-B, EGRET and AGILEhave been successful in detecting pulsed emission of high energyc-rays from pulsars. So far 7 such pulsars have been detected withhigh significance [29,54]. The energy spectra of c-rays from most ofthese pulsars [53] show dominant power at energies <1 GeV. Thereseem to be some indications for a change in the characteristics ofpulsed emission at higher energies. The emission may either totallycut off or the energy spectra may steepen at higher energies. There-fore, many attempts were made to detect these pulsars at higherenergies using the ground based Atmospheric Cherenkov Tech-nique. Recently, the MAGIC collaboration was successful in detect-ing pulsed emissions from the Crab pulsar at energies >25 GeV[51,2].

It is generally expected that charged particles accelerated in thepulsar environs radiate energy giving rise to c-rays in addition toradiation at other wavelengths. The high energy electrons emitsynchrotron radiation. The energy of some of these photons mayget boosted upwards due to interactions with the charged particlesthemselves via the Inverse Compton (IC) effect. Thus the energyspectrum of emitted photons is distinctly different due to differentprocesses of their origin. Moreover, the different sites for the accel-eration of particles: Polar Cap (PC) or Outer Gap (OG) regions also

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cause noticeable change in the energy spectra of emitted c-rays aswell.

It has been suggested that emission from Polar Cap region of thepulsar results in a steep spectral cutoff and no detectable emissionis expected at energies above 50 GeV [49,46,24,15,25]. The spectralcut off in the case of emission from the Outer Gap region is moregradual [14,45]. In this scenario it is possible that lower energy In-fra-Red (IR) photons get converted to higher energy (TeV energy)photons due to their interactions with the accelerated particlesin the gap region itself (due to the IC effect). Nel and De Jager[39] have surmised that pulsed emission is not expected beyond20 GeV from isolated radio pulsars. Though there are variationsin the specific models constructed to understand the mechanismof radiation emission from pulsars, the OG models in general havecomponents of emission of c-rays at TeV energies. Hence the moti-vation for the study of pulsars in the TeV energy region.

The Geminga pulsar has been one such interesting object. It wasdiscovered by the SAS-II satellite detector as a source of c-rays in1972 [18,52]. It was seen as the second brightest object by theCOS-B satellite detector [50]. Being a radio quiet pulsar, Gemingaremained a mysterious object for almost a decade till the discoveryof �237 ms periodicity 3 by ROSAT [23] and later by the EGRET [5].

Over the years, Geminga has been identified as a weak X-raysource from Einstein X-ray satellite data [8]. It shows weak pulsa-tions at both optical and radio wavelengths [9,22,31,36,47]. TheGeminga pulse profile as obtained from SAS-II [37], COS-B [10],

3 The earlier reported period of �59 s seen in the SAS-II data was not confirmed byter observations [13,27].

la

B.B. Singh et al. / Astroparticle Physics 32 (2009) 120–128 121

EGRET [5] and AGILE [43] data shows a double peaked profile, sim-ilar to those seen from the Crab and Vela pulsars. The phase re-solved spectra in the energy range 30 MeV–2 GeV show that thehardest emission corresponds to the second c-ray peak [19].

In the past, a few groups have observed the Geminga pulsarusing the Atmospheric Cherenkov Technique. Early observationsindicated it as a source of Very High Energy (VHE) pulsed c-rays[12,56]. But the later observations by the more sensitive imagingtelescopes (for e.g. Whipple, HEGRA, etc.) did not show any evi-dence for pulsed emission from this source [4,3]. Recently, Neshporet al. have reported detection [40,41] of periodicity (with a periodof �237 ms as well as �59 s) in the c-ray emission in their obser-vations during 1996–1997 at the Crimean Astrophysical Observa-tory. EGRET detected only pulsed emission from Geminga but apulsar wind nebula (PWN) has been observed in the X-ray/lowerenergies by Chandra and XMM-Newton detectors [42]. Recently,evidence for steady emission of c-rays have been reported by theSHALON and MILAGRO groups. The integral c-ray flux detectedby SHALON group is ð0:48 � 0:17Þ � 10�12 photon=cm2=s at ener-gies >0.8 TeV [48]. The energy spectrum of Geminga supernovaremnant is found to be harder than the Crab nebula spectrum.The MILAGRO group has also detected a 4.9r steady signal fromGeminga location during their all sky survey [1].

