We summarize here the results, most of which are preliminary, of a number of
recent observations of the Crab nebula system with the Chandra X-Ray
Observatory. We discuss four different topics: (1) The motion on long (> 1yr)
time scales of the southern jet. (2) The discovery that pulsar is not at the
center of the projected ring on the sky and that the ring may well lie on the
axis of symmetry but appears to be displaced at a latitude of about 5 degrees.
(Note that this deprojection is by no means unique.) (3) The results and
puzzling implications of the Chandra phase-resolved spectroscopy of the pulsar
when compared to observations of pulse-phase variations of similar and
dissimilar measures in other regions of the spectrum. (4) The search for the
X-ray location of the site of the recently-discovered gamma-ray flaring. We
also comment briefly on our plan to use the Chandra data we obtained for the
previous project to study the nature of the low-energy flux variations recently
detected at hard X-ray energies.Comment: 6 pages, 5 figures, submitted to the proceedings of the conference:
  "The Extreme and Variable High Energy Sky", 19-23 September 2011, Chia Laguna
  (Cagliari) - Ital

Weisskopf, Martin C.

English

arXiv

The Crab nebula and its associated pulsar have been the target of thousands of observations at all wavelengths over the years. Nevertheless, the system continues to provide new surprises and observational insights into its physical mechanisms. We shall discuss a number of new results we have obtained through Chandra observations. Results include highly detailed pulse-phase spectroscopy which poses challenges to our understanding of pulsar emission mechanisms, a new and precise look at the pulsar geometry, the results of a search for the site of the recently-discovered gamma-ray flares, and a study of the spatial and temporal variation(s) of the southern jet

NASA Technical Reports Server

SCIENCE WORKSHOPE: "The Extreme and Variable High Energy Sky”   
WEBSITE:  http://www.iasf-roma.inaf.it/extremesky_chia2011/ 
 
Title: New Results from an old Friend: The Crab nebula and its Pulsar 
Author:  Martin C. Weisskopf 
 
Abstract: The Crab nebula and its associated pulsar have been the target of thousands of 
observations at all wavelengths over the years. Nevertheless, the system continues to provide 
new surprises and observational insights into its physical mechanisms. We shall discuss a 
number of new results we have obtained through Chandra observations. Results include highly 
detailed pulse-phase spectroscopy which poses challenges to our understanding of pulsar 
emission mechanisms, a new and precise look at the pulsar geometry, the results of a search for 
the site of the recently-discovered gamma-ray flares, and a study of the spatial and temporal 
variation(s) of the southern jet. 
 
https://ntrs.nasa.gov/search.jsp?R=20120002831 2019-08-30T19:15:16+00:00Z
  
 
 
© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it 
 
New Results from an old Friend: The Crab Nebula 
and its Pulsar 
Martin C. Weisskopf 1  
NASA/ Marshall Space Flight Center 
ZP 12 
320 Sparkman Drive 
Huntsville, AL 35801 
USA 
E-mail: martin.c.weisskopf@nasa.gov 
We summarize here the results, most of which are preliminary, of a number of recent 
observations of the Crab nebula system with the Chandra X-Ray Observatory. We discuss four 
different topics: (1) The motion on long (> 1yr) time scales of the southern jet. (2) The 
discovery that pulsar is not at the center of the projected ring on the sky and that the ring may 
well lie on the axis of symmetry but appears to be displaced at a latitude of about 5°  (Note that 
this deprojection is by no means unique.) (3) The results and puzzling implications of the 
Chandra phase-resolved spectroscopy of the pulsar when compared to observations of pulse-
phase variations at similar and dissimilar measures in other regions of the spectrum. (4) The 
search for the X-ray location of the site of the recently-discovered γ-ray flaring. We also 
comment briefly on our plan to use the Chandra data we obtained for the previous project to 
study the nature of the low-energy flux variations recently detected at hard X-Ray energies. 
 
 
 
 
 
 
 
 
 
 
 
 
The Extreme and Variable High Energy Sky, 
Chia Laguna (Cagliari), Italy 
September 19-23, 2011
                                                 
1  Speaker 
Short title Speaker Name 
 
     2 
 
 
 
1. Introduction 
Over the years the Crab Nebula and its pulsar have provided observers a remarkable 
astrophysics laboratory which continues to produce new puzzles and encourage insights as 
observational accuracy has improved and wavelength coverage has expanded. For a complete 
historical perspective up to the time of its publication, the reader is referred to the review by 
Hester1. Subsequently, there have been a number of new developments. Here we discuss four 
recent results from the Chandra X-Ray Observatory. Since we have a number of different 
collaborators involved in each of these projects, each section begins with an acknowledgement 
to those colleagues.  
2. Motion of the Southern Jet 
My collaborators on this project are A.F. Tennant, S.L. O’Dell, D. A. Swartz, G.G. Pavlov, and 
K. Mori . 
 
