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1. Introduction 2. Radio Observations THE ASTROPHYSICAL JOURNAL, 516:297È306, 1999 May 1 ( 1999. The American Astronomical Society. All rights reserved. Printed in U.S.A. GAMMA RADIATION FROM PSR B1055[52 D. J. THOMPSON,1,2 M. BAILES,3 D. L. BERTSCH,1 J. CORDES,4 N. DÏAMICO,5 J. A. ESPOSITO,1,6 J. FINLEY,7 R. C. HARTMAN,1 W. HERMSEN,8 G. KANBACH,9 V. M. KASPI,10 D. A. KNIFFEN,11 L. KUIPER,8 Y. C. LIN,12 A. LYNE,13 R. MANCHESTER,14 S. M. MATZ,15 H. A. MAYER-HASSELWANDER,9 P. F. MICHELSON,12 P. L. NOLAN,12 H. O GELMAN,16 M. POHL,17 P.VRAMANAMURTHY,1,18 P. SREEKUMAR,2 O. REIMER,9 J. H. TAYLOR,19 AND M. ULMER15 Received 1998 June 12; accepted 1998 December 7 ABSTRACT The telescopes on the Compton Gamma Ray Observatory (CGRO) have observed PSR B1055[52 a number of times between 1991 and 1998. From these data a more detailed picture of the gamma radi- ation from this source has been developed, showing several characteristics that distinguish this pulsar: the light curve is complex; there is no detectable unpulsed emission; the energy spectrum is Ñat, with no evidence of a sharp high-energy cuto† up to greater than 4 GeV. Comparisons of the gamma-ray data with observations at longer wavelengths show that no two of the known gamma-ray pulsars have quite the same characteristics; this diversity makes interpretation in terms of theoretical models difficult. Subject headings: gamma rays: observations È pulsars: individual (PSR B1055[52) 1. INTRODUCTION source exposure time compared to the discovery data (Fierro et al. 1993). The gamma-ray observations of this and The Compton Gamma Ray Observatory (CGRO) tele- other pulsars are shown in a multiwavelength context. scopes have detected pulsed gamma radiation from at least Comparison of the multiwavelength properties of pulsars is seven spin-powered pulsars: Crab, Vela, Geminga, PSR important in attempting to construct models for these [ [ ] B1509 58, PSR B1706 44 PSR B1951 32, and PSR objects. [ B1055 52, with some evidence for an eighth, PSR Using ROSAT ,O gelman & Finley (1993) found pulsed ] B0656 14. For a summary of CGRO pulsar results, see X-rays from PSR B1055[52, with a pulse that changed Ulmer (1994) and Thompson et al. (1997). Upper limits have both shape and phase at photon energy about 0.5 keV. The been calculated for selected samples of radio pulsars X-ray energy spectrum requires at least two components, (Thompson et al. 1994; Fierro et al. 1995;Carramin8 ana et one thought to be emission from the hot neutron star al. 1995; Schroeder et al. 1995) and for all cataloged pulsars surface and the other likely to be from the pulsar magneto- (Nel et al. 1996). sphere (see also Greiveldinger et al. 1996, and Wang et al. The present work is a detailed analysis of the gamma-ray 1998). [ observations of PSR B1055 52, based on repeated obser- Mignani, Caraveo, & Bignami (1997) have found evi- vations during 1991È1998 that have nearly tripled the dence based on positional coincidence for an optical counterpart of PSR B1055[52 using the Hubble Space T elescope (HST ) Faint Object Camera. In the absence of a 1 Code 661, Laboratory for High Energy Astrophysics, NASA Goddard Space Flight Center, Greenbelt, MD 20771. fast photometer on HST and the presence of a nearby 2 djt=egret.gsfc.nasa.gov. bright star, Ðnding optical pulsations will be difficult, as 3 Astrophysics and Supercomputing, Mail No. 31, Swinburne Univ. of noted by the authors. Technology, PO Box 218, Hawthorn Victoria 3122, Australia. 4 Physics Department, Cornell University, Ithaca, NY. 5 Osservatorio Astronomico di Bologna, via Zamboni 33, I-40126 RADIO OBSERVATIONS Bologna. 2. 6 USRA Research Associate. 7 Physics Dept., Purdue University, Lafayette, IN. The basic pulsar parameters (Taylor, Manchester, & 8 SRON/Utrecht, Sorbonnelaan 2, 3584 CA Utrecht, the Netherlands. Lyne 1993), derived from radio observations, are shown in 9 Max-Planck-Institutfu r Extraterrestrische Physik, Giessenbachstr D- Table 1. 85748, Garching, FRG. PSR B1055[52 was on the list of nearly 300 pulsars 10 Physics Department, MIT, Cambridge, MA. 11 Department of Physics, Hampden-Sydney College, Hampden- monitored regularly by radio astronomers to assist Sydney, VA 23943. gamma-ray telescopes on the CGRO (Kaspi 1994; 12 W. W. Hansen Experimental Physics Laboratory and Department of Arzoumanian et al. 1994; Johnston et al. 1995; DÏAmico et Physics, Stanford University, Stanford CA 94305. al. 1996). High-energy gamma-ray data are sparse; weak, 13 Nuffield Radio Astronomy Laboratories, Jodrell Bank, University of short-period gamma-ray pulsars are detectable only if the Manchester, MacclesÐeld, Cheshire SK11 9DL, UK. 14 ATNF, CSIRO, Australia. timing parameters are determined independently of the 15 Physics Dept., Northwestern University, Evanston, IL. gamma-ray data. In the case