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The X-Ray Structure and Spectrum of the Pulsar Wind Nebula Surrounding PSR B1853+01 in W44

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The X-Ray Structure and Spectrum of the Pulsar Wind Nebula Surrounding PSR B1853+01 in W44 The X-ray Structure and Spectrum of the Pulsar Wind Nebula Surrounding PSR B1853+01 in W44 R. Petre, K. D. Kuntz 1 NASA Goddard Space Flight Center, Greenbelt, MD 20771 and R. L. Shelton The Henry A. Rowland Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218 ABSTRACT We present the result of a Chandra ACIS observation of the pulsar PSR B1853+01 and its associated pulsar wind nebula (PWN), embedded within the supernova remnant W44. A hard band ACIS map cleanly distinguishes the PWN from the thermal emission of W44. The nebula is extended in the north-south direction, with an extent about half that of the radio emission. Morphological differences between the X-ray and radio images are apparent. Spectral fitting reveals a clear difference in spectral index between the hard emission from PSR B1853+01 (F --_ 1.4) and the extended nebula (F --, 2.2). The more accurate values for the X-ray flux and spectral index are used refine estimates for PWN parameters, including magnetic field strength, the average Lorentz factor _ of the particles in the wind, the magnetization parameter a, and the ratio k of electrons to other particles. Subject headings: supernova remnants--ISM:individual(W 44)--X-rays:general 1. Introduction fated, old radio pulsars. Additionally, because of its high proper motion, the pulsar leaves a record The remarkable pulsar wind nebula associated of its evolution embedded in the extended neb- with PSR B1853+01 and embedded in the evolved, ula. A combination of high resolution radio and mixed-morphology supernova remnant W44 is of X-ray observations can potentially disentangle this particular interest for several reasons. It is the record. For example, radio measurements of the oldest known pulsar wind nebula (PWN), asso- extent of the nebula perpendicular to the direc- ciated with an active pulsar (only the PWN in tion of motion provide means for setting an up- IC 443 is thought to be older - Bocchino & By- per limit to the lifetime of the radio emitting elec- chkov 2001). Its age of 20,000 yr is estimated from trons (,-, 6,000 yr) unavailable from observation of the PSR B1853+01 spindown (Wolszczan, Cordes wind nebulae associated with stationary pulsars. & Dewey 1991). At the same time, PSR B1853+01 Finally, some of the properties of this PWN are is one of the 10 youngest known pulsars. Thus similar enough to those of the much younger and the nebula may allow the testing of hypotheses re- more luminous Crab Nebula (Chevalier 2000) to garding PWN evolution, and serves as a bridge invite comparisons and speculation regarding the between the young, active pulsars in supernova reason for the similarities. remnants and the preponderant population of iso- With the clearest view of the X-ray universe, es- 1Department of Physics, University of Maryland Balti- pecially above 3 keV, now available via the Chan- more County, 1000 Hilltop Circle, Baltimore MD 21250 dra X-ray Observatory, more comprehensive stud- iesof embeddedPWNsbecomefeasible.In this sar. Frail et al. interpret this structure as the paperweuseChandra'sAdvancedCCDImaging result of the pulsar's motion through the interior Spectrometer(ACIS)torevealtheX-raystructure of the remnant. Using three independent tech- andspectrumofthesynchrotronnebulasurround- niques, they derive a velocity of the pulsar of ap- ing theW44pulsar,andprovidea morecareful proximately 375 km/s. The radio emission has a lookat theX-rayspectrumandits spatialvaria- nonthermal spectrum with a spectral index of- tion.Ashasbeenshownin numerousotherworks 0.12=t:0.04, and it is 174-4 percent polarized. The (e.g.,Harrusetal. 1996;Frailetal. 1996;Toriiet spectral index distinguishes the PWN from the al. 2000), the study of the wind nebulae surround- surrounding emission associated with W44 (F -_- ing pulsars provides a means for understanding the 0.33); the spectrum and degree of polarization are energetics of pulsars, and in particular how they similar to other pulsar wind nebulae. Using a com- transfer their rotational spindown energy into a bination of the X-ray and radio properties, Frail et relativistic wind. al. estimate some key pulsar wind nebula param- W44 is one of the first remnants for which hard eters, including magnetic field strength, and the band X-ray imaging was used to isolate a pulsar Lorentz factor 7 of the electrons near the spectral and its associated PWN from the brighter, softer break between the radio and X-ray slopes. thermal X-ray emission associated with the rem- Giacani et al. (1997) presented radio and X-ray nant's shock-heated gas. Neither the pulsar nor images of