Maurizio Falanga mfalanga@issibern.ch

White Dwarfs, Neutron Stars, and Black Holes

Einführung und Überblick: Donnerstag, 20. Februar 2020

Specialist Course - Compact Objects In Galaxies

Einführung und Überblick 19. September 2017

Organisation der Vorlesung

Webseite:

Zeit & Ort: Donnerstag 10:15 – 12:00 Uhr; Physik, Hörsaal B78

Voraussetzung (ECTS 4):

• regelmässige und aktive Teilnahme an der Vorlesung • regelmässige und aktive Teilnahme an den Übungen

Lectures

General introduction to high energy astrophysics Stellar structure equations, Equation of States White dwarfs, npe-gas, Equation of states for WD Chandrasekhar limit Equation of States for dense matter Neutron Stars observational signatures Pulsars, Radio, X-ray Conditions for mass flow through an accretion disks and or wind Single aspect of General relativity Black holes observational aspects Radiation processes

-  6 Exercises for different topics to solve during the whole period -  One Numerical Project to work on. Write a simple code, using

only analytical equations, to trace (in the Schwarzschild metric) the photons emitted around a black hole as seen from an distant observer.

Exercises

Objectives

Formation and observation of compact stars and stellar black holes in the context of stellar evolution. Astrophysical phenomena related to compact objects. Properties and description of dense matter. Simple applications of the theory of relativity.

Literatur

Frank, King & Raine: Accretion Power in Astrophysics

Rosswog & Brüggen: Introduction to High-Energy Astrophysics

Shapiro & Teukolsky: Black Holes, White Dwarfs, and Neutron Stars

Seward & Charles: Exploring the X-ray Universe

Rybicki & Lightman: Radiative Processes in Astrophysics

Chandrasekhar: The Mathematical Theory of Black Holes

Sternentwicklung

S. Chandrasekhar 1910-1995

A massive star can collapse into something denser WD (1930)

R. Oppenheimer & H. Snyder predict that massive stars can collapse into black holes (1939)

Weisse Zwerge

White Dwarfs:

•  1844: F. Bessel noticed that Sirius had a slight back and forth motion, as if it was orbiting an unseen object. In 1863, the astronomer A. Clark spotted this mysterious object (Radius, period). This companion star was later determined to be a white dwarf (Radius, Mass).

Magnetic Cataclysmic Variables

Polars Intermediate Polars Classical/Dwarf Novae

(Prototype AM Her)

B ~ 108 –106 G

Energy Sources: CN: nuclear burning DN: through gravity

Non MCVs

Porb ~ 1-10 hrs

Pspin = Porb Pspin « Porb

Synchronous Rotation Asynchronous Rotation

Porb (P) < Porb (IP)

Lacc = ηMc2 η ~ 0.7% (CV); ~ 20% (NS)

(For a review see B. Warner 1995)

Types of CV binaries:

Neutronenstern

Radio Astronomy in the 30’s-60’s

Discovery (1961-63) Quasi-Stellar Radio Sources as the most energetic and distant members of a class of objects

3C273

Karl Jansky 1933

1967

Radio Signals from the Sky

Jocelyn Bell (1943-)

- They soon realized that the best clocks of the time were not accurate enough to time the object. It seemed very unnatural to receive such a perfectly regular signal from space!

Period of 1.337 s 1.3372866576 s

LGM-1 (Little Green Men)

- It could originate from extra terrestrial intelligence?

M. Falanga

A massive star can collapse into something denser (1930). He was awarded the 1983 Nobel Prize in Physics for this fundamental prediction!

In 1934, F. Zwicky and W. Baade and that a stellar collapse of a heavy star during a supernova event should lead to the formation of a dense core of neutrons (NS) at the center of the SN remnant.

S. Chandrasekhar (1910-1995)

Fritz Zwicky (1898 –1974)

In 1932, James Chadwick, then at the Cavendish Laboratory in Cambridge, England, discovers the neutron. He got the Nobel physics prize in 1935.

James Chadwick (1891-1974)

During the course of the next few months, J. Bell discovered 3 more pulsating radio sources (or pulsars). These pulsars were proposed to be rapidly rotating neutron stars.

PULSARS are NEUTRON STARS Anthony Hewish, won the Nobel Prize in Physics for the discovery in 1974.

R ~ 10 km ρ ~1014g/cm3 M ~ 1.4 Msun -  Magnetic dipole -  Electromagnetic radiation -  Pulsar slowdown

Properties of neutron stars:

Core collapse of an evolved star •  Stellar core collapse => conservation of angular momentum => fast

spinning neutron star! •  Stellar core collapse => conservation of magnetic flux => highly

magnetized neutron star

•  Pacini (1967) proposed the existence of a highly magnetized, rapidly spinning neutron star as the power source of the nebula. This would radiate a very powerful EM wave with the rotational frequency of the star. This is below the plasma frequency of the nebula, therefore all this energy will be absorbed and re-radiated by the plasma of the nebula.

1968: The discovery of PSR B0531+21 (Crab Pulsar) These extremely short bursts (~33 ms) proved the

existence of a pulsar at the center of the Nebula. (This is the compact radio source detected by

Hewish and Okoye in 1964)

PSR 0531+21

Crab Nebula In 1968, at the height of the “pulsar fever”, giant radio pulses originating in the Crab Nebula were detected.

