The solid state spectrometer on the Einstein Observatory and the GSFC cosmic X-ray spectrometer on OSO-8 observed the X-ray spectrum of SN1006. The data can be well-represented by a power-law model with alpha = 1.2, similar to the spectrum of the Crab nebula. This is in contrast to the radio and X-ray maps of SN1006 which show a shell structure more typical of SNR with thermal X-ray emission. The X-ray spectrum is suggestive of nonthermal synchrotron emission, raising the possibility that the remnant of SN1006 contains a source of relativistic electrons

Becker, R. H.

Boldt, E. A.

Holt, S. S.

Serlemitsos, P. J.

Szymkowiak, A. E.

English

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Technical Mom 80689
Is the Remnant of
SN1006 Crab-Like?
(VASA-TR-80689) IS THE &ERRANT OF SE1006	 080-26251
CRAB-LIRE. (1ASA) 14 p HC A02/5F •01
CSCL 038
Snclas
G3/93 22937
R. H. Becker, A. E. Szymkowiak,
E. A. Boldt, S. S. Holt and P. J. Sedemitsos
MAY 1980
Nat'onal Aeronautics and
Space Administration
	
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62i 2
Goddard Space Flight Center	 ti .^ ^vGreenbelt. Maryland 20771
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IS THE SIT (W 011086 C -LIKE?
R.N. Recker l, A.E. Szynkoviak l , E.A. soldt,
S.S. Holt and P.J. Serlevitsos
Laboratory for High Energy Astrophysics
WLSA/Goddard Space Flight Center
Greenbelt, Maryland 20771
ABSTRACT
The solid state spectrometer on the Einstein Observatory and
the GSFC cosmic X-ray spectrometer on OSO-8 have observed the
X-ray spectrum of SN1006. The data can be well-represented by a
power-law model with a = 1.2, similar to the spectrum of the Crab
nebula. This is in contrast to the radio and X-ray maps of
SH1006 which show a shell structure more typical of SNR with
thermal X-ray emission. The X-ray spectrum is suggestive of non-
thermal synchrotron emission, raising the possibility that the
remnant of SN1006 contains a source of relativistic electrons.
lAlso dept. of Physics 6 Astronomy, Univ. of Maryland
- 1 -
I . IM?CtIU^!
Radio astronomers hmm traditionally divided supernova
remnants (SMR) into two catagories, shell source and Crab-like
sources. The shell sources are typified by Tycho's Sift and the
Cygnus Loop, objects with steep spectral indices (a i .5) and
shell-like spatial distributions. The prototype Crat+-like (i.e.
pulsar-driven) source is the Crab nebula with identifying radio
characteristics of a filled-center spatial distribution and a
flat spectrum (a '-M.2).  In both cases, the radio emission is
synchrotron in natur e but for the shell sources the magnetic
field and the relativistic electrons are most likely swept-up
from the ISM ( van der Laan 1962a,b) while the relativistic
electrons in Crab-like sources derive from a central pulsar, as
suggested by the spatial distribution of the radio emission.
X-ray spectra of shell sources have been observed to be thermal
in origin as evidenced by profuse line emission (Becker et al.
1979,1980a , b) while the Crab nebula X-ray spectrum is a
featureless power-law with a = 1.15.
The solid state spectrometer (SSS) on the Einstein
Observatory engaged in a program of surveying the X-ray spectra
of SNR, from which results for Cas A, Tycho, and Kepler (Becker
et al. 1979, 1980a , b) have already been reported. These three
remnants can al l be catagorized as shell sources based on their
radio properties and all three have X-ray spectra dominated by
line emission. The remnant of SN1006 is approximately twice as
old as the above sources, and on the basis of radio observations
should also be considered a shell source (Stephenson et al.
- 2 -
1977). This paper presents the X-ray spectrum of SM1006, which
leads to a reconsideration of the traditional distinctions
between shell and Crab-like SNR.
11.  ORSERVATIUM.S
The SSS experiment has been described in detail by Joyce et
al. (1978) and the data analysis procedures are detailed by Holt
et al. (1979). It is important to rote that the SSS field of
view has a 6 arc min diameter, considerably smaller then
SN1006. To cagy out these observations, the SSS was pointed at
4 different fields centered at RA(1950) and 6 (1950) of (224071,
-41092), (225017, -41058), (225000, -41075), and (225019,
-41067), thus sampling both the rim (3 of the 4 positions) and
