Cosmic UV and X-Ray Sources - Max Planck Society · 2003. 3. 12. · Cosmic UV and X-Ray Sources Cornelia Wunderer, MPE. Thermal Emission of a Hot Gas • hot gas of low enough density

Cosmic UV and X-Ray Sources Cornelia Wunderer, MPETUM/MPE/MPA Seminar, 2002/2003

Cosmic Cosmic UV and XUV and X--Ray Ray SourcesSources

What‘s special about UV and X-Ray observations?The Energy RangeEmission Processes

First Discoveries

Selected objects of InterestSupernova Remnants, X-ray Binaries, AGNs, clusters of Galaxies, ....


Why observe Why observe in UV and Xin UV and X--raysrays??• UV

– Some white dwarfs– Stars at the center of planetary nebulae– Transition regions between stellar surfaces and (much hotter) coronae

⇒ study stellar evolution and energy transport in stars– Observations of „big blue bump“ in AGN spectra would be interesting– Some elements have energy transitions in the UV but are not

observeable in the optical (e.g. most sensitive observations of Si and C in stars is via UV)

• X-rays– Matter under strong gravity (inner accretion disk around a black hole) – Hot Universe– „dawn of the universe“ (strongly redshifted galaxies)


The Universe The Universe in Xin X--rays rays –– Some ExamplesSome ExamplesSu

n –

coro

nal a

ctiv

ity moon & x-ray background comets Intense SF

SNR Active Galaxies Matter under strong gravity


The Energy The Energy RangeRange

• „UV“:– 10 nm – 300 nm (10 nm – 91.2 nm strongly absorbed by H;

⇒ poorly explored „Extreme UltraViolet“)– 3·1016 Hz – 1·1015 Hz– 125 eV – 4 eV

UV radiation is strongly attenuated by gas and dust of the ISM, making even distant stars in our own galaxy inaccessible to UV observation.

• „X-Rays“:– 0.1 keV – 10 keV („hard X-rays“ up to 500 keV,

depending on definition)– 12.5 nm – 0.125 nm– 2.4·1016 Hz – 2.4·1018 Hz


Emission Emission ProcessesProcesses

• Thermal– Thermal emission of a hot gas– Blackbody spectra

• Black body of 1·104 K has peak emission at 4 eV• Black body of 2·107 K has peak emission at 10 keV

• Non-Thermal– Synchrotron– Bremsstrahlung– Inverse Compton


Thermal Emission of a Hot GasThermal Emission of a Hot Gas• hot gas of low enough density to be

transparent to its own radiation• above 105K, atoms are ionized ⇒ gas

consists of ions and electrons; thermal energy shared among them

• when an electron passes close to an ion, its trajectory is changed; the acceleration causes the electron to emit bremsstrahlung

• electrons in thermal equilibrium have well-determined (Maxwellian) velocity distribution ⇒ resulting radiation is continuum with characteristic shape determined by the temperature

• also: X-ray line emission

Intensity I at energy E:

where k is Boltzmann‘s constant and G a slowly varying function (increasing with decreasing E)

EI( kTEie ekTnnZTEGAT /2/12 )(),(), −−⋅⋅⋅⋅⋅=


Blackbody Radiation Blackbody Radiation

• Planck‘s law:

• If the surface temperature of a neutron star is ~100 000 K or higher, it is expected to emit blackbody radiation with photons in the X-ray range

[ ] 1/223 )1(2),( −−⋅= kTEechETEI

X-ray


Nonthermal Nonthermal Emission Emission -- SynchrotronSynchrotron• a fast electron traversing a magnetic field changes direction

⇒ electron is accelerated ⇒ emission of anisotropicelectromagnetic radiation

• Ephoton depends on Eelectron, B, and |v×B|• in astrophysics, generally

– velectron is isotropically distributed– Eelectron is assumed to have power-law distribution (and thus

Ephoton also follows a power-law distribution)• if the magnetic field is aligned, the observed synchrotron

radiation is polarized• synchrotron X-rays indicate the presence of very energetic

electrons (~ 104 GeV)2

02 cmcmE e ⋅== γ

22v11

c−=γ

2

220 v11

cEcm

−===γ

θ

esyn m

eB⊥= 2max 069.0 γν

Energy of e-:

