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 = 10 7 K This usually requires compact objects such as white dwarfs, neutron stars or blackholes with deep gravitational potential. V esp =(2GM/R) 1/2 approaching c Or R not much greater than the Schwarzschild radius: 2 GM/c 2 (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 K and gamma-rays > 10 9 K Eddington Luminosity: 1.3x10 38 erg/s for 1 M o. Optically thick blackbody radiation in x-ray requires a compact object! (derive the Eddington limit) Thus if the radiation is black-body and luminosity is close to Eddington, Then x-ray temperature is reached provided that R\sim R_s and M is not much greater than M_sun. This result is violated, as it often is, when the radiation is non-thermal. 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, V esc ~0.02 c, density~ 10 6 g/cc (Nuclear reaction is more efficient source of energy than the PE release of in-falling gas on WDs) : Adams-- Sirius B has M~ 1M o, T~ 8000 K, R~10,000km : Adams confirmed M & R by measuring gravitational redshift -- z ~ GM/(R c 2 )= : F-D statistics discovered. Fowler applied it to model WDs : Chandrasekhar: WD model including relativity; mass limit : Nobel prize to Chandrasekhar.

Neutron Stars : Chadwick --discovers neutrons :Baade & Zwicky suggested neutron-stars, and postulated their formation in supernovae : Hewish, Bell et al. Discover radio pulsars : Gold proposed rotating NS model : Nobel prize to Ryle (aperture synthesis) Hewish (pulsars) : Hulse & Taylor discover binary pulsar PSR : Nobel prize to Hulse & Taylor. Neutron stars: R~15 km, V esc ~0.32 c, density~ g/cc (Nuclear reaction is much less efficient source of energy than 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 : Miyoshi et al. -- NGC : Eckart & Genzel -- (Sgr A*) Galactic center. 2002: Nobel prize in physics to Giacconi (x-ray astronomy). Schwarzschild radius = 2.95 km M/M o 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 energy photons are typically compact objects such as 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 A o = 12.5 kev)

(SOHO) at 171 A = 74 ev EUV picture of the Sun Corona & several Active regions are visible Coronal luminosity: ~ erg/s X-ray luminosity of corona for O-stars is ~ 10^33 erg/s (~10^{-6} the optical/UV luminosity of the star). 1. The coronal emission is free-free radition from hot plasma at a few million K. 2. The corona is heated by magnetosonic waves, and magnetic reconnections. Mention that: 3. Stellar corona are the most numerous source of x-rays in our galaxy.

SOHO EUV picture of the Sun at 195 A = 65 ev from Corona, active regions and a flare is visible

at 195 A = 65 ev

1. This is a low mass x-ray binary system (the companion star is low mass which Supplying gas to the compact star via Roche-lobe overflow). 2. X-ray binaries are some of the most luminous x-ray sources in the Galaxy; Some of these have luminosity close to 10^{38} erg/s or the Eddington Lum. 3. The luminosity is generated by a combination of emission from the disk and thermal emission from hot gas accreted by the n-star (nuclear reactions provide ~ 10 times less energy than PE release when the compact object is a neutron star.

Crab nebula Blue: x-ray Red: optical Green:radio Luminosity ~ erg/s (mostly x-ray & gamma) Synchrotron radiation: (linear polarization of 9% averaged over nebula). Electrons with energy > 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) Plerion: is derived from the Greek word “pleres” which means “full”. Crab nebula is the remnant of Sne explosion (perhaps type II) observed by the chinease Astronomers in 1054 (July 4th). The pulsar at the center has a period of 33milli-sec. Crab shows pulsed emission from radio to optical to >50 Mev! And moreover The pulse shape is nearly the same over this entire EM spectrum, suggesting A common origin for the radition which is believed to be synchrotron (curvature radiation). The radio is produced not too far away from the Neutron star (within 5-10 radaii) and high energy pulsed radiation is Likely produced near the light cylinder. The bolometric luminosity is pulsed radiation is about a factor 100 smaller Than nebular radiation; pulsed radio is smaller than total pulsed radiation By a factor of 10^4.

SN remnant: Cas A (3-70 kev; Chandra) Age 300 yr (1670 AD) SNe II remnant Mass of x-ray gas solar mass. (Plerion) X-ray luminosity: 3.8x10 36 erg/s

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

SN remnant G in x-ray (Chandra) X-ray luminosity: ~ erg/s. The radiation is produced by shock heated gas at ~ 10 9 K via bremsstrahlung. Calculate the temperature of the shocked gas (speed~ 5,000 km/s) and Bremsstrahlung luminosity using: Emissivity/volume = 1.4x10^{-27} n_e n_I T^{1/2} z^2 erg/s/cc Also describe the Bremsstrahlung spectrum. Note the bright (blue) Pulsar nebula at the Center. Produced in SN of 386 AD

Gamma-ray burst: note the relativistic jet, and supernova explosion.

AGN jet from the quasar GB (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) Cen A (Centaurus A) is our NEAREST Active galaxy (distance of 2-3 Mpc). It is a giant elliptical galaxy that has got a dust lane usually associated with Spiral gamaxies (Cen A has properties of both elliptical & spirals). Radio lobe size ~ 200 kpc! The radio lobes are fed by relativistic jets; we see only one sided jet due to relativistic beaming. The energy in the radio lobe is ~ 10^{55} ergs (need to check this).

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 the cluster (A, B, D & E) is in fact a quartet.

Cluster x-ray & optical Chandra x-ray; ~ 2 kev HST - optical image (note lensing of background gals) Abel Gpc MS (1 Gpc)

SN remnant G

M87 jet