High Energy Astrophysics

High energy astrophysics typically deals with x-rays and higher energy radiation. It also deals with high energy

neutrinos and other particles such as protons, electrons, positrons etc.

High energy radiation is produced by objects at high temperatures and/or relativistic particles.

1 ev = 10,000 K, 1 kev = 107 K

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This usually requires compact objects such as white dwarfs, neutron stars or blackholes with deep gravitational potential.

Vesp=(2GM/R)1/2 approaching c

Or R not much greater than the Schwarzschild radius: 2 GM/c2 (2.95 km for a solar mass object).

X-ray astronomy: 0.1 to 100 kev

Gamma-ray astronomy: >100 kev.

E=h \nu = k T ==> x-rays probe 106 -- 109 K and gamma-rays > 109 K

Eddington Luminosity: 1.3x1038 erg/s for 1 Mo.

Optically thick blackbody radiation in x-rayrequires a compact object!

(derive the Eddington limit)

T as a function of object mass, radius (in units of Schwarzschild radius) and Luminosity (in units of Eddington luminosity), is given by:

T ~ 7 kev (L/L_Edd)^{1/4} (R/R_s)^{-1/2} (M/M_sun)^{-1/4}

Brief Property and History of Compact Objects

White dwarfs: R~10,000 km, Vesc~0.02 c, density~ 106 g/cc

(Nuclear reaction is more efficient source of energythan the PE release of in-falling gas on WDs).

1. 1914: Adams-- Sirius B has M~ 1Mo, T~ 8000 K, R~10,000km2. 1925: Adams confirmed M & R by measuring gravitational

redshift -- z ~ GM/(R c2)=0.0003.

3. 1926: F-D statistics discovered. Fowler applied it to model WDs.

4. 1930: Chandrasekhar: WD model including relativity; mass limit.

5. 1983: Nobel prize to Chandrasekhar.

Neutron Stars1. 1931: Chadwick --discovers neutrons. 2. 1934:Baade & Zwicky suggested neutron-stars, and

postulated their formation in supernovae.

3. 1967: Hewish, Bell et al. Discover radio pulsars.

4. 1968: Gold proposed rotating NS model.5. 1974: Nobel prize to Ryle (aperture synthesis)

Hewish (pulsars).

6. 1975: Hulse & Taylor discover binary pulsar PSR 1913-16.

7. 1993: Nobel prize to Hulse & Taylor.

Neutron stars: R~15 km, Vesc~0.32 c, density~ 1014 g/cc

(Nuclear reaction is much less efficient source of energythan the PE release of in-falling gas on NSs).

Black Holes

1795: Laplace noted the possibility of light not being able to escape.

1915: Einstein’s theory of general relativity.

1916: Schwarzschild -- metric for a spherical object

1963: Kerr --metric for a spinning BH.

1972: Discovery of Cyg X-1

1995: Miyoshi et al. -- NGC 4258.

1997: Eckart & Genzel -- (Sgr A*) Galactic center.

2002: Nobel prize in physics to Giacconi (x-ray astronomy).

Schwarzschild radius = 2.95 km M/Mo

Efficiency of energy production 6% to 42%.

Summary of last lecture

1. Derivation of the Eddington limit.

2. We showed that bright sources of high energyphotons are typically compact objects suchas WD, NS or BH.

High speed, strong, shocks are another way of generating high energy photons; however

high speed shocks are usually produced when compact objects form eg. SNe, GRB etc.

(an exception is x-rays from clusters.)

(1 Ao = 12.5 kev)

(SOHO) at 171 A = 74 ev

EUV picture of the Sun

Corona & severalActive regions

are visible

Coronal luminosity:~ 1026 erg/s

SOHO

EUV picture of the Sun at 195 A = 65 ev

from

Corona, active regionsand a flare is visible

at 195 A = 65 ev

QuickTime™ and aGIF decompressor

are needed to see this picture.

Crab nebula

Blue: x-ray

Red: optical

Green:radio

Luminosity ~ 1038 erg/s(mostly x-ray & gamma)

Synchrotron radiation:(linear polarization of 9%averaged over nebula).

Electrons with energy > 1014 ev are needed for emission at 10 kev;lifetime for these e’s < 1 year. So electrons must be injected

continuously & not come from SNe.

(Plerion)

SN remnant: Cas A (3-70 kev; Chandra)

Age 300 yr (1670 AD)

SNe II remnant

Mass of x-ray gas10-15 solar mass.

(Plerion)

X-ray luminosity:3.8x1036 erg/s

Pulsar wind nebula G292(Chandra 3-80 kev)(Plerion)

SN remnant G11.2-0.3 in x-ray (Chandra)

X-ray luminosity:~ 1036 erg/s.

The radiation is produced by shock heated gas at ~ 109 Kvia bremsstrahlung.

Note the bright (blue)Pulsar nebula at the Center.

Produced in SN of 386 AD

AGN jet from the quasar GB 1508+5714 (distance 4Gpc)

Chandra x-ray obs.

(x-ray produced by IC of CMB-photons with jet e-s)

Obs. jet size~30 kpc

Cen A

HST & 6 cm VLA

VLA: 6 cm

(distance ~ 2.5 Mpc)

Radio lobe size ~ 200 kpc!

The radio lobes are fed by relativistic jets; we see onlyone sided jet due to relativistic beaming.

Stephan’s Quintet

Blue:Chadra x-ray

SDSS optical

Yellow:

Compact group of interacting galaxies. Gas is stipped and shock heated to 6 million K produces x-rays.

F is a foreground galaxy. So thecluster (A, B, D & E) is in fact a quartet.

Cluster x-ray & optical

Chandra x-ray; ~ 2 kevHST - optical image

(note lensing of background gals)

Abel -2390.5 Gpc

MS2137.3-2353(1 Gpc)

SN remnant G11.2-0.3

M87 jet