The Crab Nebula.pdf

The Crab Nebula:

An Astrophysical Chimera

Further

ANNUAL

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J. Jeff Hester

School of Earth & Space Exploration, Arizona State University, Tempe, Arizona 85287;

email: jhester@asu.edu

Key Words

pulsar wind nebulae, supernova remnants, historical supernovae

Annu. Rev. Astron. Astrophys. 2008. 46:127–55

First published online as a Review in Advance on

May 19, 2008

Abstract

The Crab Nebula, henceforth the Crab, the remnant of the historical super-

nova of 1054 AD, has long been of intense interest. The pulsar at the center

of the Crab has a spin-down luminosity

The Annual Review of Astronomy and Astrophysics is

online at astro.annualreviews.org

This article’s doi:

10.1146/annurev.astro.45.051806.110608

10 5 times that of the Sun. The

outer nebula holds several solar masses of material ejected by the explosion.

Between the two lies the trapped pulsar wind, visible as synchrotron radia-

tion at radio wavelengths through X-ray wavelengths. Recent observations

with the Hubble Space Telescope , the Chandra X-ray Observatory , and a host

of other instruments have provided a wealth of information about the ex-

traordinary structure and dynamics of the Crab. Understanding those data

requires thinking of the Crab not in terms of its individual components, but

instead as a single interconnected physical system formed as the axisymmet-

rical wind from the pulsar pushes its way outward through a larger freely

expanding supernova remnant.

∼

2008 by Annual Reviews.

0066-4146/08/0922-0127$20.00

127

1. INTRODUCTION

The Crab Nebula is the remnant of a supernova explosion witnessed by Chinese astrologers on

July 4, 1054 AD. Even at a distance of

2 kpc (Trimble 1968), the Chinese astronomer Wang

Yei-te reported that the “guest star” was visible during the daytime for three weeks, and was

visible at night for 22 months (Clark & Stephenson 1977). Following its rediscovery in 1731 by

the English astronomer John Bevis, the Crab Nebula was observed in 1758 by Charles Messier

and became the ﬁrst object in his famous catalog of nebulous noncometary objects. The Crab was

given its name circa 1850 by William Parsons, third Earl of Rosse, who observed the appearance

through his 72-inch reﬂecting telescope. Lundmark (1921) suggested a connection between the

Crab Nebula and the event of 1054 AD, but it was not until 1942 that Duyvendak (1942) and

Mayall & Oort (1942) presented complete studies of modern observations of the expanding nebula

and of the early Chinese records. It was this work that established unambiguously that the Crab

is the remnant of SN1054.

Today the Crab Nebula ( Figure 1 ) is perhaps the most observed object in the sky beyond our

own solar system and is related to almost every branch of astrophysics. This review of the Crab

is timely given the amount of recent observational work on the object using facilities such as the

Hubble Space Telescope (HST) (Hester et al. 1995, 1996; Blair et al. 1997; Sankrit & Hester 1997;

Hester 1998; Sankrit et al. 1998; Loll et al. 2007; Hester, 2007), the Chandra X-ray Observatory

( Chandra ) (Weisskopf et al. 2000; Hester et al. 2002; Mori et al. 2004; Seward, Gorenstein & Smith

2006; Seward, Tucker & Fesen 2006) the Spitzer Space Telescope ( Spitzer ) (Temim et al. 2006), and

the Very Large Array (VLA) (Frail et al. 1995; Bietenholz, Frail & Hester 2001; Bietenholz et al.

2004). The current importance of the Crab Nebula to astrophysics stems from the quality of

these observations, because they allow perhaps a more complete and detailed comparison between

theory and observations than is possible with any other nonstellar astrophysical object.

The volume of the literature on the Crab Nebula is large. An ADS abstract search on the name

turns up over 5000 references. A comprehensive review of this wealth of literature is virtually

impossible and would do little to clarify understanding of this remarkable object. (For that reason

references in this review are often representative rather than exhaustive. I apologize in advance

to those whose excellent work may not be cited.) As a consequence of the wealth of observational

and theoretical work on the Crab, what is often missing is synthesis. Observers and theorists

who are working on one aspect of the Crab or related topics may be unaware or unfamiliar with

aspects of the remnant that are relevant to their studies. With this audience largely in mind, this

review approaches the literature with an eye toward forming such a synthesis. It offers an overall

physical description of the Crab, which, even where it turns out to be incomplete or incorrect,

might still provide a common physical context in which current and future work can be discussed.

That picture will not be of a freely expanding remnant of a possibly underluminous explosion, as

is often proposed. Instead, this review concludes that the Crab is best thought of as a pulsar wind

nebula (PWN) conﬁned by and pushing out into a much larger, freely expanding remnant, as ﬁrst

proposed by Chevalier (1977). It is likely that this larger but almost unseen remnant contains most

of the mass and kinetic energy of the explosion. Were it not for the spin-down luminosity of the

pulsar, the pressure of the PWN that has compressed some of the ejecta into dense ﬁlaments, and

the synchrotron radiation from the PWN that photoionizes that ejecta, the Crab Nebula might

be known today by little more than obscure historical references to an especially bright “guest

star” observed in 1054 AD.

