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XMM-Observations of Pulsars: a Study of Thermal- Vs. Non-Thermal Emission

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XMM-Observations of Pulsars: a Study of Thermal- Vs. Non-Thermal Emission XMM-Observations of Pulsars: A study of thermal- vs. non-thermal emission W. Becker &J.Trump er Max-Planck-Institut fur extraterr. Physik, Giessenbachstrasse 1, D-85740 Garching ABSTRACT Recent X-ray observatories likeROSAT and ASCA have achieved imp ortant progress in neutron star and pulsar astronomy. The identi cation of Geminga, the discovery of X-ray emission from millisecond pulsars and the identi cation of co oling neutron stars are only few of the fascinating results. In the following we will give a brief review on the X-ray emission prop erties of rotation-p owered pulsars and outline brie y the prosp ects exp ected from a mission like XMM and AXAF for neutron star astronomy. 1. Why are neutron stars targets for X-ray observatories ? Neutron stars are among the most fascinating astronomical ob jects in the universe. Born in the implo ding core of a sup ernova, they provide a unique class of stellar ob jects with prop erties that make them nearly ideal prob es for investigating a wide varietyofphysical problems. A primary goal to study rotation-p owered pulsars at X-ray energies is to search for thermal X-ray emission from co oling neutron stars. The close link b etween the thermal evolution of neutron stars and the physical characteristics of neutron star material at sup er-nuclear densities provides an imp ortant clue to the empirical study of matter at extreme energies and sup er-nuclear densities. Comparing measured neutron star temp eratures with the theoretical predictions based on di erent equations of state thus provides the empirical basis essential for the veri cation of neutron star mo dels and co oling theories. Another p oint of interest is the physical mechanism op erating in the pulsar magnetosphere to pro duce the intense, broad-band b eamed radiation. It is well known that the radio emission of pulsars is due to coherent pro cesses, and coherent curvature radiation has b een identi ed as the most promising mechanism see Michel 1991 and references therein. On the other hand, the optical, X-ray and gamma-ray emission observed in pulsars must be incoherent. Therefore, the uxes in these energy bands are directly prop ortional to the densities of the radiating high energy electrons in the acceleration regions, no matter what radiation pro cess synchrotron radiation, cyclotron radiation or inverse Compton e ects is at work. High energy observations thus should provide the key for an understanding of the pulsars' emission mechanisms. {2 { 2. What is known on the pulsars' X-ray emission prop erties ? Studying neutron stars with ROSAT and ASCA for more than eight years has increased the 1 number of X-ray detected pulsars to 34 by now. Although this is less than 5 of the known radio-pulsar p opulation, the detected ob jects cover a wide range of ages, magnetic eld strength and spin p erio ds. The observed X-radiation can b e attributed to various thermal an non-thermal emission pro cesses, including Non-thermal emission from relativistic particles accelerated in the pulsar magnetosphere. The emission is characterized bya power-law sp ectrum and sharp X-ray pulses. Extended emission from a pulsar driven synchrotron nebula and emission from a relativistic pulsar wind interacting with the interstellar matter. Photospheric emission from the hot surface of a co oling neutron star. In this case a mo di ed black-body sp ectrum and smo oth, low amplitude variations with rotational phase are exp ected. Thermal emission from the neutron star's p olar caps which are heated by the b ombardment of relativistic particles streaming back to the surface from the pulsar magnetosphere. Investigating the observed X-ray emission prop erties it is instructive to group the whole sample of detected ob jects according to their spin-down age. Young pulsars like the 1000 year old Crab pulsar emit sharp X-ray pulses with a fraction of pulsed photons up to 75. The X-ray emission of these ob jects is connected with the acceleration of charged particles in the pulsar magnetosphere. No thermal X-ray emission from the surface of these young neutron stars could be detected by ROSAT. The intense X-ray emission from synchrotron nebula surrounding the young pulsars buries the thermal co oling emission Harnden & Seward 1984; Becker & Aschenbach 1993. The Vela pulsar, which has a characteristic age of ab out thirty-thousand years, is lo cated in the center of the Vela sup ernova remnant. Its characteristic pulsar radiation is observed in the