the center of the source. However, the data from the central
region is of relatively poor quality and will not be discusser:.
The composite spectrum for the rim of SN1006 is shown in
Figure 1. (This represents only a fraction of the total emission
due to the small field of view) . The data can be well-
represented by either a power-law with a spectral index of 1.2 t
0.2 with a equivalent hydrogen column density N Ei of 9 t 3 x 1020
cm-2 or a featureless thermal bremsstrahlung model with kT = 2.7
t 0.3 keV and NEj = 6 t 2 x 10 20 cm-2 (errors are 90% confidence
limits). Both models are improved by the inclusion of an
emission feature at 1.85 keV with an equivalent width of 50 eV,
but the line is significant at only the 2a level.
The remnant of SN1006 was also observed by the GSFC Cosmic
X-ray Spectrometer (CXS) xenon detector (see Pravdo et al. 1976
for description) on OSO-8 for six days. The data were consistent
- 3 -
with either simple thermal bremestrahlung emission of kT - 6 * 2
keV or a power law with spectral index a - 1.2 * .S. In either
case, there was no evidence for Fe line emission at 6.7 keV and
we can place a 901 confidence upper limit to the equivalent width
of 600 eV for such a feature. (This can be compared to the Cas A
X-ray spectrum which shows an Fe line with an equivalent width of-
1270 eV (Pravdo et al. 1976)). It would appear, therefore,
that the same power law can be reconciled with both the SSS and
CXS data, while a single bremsstrahlung cannot.
III. DISCUSSION
The spectra of SN1006 measured by the SSS and the CXS are in
excellent agreement with previous observations (see Toor 1980 and
references therein). Although Toor (1980) noted the consistency
of the spectrum with a power-law like that of the Crab nebula,
his results could not rule out a two-component thermal model or
the presence of emission lines. The present data place limits on
the line emission of Si XIII and Fe XXIV which are substantially
below the line strengths observed in young shell remnants.
Although the spectrum of SN1006 may be consistent with a single
high temperature thermal component (although the SSS and the CXS
disagree in detail) it lacks a low-temperature ( - 500 eV)
thermal component typically found in other remnants. The
presence of a 500 eV component would be clearly discernible with
the high spectral resolution of the SSS. For all practical
purposes, the X-ray spectra of SN1006 and the Crab nebula are
indistinguishable above 0.5 keV, differing only by the amount of
absorption due to interstellar material.
- 4 -
Itob (1979) bas questioned the reality of equilibrium low
temperature X-ray components in SNR. He points ouc that if the
hot plasma is not in collisional equilibrium, emission lines
typical of a 0.5 keV plasma could be present even in the
absence of such a plasma. Therefore, it follows that the absence
of such lines in SN1006 may be indicative of the extent of
departure from equilibrium rather then the absence or presence of
a true low temperature component.
It is important to note that the imaging proportional
counter (IPC) on the Einstein Observatory has detected, in
addition to a hard (E > 0.5 keV) component distributed in a
partial shell, a very soft (E < .5 keV) component distributed
fairly uniformly across the remnant (Pye et al. 1980). This soft
component_ cannot be an artifact of non-equilibrium conditions
because it does not originate in the same volume of space as the
harder emission. If this soft component is an analog to the soft
components found in other SNR, its unusually low temperature
could explain the absence of strong lines below 4 keV. Even so,
some line emission should be associated with the high temperature
component, particularly a 6.7 keV line from ionized Fe. The
observations presented here show that if such a line is present,
it is significantly weaker than those found in other young
remnants.
The observations of SN1006 by the SSS differ from those of
other young remnants because the SN1006 observations isolated the
bright rim from the interior while the other remnants were
comparable in size to the field of view. This raises the
- 5 -
! -
possibility that the bright rim of such remnants might also be
lacking in line emission. This seems very unlikely in so far as
