The Astrophysical Journal, 577:L49–L52, 2002 September 20 䉷 2002. The American Astronomical Society. All rights reserved. Printed in U.S.A.
HUBBLE SPACE TELESCOPE AND CHANDRA MONITORING OF THE CRAB SYNCHROTRON NEBULA J. J. Hester,1 K. Mori,2,3 D. Burrows,2 J. S. Gallagher,4 J. R. Graham,5 M. Halverson,4 A. Kader,1 F. C. Michel,6 and P. Scowen1 Received 2002 July 14; accepted 2002 August 9; published 2002 August 27
ABSTRACT We report the results of coordinated Hubble Space Telescope and Chandra observations of the Crab synchrotron nebula. Similar dynamical structures, including equatorial wisps moving outward at ∼0.5c, are seen in both passbands. The inner X-ray ring is a variable, irregular structure from which wisps and diffuse emission emerge. The X-ray/visible jet to the southeast of the pulsar is a true jet. The data support the interpretation that the inner ring and a highly dynamical feature at the base of the jet are unstable quasi-stationary shocks in the “cold” equatorial wind and polar jet from the pulsar. Subject headings: ISM: individual (Crab Nebula) — ISM: jets and outflows — pulsars: individual (Crab Pulsar) — supernova remnants On-line material: mpeg animation 1. INTRODUCTION
2. OBSERVATIONS
The inner region of the Crab synchrotron nebula was observed on 24 occasions between 2000 August and 2001 April using the HST WFPC2. Observations were obtained on 11 day intervals through F547M, a relatively line-free filter centered ˚ . Unfortunately, the ninth exposure in the sequence near 5500 A failed as the result of a telescope safing event. To keep the time sequence clear, we refer to each HST visit by an H followed by its planned sequence number. Chandra Advanced CCD Imaging Spectrometer (ACIS) observations were obtained on eight visits (referred to as C1–C8). The ACIS-S was used as a focal plane detector without grating. A 150 # 150 subarray was used with 0.2 s frame times. This provided coverage of the region of the torus, jet, and counterjet, while reducing problems due to pileup. There were 25 ks of on-target time for each visit, with exposure efficiencies of about 11%. Data were reduced using the standard tools distributed by the Chandra X-ray Center (CXC; the CIAO software package and the CALDB calibration database). Trailing along CCD columns due to the brightness of the pulsar was subtracted from the reduced images. Each Chandra visit was separated from its neighbors by ∼22 days and was scheduled to fall within a day of one of the HST visits.
It has long been known that the inner part of the Crab synchrotron nebula is dynamic, but the early study by Scargle (1969) lacked the spatial or temporal resolution to make sense of the changes that were seen there. Hester et al. (1995) used the Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2) and ROSAT to clarify the structure of the region around the pulsar, showing that the nebula is symmetrical about an axis tilted by about 25⬚ with respect to the plane of the sky. Subsequent HST observations sampling time intervals as short as 6 days (Hester 1998) show that wisps in the Crab move outward in the equatorial plane at speeds of around 0.5c. Outward motion of wisps is also seen in radio images of the Crab (Bietenholz, Frail, & Hester 2001). When viewed in X-rays, the Crab synchrotron nebula is much smaller than when viewed at other wavelengths. This is due to the rapid synchrotron burn-off of the energetic particles responsible for X-ray emission. Observations of the Crab obtained with the Chandra X-Ray Observatory (Weisskopf et al. 2000) also show the presence of sharp features much like those seen at visual wavelengths. These observations also reveal the presence of an inner X-ray ring, thought to be associated with the conversion of the cold relativistic wind into a more slowly moving synchrotron-emitting plasma. We have recently obtained coordinated HST and Chandra observations of the Crab to study the dynamics of the inner nebula. In this Letter, we present the observations and brief highlights of what can be seen in these extraordinarily rich data sets. More detailed analyses will be presented in subsequent papers.
3. RESULTS AND DISCUSSION
Figures 1a and 2a show the central regions of the HST and Chandra images for the coordinated visits H23 and C8. The basic axisymmetrical geometry of the system is as described by Hester et al. (1995). While the relative brightness of features can vary considerably between the two bands, virtually all features that can be picked out of the X-ray images have visual counterparts that share the same motion. A feature-by-feature comparison of the difference images in Figures 1b and 2b supports this conclusion. The most dynamic features in the HST images fill roughly the same volume as the X-ray nebula. The torus is largely a collection of moving wisps. Prominent equatorial wisps on the front (northwest) side of the torus have projected motions on the plane of the sky of ∼11⬙–15⬙ yr⫺1, corresponding to apparent motions of ∼0.35c–0.5c. Prominent wisps on the back side of the torus appear to move more slowly (∼0.03c–0.1c), last longer, and are also more bunched up than on the front
1 Department of Physics and Astronomy, Arizona State University, Box 871504, Tempe, AZ 85287-1504;
[email protected]. 2 Department of Astronomy and Astrophysics, Pennsylvania State University, 525 Davey Laboratory, University Park, PA 16802. 3 Department of Earth and Space Sciences, Osaka University, Osaka 5600043, Japan. 4 Department of Astronomy, University of Wisconsin at Madison, 5534 Sterling Hall, 475 North Charter Street, Madison, WI 53706. 5 Department of Astronomy, University of California at Berkeley, 601 Campbell Hall, Berkeley, CA 94720-3411. 6 Department of Physics and Astronomy, Rice University, MS 61, 6100 South Main, Houston, TX 77005-1892.