Opening angle:

Continuum spectrum with peak at:


Nonthermal Nonthermal Emission Emission -- BremsstrahlungBremsstrahlung

• process identical to that responsible for thermal emission from a hot gas

• however the distribution function for electrons differs from the Maxwell-distribution

• a non-thermal distribution of electron (kinetic) energies often follows a power law (e.g. from Fermi acceleration)

• if this is the case, then the resulting photon emission also follows a power law


Nonthermal Nonthermal Emission Emission ––Inverse Compton ScatteringInverse Compton Scattering

Interactions of low-energy photons with free, high-energy electrons (photons receive energy and are upscattered into theX-ray or γ-ray regime)

Examples:• X-rays in jets• corona of accretion disks• Sunyaev-Zeldovich effect

(scattering of 3K-radiation on X-ray gas of galaxy clusters)

( )

( ) ( )

( )( )

++

−+

+

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++

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2

3

2131

221ln

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121

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αα

αα

αααα

αα

σνσ Tc

electron

photon

EE

=α


First First Discoveries Discoveries in UV and Xin UV and X--RaysRays

• Sun revealed as powerful source in UV and X-rays (H. Friedman, ~1950)

• In a search for X-rays from the moon, R. Giacconi (Nobel Prize 2002) et al. discovered X-ray emission from Sco X-1 (rocket flight 1962)

• Subsequently discovered X-ray sources include the Crab nebula, Cyg X-1 („1st x-ray source in cygnus“)


Objects Objects of of InterestInterest• sun & solar system

– sun, moon, planets• our galaxy

– supernova remnants– X-ray binaries– Pulsars– ...

• and beyond– AGNs– clusters of galaxies– the diffuse X-ray background (ROSAT)– Chandra „Deep Fields“– ...


Some Some UV UV ResultsResults((hard hard to to observeobserve, , but interestingbut interesting!)!)

• ISM is hot• scattering of light by interstellar dust

reaches a maximum at 217.5 nm, this is attributed to carbon or carboneous particles in the ISM

• stellar winds are „warm“; Doppler-shifts of UV absorption lines e.g. of highly ionized C, N, and Si allow measurements of the material‘s velocity

• plasma diagnostics of e.g. SNRs and HII regions (together with optical observations)

10000 1000

Abso

rptio

n Aλ

[mag

]

12

6

0

λ [A]

λ-1


Supernova Supernova RemnantsRemnants• Phase 1: free expansion

– mass of original ejecta dominates– interior: low density region

• Phase 2: „blast wave“– expansion slows; reverse shock

propagates inwards– mass of swept-up material large

compared to original ejecta– energy radiated by material small

compared to its internal energy• Phase 3: „radiative phase“

– efficient radiation via UV line emission

– bright optical filaments

Illustration of a SN explosion

X-ray observations of SNRs

Vela Jr and Vela SNRs Cas A SNR


XX--ray ray Emission Emission from from StarsStars• X-ray emission correlated with stellar activity

(coronal loops contain hot plasma which emitsX-rays)

• average X-ray emissivity decreases with age• X-ray luminosity is correlated with the

rotational velocity of a star• X-ray emission is also correlated with the

magnetic field of the star

• Massive (O-type) stars are relatively strong X-ray sources, especially during later evolutionary stages (Wolf-Rayet stars) – their strong and fast stellar winds generate a „bubble“ partially filled with hot, X-ray emitting gas


XX--Ray Ray Binaries Binaries I I ––LMXBs LMXBs ((LowLow--MassMass XX--ray Binariesray Binaries))

• Binary System• Evolved low-mass star fills its Roche

Lobe and looses mass via the L1 point towards a compact object (NS or BH)

• The infalling matter is heated by the viscosity to the point where X-rays are emitted


XX--Ray Ray Binaries Binaries II II ––HMXBsHMXBs (High(High--MassMass XX--ray Binariesray Binaries))

• Binary system• Powerful stellar wind from massive star

(omnidirectional) is accreted by a compact object „ploughing“ through this matter; a shock front is formed