Jackson et al. reported [27] a glitch in the frequency of rotationof the Geminga pulsar from their study of a phase-connectedephemeris spanning the years 1973–2000. The glitch correspondsto the observation period of late 1996. It was confirmed from longterm observations of the hard X-ray pulse profile with ASCA satel-lite data. Jackson and Halpern have derived a phase-connected,post-glitch ephemeris [28] spanning the period from 22 April1991 to 13 March 2004 using data from X/c-ray satellite detectorslike EGRET, ASCA and XMM-Newton.

We have observed the Geminga pulsar in the TeV energy bandduring the 6 year period spanning 2000–2006 using the PachmarhiArray of Cherenkov Telescopes (PACT). The earlier possible evi-dence [57] for pulsed emission at energy >1.6 TeV presented usingPACT data were analysed with a pre-glitch ephemeris as the pres-ence of a glitch was not known to us then. We have added moredata since then. In the present analysis, a long stretch of data,new computer codes, the ‘‘Tempo” package and the post-glitchpulsar ephemeris obtained by Jackson and Halpern have been usedfor searching for evidence of pulsed emission of c-rays from theGeminga pulsar.

In this paper we describe our setup, data and analysis tech-niques. We did not see any significant evidence for pulsed emissionof c-rays from Geminga pulsar at a threshold energy of 825 GeV.We discuss our results on the light curve in the TeV energy bandand compare our upper limit on the flux of time averaged pulsedemission of c-rays with the results of other ground basedobservations.

2. PACT setup

The Pachmarhi Array of Cherenkov Telescopes (PACT) is aground based atmospheric Cherenkov system located at the HighEnergy Gamma Ray Observatory, Pachmarhi in central India(22.47N, 78.43E, 1075 m asml) [6]. It has been used to study VHEc-rays from celestial sources based on the wavefront samplingtechnique. It consists of an array of 24 Cherenkov telescopes de-ployed over an area of 100 m� 80 m in the form of a rectangularmatrix. Fig. 1 shows the layout of the PACT array. These telescopesare separated by 20 m in the E–W direction and 25 m in the N–Sdirection from their neighbouring telescopes.

Each telescope consists of seven para-axially mounted parabolicreflectors of diameter 0.9 m and f=d � 1. These reflectors were fab-

ricated indigenously by using commercially available 6 mm thickfloat glass and forming them into a parabolic shape by slumping.The back surface of the mirrors were coated with Al and a layerof paint was applied for protection from the elements. The reflec-tivity of these mirrors in the visible range was about 80% whichwas degraded to � 50% over several years of operation. The sizeof the image of a point source is 6 0�:2. Seven reflectors aremounted paraxially on a single equatorial mount. The total reflec-tor area per telescope is 4:45 m2. A fast Photo Multiplier Tube(PMT) of type EMI9807B is mounted at the focus of each reflector,behind a mask of diameter 3� to limit the field of view. A motorcontrolled shutter is used to expose the PMTs only during observa-tion. A guiding telescope (called the elbow-telescope), is co-alignedwith the seven mirrors at the northern apex of each telescope. Alltelescopes are independently steerable in both E–W and N–S direc-tions up to �45� from zenith. For the safety of telescope structures,4 limit switches were provided in each telescope at the extremepoints to prevent its movement beyond �45� in N–S and E–Wdirections. The alignment of mirrors within a telescope and orien-tation/tracking of telescopes were calibrated periodically usingbright stars.

A remotely controlled Automated Computerized TelescopesOrientation System (ACTOS) [21] is used for the slow, fast move-ment and tracking of all telescopes. The hardware of ACTOS is de-signed in-house and consists of a semi-intelligent closed loopfeedback system with built-in safety features. The angular positionsensors, which are basically gravity based transducers (LUCASsensing system, USA) called clinometers, are used as absolute angleencoders. The DC output of these clinometers are linearly propor-tional to their offset angles from vertical (60 mV/degree). In eachtelescope, two clinometers are used, one to get the telescope anglein the N–S direction and the other for the E–W direction. The cli-nometer outputs are fed to a low pass filter and an integration typeADC which is readout by the host PC. These clinometers are cali-brated by aligning telescopes to bright stars at various anglesand measuring their output voltages. This system can orient thetelescopes to a known source direction in the sky from an arbitraryinitial position with an accuracy of 0�.05. The position angles of alltelescopes are monitored sequentially through out the observation.The time for a typical monitoring cycle is about 5 min. Correctionsare applied in real time if any telescope is found to deviate fromthe desired position by more than 0�.05.

