Osse Observations of the Crab Pulsar

We present preliminary results of the Compton Gamma Ray Observatory OSSE observations of the Crab pulsar. The pulsar energy spectra and light curves are in general agreement with previous observations, validating the OSSE pulsar data acquisition modes and data analysis algorithms. The data suggest that the spectrum of the pulsar varies throughout the light curve. The \interpulse" region has a slightly atter spectrum in the 60{250 keV region and a slightly steeper spectrum at higher energies than the two main pulses. We nd no evidence for any lines in the spectra with a typical sensitivity of about 10 4 photons cm 2 s . INTRODUCTION The Oriented Scintillation Spectrometer Experiment (OSSE; Johnson et al. 1989) observed the Crab pulsar during instrument activation and also during observation period 1. In this paper we report on the preliminary analysis of the data acquired during viewing period 1 (17 May 1991 to 30 May 1991). The OSSE pulsar data modes permit acquisition of time-tagged gamma-ray energy losses for the study of fast pulsars. The entire event stream for the detectors cannot be accommodated on an event-by-event basis in telemetry; consequently, the pulsar processing includes event selection and compression for telemetry formatting. The pulsar processing permits the de nition of up to eight energy bands to be included in the transmitted pulsar data. These energy bands, as well as the rest of the pulsar data collection con guration, can be de ned by OSSE mission operations activities and uploaded into the experiment via command. The pulsar data can therefore be optimized to the speci c observing strategy and energy range of particular interest while limiting the event rate to that which can be handled in the OSSE telemetry. Northwestern University, Evanston, IL Universities Space Research Association, Washington DC NRC/NRL Resident Research Associate Naval Research Laboratory, Washington DC Clemson University, Clemson, SC Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE 1991 2. REPORT TYPE 3. DATES COVERED 00-00-1991 to 00-00-1991 4. TITLE AND SUBTITLE OSSE Observations of the Crab Pulsar 5a. CONTRACT NUMBER 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 6. AUTHOR(S) 5d. PROJECT NUMBER 5e. TASK NUMBER 5f. WORK UNIT NUMBER 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) Naval Research Laboratory,4555 Overlook Avenue, SW,Washington,DC,20375 8. PERFORMING ORGANIZATION REPORT NUMBER 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR’S ACRONYM(S) 11. SPONSOR/MONITOR’S REPORT NUMBER(S) 12. DISTRIBUTION/AVAILABILITY STATEMENT Approved for public release; distribution unlimited 13. SUPPLEMENTARY NOTES 14. ABSTRACT 15. SUBJECT TERMS 16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF ABSTRACT 18. NUMBER OF PAGES 9 19a. NAME OF RESPONSIBLE PERSON a. REPORT unclassified b. ABSTRACT unclassified c. THIS PAGE unclassified Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std Z39-18 Table 1: Crab pulsar ephemeris 29:9493820573442 s 1 _ 3:77660 10 10 s 2  7:45 10 21 s 3 T0 8367.0 (TJD) RA 53431: 973 Dec 22 0052: 06 Gamma-ray events quali ed as being in one of these eight energy bands are then processed in one of two modes: (1) event-by-event mode, where selected events are time-tagged, and both energy loss and arrival time of the event are transmitted in the telemetry, or (2) rate mode, where high time resolution rate samples are taken in each of the eight energy bands. The event-by-event mode of pulsar data provides the highest time resolution for the study of fast pulsars. This mode time-tags events accurate to 0.125 milliseconds at its highest resolution, and transmits the arrival times, detector identi cations, and encoded energy losses. Depending on the OSSE telemetry format, a maximum of 290 events per second is supported in the event-by-event pulsar mode. The pulsar rate mode can accommodate a much higher event rate but at the sacri ce of spectral resolution. This mode records the number of events in each of the de ned energy bands at a speci ed sample frequency. The highest sample rate in this mode provides a resolution of 4 milliseconds. Sample times from 4 msec to 512 msec can be selected. In this mode OSSE can achieve its best sensitivity to a continuum ux. The data presented here were collected in the 1 millisecond event-by-event (or EBE) mode, and various energy ranges were selected at di erent times for di erent detectors in order to cover nearly the entire 50 keV{10 MeV range with optimal energy resolution. ANALYSIS METHOD Using a version of TEMPO (Taylor and Weisberg 1989) and a position of the Crab pulsar based on an ephemeris provided by Taylor and Nice (private communication), we corrected the arrival time of the time-tagged photons to the solar system barycenter. Next, we epoch-folded the data using the relative phases of these events calculated from the ephemeris provided by Taylor and Nice (Table 1). The resulting light curve for the 60{ 246 keV energy range integrated over the observations made from TJD 8393.6 to 8399.5 is shown in Figure 1. In order to compare the phase of our pulse with that seen at radio frequencies, further phase corrections must be applied to the light curve (Taylor, private communication). However, even after applying these corrections, there is still a net o set of approximately 6 milliseconds between the two light curves. This o set is almost certainly not real, but is probably caused by insu cient accuracy in extrapolating the radio phase to in nite frequency. Further study of this problem is required. PRELIMINARY RESULTS In Figure 2 we show the light curves in several broad energy channels and in Figure 3 we show the hardness ratio as a function of phase. From these two gures we see that in the lower energy range, the spectrum of the interpulse region (phase bins 20 to 29) is atter than the spectra of the pulse peaks (phase bins 14 to 19 and 35 to 41). This is in basic agreement with Knight (1982), Mahoney et al. (1984), and Agrinier et al. (1990). We have produced several preliminary spectra, but it is too early to report a detailed spectral comparison between the di erent regions. The average spectrum is shown in Figure 4 where we see very good agreement with previous experiments. Note, this is only an approximate photon spectrum since we used a simpli ed instrument response model which ignored Compton scattering. We cannot conclusively exclude the existence of the 440 keV line in the spectrum of the second peak reported by the FIGARO group (Agrinier et al. 1990, Massaro et al. 1991) of 0:86 0:33 10 4 photons cm 2 s . The FIGARO result is based on combining data from balloon ights made on 11 July 1986 and 9 July 1990. In our data, we nd that no features are obvious at 440 keV or elsewhere. Our 2 upper limit to a line that is intrinsically much narrower than OSSE's 8:5% detector resolution at 440 keV is 0:8 10 4 photons cm 2 s . On average, between 200 and 500 keV, our 2 sensitivity to a narrow line for this observation is 10 4 photons cm 2 s . CONCLUSIONS The OSSE pulsar mode is performing as designed. For the Crab pulsar, we nd general agreement with previous experiments in terms of average spectra and pulse shape. We do not yet have any evidence for line features in the spectra, but further analysis must be done. The di erence in spectral shape between the interpulse region and the peaks exhibited in our data is consistent with that reported by Knight (1982). This spectral di erence suggests that the interpulse emission is coming from a di erent site than the peak emission. Comparison with concurrent radio observations made by Phase I Guest Investigators from Cornell University may help elucidate the nature of the emission sites (Lundgren and Cordes 1991). REFERENCES Agrinier, B., et al., 1990, Ap. J. Letters, 355, 645. Johnson, W.N., et al., 1989, Proc. of the GRO Science Workshop, ed. W.N. Johnson, pp. 2{22. Knight, F.K., 1982, Ap. J., 260, 578. Lundgren, S., and Cordes, J.M., 1991, this proceeding. Mahoney, W.A., Ling, J.C., and Jacobson, A.S., 1984, Ap. J., 278, 784. Massaro, E., et al., 1991, Ap. J. Letters, in press. Taylor, J.H., and Weisberg, J.M., 1989, Ap. J., 345, 434.