Discovery of Emission Lines in the
X-ray Spectrum of the Perseus Cluster
J. L. Culhane
University College London
Mullard Space Science Laboratory
Summary
•
Nature of the Solar Corona and properties of its high temperature plasma are briefly outlined
•
Ariel-I result (1962) on the temperature of the high temperature plasma produced during solar
flares is presented
•
The importance of Bragg crystal spectroscopy with SMM (1980-89) and Yohkoh (1991- 2001)
is emphasised
- high resolution Fe XXV and Fe XXVI spectra that illustrate the many possible constituents of the
Perseus emission feature are shown
- for 7 keV photons, (Bragg) ~ 4000 while E/E (Proportional Counter) ~ 7!
•
After a summary of the Uhuru (1971) discovery of extended X-ray emission sources in
clusters of galaxies, the Ariel V data on the 7 keV Perseus emission feature is presented
- statistical significance of the feature is assessed and the measured line-to-continuum ratio is given
- physical significance of the result as assessed at the time of the observation is discussed
•
Confirmation and follow-up work is described and a dark matter-related result is outlined
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Understanding of the Solar Corona Prior to the Space Age
• Coronal brightness studies
- during total solar eclipses
- using coronagraphs
• White light emission is from
scattered photospheric radiation
– absence of Fraunhofer lines due to
high T e value in corona
• Identification of forbidden
Fe XIV (green) and Fe X (red)
emission lines by Edlén (1942)
established the existence of a
high Te corona
(Dust)
(Electrons)
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Solar Corona - US/UK Ariel I Spacecraft
•
Ariel I was launched from Cape
Canaveral on 10th April, 1962
– the world’s first international
collaborative Space Science mission
•
Solar coronal plasma was known to have
Te ~ 1 MK from presence of Fe X and
Fe XIV visible emission lines
•
Early US V2 rocket flights had registered
X-rays with photographic film behind thin
Be filters and with geiger counters
•
UCL/Leicester proportional counter
spectrometer gave the first direct
evidence of high temperature coronal
plasma in solar flares
(Culhane, Pounds, Willmore, Sanford, 1964)
Proportional
Counter
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Ariel I Solar Flare X-ray Spectrum: 3-May-1962
•
Te values derived
from thermal
bremsstrahlung fitting
- no line emission used
in fitting process
•
Quiet Sun plasma
at Te ~ 2 MK
•
Flare plasma at
Te~ 10 MK
- possible non-thermal
contribution discussed
•
Flare development
Quiet Sun - post-flare spectrum
Essential to resolve
emission lines for
accurate estimation
of plasma properties
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Bragg Crystals for High Resolution X-ray Spectroscopy
•
Crystal spectrometers follow Bragg’s law: nλ = 2d Sin 
•
SMM (1980) and Yohkoh (1991) curved
spectrometers enabled major progress in
flare plasma observations
- is varied by crystal rotation or continuous curvature
- bent crystals acquire simultaneous line data
over a defined λ range; good for flare studies
•
Yohkoh BCS was optimised for flares and not
collimated
~ x10 greater sensitivity than the SMM instrument
•
Four Ge crystals cover emission lines from highly
ionised Fe, Ca and S
- diffracted radiation registered by 1-d position sensitive
proportional counters
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Yohkoh BCS Ions and Wavelengths
•
Table shows the ions whose emission lines are registered by the spectrometer
Photon
Energy
7.1 keV
6.7 keV
3.9 keV
2.5 keV
•
Fe II K Fe XXV and Fe XXVI lines are possible contributors to the Perseus emission feature
- Fe XXV and Fe XXVI lines emitted by high temperature coronal solar flare plasma
- Fe II K line excited by high energy flare electron impacts in photosphere/chromosphere
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Fe Emission Lines that Contribute to the Perseus Feature
•
Fitting the Perseus X-ray spectrum with a simple thermal continuum gave Te = 66 MK
- Fe XXV and Fe XXVI resonance lines and associated satellite lines should be emitted by plasma at
this temperature
•
Spectra shown are typical for solar flare thermal plasma with Te ~ 20 MK – 30 MK
SMM BCS
Te ~ 20 MK
•
Yohkoh BCS
1991, December 16
04:56 – 04:59 UT
Te ~ 30 MK
Detection of an emission feature at ~ 7 keV (1.77 Å) would therefore strongly suggest the
presence of high temperature plasma in the Perseus intracluster medium
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X-ray Astronomy with Ariel V
•
Ariel V was launched from the Indian Ocean
San Marco platform on 15th October, 1974
- spin-stabilised spacecraft
- orbit had 2.80 inclination for low background
- mission ended on 14th March 1980
• Instruments aligned with spin axis were
- rotation modulation collimator (0.3 – 30 keV)
- proportional counter spectrometer (1.3 – 25 keV)
- Bragg crystal polarimeter/spectrometer
(Si XIV/XIII, S K, SXV/XVI, Fe K, Fe XXV/VI)
- X-ray scintillation detector
•
Instruments viewing 900 to the spin axis were
- two sky survey proportional counters (1.5 - 20 keV)
- all-sky X-ray transient monitor (3 – 6 keV)
• The x-ray emission line discovery in the Perseus Cluster spectrum was made by the on-axis
proportional counter spectrometer
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Extended X-ray Sources and Clusters of Galaxies
First detected by Gursky et al., 1971 in the Uhuru X-ray sky survey
•
Extended (~ 45 arc min) source found associated with the Coma cluster
•