such a large percentage of the total flux of Cas A, Tycho, and
Kepler are in line emission that some of line flux must originate
in the bright rims.
The arguments against a Crab-like description of SN1006 are
strong. Firstly, the radio and hard X-ray emission are
distributed in a shell. Secondly, the radio spectrum is steep,
typical of shell remnants. Thirdly, there is no direct evidence
for the presence of a central pulsar (Fesen, Kirshner, and
Winkler MIN*; Helfand 1980). And, lastly, the optical filaments
observed in SN1006 are similar to those seen in Tycho's SNR
(Schweizer and Lasker 1978) suggesting the presence of an
expanding shock. If the SN1006 X-ray emission is attributed to
thermal processes, either the composition or the state of the
responsible plasma must be quite different from that of
comparable objects.
On the other hand, the X-ray spectrum of SN1006 appears
quite similar to that of the Crab. In particular, the absence of
significant line emission above 0.5 keV is in sharp contrast to
the X-ray spectrum of any other young shell remnant. We raise
the possibility that the X-ray emission is a continuation of the
radio synchrotron spectrum. If so, than the relativistic
electrons emitting the radiation must originate in the remnant,
and not from the ISM. The close agreement between the radio and
X-ray spatial distribution (Pye et al. 1980) is consistent with
such an interpretation. The two spectra (radio and X-ray) are
- 6 -
consistent if there is a break cprresponding to an increase in a
of 0.5 at 4 x 1013
 Ha. If the relativistic electrons have been
ejected continuously since an earlier time ( - 500 yrs), the
break energy would imply a magnetic field of - 10 -4
 G. fairly
typical for a SNR.
In summary, the X-ray spectrum of SH1006 between 0.5 and 4.5
keV is consistent with that of the Crab nebula. However, in most
other aspects, SN1006 appears to be a shell remnant. In order to
determine if the hard X-ray spectrum of SN1006 is thermal or
synchrotron in nature, a more sensitive search for X-ray Fe line
emission at 6.7 keV should be made. A search for polarized X-ray
emission would also differentiate between these two possible
mechanisms. If the X-ray emission from SN1006 is found to be
non-thermal, it would provide strong circumstantial evidence for
the existence of a continuing source of relativistic electrons
such as a central neutron star. In addition, the classical
distinctions between Crab-like and shell SNR would have to be
reevaluated.
7 -
REFERENCES
Becker, R.H., Holt, S.S., Smith, B.W., White, N.E., Boldt, E.A.,
Mushotzky, R.F., and Serlemitsos, P.J. 1979, Ap. J. (Letters)
234, L73.
Becker, R.H., Holt,S.S., Smith, B.W., White, N.E., Boldt, E.A.,
Mushotzky, R.F., and Serlemitsos, P.J. 1980a, Ap. J. (Letters)
235, L5.
Becker, R.H., Boldt, E.A., Holt, S.S., Serlemitsos, P.J., and
White, N.E. 1980b, Ap. J. (Letters) in press.
Feser, R.A., Kirshner, R.P., and Winkler, P.F. 1980, preprint.
Helfand, D. 1980, invited talk at AAS-HEAD meeting, Cambridge,
Mass.
Holt, S.S., White, N.E., Becker, R.H., Boldt, E.A., Mushotzky,
R.F., Serlemitsos, P.J., and Smith, B.W. 1979, Ap. J. (Letters)
234, L65.
Itoh, H. 1979, Publ. Astron. Soc. Japan 31, 541.
Joyce, R.M., Becker, R.H., Birsa, F.B., Holt, S.S. and Noordzy,
M.P. 1978, IEEE Trans. Nuc. Sci. 25, 453.
Pravdo, S.H., Becker,R.H., Boldt, E.A., Holt,S.S., Rothschild,
R.E., Serlemitsos, P.J., and Swank, J.H. 1976, Ap. J. (Letters)
206, L41.
Pye, J.P., Pounds, K.A., Rolf,D., Seward, F.D., Smith, A.,
Willinggale, R. 1980, presented at AAS-HEAD meeting, Cambridge,
Mass.	 %
Schweizer, F., and Lasker, B.M. 1978, Ap. J. 226, 167.
Stephenson, F., Clark, D., and Crawford, D. 1977, MNRAS 180, 567.
Toor, A. 1980, Astron. Astrophys., in press.
Van der Laan, H. 1962a, MNRAS 124, 125.
Van der Laan, H. 1962b, MNRAS 124,179.
FIGURE CAPTIONS
Figure 1 - The X-ray spectrum of the bright rim of SN1006 as
observed by the SSS onboard the Einstein Observatory.
Superimposed upon the data is the best fit power-law model (a =
+1.2).
Figure 2 - The X-ray spectrum of SN1006 as observed by the CXS
onboard the OSO-8 satellite.
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Is the remnant of SN1006 crab-like?

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