of PSR B1055[52, this 16 Physics Dept., University of Wisconsin, Madison, WI. monitoring, carried out at Parkes, has continued. The 17 Danish Space Research Institute, 2100 Copenhagen O, Denmark. pulsar exhibits considerable timing noise. For this reason, 18 National Academy of Sciences-National Research Council Senior Research Associate. Present address: S. T. E. Laboratory, Nagoya Uni- the timing solutions used for the gamma-ray analysis were versity, Nagoya, Japan. developed piecewise over time intervals for which the pulse 19 Physics Department, Princeton University, Princeton, NJ. phase could be adequately modeled using only a simple 297 298 THOMPSON ET AL. Vol. 516 TABLE 1 hood method (Mattox et al. 1996). The timing and spatial BASIC DATA approaches can also be combined to produce phase- resolved maps and energy spectra. Parameter Value COMPTEL, the Imaging Compton Telescope, is another Names ........................... PSR B1055[52, PSR J1057[5226 of the CGRO telescopes, operating in the energy range Period P ........................ 0.1971 s 0.75È30 MeV(Scho nfelder et al. 1993). Like EGRET, Period Derivative P0 ........... 5.8 ] 10~15 ss~1 COMPTEL uses both spatial and timing analysis, and Timing Age q ................... 537,000 yr because the COMPTEL Ðeld of view is larger than Spin-down luminosity E0 ...... 3.0] 1034 ergs s~1 EGRETÏs and the two telescopes are co-aligned on the Magnetic Field B .............. 1.1 ] 1012 gauss spacecraft, PSR B1055[52 was viewed by COMPTEL at the same times as by EGRET. OSSE, the Oriented Scintillation Spectrometer Experi- spin-down law in terms of l,l5 , andl . Table 2 lists the ment, is a third of the CGRO telescopes, operating in the solutions relevant to the CGRO viewings, given in terms of energy range 0.05È10 MeV (Johnson et al. 1993). Like frequency l and its derivatives instead of period, and valid EGRET and COMPTEL, OSSE uses both spatial and at time T .20 timing analysis, with the spatial analysis coming from an 0 on-source/o†-source analysis. Because of its smaller Ðeld of view, OSSE observes individual targets. PSR B1055[52 was observed by OSSE and simultaneously by EGRET and 3. GAMMA-RAY OBSERVATIONS COMPTEL 1997 September 2È9 and 1997 September 23È All the telescopes on the CGRO have pulsar timing capa- October 7. bility. Time in universal coordinated time (UTC) is carried RESULTS on board the spacecraft to an accuracy of better than 100 4. km. The conversion of gamma-ray arrival time at the loca- 4.1. L ight Curves tion of the CGRO to pulsar phase is carried out using a Figure 1 shows the EGRET light curve for PSR modiÐcation of the TEMPO timing program (Taylor & B1055[52 combining data from all 24 viewings, derived in Weisberg 1989) and the Jet Propulsion Laboratory DE200 two di†erent ways: (Top) The gamma-ray selection was ephemeris. based on maximizing the signiÐcance of the pulsed signal in EGRET is the high-energy gamma-ray telescope on the light curve, as characterized by the (binned) s2 or CGRO (Thompson et al. 1993), operating from about 30 (unbinned) H-test value (De Jager, Swanepoel, & Rauben- MeV to over 20 GeV. The Ðeld of view mapped by EGRET heimer 1978). The strongest signal was obtained for energies extends to more than 30¡ from the instrument axis. PSR above 240 MeV with gamma rays selected within1¡.7 of the [ B1055 52 was within 30¡ of the telescope axis during 24 of known pulsar position. As found by Ramanamurthy et al. the CGRO viewing intervals to date. No additional EGRET (1995a) for PSR B1951]32, a Ðxed cone can produce an [ observations of this pulsar are scheduled. PSR B1055 52 is improved signal to background for relatively weak signals, not a particularly bright source compared to many others because the Ðxed cone selects photons from the narrow seen by EGRET (see the second EGRET catalog, Thomp- component of the point-spread function of the EGRET son et al. 1995). Its gamma-ray count rate is low, about 4 instrument (Thompson et al. 1993) for this energy range, photons (E [ 100 MeV) per day when the source is within eliminating the broad wings that contribute to the standard 10¡ of the EGRET axis. Data processing for EGRET relies energy-dependent event selection. The optimization was on two principal methods: timing analysis and spatial done iteratively on angle and energy, involving about 20 analysis. The spatial analysis compares the observed trials; (Bottom) The alternate gamma-ray selection used gamma-ray map to that expected from a model of the photons selected within an energy-dependent cone of radius di†use Galactic and extragalactic radiation (Hunter et al. ~0.534 1997; Sreekumar et al. 1998). Source location and Ñux as a h ¹ 5¡.85(Ec/100 MeV) , (1) function of energy are determined using a maximum likeli- with respect to the pulsar position(Ec in MeV).
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