The PACT array is sub-divided into four sectors to reduce thelength of signal cables thereby minimizing the distortion andattenuation of pulses from photo-tubes. Six telescopes are groupedto form a sector for data acquisition purposes. Each sector can beoperated as an independent unit and has its own Data Acquisitionsystem. A field signal processing center (FSPC) that collects, pro-cesses and records information from the nearby six telescopes ofthat sector is located at the centre of each sector. Pulses/signalfrom photo-tubes are brought to the respective FSPC through lowattenuation RG213 cables of length � 40 m. Pulses from individualPMT’s are processed and the information regarding pulse heightand arrival time of the shower front at each telescope are recordedin the FSPC. A central control room located at the centre of the ar-ray houses the master signal processing centre (MSPC). A real timeclock (based on a 5 MHz crystal Oscillator), synchronized to a 1 Hzpulse from a GPS clock, is used for recording absolute time with aresolution of 1 ls. This 1 Hz GPS pulse is also used to reset the msand ls counters.

Pulses from the individual PMTs of a telescope are linearlyadded to form a Telescope pulse. The trigger for data acquisitionis obtained from a coincidence (75 ns) of 4 out of 6 Telescopepulses of a sector. For each trigger information regarding the pulseheight and arrival time of pulses from all PMTs of a sector are re-corded in the FSPC using a CAMAC based system. Similarly, infor-

Fig. 1. A schematic layout of the PACT. Each telescope consists of seven mirrors and each sector consist of six telescopes and their data acquisition centers.

4 The Cherenkov photon bunch size is chosen to be equal to one.

122 B.B. Singh et al. / Astroparticle Physics 32 (2009) 120–128

mation relevant to entire array such as arrival time of the showerfront at individual telescopes, absolute arrival time of the event arerecorded at the MSPC along with the trigger information from allsectors. A trigger from any one sector initiates the data recordingat the MSPC. The typical trigger rate was about 2–3 Hz at the sec-tors while it was about 5–10 Hz at MSPC. The distributed dataacquisition system and telescope control and monitoring systemsof the FSPC and the MSPC are all networked through a local areanetwork (LAN) [7,55].

3. Simulation of PACT detector response

The performance of this array has been studied by Monte Carlosimulations which involve the generation of both c-ray and cos-mic-ray showers in the atmosphere and the subsequent responseof the detectors to the Cherenkov light in the showers. Specialattention has been paid to wavelength dependent detection prob-ability, as well as the Fourier frequency dependent attenuation ofthe PMT signal in the cable. Detailed simulations of the array re-sponse for c-ray and cosmic ray induced showers were made andcompared with the measurements [44].

We have divided the complete Monte Carlo simulation proce-dure into two steps in our calculations. The first step was used togenerate an extended set of simulated air showers, induced by c-rays and cosmic-ray primaries. In the second step the response ofthe array was applied to all generated events. The most time con-suming step is evidently the first one, whereas the second step isrelatively fast. This division allowed us to apply the detector sim-ulation procedure several times to the generated showers in orderto tune the Monte Carlo simulations to the observed features of theexperimental set up, like the pulse height spectrum of PMT pulses,

trigger rate for various trigger logics, number of triggered PMTs pertelescope, trigger rate as a function of zenith angle etc. The varia-tion of trigger rate with zenith angle, distributions of QDC andTDC values, space angle distribution of events etc. as obtained fromsimulations were found to be consistent with data.

