γ-ray observation of the crab pulsar with the sigma telescope

Adv. Space Res. Vol.11, No.8, pp. (8)79.-(8)82,1991 Printed in Great Britain. All rightsreserved.

0273—1177/91 $0.00 + .50 Copyright © 1991 COSPAR

Xiy-RAY OBSERVATION OF THE CRAB PULSAR WITH THE SIGMA TELESCOPE L. Natalucci,* L. Bouchet,* M. Denis,* A. Goldwurm,** P. Laurent,** F Lebrun,** S. Mereghetti,*t L. Salotti,** R. A. Sunyaev,*** D. Stepanov,*** S. Iounin,*** I. Tchoulkov,*** A. Kuznetsov,***A. Diachkov,*** N. Khavenson*** and B. Novikov*** *Cenz~.ed’Etude Spatiale des Rayonnements, Toulouse, France ** Service d’Astrophysique, CEA Saclay, France ***JJ(J, Moscow, U.S.S.R. ABSTRACT The region of the sky containing the Crab Nebula and its pulsar was observed by SIGMA several times during February and March, 1990. Data in the energy range 30-1300 keV were recorded in different functioning modes. One observation of — 2 hours in timing mode allowed us to study the pulsed emission from PSR 053 1+21. The pulsedsignal has been detected up to - 500 keY. We report here the results of the timing and spectral analysis ofthe data recorded in this observation. INTRODUCTION Since its discovery in 1968, the Crab pulsar has been extensively observed at energies from radio to y-rays, along with the emission from the surrounding nebula. The pulsed X/y-ray spectrum is generally harder than the total spectrum. The ratio of the pulsed intensity to the total emission increases with energy, spanning from 10 to 20 % in hard X-rays /1,2,3,4/ and reaching about 50 % at 1 MeV /5,6/. The pulse profiles at all energies present a typical double-peaked structure, with the peak positions coincident in phase. Profiles and intensities do not show long time scale variations, with only one exception in the high energy ‘i-rays (E 50 MeV) 17/. An interpeak component is detected in the range from optical to low-energies y-rays (E 1 MeV), and represents more than 20 % of the total pulsed emission in hard X-rays. The presence of additional structures in the light curve above -500 keV has recently been reported/S/. INSTRUMENTATION AND OBSERVATION The X/y.ray telescope SIGMA (35 key - 1.3 MeV) has been launched on Dec. 1st, 1989 on board the GRANAT spacecraft from the launching site of Baliconour(USSR). Itconsists of an URA type coded mask associated with an Anger camera position sensitive detector (for instrumental details see /9/), giving a totally coded field of view of4°45’x 4°20’.SIGMA is designed to make high resolution images (—2 arc mm) ofthe sky as well as timing studies of the high energy emission. The instruments on board GRANAT observed the Crab region several times during February and March, 1990/10,11/. Among these observations, one pointing in fast timing mode was carried out by SIGMA on March 14, 1990, starting at 13h45m UT (3D 2447965.073) for a total of 1h45m. In this paper we report results from this observation. When working in fasttiming mode, SIGMA records the total pulse heigth (coded in 128 channels) and the arrival time of each photon with a precision of 1 ms, while no imaging is performed. Timing is provided by an on board 1024 Hz clock, which is constantly monitored and found tobe stable to < 108. The total counting rate registered dining the observation was 430 count~fsin the 30-1300 keV energy band. Due to the lack of imaging, it is impossible to make a background subtraction via the standard process of image deconvolution. We can, however, obtain the characteristics ofthe pulsed signal from PSR 0531+21 by performing a time profile analysis.

