X-ray properties of galactic supernova remnants [Elektronische Ressource] / vorgelegt von Daniel Schaudel

Schaudel, Samuel Daniel

Sujets

• Physik
• Astronomie

Informations

• Publié par: ludwig-maximilians-universitat_munchen
• Publié le: 01 janvier 2003
• Nombre de lectures: 29
• Langue: English
• Poids de l'ouvrage: 22 Mo

Extrait

X-ray Properties of
Galactic Supernova Remnants
DISSERTATION
der Fakultat fur Physik der Ludwig-Maximilians-Universitat Munc hen
zur Erlangung des Grades
Doktor der Naturwissenschaften
Dr. rer. nat.
vorgelegt von
DANIEL SCHAUDEL
aus Ettenheim
Munc hen, den 18. Februar 20031.Gutachter: Prof. Dr. Joachim Trump er
2.Gutachter: Prof. Dr. Ralf Bender
Tag der mundlic hen Prufung: 8. Juli 2003Summary
Galactic supernovae (SNe) are rare events, believed to occur at intervals of 30-50
years. However, in the past 2000 years, only seven Galactic SNe have been observed:
SN 185 (RCW86), SN 386 (G11.2-0.3), SN 1006, SN 1181 (3C58), Crab SN, Tycho SN
and Kepler SN. Most Galactic SNe go unobserved because of visible-band extinction
by interstellar dust. Due to an average lifetime of supernova remnants (SNRs) of a
few 10,000 to 100,000 years, about 15000 SNRs are expected in our Galaxy, which
exceeds the number of identi ed radio SNRs by almost a factor 70. These identi ed
Galactic SNRs comprise an incomplete sample of SNR population due to various se-
lection e ects. ROSAT performed the rst all-sky survey (RASS) with an imaging
X-ray telescope, providing another window for searching for SNRs.
Performing a search for extended X-ray sources in the RASS database, 373 objects
were identi ed as SNR candidates in recent years (Busser 1998). One of the main
objectives of this work was to perform an identi cation campaign of these Galactic
SNR candidates. The low exposure ( a few hundred seconds) and spatial resolution
00
( 96 ) of the RASS data did not, in most cases, allow for a quantitive analysis of
the SNR candidates in order to identify them by their X-ray morphology and spectral
source properties. A few candidates from the sample were additionally located in the
eld-of-view of pointed ROSAT HRI and PSPC as well as Einstein and ASCA obser-
vations. The spatial resolution of the pointed ROSAT and observations and
the spectral resolution capability of ROSAT PSPC, Einstein and ASCA allowed for
a more detailed analysis and an identi cation of the sources in most cases. Studying
their X-ray and radio morphology and correlating the results with radio databases like
the NRAO/VLA Sky Survey (NVSS), Parkes-MIT-NRAO (PMN) survey, Molonglo
and E elsb erg Galactic Plane Surveys and the optical DSS2red as well as with SIM-
BAD and NED allowed for discrimination between extragalactic background objects
and SNR candidates, leaving 215 targets for subsequent identi cation. Fifty-nine of
the 373 candidates (16%) turned out to likely be galaxies or cluster of galaxies, and
99 targets (27%) were found to be spurious background features. The most promising
candidates were subject of follow-up observations in the X-ray and radio band because
of their distinct X-ray and radio morphology.
The SNR candidates 1RXSJ161411.3-630657 and 1RXSJ104047.4-704713 have
been observed in the X-ray band with the Chandra observatory. These data do not
support to further interpret this sources as SNR candidates. 1RXSJ161411.3-630657
has a slight elliptical shape in X-rays and habours a central cD galaxy. The entire
spectrum is best described by the thermal bremsstrahlung model MEKAL which yields
7a temperature of 5.4 10 K, abundances of 0.35 times solar and a redshift
of the hot intracluster gas of 0.048, which is consistent with the measurements
of optical redshifts of two galaxies in the same eld within the errors. Optical data
show many galaxies in the region where the X-ray emission peaks. The other source
1RXSJ104047.4-704713 also has a slight elliptical shape and is centrally brightened
in X-rays. In this case, a thermal bremsstrahlung model is also found to t the data,
17with a temperature of 4.0 times 10 K, abundances of 0.75 solar and a redshift
of the hot intracluster gas of 0.075. Both objects are therefore identi ed as clusters
of galaxies.
The six SNR candidates G6.1-1.3, G38.7-1.4, G39.9-2.8, G75.8+8.1, G80.7+6.8
and G178.2-3.3 were selected for radio-continuum follow-up observations at 6 cm due
to their radio morphology in the E elsb erg Galactic Plane survey. Only G38.7-1.4
is identi ed as a SNR due to its polarized radio emission which has a spectral index
-0.79. The remaining sources are identi ed as a probable Hii region (G6.1-1.3),
part of the large SNR W50 (G39.9-2.8), a spurious radio feature (G75.8+8) and
faint polarized radio sources with steep non-thermal spectral shapes (G80.7+6.8 and
G178.2-3.3).