BondiBondi--Hoyle AccretionHoyle Accretion((accretion from accretion from a a moving moving wind wind by by a a compact objectcompact object))

aGMvandvvv

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nwrel

rel

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2/32

442

2/3232

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)(44

4

ww vv

w

n

x

vv

ww

n

n

x

ww

x

relw

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MM

vvvaMG

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varv

MM

+

=

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ρπρπ

&

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• accretion if kinetic energy of material is less than potential energy of the NS at a given racc

• mass inside cylinder with r = racc is accreted;assume uniform wind to calculate ρ

• thus


XX--Ray Emission Ray Emission from Galaxiesfrom Galaxies

• X-ray emission extends beyond the optical limits („X-ray halo“)

• Interpreted as thermal radiation from a diffuse hot gas forming a halo around the galaxy

• While the hot gas itself does not contain much mass, it requires more than the galaxy‘s optically determined mass to be gravitationally bound ⇒ evidence for dark matter (which extends beyond the optical limits of the galaxy)

Starburst galaxies: hot gas driven out of the galactic plane by SN explosions


Clusters Clusters of of Galaxies Galaxies ––XX--ray emission as evidence for dark ray emission as evidence for dark mattermatter

Virgo cluster at X-rays

Emission dominatedby hot gas T ~ Million K

The total visible mass of the cluster(galaxies and gas) accounts for only ~ 30% of the mass calculated from the velocity dispersion of individual galaxies. Thus 70% of the mass isnot emitting electromagnetic radiation(dark matter)


XX--Ray Emission Ray Emission from AGNsfrom AGNs

• „Active Galactic Nuclei“• Massive black hole with accretion

disc and jets• Nearest active galaxy: Cen A• X-ray and radio images of the jets

reveal similar structure, with close correlation of details such as knots

3C 273 Pictor

Narrow-Line Region

AGN AGN ComponentsComponents

hot coronaPower-law emission

Accretion diskthermal emission

Black Hole

Broad-Line Region


AGNs AGNs –– Spectral ComponentsSpectral Components


AGN AGN SpectraSpectra –– Soft Soft Excess Excess

hotter disk ?hotter disk ?ROSAT results

extre

me s

o ft e

x ces

s

~ velocity dispersion in the BLR

mod

erat

e

• soft excess: thermal emission of the accretion disk

• Soft excess is related to the width of optical lines (and thus to the velocity dispersion in the broad line region (BLR))

⇒ Narrow optical lines indicate a hotter accretion disk


AGN AGN Spectra Spectra –– Fe K LineFe K Line

• Photons from the corona hit the accretion disk

• Result: reflection spectrum showing Fe and other lines

• Shape and intensity of Fe lines vary with the ionization state of Fe in the disk (and thus with disk temperature)


XX--ray ray BackgroundBackground

• Cosmic X-ray background (XRB) observed as diffuse emission by ROSAT

• Deep X-ray surveys show this to be due largely to accretion onto supermassive black holes

• Hard spectrum explainable by gas and dust absorption

• Total AGN light inferred from the XRB is consistent with the total mass in dark remnant BHs in nearby galaxies


Building Building up a 5 up a 5 Msec Chandra Msec Chandra database database of of the the HDFN HDFN

1 Ms of exposure

400 detected sources

At the faintest flux levels:population of normal galaxies and AGNs similar to the local population found

excellent correlation between X-ray sources and ISO sources(soft-X-ray absorbed AGNs produce reemission in the IR, e.g. NGC6240)


ChandraChandra/XMM/XMM--Newton Newton Deep Deep FieldsFields

line = SloanDSS quasars

as of 2001


BibliographyBibliography

• Exploring the X-Ray Universe, P. Charles & F. Seward, Cambridge University Press 1995

• The Physical Universe, Shu, 1982• Lectures on AGNs at Univ. Padova by Th. Boller

(May 2002, www.xray.mpe.mpg.de/~bol/padova)• Lectures on Astrophysics at Univ. Frankfurt by

Th. Boller• Hasinger, in Proc. „XEUS-studying the evolution

of the Universe“, 2002 (MPE Report 281)

Documents

Cosmic UV and X-Ray Sources - Max Planck Society · 2003. 3. 12. · Cosmic UV and X-Ray Sources Cornelia Wunderer, MPE. Thermal Emission of a Hot Gas • hot gas of low enough density