X-ray Binaries - An Overview

X-RAY BINARIES
M.C. RAMADEVI
ISRO Satellite Centre
Bangalore.
YAM – Jan, 2007

Discovery of X-ray Binaries
Historic discovery of a bright X­ray source Sco X­1 in 1962
by Riccardo Giacconi and team; first extra-solar X-ray detection
Detected during a rocket flight which was lauched to look for
X­rays from Moon
The source was extremely bright in X­rays.
An optical star of 13 th magnitude was found at this location
Was Quite Intriguing...
TRIGGERED
X-RAY
Giacconi et al., 1962
Sco X-1
ASTRONOMY
X­ray background

THIS IS A BINARY SYSTEM WITH NEUTRON STAR AND A
COMPANION STAR (OPTICAL COUNTERPART): X-RAY BINARY
Later in 1970, the first X-ray satellite, UHURU, discovered another
X-ray source Cen X-3
counts
Found to have PULSATIONS in X-rays; 4.84 sec
Only a neutron star can produce such pulsations
time
Continuous monitoring showed slight variation in the pulse period
These variations - recognised as due to DOPPLER SHIFT; the star is
moving in an ORBIT.
X-rays found to disappear for 11 hours in every 2.09 days; ECLIPSES
counts
Ahhhh….. ECLIPSING BINARY SYSTEM !!!!
time
Here we go…

What are X-ray Binaries?
Special class of binaries which emit predominantly in
X-rays
The Most luminous galactic X-ray sources
Lx ~ 10 36 to 10 38 ergs/s
Consist of a compact object and a companion star
orbiting about a common centre of mass.
The compact object can be a white dwarf (cataclysmic
variables), a neutron star or a black hole.
Companion star can be a normal star or a white dwarf.

X-ray emission
What could possibly give rise to such high X-ray luminosities ?
GRAVITY
Compact star accretes matter from the companion star
The gravitational potential energy of the in-falling matter is
converted to kinetic energy eventually giving rise to radiation
Gravitational potential energy
Kinetic energy
Heat (T ~ 10 7 K)
(accreted matter swirls in)
(friction between layers)
(viscous heating)
Radiation (X-rays, UV)

How are X-ray Binaries
formed?
•Start off with Binary Stars:
2 stars, gravitationally bound
to each other in an orbit, about
a common centre of mass

Two mechanisms of mass transfer in a
binary system
Accretion from stellar wind
Accretion through
Roche lobe outflow

Accretion from stellar wind
Accretion through
Roche lobe outflow

Classification of X-ray Binaries
High-Mass X-ray Binaries (HMXB)
HMXB :
•Mass of the companion star , very
massive, >~ few solar masses.
•Usually NS systems accreting
mass from the wind of companion,
a Be star.
Low-Mass X-ray Binaries (LMXB)
LMXB:
•Mass of the companion star

•Hard State
Observations from X-ray binaries
X-ray binaries are observed as transient sources which
suddenly brighten up in X-rays by a factor of 100 to 1000
Light Curve has different profiles
•Pulsations
•Type I bursts
•Type II bursts
•Eclipses
•Persistent outbursts
Spectra suggests different emission processes
•Different spectral states
•High/Soft State
•Very High State

X-ray Pulsars - Pulsations
THIS IS NOT OBSERVED IN BLACK HOLE SYSTEMS
Accretion onto a
magnetized Neutron
Star
Accretion powered
Pulsars
B ~ 10 12 G
Cen X-3, observed by UHURU;
4.8 sec (Giacconi et. al., 1971)

Accretion onto weakly magnetised NS or WD
Thermonuclear explosion on the surface of a WD or NS
X-ray bursts
(Type I Bursts)
X-ray bursts – H burning
15 s
(1735-444)
X-ray Superbursts – He burning

Type I Bursts
Observation of thermonuclear energy
Unstable, explosive burning in bursts (release over short time)
Burst energy
thermonuclear
(from the surface of
NS or WD)
Persistent flux
gravitational energy
(from the accretion disk)
THIS IS NOT OBSERVED IN BLACK
HOLES, since they don’t have a surface.