Figure 1 shows two Chandra images of the Crab system taken ~8 years apart and demonstrates 
that the jet to the south-east has evolved with time. For example, the tail of the jet points in a 
somewhat different direction and the number of “kinks” (suggestive of a corkscrew) has 
changed. Presumeably, there is similar behavior in the northern jet, but this is more difficult to 
discern as that jet is masked by the radiation from the synchrotron nebula. We are currently 
completing a detailed study of all of the relevant Chandra observations of the southern jet to 
quantitiatively characterize the time evolution of changes in the mean position, the width, and 
the spectrum of the jet as as a function of distance from the pular and of time.  
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 1. Chandra Low Energy Transmission Grating zero order images of the Crab 
system taken in Feb 2000 (left) and Jan 2008 (right). North is up and east to the left. The 
line that extends mostly N-S is the grating-dispersed spectrum of the pulsar. The 
orthogonal line is the crossed-dispersed spectrum produced by support bars in the LETG 
grating  facets. 
Short title Speaker Name 
 
     3 
 
 
3.The Pulsar is not necessarily at the Center of the Inner Ring 
My collaborators on this project are R.F. Elsner, J. Kolodziejczjak, S.L. O’Dell, and A.F. 
Tennant.  
 
The Chandra images resolve the detailed structure of its “inner ring”, most likely a termination 
shock where pulsar-accelerated relativistic particles begin to emit X-rays 2,3. We have analyzed 
such images and we find that the center of the apparent ellipse (Fig 2) —presumably a circular 
ring in projection—lies ~ 0.9″ (10 light-days at 2 kpc) from the pulsar’s image, at a position 
angle of ~300° (East of North). This analysis also measures properties of the ellipse: The 
position angle of the semi-major axis is ~210° (East of North); and the aspect ratio 0.49. 
 
A consequence of this discovery, i.e. that the pulsar is not symmetrical with respect to the “inner 
ring”, is that that in a simple—albeit, not unique—de-projection of the observed geometry, a 
circular ring is centered on the axis of symmetry of the pulsar wind nebula. In this deprojection, 
however, although the ring is on the axis of symmetry, it is not on the equatorial plane of the 
pulsar, but rather lies at +4.5° latitude in pulsar-centered coordinates (Fig 2.). 
 
Figure 2. Left: Image of Chandra LETG/HRC-S zeroth-order image of the Crab Nebula 
(ObsIDs 758 and 759) over a 40″¯40″ field centered on the pulsar. The oval line marks 
the peak brightness of the ellipse, which is obviously not centered on the pulsar. North is 
along the +Y axis; East, along the –X axis. The units are HRC pixels (0.132”).  Right:  
Possible de-projected geometry. A ring of radius R is oriented to, and centered on, the 
symmetry axis z of the pulsar wind nebula. However, the ring lies in a plane displaced an 
axial distance ξ from the pulsar at “P”, corresponding to a latitude λ = tan-1(ξ/R). The 
projection z⊥ of the symmetry axis onto the sky lies at an angle ψ East of North. The 
direction to the observer “O” is at an angle θ from the pulsar’s axis of symmetry z. 
Short title Speaker Name 
 
     4 
 
 
4. Phase resolved spectroscopy of the pulsar at all phases 
My collaborators on this project are A. F. Tennant , D.G. Yakovlev, A. Harding, V.E. Zavlin, 
S.L. O’Dell,  R.F. Elsner, and W. Becker. 
 
In 20044 we presented the first Chandra-LETGS phase-resolved X-ray spectroscopy of the Crab 
Pulsar. More recently we made a far more efficient observation, the details of which are about to 
be published.5 We show in Fig 3 one of the most interesting consequences of this research. Fig 3 
compares the variation of the X-ray spectral index measured with Chandra to a number of other 
different parameters measured in very different spectral ranges as a function of pulse phase. The 
fact that the soft X-ray and hard gamma-ray spectra 6 are part of two seemingly different 
radiating components, and thus most likely have different emission mechanisms, raises the 
question of why their spectral index variation with phase should be so similar. They must share 
a common property, such as the same radiating particles or the same locations in the 
magnetosphere. This in itself poses a challenge to the theorists. 
 