The candidates G296.7-0.9 and G308.3-1.4 have been observed with the Australia
Telescope Compact Array (ATCA) at 13cm and 20cm. No polarization could be de-
tected from either source, but G296.7-0.9 shows bright extended radio emission with
a spectral index -0.30 at the location of an incomplete X-ray shell that was found
in pointed ROSAT HRI and PSPC data. The X-ray spectrum is well described by a
Raymond-Smith model with a temperature of 0.220.13 keV and a galactic absorp-
22 2tion N =1.4210 cm . Based on these data G296.7-0.9 is identi ed as a GalacticH
SNR. G308.3-1.4 shows two radio arcs with spectral index -0.71 matching well
with a region of extended X-ray emission. But the nature of the X-ray source is still
unclear. G308.3-1.4 is still counted among the most promising SNR candidates.
The following summary gives a brief overview of the results from the re-analysis
campaign of the original 373 RASS candidates:
2 targets can be identi ed as SNRs (G38.7-1.4 and G296.7-0.9)
9 targets are very promising SNR candidates
99 targets emerge as spurious background features
59 targets can be assigned mainly to extragalactic
objects like clusters of galaxies
90 targets have extended X-ray emission but lack typical
shell structure in X-rays/radio
114 targets show poor evidence of typical properties of SNRs and are
located in crowded regions of X-ray and radio point sources.
Rejecting these 114 targets, only 2 candidates are found to be SNRs, leaving still 99
of the original 373 RASS targets as SNR candidates. A simulation of the theoretical
distribution of SNe and their remnants in the Galactic plane was performed by Busser
(1998) using canonical input values. His simulation predicted somewhat more than
200 SNRs to be detected in the RASS. A comparison of the results from the identi -
cation campaign with that of the simulation shows that in deed less SNR candidates
are found in the RASS than is expected from his simulation. If the simulation accu-
rately describes the physics of SNe and SNRs, indicating the distribution of Galactic
2SNRs, the smaller number of Galactic SNRs signify either a lower Galactic SN rate,
a lower explosion energy, higher interstellar density and/or lower ambient density at
the location where the star exploded. However, these results do not allow for the de-
termination of the individual physical parameters which have to be adopted to bring
his simulation in agreement with the results of this work.
The second part of this thesis deals with the spatial and spectral analysis of three
selected Galactic SNRs. They are RCW 103, G21.5-0.9 and G65.3+5.7.
XMM-Newton observation of RCW 103:
RCW 103 is a shell-like SNR with a central compact X-ray source. The thermal
X-ray spectrum, with prominent He-like lines of Neon, Magnesium, Silicon,
Sulphur and the Iron L complex, shows a low temperature component of
0.3 keV and a high component of 0.7 keV, varying slightly across the rem-
nant. A belt of faint X-ray emission across the remnant is found to emerge from
absorption and not from an intrinsic emission mechanism. It is not clear yet if
RCW 103 originated from a SN Ia or a core-collapse SNR, but there are some
indications supporting the core-collapse event. Constant radii of RCW 103 in
di eren t emission lines, as well as the low expansion velocity of light elements like
Magnesium, are found, which are rather expected for core-collapse SNe. Finally,
the existence of the central point source, which is a binary consisting probably
of a neutron star and a low mass companion, is a strong argument for a core-
collapse scenario (Garmire et al., 2001).
Deep optical observations are necessary to detect and characterize the as-
sumed low mass companion of the neutron star.
XMM-Newton observation of G21.5-0.9:
G21.5-0.9 was found to harbour a central compact object, probably a pulsar,
which is embedded in a more extended synchroton nebula. Recent Chandra data
let assume that the faint extended halo further outside corresponds to an outer
shell that was formed from the expanding ejecta (Slane et al. 2000). Data from
a subsequent XMM-Newton observation show that the nebula of G21.5-0.9 is
best described by a power law continuum with an increase in photon spectral
index from values of 1.72 at the center of G21.5-0.9 to 2.43 at the edges of its
halo.
In the case of thermal halo emission, the lack of line emission implies that
the plasma is far from ionization equilibrium. Modelling with the thermal model
8 3
NEI, a low ionization state of n t 310 cm s was found which corresponds toe
3an electron density n < 0.1 cm , assuming a expansion velocity of 10,000 kme
1s and a distance of 5 kpc to G21.5-0.9. This low value of the electron density
is not in agreement with the electron density determined from the observed X-
ray luminosity, which is required for thermal bremsstrahl