Type II Bursts
•Transient outbursts
•Likely to originate due to
instabilities in the
accretion disk
•Observed in both NS and
BH systems
120 days

Accretion onto black holes
There is no hard surface. How can we detect it?
Will there be any radiation from the infalling
matter??
Yes, from the accretion disk around a BH

Black Hole X-Ray Binaries
Accretion disks around black holes
Strong X-ray sources
Rapidly, erratically variable (with flickering
on time scales of less than a second)
Sometimes: Quasi-periodic oscillations (QPOs)
Sometimes: Radio-emitting jets

Variabilities observed in Black Holes

Understanding Accretion
Evolution of the accretion disk
Spectra suggests different emission processes
.
m
disk
corona
disk
corona
corona
X-ray Properties
Radio Properties

Soft state

Hard state

Example
An outburst observed in 4U 1543-47
June-August 2002, 2-10 keV LC
Spectral and
temporal
analysis is
required
to understand
accretion

Spectral-
Temporal
States:
HSS:
VHS:
LHS:
(Remillard &
McClintock, 2004;
Park et al 2004)

Why study X-ray Binaries?
To understand accretion:
Accretion - known to power the most luminous objects in the
Universe.
Understanding accretion around the compact objects like the
neutron star and the stellar mass black holes can help
understand accretion around supermassive black holes which
form the heart of Active Galactic Nuclei, Quasars etc.
Accretion is everywhere: be it the formation of stars or planetary
systems or galaxies.
Also to understand behavior of matter very close to the event
horizon of a black hole where General Theory of Relativity is
applicable, Black Hole X-ray Binaries are good candidates.
X-ray Binaries form good laboratories for
these studies.

Evolved picture of a Low-Mass X-ray binary:
accretion through Roche-lobe overflow

India’s first multi-wavelength
ASTROnomy SATellite
ASTROSAT
INDIA’S FIRST DEDICATED SATELLITE FOR ASTRONOMY
UV/Optical 130-6000 nm), Soft X-rays (0.2 - 10 keV) , Hard X-rays (10 – 150

Thank You

LMXB
•For NS, there is emission from boundary layer where disk
meets NS and surface of NS (for a less magnetized NS).
•Optical emission arises from outer disk, companion star, and
X-rays reprocessed by disk or companion.
•For BH, X-ray emission is from disk.

HMXB
Be star
Neutron star
•Mostly the compact object is a neutron star, in eccentric orbit around the
companion
•X-rays transients occur when the neutron star accretes matter from the wind
of the companion star (Be star)
•No prominent disks emission

Binary Orbit
CM
M2
M1
a
Compact star mass = M 1
M
Normal star mass = M 2
M
Binary separation = a, mass ratio q = M 2
/M 1
+
Kepler' s law :
2
4π
a
3
=
G(
M
1
+
M
2
) M
Θ
P
2
10
1/3
1
a = 3.5×
10 M (1 +
q)
1/3
P
2/3
hr
cm

Geometry
Observed phenomenology depends on viewing angle.

Typical X-ray bursts:
• 10 36 -10 38 erg/s
• duration 10 s – 100s
• recurrence: hours-days
• regular or irregular
Frequent and very bright
phenomenon !
(stars 10 33 -10 35 erg/s)

10 s
Discovery
First X-ray pulsar: Cen X-3 (Giacconi et al. 1971) with UHURU
T~ 5s
Today:
~50
First X-ray burst: 3U 1820-30 (Grindlay et al. 1976) with ANS
Today:
~40
Total ~230 X-ray binaries known

X-Ray Pulsar Cen X-3
Pulses are
modulated
at orbital
period of
2.09 days

Formation of Accretion disk
around the compact object