Interaction of Proton, Helium and c-ray primaries incident onthe top of the atmosphere within a circular aperture of 3� in thedirection of the telescope axis and those with the impact parame-ter of their axis on the ground (i.e.shower core position) within acircular area of radius 300 m from the center of the array have beensimulated. CORSIKA (version 6.019), has been used to simulateCherenkov light emission in the earth’s atmosphere by the second-aries of the extensive air showers [30,26]. Hadronic interactionsabove 80 GeV are simulated with VENUS while GHEISHA is usedfor interactions of below 80 GeV. EGS4 code is used for simulationof the electromagnetic component of air showers. Multiple scatter-ing length for electrons and positrons is decided by the parameterSTEPFC in the EGS code which has been set to 0.1 in the presentstudy [20]. The development of an air shower in the atmospheredepends on the density profile while the attenuation of Cherenkovphotons depends on the local refractive index. We have used theUS standard atmosphere parameterized by Linsley [34]. The Cher-enkov radiation produced within the specified band width (300–650 nm) by the charged secondaries is propagated to the ground.The observation level is set to the altitude of Pachmarhi and thegeomagnetic field appropriate for this location is used. We havetracked single photons for each primary at all energies.4 The result-ing Cherenkov pool is sampled by all 24 telescopes which are usedto study the core distance dependence of the parameters.

Fig. 2. The differential trigger rate as a function of c-ray primary energy for the assumed energy spectrum of the form JcðEÞ ¼ 3:2� 10�7ðETeVÞ�2:49.

5 During the year 2001–2002, on some occasions, observations were carried ouwith 2 sectors pointing to the source direction and the other 2 to the backgroundregion.

B.B. Singh et al. / Astroparticle Physics 32 (2009) 120–128 123

The detector simulation procedure accounts for all steps in-volved in the process of conversion of Cherenkov light into electri-cal signals, which starts with the incidence of the Cherenkovphoton on the mirror and ends with an analog PMT voltage pulseat the front end electronics. This program takes into account vari-ous site and instrument related details as well. We have adoptedElterman’s attenuation model [16], which provides the attenuationcoefficients for the Rayleigh and aerosol scattering as well as ozoneabsorption in an altitude dependent form for the wavelength range270–1260 nm, for calculating the transmission of Cherenkov pho-tons in the atmosphere. The important aspects like, Night SkyBackground (NSB) light, mirror reflectivity, the cathode mask (solidangle), Quantum efficiency and gain of the PMT, attenuation of thePMT pulse in the cable etc. were all taken into account. The NSBlight at the PACT site has been measured directly using the sameEMI 9807 PMT on few nights and the flux of NSB was found be� 3� 108 photons cm�2 str�1 s�1 in the wavelength range 300–650 nm. Analog signals from the 7 PMT’s of a telescope were addedin phase to generate the telescope signal as done in the experi-ment. The hardware trigger condition of the presence of at least4 telescope signals in a sector of 6 telescopes has been imposedto generate the event trigger.

Using the above procedure the trigger efficiencies, effective col-lection area and the differential trigger rate as a function of pri-mary energy have been estimated in the energy range 100 GeVto 10 TeV for c-rays and 250 GeV to 20 TeV for protons and alphaprimaries. We have assumed a hypothetical source of c-rays inthe vertical direction having the energy spectrum of the Crabnebula as measured by the Whipple group, JcðEÞ ¼ 3:2�10�7ðETeVÞ�2:49 m�2 s�1 TeV�1 for estimating the trigger rate dueto c-ray showers. The trigger rate due to cosmic-rays is estimatedusing the following energy spectrum for proton and helium nuclei:JpðEÞ ¼ 8:67� 10�2 ðETeVÞ�2:7 m�2 s�1 Sr�1 TeV�1 and JaðEÞ ¼ 5:95�10�2 ðETeVÞ�2:7 m�2 s�1 Sr�1 TeV�1 cosmic-ray showers are simu-lated with arrival angles within a 3� aperture around the telescopeaxis in accordance with their isotropic nature.

The estimated differential trigger rate for c-rays is plotted as afunction of primary energy in Fig. 2. The energy corresponding tothe peak of this distribution is taken as the threshold energy. The

estimated threshold energy of PACT [11] for c-rays incident verti-cally is around 750 GeV while that for cosmic-ray protons isaround 1.65 TeV. The corresponding effective collection area is1:38� 105 m2 for c-rays. The collection area and the threshold en-ergy increases with zenith angle. The calculated trigger rate of�7 Hz (5.4 Hz due to protons and 1.5 Hz due helium nuclei) dueto cosmic-ray primaries agrees well with the observed averagetrigger rate with all telescopes stationary and pointing to zenith.The response of the array has been calculated for various off-linetrigger conditions as well. These values were verified by an inde-pendent but slightly approximate Monte Carlo simulation usingthe average properties of shower parameters. A large number ofshowers were generated by fluctuating the average lateral densitydistribution of Cherenkov photons to increase the statistical signif-icance of the predictions.