To avoid confusion it is worth prefacing a review of the Crab with a few notes on the nomen-

clature that has grown up around the object. Moving from the inside out, the Crab consists of the

Crab pulsar, the Crab synchrotron nebula, a bright expanding shell of thermal gas, and a larger

∼

128

Hester

Figure 1

A composite Hubble

Space Telescope image of

the Crab Nebula.

Thermal ﬁlaments

composed of ejecta

from the explosion

appear around the

outer part of the

nebula. [O III]

5007

is shown in red,

[S II]

6717,6731 in

green, and [O I]

λλ

6300

in blue. Note that

much of the structure

breaks up into inward

pointing ﬁngers of

emission. Note also

the increasing

ionization moving

outward and the

scalloped appearance

of the nebula. The

visible synchrotron

nebula ﬁlling the

interior of the remnant

is shown in blue. The

synchrotron nebula is

bounded and conﬁned

by the thermal ejecta.

Note the presence of a

red [O III] “skin”

around the SE portion

of the remnant, which

is interpreted as

emission behind a

shock driven by the

pressure of the

synchrotron nebula

into a larger, freely

expanding remnant

surrounding the Crab.

very faint freely expanding supernova remnant. By convention, features in the synchrotron nebula

are referred to as wisps. Structures seen in the light of emission lines from thermal gas are referred

to as ﬁlaments. The wind shock is located at the interface between the wind and the synchrotron

nebula. There is a second shock at the outer boundary of the synchrotron nebula driven by the

pressure of the synchrotron nebula into surrounding thermal gas. The synchrotron nebula and

thermal ﬁlaments lie within a larger freely expanding supernova remnant. All of these components

have been observed either directly or indirectly. An as yet unseen outer shock presumably lies well

beyond the visible boundary of the Crab at the leading edge of the expanding cloud of ejecta.

2. THE CRAB CONSISTS OF FOUR OBSERVABLE COMPONENTS

2.1. The Crab Pulsar Powers the Nebula

The Crab Nebula consists of four observable components. At the center of the Crab lies the Crab

pulsar. Since its discovery at radio wavelengths (Staelin & Reifenstein 1968, Comella et al. 1969),

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The Crab Nebula

129

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the Crab pulsar has been studied intensively in all wavelength regimes from gamma rays to radio.

It was the ﬁrst pulsar to be observed at visible wavelengths (Cocke, Disney & Taylor 1969).

The Crab pulsar has a period of 33 ms, and

10 −13 . Assuming a uniform sphere

with a radius of 10 km and a mass of 1.4 M , the moment of inertia of the pulsar is I

P 3

10 45

gcm 2 . The pulsar then has a spin-down luminosity of L spin =

2 I

10 38

erg s −1 ,or

130,000 L . Using a braking index n

51 (Lyne, Pritchard, & Smith 1988), Bejger & Haensel

2003 estimate that the pulsar was born with a period of

∼

19 ms, so it has lost

∼

10 49

ergs

since its birth. This corresponds to an average luminosity of

10 39 ergs s −1 over the lifetime

of the remnant, or about twice the current value. Although a small fraction of this energy goes

into pulsed emission, the majority of the pulsar’s spin-down luminosity is carried away by some

combination of magnetic dipole radiation and an ultrarelativistic wind. Kennel & Coroniti (1984a)

point to the high efﬁciency with which this energy is converted into synchrotron emission in the

Crab. Since this work, it has typically been assumed that the pulsar’s spin-down luminosity is

carried for the most part by a highly magnetized pair plasma, perhaps with some admixture of ions

from the surface of the neutron star itself (Gallant & Arons 1994). (This review makes the same

assumption throughout.) This is the source of the energy that powers the Crab.

∼

2.2. The Shocked Pulsar Wind Fills the Crab Synchrotron Nebula

The pulsar’s ultrarelativistic wind is conﬁned by the thermal ejecta from the explosion into which it

is expanding. Little or no synchrotron emission is seen from the wind in the volume immediately

surrounding the pulsar (Schmidt, Angel & Beaver 1979), indicating that the wind itself is not

luminous. The transition from this cold fast wind (dominated by the kinetic energy of the ﬂow)

to the hot plasma that ﬁlls the nebula occurs in a shock. Momentum balance between the wind

and the synchrotron nebula suggests that the shock should be located about 3

10 17 cm from

the pulsar (Rees & Gunn 1974, Kennel & Coroniti 1984a), and it is often pointed out that this is

roughly the projected distance between the pulsar and the wisps observed by Scargle (1969). As

discussed below, more recent imaging of the Crab synchrotron nebula shows that this picture is

overly simplistic and that the pulsar wind is far from spherically symmetrical. Even so, there are a

number of quasi-stationary features (e.g., the X-ray ring from Weisskopf et al. 2000 and the sprite

and halo from Hester et al. 2002) that are plausibly identiﬁed with the shock. In no direction does

the energy ﬂow from the pulsar appear unconﬁned.