radio, optical, X- and gamma-ray domain. With ROSAT it b ecame p ossible to show that its X-ray emission consists of two di erent comp onents: a pulsed soft comp onent likely emitted by the 1.5 million degree hot neutron star surface and dominating the emission in the soft band 0.1-0.6 keV, and a somewhat harder and steady comp onent, emitted from the pulsar driven synchrotron nebula Ogelman et al. 1993. The mo dulation of the neutron stars co oling emission is explained by non-uniformities in the surface temp erature distribution due to the presence of the strong magnetic eld which gives rise to an anisotropic heat ow in the neutron star's outer layers. A second sup ernova remnant, Puppis-A is lo cated b ehind the Vela sup ernova remnant and stronger 1 For a tabulated summary of the X-ray detected rotation-p owered pulsars, their X-ray luminosities and parameters see http://www.xray.mp e.mpg.de/web/bt97 up date.html {3 { Fig. 1.| a. False-color representation of the Vela sup ernova remnant curtesy of B. Aschenbach. The red color represents the soft emission 0.1-0.6 keV and the blue color the emission within 0.6-2.0 keV. The lo cation of the Vela pulsar is represented by an arrow. Its X-ray emission b elow2 keV is dominated by the intense emission from the pulsar's synchrotron nebula. Co oling emission from the young neutron star is only visible b elow 0:5 keV. b. The SNR Puppis-A is lo cated in the north-west of the Vela remnant. A zo omed image of Puppis-A curtesy B. Aschenbach is given in the right image. The arrow indicates the p oint source RX 0820+42 which is regarded to be very go o d candidate for a young neutron star. absorb ed than Vela. The faint p oint source RX J0820+42 in the center of the remnantisa very go o d candidate for a young neutron star showing photospheric emission at a temp erature of 1:5 million degree Zavlin, Trump er & Pavlov 1999. It has b een found recently that its emission is mo dulated with a p erio d of 75.3 ms. At the same time the p erio d derivative could b e determind. 3 12 The corresp onding age and magnetic eld is 8:0 10 years and 3:4 10 Gaus, resp ectively Pavlov, Zavlin & Trump er 1998. Middle aged neutron stars which are hundred thousand to million years old show soft mo dulated X-ray emission with a pulsed fraction of typically 15-30 Ogelman 1993. Their X-ray emission is found to b e of thermal origin and b eing a relic of the initial heat content from the neutron star birth. The b olometric luminosities and luminosity upp er limits deduced from the ROSAT data are found to be in good agreement with the predictions of standard co oling mo dels for young neutron stars Crab, PSR 1509-58, Vela, neutron star candidates in sup ernova remnants RCW 103, 1E1207-51, Puppis-A as well as for the middle aged pulsars Geminga, PSR 1055-52 and {4 { RCW 103 0820+42 Crab 1509-58 1055-52 1207-51 Vela 0656+14 Geminga Becker & Trump er 1997, A&A, 326,682 Fig. 2.| Empirically derived b olometric luminosities and predicted co oling curves for a neutron star with a realistic equation of state cf. Becker &Trump er, 1997. Solid lines represent the standard co oling, dashed and dotted lines the so called fast co oling mo dels. The latter are predicted by 5 assuming an enhanced neutrino co oling in the rst 10 years in the neutron stars' live. PSR 0656+14. The latter have b een identi ed to b e archetypal co oling neutron stars see Becker & Trump er 1997 and references there in. The pulsars b elonging to the class of middle aged neutron stars also show another somewhat harder emission comp onent which is asso ciated with non-thermal emission from the neutron star magnetosphere Becker & Trump er 1997. Millisecond pulsars are b elieved to represent an end-p ointinthe evolution of accretion-p owered neutron stars. They are distinguished by their small spin p erio ds of less than 20 ms, a relatively 8 9 weak magnetic eld of 10 10 Gauss and a very old characteristic age of several billions of years. More than 75 of the known disk millisecond pulsars are in binaries with a compact companion star, compared with the 1 of binary pulsars found in the general p opulation. This gives supp ort to the idea that their fast rotation has b een acquired by angular momentum transfer during a past mass accretion phase. X-ray emission from millisecond pulsars was rst discovered by ROSAT Becker & Trump er 2 1993 . However, although ten of the 34 detected rotation-p owered pulsars b elong to the small 2 A recent review on the X-ray emission prop erties of millisecond pulsars which summarizes the current emission prop erties and observational status in a much wider and more complete way than here p ossible is given by Becker & Trump er 1998.
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