1054

Schwarze Löcher

The existence of “dark stars” (in Newtonian mechanics)

Pierre-Simon Laplace (1749–1827)

John Michell (1724–1793)

V =

1/2

R 2GM

A Treatise of the System of the World, London (1728)

Escape Velocity

Earth: 11.2 km/s Moon: 2.3 km/s Sun: 600 km/s

Isaac Newton (1643-1727)

K. Schwarzschild 1873-1916

The event horizon Rs = 2 M

R. P. Kerr 1934 -

Finds the solution for rotating black holes (1963)

Finds black holes as a solution to Einstein’s equations (1916)

J. A. Wheeler 1911-2008

No Hair Theorem

1967 Black Holes

Black Escape velocity c Hole singularity in space-time

Black Holes

J. A. Wheeler

BH have NO HAIR

X-ray Astronomy 1962

1970

1972

§  Bright X-ray emission §  Rapid X-ray variability

Sloa

n D

igita

l Sky

Sur

vey

§  30 M¤ Blue supergiant main-sequence star (optically bright, X-ray dim)

Optical Astronomy

§  Orbits, 5.6 days, an unseen optically (but bright X-ray) object

X-ray Binary System §  The companion has a mass between of ~ 10 M¤

What is it?

Cygnus X-1

§  Can’t be a Neutron star because M > 3 M¤

By elimination, we are left with a Black Hole

§  A red giant would be easily seen

§  A main-sequence star would be seen with a little effort

§  Can’t be a White Dwarf because M > 1.4 M¤

§  The companion has a mass between of ~ 10 M¤

Black Hole in our Galactic Center ?

NIR Evidences of a SM-BH at the GC

h  Enclosed Dark Mass ≈ 3-4 106 M¤ within 124 AU = 17 l. h. ≈ 2000 RS

NIR adaptive optics at VLT & Keck h  Proper motions of the stars of the central

cluster h  Orbital parameters of the closest star S2 to

the GC: P ≈ 15.2 yr, V ≈ 5000 km s-1 h  Dynamical center in Sgr A*

Stellar-mass Must be at least 3 solar masses (~1031 kg)

Supermassive Millions to billions of solar masses; located in

the centers of galaxies

We cannot see black holes directly, but their influence on the matter around them reveals their presence

Types of Black Holes

Intermediate mass A few thousand to a few tens of thousands of solar

masses; possibly the agglomeration of stellar mass holes

Energy released onto the Black Hole as X-ray Luminosity

LX ≈ ~1035 -1038 erg s-1 RNS

M G MNS

Disk Accretion Shakura & Sunyaev, 1973, A&A

Artist impression

(Kepler’s Law)

€

P =2π a 3/2

G1/2 [m1 +m2 ]1/2

v1 = 2πa1P , m1a1 = m2a2, a = a1 + a2

v1 =2πa m2

P(m1 +m2 )

v1,obs = v1 sini =2πa m2 sin iP(m1 +m2 )

P =P 3/2 (m1 +m2 )

3/2 v1,obs3/2

(2π )1/2 (m2 sin i)3/2G1/2 [m1 +m2 ]

1/2 =P 3/2 (m1 +m2 ) v1,obs

3/2

(2π )1/2 (m2 sin i)3/2G1/2

Pv1,obs3

2πG = (m2 sin i)3

(m1 +m2 )2

a: semi-major axis P: orbital period i: orbital inclination angle v1,obs: line of sight speed

With the mass ratio (m1/m2); we still need the orbital inclination in order to determine the masses individually.

Mass determination in binary systems

Center of the Gravity

a1

a2

©http://mc2.gulf-pixels.com/ 35

What is X-ray?

•  X-ray = high-energy photon o hν = 0.1 keV – 100 keV o λ = 0.1 – 100Å (λ1keV = 12.4Å) o kT = 106 – 1010 K (kT1keV = 107K)

•  We can see o Extremely high temperature o Non-thermal particle acceleration o Atomic process

Radiation processes

Bremsstrahlung:

Synchrotron radiation:

Compton Scattering:

Multi temperature and density post-shock model for CVs

Magnetic Field

NS geometry of the emission region

XTE J1807-294

Thermal disk emission

The plasma is heated by the accretion shock as the material collimated by the hotspot on to the surface. The seed photons for Comptonization are provided by the hotspot.

Seed photons from the hotspot

Thermal Comptonization in plasma of Temperature ~ 40 keV

B ~ 108 G

Rm

θ

Detected in 1969 Cen X-4 with Vela 5b (Belian et al. 1972)

Discovery paper 3U 1820-30 with ANS (Grindlay et al. 1976)

Kuulkers, in ’t Zand & Lasota 2009 Falanga et al. 2008

Type of X-ray Bursts

M Iron reflection line

M

Relativistic Iron line

Accretion disk model

X-ray Sources in the Galaxy observed with INTEGRAL

Over 700 hard X-ray sources ranging from CV to AGN

White dwarfs: R~10,000 km, Vesc~0.02 c, density~ 106 g/cc Neutron stars: R~15 km, Vesc~0.32 c, density~ 1014 g/cc Schwarzschild radius = 2.95 km M/Mo Efficiency of energy production 6% to 42%.

Energy released onto the Black Hole as X-ray Luminosity

LX ≈ ~1035 -1038 erg s-1 RNS

M G MNS

Disk Accretion Shakura & Sunyaev, 1973, A&A

Artist impression

Stellar-mass Must be at least 3 solar masses (~1031 kg)

Supermassive Millions to billions of solar masses; located in

the centers of galaxies

We cannot see black holes directly, but their influence on the matter around them reveals their presence

Types of Black Holes

Intermediate mass A few thousand to a few tens of thousands of solar

masses; possibly the agglomeration of stellar mass holes