4. Observations and data collection

The observations were carried out by pointing all telescopes inthe same direction.5 Cosmic-ray data were collected with all tele-scopes stationary and pointing to zenith as well as other fixed zenithangle directions for calibration and comparison with Monte Carlosimulations.

Observations of the Geminga pulsar spanning 6 years betweenDecember 2000 and February 2006 were carried out with a mini-mum of 2 sectors in operation. The total time of observations onON-source and OFF-source regions were � 94 h and � 58 h, respec-tively. The J2000 co-ordinates of the Geminga pulsar and otherparameters shown in Table 1 were used.

Data were collected at a stretch, lasting for 1–3 h, first eitherON-source and followed by OFF-source region with equal exposureor vice versa during the same night. The OFF-source region is cho-sen to have the same declination as that of the source with an offset(about 5�–20�) in Right Ascension such that same zenith angle

t

Table 1Ephemeris of Geminga pulsar.

PSR 0633+1746

RA 06h33m54:153s

DEC 17d46m12:910s

EPOCH 2000.0PMRA ðlaÞ 138.0 ± 4 mas/yearPMDEC ðldÞ 97.0 ± 4 mas/yearPX 6.3694 mas/yearDistance 157 pc

Table 2Observation log of Geminga source.

Year ON-source # Runs OFF-source # RunsDuration (h) Duration (h)

2000 13.2 10 6.3 42001 21.2 13 1.8 12002 10.8 5 7.0 42003 10.0 4 7.9 72004 19.8 10 15.3 102005 9.2 4 8.6 62006 9.6 6 11.0 6Total 93.8 52 57.9 38

124 B.B. Singh et al. / Astroparticle Physics 32 (2009) 120–128

range is covered for both ON-source and OFF-source runs. Only oneset of observations (ON-source and OFF-source) was carried out oneach clear moonless night most of the time. On some nights datawere collected only from the ON-source region. During 2001–2002 observations of the source and background were carriedout simultaneously with half of the telescopes pointing to thesource and the other half to a background region. The observationlog of Geminga is given in Table 2. The night sky condition, rates ofindividual PMT’s, the orientation of telescopes etc. were monitoredthroughout the observations.

5. Data reduction and analysis

Preliminary cuts and boundary limits on the data were applied,such that good ON-source and OFF-source data were available overthe same zenith angle region. We did not use the data collectedwith half the array on source and the other half on backgroundas the geometry of the array was different for source and back-ground data in this configuration.

5.1. Arrival direction of shower

The relative time of arrival of Cherenkov photons is fitted to aplane shower front and the arrival direction of the shower is esti-mated for each event [35]. The space angle ðwÞ between the direc-tion of arrival of the shower and the source direction, is obtainedfor each event. Fig. 3 shows the typical space angle distributionsfor two hours of data from the direction of the Geminga pulsar.

Cuts are applied on the number of telescopes (NDF) with valid‘timing’ data as well as on the goodness-of-fit parameter ðv2Þ ofthe timing fit. A minimum of 8 telescopes are requiredðNDF P 8Þ in each event and those events greater than 1:0r fromthe expectation for the goodness-of-fit are rejected.6 For furtherperiodicity analysis, only those events for which the space angleis 6 2�:5 were used.

6 Events with v2 P ðhv2i þ 1:0rÞ are rejected.

5.2. Steady emission

The Space angle distribution of ON-source events is comparedwith the corresponding OFF-source events. The shape of the tailof the space angle distribution beyond 2�:5 were used for normal-ising the OFF-source distribution with the ON-source one as ex-plained in Bose et al. [11]. The normalised backgrounddistribution is subtracted from the ON-source distribution and theexcess/deficit of events in the space angle region < 2�:5 is usedas c-ray signal. The procedure is carried out for the 24 pairs ofsource and background observations which correspond to 35.71 heach of ON-source and OFF-source data. The excess/deficit of indi-vidual ON–OFF pairs is shown in Fig. 4 as a function of MJD corre-sponding to the middle of the ON-source observation time. Theoverall excess is 730� 617:6 counts and is consistent with no sig-nal (at the level of 1:18r significance). The excess of events corre-sponds to 0:34� 0:29 events per minute and is consistent withbackground fluctuations.