W44 as a whole. The PWN is apparent the nebula is apparent in low energy X-ray images, but inconspicuous in the radio. A line of Ha fil- such as that from the ROSAT PSPC (Rho et al. aments lies along the eastern edge of the PWN, 1994). Using ASCA and its broader band imaging, but there is no general correspondence with radio however, Harrus et al. (1996) showed that while features, and it is unclear whether this emission the PWN is invisible in the broad band image, it is associated with the PWN or with shock heated becomes the dominant feature above 4 keV. The material near the PWN only in projection. centroid of the X-ray emission is consistent with The distance to W44 has generally been taken the location of the pulsar, PSR B1853+01 (Wol- to be around 3 kpc, based on measurements of H I szczan, Cordes & Dewey 1991). ASCA's modest absorption and 1720 MHz maser lines (Caswell et angular resolution precluded spatially distinguish- al. 1975; Claussen et al. 1996). The analytical ing the synchrotron nebula from the surrounding modeling of Cox et al. (1999) refines this distance diffuse emission, but Harrus et al. showed that the to be between 2.5 and 2.6 kpc. We use a distance spectrum of the region including the PWN has of 2.6 kpc, and scale parameters in terms of d2.6. a hard continuum component not detected else- The implications of using this refined value are where in W44. The techniques pioneered in Har- minor. A luminosity estimate, for instance, is re- rus et al. (1996) have subsequently been used to duced by 25 percent, which is probably well within identify stellar remnants or synchrotron nebulae in the uncertainty of the estimate. other remnants (e.g., IC 443 - Keohane et al. 1997; MSH 15-56- Plucinsky 1998; G292.0÷1.8- Torii, 2. Observations and analysis Tsunemi, & Slane 1998). The most important con- sequence of this approach is the dramatic increase W44 was observed using ACIS-S on 31 Octo- in the number of supernova remnants with iden- ber, 2000 for 45.5 ks. As the primary goal of the tified synchrotron nebulae and/or compact stellar observation was to understand the nature of the remnants. centrally peaked thermal emission in this mixed- The discovery of the X-ray synchrotron nebula morphology SNR, the $3 chip was pointed at the occurred contemporaneously with the mapping of remnant center. The results of that investigation the pulsar wind nebula in the radio (Frail et al. will be reported elsewhere. A fortuitous observa- 1996). At 1.4 GHz it appears cometary in shape tion date made it possible to orient the spacecraft with an extent of -_2.5 arc minutes. The pulsar roll angle to place the pulsar squarely within the is located at the southern extremity. The radio front-illuminated $2 chip. Although the calibra- surface brightness peaks at the widest part of the tion of $2 is not as complete as that of $3, it is tail, approximately 1 arc minute north of the pul- adequate for the analysis we perform here. Thedatawereanalyzedusingacombinationof 01 ° 13' 20".6. It should be noted that the off- publicandcustomsoftwaretools.WeusedCIAO set cannot be a result of proper motion, which has tools version2.2 and the calibrationsavailable been estimated to be 25 mas/yr (Frail et al. 1996). withtheCIAOcalibrationdatabase2.9. An initial In Figure 2 we show a close up of the hard image of the pulsar nebula in the 2.0-8.0 keV band diffuse emission surrounding PSR B1853+01, and was formed from the events file after point source compare it with the radio map of Frail et al (1996). removal, and smoothed with a 4'.'4 (9 ACIS pixel) Both show diffuse emission, cometary in shape, HWHM Gaussian. Spectra were extracted using trailing toward the north. The resemblance is custom IDL software designed to extract counts close enough to suggest that this feature is the within predefined brightness contours. Spectral X-ray PWN. Nevertheless, clear differences exist fitting was performed using XSPEC v.ll.l.0. between the X-ray and radio images. Unlike the radio emission, the X-ray emission peaks at the 3. Images pulsar. Additionally, the X-ray extent is about half the radio. The real X-ray extent is slightly In Figure 1 we show images of the region of (about 20 percent) less than what is apparent in W44 in the $2 field of view. The soft band (0.3- the figures. Due to the lack of counts, the image 2.2 keV) image (Fig. la) is dominated by diffuse has been smoothed, thus increasing the apparent thermal emission from W44. Several unresolved X-ray extent. sources are apparent; the pulsar is one of these, An elliptical region of intermediate X-ray sur- though not the brightest. The pulsar is located face brightness extends 1-2' in all directions be- along the edge of an emission plateau stretching yond the radio nebula, and a brighter ridge ex- approximately southeast to northwest.
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