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Fig. 1.—(a) HST image H23 obtained on 2001 April 6. (b) Difference between image H23 (dark) and image H13 (light), taken 109 days earlier. The dark/light patterns show changes in morphology, including wisp motion. The arrows indicate nonradial motions of features A and B.
side. These differences may be explained by differences in light delay times in features moving toward or away from the observer with typical velocities of about 0.5c (this is the same phenomena responsible for superluminal motion in active galactic nucleus jets). Wisps can brighten very quickly and then might fade over a month or so. We tentatively interpret moving wisps as magnetic flux tubes undergoing unstable synchrotron cooling (Hester 1998). Rapidly moving wisps in the inner nebula merge to form less dynamic structures that move outward at speeds of ∼0.03c near the outer edge of the torus. Weisskopf et al. (2000) noted what appeared to be poloidal loops around the torus in an early Chandra image. These features, which expand perpendicular to striations at ∼0.03c– 0.04c, are also seen in the HST images (A in Fig. 1b). Rather than closed poloidal structures, in HST difference images these appear to be field lines that are draped around the expanding torus. We interpret these as evidence of the confinement of the torus by the magnetic field of the surrounding nebula. Motions in the outer part of the torus that are perpendicular to the radial direction (e.g., the upward motion of feature B in Fig. 1b) provide other evidence of pressure-driven expansion of the torus. While some structures are relatively brighter in X-rays or visual light, the most prominent differences involve the inner X-ray ring noted by Weisskopf et al. (2000). Figure 3a shows a high-pass–filtered version of the average of Chandra visits C2 through C8. Figure 3b shows the difference between this image and a similarly filtered average of the corresponding HST observations. In the difference image, the sharp feature seen in projection along the northwest edge of the inner ring and a faint feature located immediately to the northwest of the
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Fig. 2.—(a) Chandra image C8. (b) Difference between Chandra images C8 (dark) and C3 (light). These images were each taken within a day of the HST images shown in Fig. 1.
Fig. 3.—(a) High-pass–filtered version of the average of Chandra visits C2 through C8. This is a negative image. (b) High-pass–filtered version of the average Chandra image minus the high-pass–filtered version of the average of the corresponding WFPC2 images. The WFPC2 image was smoothed to match the resolution of the Chandra image.
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Fig. 4.—(a) Detail of the jet from X-ray image C8. (b) HST difference image of the jet between visits H13 and H23. The dark/light patterns are due to the motion of structure along the jet.
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pulsar oversubtract. These sharp X-ray wisps are not associated with the inner ring but are instead X-ray counterparts of the “halo” identified by Hester et al. (1995). (The visual halo as well as the knot ∼0⬙. 5 southeast of the pulsar remain persistent features throughout the sequence of HST images.) After subtraction of visual nebulosity, the inner X-ray ring consists of a set of roughly two dozen knots. These knots typically do not move outward as do the wisps. Instead, they form, brighten, fade, dissipate, move about, and occasionally undergo outbursts, giving rise to expanding clouds of nebulosity. While the knots in the inner ring are much brighter in X-rays, they can still be seen as faint features in the HST images that bound an inner underluminous region. Faint, mottled emission streams away from the location of the inner ring around its entire circumference. Some prominent wisps are also seen to emerge directly from the vicinity of the ring. The data leave little doubt that the inner X-ray ring is, indeed, associated with the shock that turns the “cold” ultrarelativistic pulsar wind into a synchrotron-emitting plasma. Figure 4a shows an X-ray image of the jet to the southeast of the pulsar. The jet is also prominent in visual light. Figure 4b, the difference of HST images H23 and H13 taken 109 days apart, demonstrates that this feature is a true dynamical jet. Features move along the length of the jet at characteristic speeds of ∼0.4c. Jet features include both diffuse features and discrete bow-wave–like structures. The jet can also be seen to be pushing into the surrounding syn-
Fig. 5.—Chandra and HST images of the shock at the head of the jet (the “sprite”). There are approximately 44 day separations between adjacent images. This figure is also available as an mpeg animation in the electronic edition of the Astrophysical Journal.
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chrotron nebula at speeds of ∼0.03c. Structure associated with the counterjet can be seen as well (e.g., Fig. 3b). Figure 5 shows images of an extremely dynamical feature located at the base of the jet. This feature, dubbed the “sprite” by Hester (1998), is interpreted as the shock in the polar jet from the pulsar. Features in the sprite are often seen in both X-rays and visual emission. The sprite appears center-filled in X-rays at some times (e.g., H21/C7), but it is difficult to be certain that this is not the projection of a knot associated with the inner X-ray ring. It is clear, however, that the synchrotron plasma emerging from the sprite is more centrally concentrated in X-rays than in visual emission. The sprite occasionally un-
dergoes outbursts that send pulses of material and bow-wave structures downstream into the jet. We note the similarities in the behavior of the sprite and the behavior of knots in the inner ring, consistent with the identification of both with unstable quasi-stationary shocks. Warm and sincere thanks go to Beth Perriello of the STScI, Pat Slane and Paul Plucinski of the CXC, and countless others at both facilities who made it possible to carry out this extremely challenging suite of coordinated observations. This work was supported by STScI grant HSTGO0740701 and CXC grant GO12076.
REFERENCES Bientenholz, M. F., Frail, D. A., & Hester, J. J. 2001, ApJ, 560, 254 Hester, J. J. 1998, in Neutron Stars and Pulsars: Thirty Years after the Discovery, ed. N. Shibazaki et al. (Tokyo: Universal Academy Press), 431
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Hester, J. J., et al. 1995, ApJ, 448, 240 Scargle, J. D. 1969, ApJ, 156, 401 Weisskopf, M. C., et al. 2000, ApJ, 536, L81