Further reduction in the background events to increase the sen-sitivity to a c-ray signal is possible in the case of a pulsed signalsearch as the c-ray signal is confined to a few phase bins. We couldenlarge the data set for this search and have used 56.4 h of data.

5.3. Calculation of pulsar phase

The TEMPO code (developed by the Princeton group) were usedfor the periodic analysis. In this procedure, the observed arrivaltimes ðtobsÞ of recorded events from the isolated pulsar were con-verted to their barycentric position ðtssbÞ. The corrected barycentrictimes ðtssbÞ were folded at the pulsar rotational frequency relativeto epoch T0 to get the phase / as

/ðTÞ ¼ /ðT0Þ þ f0ðT � T0Þ þ12

_f ðT � T0Þ2 þ16

€f ðT � T0Þ3 ð1Þ

where /ðT0Þ is the phase at epoch T0 and is taken as a referencephase.

The absolute phase of each event was obtained using TEMPOcodes in ‘‘prediction mode” corresponding to the PACT site usingcontemporaneous pulsar parameters [38] as given in Table 1. Inprediction or ‘tz’ mode, reference phases are calculated over a per-iod of 3 h centered on the transit time of the Geminga pulsar at thePachmarhi observatory in steps of 20 min interval for each nightsdata. By using TEMPO polynomial coefficients and reference phaseðUoÞ, the absolute phase of an event at arrival time T is obtained byinterpolation.

A monthly timing ephemeris of the Geminga pulsar has beenderived using a refined phase-connected, post-glitch ephemeris[28]. The reference data correspond to MJD 50497.718748124877and has been extrapolated to the period of the PACT observations(2000–2006). The period T0 ¼ 0:23709944169 s and it’s first deriv-ative _T ¼ 1:0970874194e� 14 ss�1 are used to compute the Ge-minga pulsar period and its derivative for the observation spanof PACT. Using these results, the derived ephemeris for theMJD51544.0 (January 1, 2000) are T0 ¼ 0:23710043345 s;_T ¼ 1:0970965974e� 14 ss�1 and for MJD 53736.0 (January 1,2006), T0 ¼ 0:23710251125 s; _T ¼ 1:0971158260e� 14 ss�1. Simi-larly monthly ephemerides for the Geminga pulsar were calculatedfor each observation period and used in the phase analysis.

The distribution of pulsar phases (Phasogram) is formed foreach observation Period with 20 phase bins and then added epi-sodically. The phasograms of the co-added data of the Gemingapulsar events are shown in Fig. 5 (panel (a) for space angleðwÞ 6 2:5� and panel (b) for w 6 1:0�). Any source pulsations willshow up as peaks in the phasogram. The details of phasogramare also given in Table 3.

Fig. 3. A typical space angle distribution of events from the Geminga direction. The data corresponds to 2 h of observation.

Fig. 4. The rate of Excess/Deficit events per minute in 24 ON–OFF pairs as a function of MJD corresponding to the middle of the ON-source observation time.

B.B. Singh et al. / Astroparticle Physics 32 (2009) 120–128 125

5.4. Light curve of Geminga

Pulsed emission of radiation in the VHE band is expected atphases corresponding to the radio main pulse (P1) and the interpulse (P2). The phasogram is divided into 4-regions, followingthe EGRET group [19], as shown in Table 4. The First pulse (P1) cor-responds to phase interval 0.565–0.765, Inter-region orBridge:0.765–0.065, second pulse (P2):0.065–0.260 and back-ground:0.260–0.565. The number of events with phases withinthe P1 and P2 intervals constitute the signal events ðNONÞ. Thebackground events ðNOFFÞ are obtained by adding the number ofevents in the background region. The background events are nor-malized by multiplying the ratio of phase ranges spanned by thepulse and non-pulse regions [32] to get NOFF events. The statisticalsignificance ðrÞ of the excess count is calculated as [33]