In the simple model, the wind shock thermalizes ﬂow energy, accelerating electrons and

positrons to energies as high as

10 4 TeV. It is this magnetized relativistic plasma that gives

off the synchrotron emission that we see. Alfv en & Herlofson (1950) were the ﬁrst to suggest

synchrotron emission as a mechanism for bright radio emission from radio stars. The Crab syn-

chrotron nebula was the ﬁrst conﬁrmed astrophysical source of synchrotron radiation (Shklovsky

1953, Dombrovsky 1954) and is the deﬁning member of the class of center-ﬁlled ﬂat spectrum

supernova remnants called plerions (Weiler & Panagia 1978). Although the term plerion remains

a morphological designation, most plerions are now identiﬁed with PWN. The Crab synchrotron

nebula ﬁlls a roughly ellipsoidal volume with a major axis of 4.4 pc and a minor axis of 2.9 pc,

tilted into the plane of the sky by 30 ◦

∼

(Lawrence et al. 1995, Loll et al. 2007), and ﬁlls a volume

30 pc 3 .

The Crab Nebula is remarkably efﬁcient at converting the energy of the shocked pulsar wind

into synchrotron emission. The synchrotron emission from the Crab Nebula has an integrated

luminosity of

∼

10 57

cm 3

∼

10 38 ergs s −1 currently being injected

into the nebula by the pulsar. A comparable fraction of the energy lost by the pulsar has gone into

PV (pressure-volume) work done on the ﬁlaments, leaving perhaps half of the energy lost by the

∼

10 38

ergs s −1 , or about 26% of the 5

130

Hester

Figure 2

The integrated

spectrum of the Crab

synchrotron nebula,

from Atoyan &

Aharonian (1996),

assembled from

sources cited in that

paper. The electron

energies shown

correspond to peak

synchrotron emission

assuming a magnetic

ﬁeld of 300 μ G. Most

of the emission from

the Crab is emitted

between the optical

and X-ray bands. The

highest energy γ -rays

are due to inverse

Compton radiation.

10 38

Soft X

10 37

FIR

HEAO A4

COMPTEL

10 36

EGRET

10 35

10 GeV

300 GeV 10 TeV

Electrons responsible for the radiation

100 TeV

3 × 10 3 TeV

10 34

10 33

log ν Hz –1

pulsar (

10 49 ergs) still resident within the synchrotron nebula. Averaged over the volume of

the synchrotron nebula, this energy density corresponds to a pressure of

∼

dyne cm −2 ,

∼

10 −9

very close to the canonical value assuming equipartition and B

300 μ G (Trimble 1968).

The overall spectrum of the Crab Nebula peaks in the range between 10 14 –10 18 Hz in the

optical through the X-ray part of the spectrum (see Figure 2 , from Atoyan & Aharonian 1996).

Assuming a magnetic ﬁeld of 300 μ G, this radiation is associated with emission from electrons with

energies between a few hundred GeV and a few tens of TeV. The very highest energy emission

from the Crab above frequencies of

∼

10 23 Hz is thought to be due to inverse Compton radiation

(Atoyan & Aharonian 1996). The bump around 10 13 Hz in the far infrared part of the spectrum is

the result of thermal emission from dust in the nebula. This dust, which condensed from material

ejected in the explosion, is heated to a temperature of about 80 K (Marsden et al. 1984).

Figure 3 shows a color composite image of the Crab synchrotron nebula. An X-ray image

of the Crab obtained with Chandra is shown in blue. An optical continuum image is shown in

green. Red shows a radio image of the Crab obtained with the VLA. The ﬁrst thing that is

immediately apparent in these images is the difference in spatial extent of the Crab synchrotron

nebula when viewed at different wavelengths. The nebula is smallest in size when viewed at

high energies, and grows progressively larger when viewed at lower energies. This basic trend is

relatively easy to understand. High-energy particles injected into the nebula at the wind shock

experience both synchrotron burn off and energy loss owing to adiabatic expansion as they move

outward through the nebula. Even so, faint X rays are still seen close to the boundary of the nebula

(Hester et al. 1995; Seward, Tucker & Fesen 2006), indicating that the real situation is more

complex.

The synchrotron nebula shows a wealth of ﬁne-scale structure that can be extraordinarily

dynamic, varying appreciably on timescales of days. The standard nomenclature for these features

comes from Scargle (1969), who identiﬁed a number of arcuate features or wisps located along and

generally perpendicular to a line going from the SE to the NW through the pulsar. (By convention,

features in the Crab synchrotron nebula are referred to as wisps, while the term ﬁlament is reserved

∼

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