Many similar extended cluster X-ray sources e.g. Perseus, Virgo, were later discovered
•
Uhuru data could not distinguish between power law and thermal bremsstrahlung spectra
•
If the latter mechanism is chosen,Te ~ 73 MK and mass of emitting gas is ~ 3.1013 Mʘ
•
Gas mass is ~ 1% of that required to stabilize the cluster
Two possible mechanisms were proposed
•
Inverse Compton radiation from 3 K microwave background photons interacting with a
relativistic electron population in the cluster (Brecher and Burbidge, 1972)
•
Thermal bremsstrahlung from a hot isothermal gas sphere (Lea et al., 1973)
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X-ray Emission Line Detection in the Perseus Cluster Spectrum
•
Following discovery of extended cluster X-ray emission, emphasis placed on identifying the
emission mechanism
•
Ariel V on-axis proportional counter spectrometer well placed for detailed X-ray spectral studies
- covered 1.3 – 16 keV with 32-channel analyser with E/E ~ 7 at E = 6 keV (4000 for Bragg)
- observed Perseus from 05-SEP-1975 to 07-SEP-1975 for ~ 44 hr
- detector operated in high and low gain modes with data from each mode fitted separately
•
Spectrum plot shows points from both gain settings
•
Emission feature present at E ~7 keV (Fe XXVI)
•
Solid line shows continuous spectrum for Gull and
Northover (GN; 1975) adiabatic gas sphere model with
central temperature T(0) = 32 keV and T∞ = - 4 keV
- assumed ISM column density NH = 5.1020 atom cm-2
consistent with 21 cm radio data for Perseus Cluster
• High and low gain spectral parameters differ
- suggests range of Te values for thermal plasma
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Significance of X-ray Emission Line Detection
Statistical Significance and Measurement
•
Deviation of flux in each channel from best fit
thermal continuum plotted for 3 – 12 keV range
- continuum I (E) = 0.205 exp (-E/6) g E-1 where g is
the Gaunt factor
•
For the emission feature
- high gain data show a 5.4excess above continuum
- low gain data show a 6excess
• Both measurements average to a 7 keV feature
intensity of 3.35 ± 0.4 x 10-3 photons cm-2 s-1at 7.5
•
Using the best fit GN adiabatic gas sphere model
and integrating the 1.5 – 6 keV continuum flux
gives a measured feature-to-continuum ratio
of 0.02
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Significance of X-ray Emission Line Detection (Cont.)
Physical Significance
•
Detection of an emission feature at 7 keV gives good evidence for presence of hot plasma
- suggests exclusion of the Inverse Compton hypothesis
• Best fitting continuum temperature is 66 MK and for this value the 7 keV emission feature
should include contributions from
- resonance, intersystem and forbidden transitions of He-like Fe XXV
- several satellites to the resonance line
- Lyman- line of H-like Fe XXVI
• Based on the sum of all these contributions, calculated line-to-continuum ratio for plasma of
Te ~ 60 – 70 MK should be ~ 0.07 for a cosmic Fe abundance N(Fe)/N(H) = 5 x 10-5
- observed value of 0.02 suggests Fe under-abundant by a factor 4 for the Perseus Cluster
- GN adiabatic gas sphere model suggests hot plasma should be mainly intergalactic material that has
fallen into the cluster potential well hence significant heavy element presence is hard to explain
• Active galaxies in the cluster could enrich gas in their neighbourhoods with heavy elements
- Fabian et al., 1974 showed that ~ 10 – 20% of total cluster flux is associated with NGC 1275
- if this were the only enriched material source, Fe abundance near NGC 1275 could be 2.5 times
the cosmic value
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Original Paper on Perseus Cluster X-rays
•
Submitted as a Letter to Monthly Notices RAS
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Follow-up to the Perseus Cluster X-ray Feature Discovery
• Confirmation of the result quickly followed
• Serlemitsos et al (1977) using the proportional counter spectrometer on OSO-8 confirmed the
presence of an emission feature in Perseus
- found similar results for the Virgo and Coma clusters
•
HEAO-I study of 30 cluster sources (Mushotzky, 1984) showed presence of Fe with
abundance ~ 0.3 – 0.5 times the solar value
•
ASCA, launched 1993, was first to detect intra-cluster line emission from elements in addition
to Fe, eg. O, Ne, Mg, Si, S, Ar, Ca and Ni
•
ASCA showed Fe abundance peak at Centaurus cluster cooling core (Fukazawa et al.,1994)
- difference in metallicity gradients for clusters with and without cooling cores (Allen & Fabian, 1998)
•
XMM and Chandra observations leading to continuing advances in the field are still in progress
•
Where will the 3.6 keV emission line in 73 stacked XMM spectra lead (Bulbul et al., 2014) ???
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New X-ray Emission Feature in Cluster Spectra
•
Bulbul et al., 2014 have reported a weak emission feature at E ~ 3.55 – 3.57 kev from the
stacked XMM spectra of 73 clusters
- paper states that there are no atomic transitions of this energy in thermal plasma
- Perseus spectrum from Chandra deviates from the rest possibly due to presence of an
anomalously bright Ar XVII line at E = 3.62 keV whose emissivity is 30 times the
expected hot plasma value
•
Weak feature could be due to decay of sterile neutrinos – a possible dark matter candidate
XMM stacked
Spectrum result
•
Astro-H 1 Ms
Perseus
simulation
Perseus Cluster spectral studies still strongly in play!
- further progress awaits Astro-H launch in 2016
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