r ¼ ðNON � c � NOFFÞffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðNON þ c2 � NOFFÞ

p ð2Þ

where c is the normalization constant. The number of events in themain pulse (P1), inter-region, inter pulse (P2) and background re-gions are obtained for the respective phase intervals and are listedin Table 3. The second column is for w 6 2:5� and the subsequentcolumns in this table correspond to events of space angle6 2:0�;6 1:5�;6 1:0�, respectively. The NON and NOFF counts werecalculated after normalization by the ratio of ON to OFF phase bins.The rate of pulsed c-rays per minute is estimated from the excesscounts in the P1 and P2 interval regions over the background. Thesignificance of excess counts ðrÞ is plotted against the space angleðwÞ in Fig. 6. It is seen that the significance is marginal but seemsto improve as near-axis events are selected.

A v2 test for the null hypothesis (uniform phasogram) is con-ducted on the binned data. The v2 of the phasogram for each indi-vidual run is calculated as

v2 ¼Xn

i¼1

ðxi � �xÞ2

r2 ð3Þ

Fig. 5. Phase histogram for Geminga pulsar events (a) w 6 2:5� and (b) w 6 1:0� .

Table 3Details of phase analysis of Geminga.

Parameters w 6 2:5� w 6 2:0� w 6 1:5� w 6 1:0�

Total events 264,057 219,222 155,636 82,738Duration (min.) 3384.9 3384.9 3384.9 3384.9Main pulse (P1) 52,775 43,945 31,406 16,820Inter-region 79,013 65,710 46,661 25,049Inter pulse (P2) 53,085 43,861 31,097 16,440Background 79,184 65,706 46,472 24,429NON 105,860 87,806 62,503 33,260NOFF 105578.7 87608.0 61962.7 32572.0P2/P1 1.01 1.00 0.99 0.98Rate/min. ± (rms) 0.083 ± 0.147 0.059 ± 0.134 0.160 ± 0.113 0.203 ± 0.082Significance ðrÞ 0.57 0.44 1.42 2.49

Table 4Geminga pulsar phase intervals.

Region Phase interval

First pulse P1 0.565–0.765Inter-region (Bridge) 0.765–0.065Second pulse P2 0.065–0.260Background (Off pulse) 0.260–0.565

126 B.B. Singh et al. / Astroparticle Physics 32 (2009) 120–128

where n is the number of bins, xi is the number of counts in the ithbin and r is the standard deviation of the events in the 20 phasebins. The v2 corresponding to the first pulse (P1) and second pulse(P2) regions are also checked and compared with that of the back-ground region. The average reduced v2 corresponding to P1, P2 andbackground regions are 1.09, 0.88 and 0.83 for w 6 1:0�. The resultsof the v2 test on these phase bins (P1&P2) do not indicate the pres-ence of any structures in the light curve, constructed using theproperly extrapolated rotational period of Geminga pulsar for thephase determination.

We have analysed the OFF-source data in the same way as weanalysed the ON-source Geminga data. The corresponding signifi-cance of the excess events are shown in the Fig. 6.

6. Results and discussion

We have analysed the data from the Geminga pulsar collectedusing PACT and a similar amount of Background data. We havesearched for steady and pulsed emission of c-rays. We did notsee any significant evidence for the presence of a steady flux ofc-rays in our data. For the pulsed emission, we have searched forthe signal assuming the well established rotational period of� 237 ms. No significant evidence for pulsed emission has beenseen in our data. The significance (pre-trail) of excess events overthe background is marginal and is 2:49r for w 6 1:0�. We have de-rived 3r upper limits on the time averaged integral flux of pulsedevents for which w 6 1:0�. The upper limits have been calculatedfor the phase regions (P1 and P2) defined according to the EGRETpulse profile. The energy threshold corresponding to the averagezenith angle of the Geminga pulsar during our observations is825 GeV and the collection area is 1:45� 105 m2. The time aver-aged gamma-ray flux upper limit 3ðrÞ obtained for the Gemingapulsar is 28:3� 10�13 photon=cm2=s which is consistent with otherrecent observations. This upper limit is compared with groundbased observations of the Geminga pulsar by various other groupsand shown in Fig. 7 as a function of energy.