DATA ANALYSIS AND RESULTS Pulse profile analysis As a first step, the photon arrival times were corrected for satellite orbital motion and then converted to the solar system barycenter by using the standard JPL Export Planetary Ephemeris Package DEl 18 /12/. The accuracy of these corrections is — 0.5 ms, the dominating source of error being the knowledge of the satellite position along the orbit. We then used the converted arrival times to 2for a set construct pulse phase profiles. A period search was performed by maximizing of ,~ with the of trial periods. The best period obtained is 33.375035 ±0.000012 ma, whichthe is value consistent value of 33.3750335 obtained from radio ephemerides (A.G. Lyne, Personal communication). The values of the pulsar period and its first derivative, extrapolated by the radio measurements of March 15th, 1990, have been used to obtain pulsar light curves in different energy bands. The pulse profile +

On leavefrom Isdiuto di FLsica Cosmica dcl CNR. Milano (Italy)

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integrated over the 30-1300 keY range is shown in Fig. 1. The shape of the ligth curve is characteristic of the hard X-ray domain /13/, showing a clear interpeak emission with the 2nd peak as the main component within the overall pulsed emission. The position ofour 1st peak coincides with the arrival time of the radio pulse obtained by the radio ephemerides within the uncertainties determined by event timing and satellite position.

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Fig. 1: The light curve of PSR 0531+21 in the 30-1300 keY energy range. The pulsed phase region used for the spectral analysis is also shown (channels 10-28).

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2 analysis showed that the pulsed signal is A x detected in the profiles up to —400 keY. In Fig. 2 we present the profiles obtained in 3 different energy bands within the range 30-400 keY. The ratio of the counts for the 2nd to the 1st pulse after subtraction of the non-pulsed component is (1.30±0.15) in the range 30-fl keY and (1.70 ± 0.35) in the range 72-160keV. This supports the already existing evidence that the ratio P2/Pl increases up to several hundreds ofkeY (see/8/).

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Fig. 2 : Light c~e profiles in ~e

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energy bands between 30 and 400 keY. 160-400

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X/~’-RayObservation of the Crab Pulsar

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Snectral analysis The lightcurve profiles up to 500 keY have been used to obtain an energy loss spectrum ofthe pulsed component. This is computed by subtracting the count rate measured in the non-pulsed region from that ofthe pulsed region (see Fig. 1). In order to estimate the original photon spectrum we used the energy response function obtained by ground calibrations. The calculations were performed using two different methods: (a) fitting a model spectrum to the data by convolving it with the response function ; (b) by means of an iterative procedure which makes use of an entropy maximization criterion, as described by Gull /14/. The spectral fits were performed using a simple power-law model 2 sec~’keV~.The values ofthe best-fit parameters of = A100 (E/lOOkeY).B ph cm forthe thetype totalF(E) pulsed flux are shown in Fig. 3 with the associated confidence contours. The power law obtained, 0.8 x E1~9ph cm2 sec2 keV~,is shown in Fig. 4 along with the flux values calculated by meansof the procedure (b). Itis seen that the power law gives an acceptable description of the data. The intensities are Consistent with most measurements in the hard X-rays /1,2,3,4/ and also with the extrapolation towards low energies of the> 50MeV spectrum ofCOS-B /15/. —‘—r

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Fig. 3 : Result of fitting a single power-law model for the total pulsed emission. The cross represents the best fit, and the lines are 68% and 90% confidence contours. 10

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ENERGY (keV) Fig. 4 : The energy spectrum of the total pulsed emission as measured by SIGMA. Data are in the energy range 45-500keY.

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In order to study the variation of the spectral index with pulse phase we performed the spectral fits after having selected counts in 3 phase regions corresponding to the first peak, interpeak and second peak. The results are listed in Table 1. No evidence of spectral variation with phase is found within the quoted enra~

TOTAL PULSED

FIRST PEAK

INTERPEAK

SECOND PEAK

1.2 ±0.3

1.5 ±0.1

0.9 ±0.4

1.8 ±0.4

B

1.9 ±0.5

2.1 ±0.1

0.9 ±0.4

1.8 ±0.4

red.X2

0.8

1.1

0.8

0.8

4)

A100(x10~

Tab. 1 We gratefully acknowledge the continuos support of the SIGMA Project Group from the CNES Toulouse and of the personnels of the Lavotchine Space Company, the Babakin Space Center, the Balkonour Space Center and the Evpatoria ground station. L. Natalucci is grateful to the European Space Agency for the supportof a post-doctoral fellowship. REFERENCES 1.

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