We also note the change of behavior in the X-ray power law index just before the rise of the 
light curve to primary pulse maximum (phases 0.83 to 0.95), the hint of a similar behavior in the 
γ-ray spectral index variation 6, and the abrupt change of optical polarization and position angle 
7 in this same phase interval. These would appear to be correlated phenomena and thus present a 
further challenge to theorists.  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 3. Upper: the measured variation of the X-Ray powerlaw index. The next 
panel is the X-ray light curve (background not subtracted). Middle: the variation 
of the gamma-ray spectral index 6 Bottom: the two panels show the variation of the 
optical degree of polarization and position angle 7. One pulse cycle is repeated 
twice for clarity.  
Short title Speaker Name 
 
     5 
 
 
5. Search for the origin of the γ-ray flares 
The collaborators on this project, led by A.F. Tennant, are M. Tavani, E. Costa and other 
members of the AGILE team, R. Buehler, R. Blandford and other members of the Fermi-Lat 
Team, D. Horns, C. Ferrigno and other members of the Crab Team. 
 
Another new finding in recent times was the discovery 8 ,9 using the AGILE and Fermi satellites, 
of γ-ray flaring from the Crab Nebula. Flaring was detected at energies from 100 MeV up to ~1 
GeV at high significance. A subsequent episode in April of 2011 was even more intense and 
reached record levels. The γ-ray observations showed that the flaring emission was not pulsed. 
Since radio pulsar observations did not find any evidence for a glitch10, the flaring must have 
arisen from the Nebula. 
 
Chandra was used to look at the observations prior to, and during, the flaring in April (Fig 4.) in 
an attempt to localize the region of the nebula wherein the flare arose. 
 
 
 
 
 
 
 
Figure 4. Fermi-LAT light curve of the flaring emission of the Crab Nebula in 
2011, March and April 11.  The vertical lines bound the times of four simultaneous 
Chandra observations with their ObsIDs listed at the top of the figure. 
Figure 5 shows a representative Chandra ACIS image. Also shown are a number of light 
curves derived from the Chandra observations that begin in September, 2010, 
subsequent to the announcement of the original discoveries. The closely spaced cluster 
of points in these light curves are from 2011, April. We emphasize that all of the X-ray 
light curves, not only those shown, exhibit some level of variability, and none that may 
be obviously associated with the γ-ray flaring. Thus, there is no clear “smoking gun”. Of 
course, depending on the details of the emission mechanism(s), there is no guarantee 
that any X-Ray signature must precisely coincide with the γ-ray flaring. However, that 
any model that requires a coincidence could/should be suspect. We hasten to add that, 
because of pileup in the CCDs, the pulsar itself and some of the region including and 
Short title Speaker Name 
 
     6 
 
 
very close-by (on the scale of an arcsec or so) cannot be observed. This is why the 
pulsar appears as a hole in Fig. 5. 
6.  
 
 
 
 
 
 
 
 
 
 
 
 
Figure 5. Chandra image of the Cran Nebula with the pulsar appearing as a hole 
due to pileup in the ACIS CCD camera. The four light curves shown are discussed 
in the text. 
 
Finally, we mention the variability of the integrated X-ray emission reported by Wilson-
Hodge et al. 12. We are using the monitoring observations of the Crab that we began in 
late 2010 and which will continue for at least two years to study and characterize the 
soft X-ray variations. 
References 
                                                 
1 Hester, J. J. 2008, ARA&A, 46, 127 
2 Rees, M. J., & Gunn, J. E. 1974, MNRAS, 167, 1 
3  Kennel, C. F., & Coroniti, F. V. 1984, ApJ, 283, 694 
4  Weisskopf, M.C. et al. 2004, ApJ, 605, 360  
5  Weisskopf, M.C. et al. arXiv:1106.3270 
6  Abdo, A. A. et al. 2010, ApJ, 708, 1254 
7 Slowikowska, A., Kanbach, G., Kramer, M. & Stefanescu, A. 2008, in HIGH TIME 
RESOLUTION ASTROPHYSICS: The Universe at Sub-Second Timescales. AIP Conference 
Proceedings, 984, 51 
8 Tavani, M. et al. 2011, Science, 331, 736 
9 Abdo, A.A. et al. 2011, Science, 331, 739 
10  Espinoza, C.M. et al. 2010, ATEL 2889. 
11 Courtsey R. Buehler 
12  Wilson-Hodge, C. et al. 2011, ApJ, 727, L40 


New Results from an old Friend: The Crab Nebula and its Pulsar

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