The positive flux reported by the Durham group [12] and theOoty group [56] pertain to observations carried out long ago, at

Fig. 6. Significance ðrÞ as a function of the space angle.

Fig. 7. Prior results on the pulsed flux of c-rays from Geminga pulsar and the results presented in this work.

Fig. 8. A comparison of significances (for w 6 2:5�) obtained with two post-glitch ephemeris.

B.B. Singh et al. / Astroparticle Physics 32 (2009) 120–128 127

128 B.B. Singh et al. / Astroparticle Physics 32 (2009) 120–128

much earlier epochs. The Ooty observations corresponds to just 8nights and only from the direction of source. The significance ofthat result is marginal and is 2:8r and 1:3r, respectively. Whereas, the present observations were carried out with much im-proved hardware and about 5 times more light collector area.Also, in the present case, data were acquired from the source aswell as background regions (for control) spanning 6 year period.The detection of a pulsed signal at energy >1 TeV by the CrimeanAstrophysical Observatory group at 4:4r confidence level corre-spond to observations carried out during the years 1996–1997.They had searched for the periodicity by the epoch-folding tech-nique in a narrow interval near the period inferred in other energybands. We notice that their observations are closer to the glitchperiod. Our data corresponds to the period well after the glitch.The gap between the Crimean and PACT observation periods andthe glitch in the pulsar make any direct comparisons betweenthe two observations difficult. Moreover Jackson and Halpernhave suggested [28] that there seem to be another glitch duringyear 2003 and 2004 [28]. Also, the possibility of source variabilitywith time cannot be ruled out.

We have not analysed our data for short time flares, lasting fora few minutes to hours. We have looked for a significant emis-sions on run by run basis but did not do so within a run. The lightcurve of none of the individual runs stands out and we do nothave evidence for episodic emission of c-rays in the time scaleof 1–3 h.

The improvement in the excess counts at P1 and P2 of aphasogram with a refined ephemeris [28] compared to an ear-lier ephemeris [27] is as expected. A comparison of significancesobtained with these two ephemerides show good agreement be-tween the two results for PACT data up to year 2003 as shownin Fig. 8. After this, the difference increases due to the possibil-ity of another glitch during years 2003 and 2004 as suggestedby Jackson and Halpern [28]. This make the ephemeris dataprovisional for use after 2003. About half of our data were col-lected during 2004 to 2006. Improved and further refinedephemeris data are required for analysing these data for pulsedemissions.

The recent measurement of the Geminga light curve by AGILE[43] indicate sharper P1 and P2 peaks at higher energies. The widthof the peaks is of the order of 0.1 in Phase compared to 0.2 as men-tioned in Table 4. If we redefine the phase bins for P1, P2 and back-ground as (0.55–0.65), (1.00–1.10) and (1.20–1.50) followingAGILE, and assuming an offset of 0.05 in absolute phase (i.e. onebin), the significance (pre-trial) for the signal goes up to 4:4r (forevents within 1� of space angle) but this would also increase thenumber of trials. The recently launched Fermi-GLAST [17] satelliteis expected to provide improved light curve and aid the analysis ofground based data.

7. Conclusions

We have observed the Geminga pulsar using PACT during theyears 2000–2006. and have searched for steady and pulsed emis-sion of c-rays from this source. We did not see any significant evi-dence for the presence of a steady flux of c-rays in our data at anenergy threshold of 825 GeV. For the pulsed emission, we havesearched for evidence assuming the well established rotationalperiod of � 237 ms. No significant evidence for pulsed emissionhas been found in our data. The time averaged gamma ray fluxupper limit 3ðrÞ was calculated to be 28:3� 10�13 photon=cm2=sand is consistent with other recent observations.

Acknowledgements

We thank Mahesh Pose, A.I. D’souza, A.J. Stanislaus, PratikMajumdar and P.N. Bhat for help during observations, calibrationsand maintenance of PACT system. Also, we thank J.H. Taylor, R.N.Manchester, D.J. Nice, J.M. Weisberg, A. Irwin, N. Wex, A.G. Lyneand Yashwant Gupta for providing TEMPO code and clarificationabout pulsar data analysis.

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