X-ray binaries
Based on: Compact Stellar X-Ray Sources', eds. W.H.G.
Lewin and M. van der Klis, Cambridge University Press
Tauris & van den Heuvel: arXiv:0303456
Mc Clintock & Remillard: arXiv:0306213
Van der Klis arXiv:0410551, Hasinger & van der Klis 1989 A&A
Psaltis arXiv: arXiv:0410536
Fender+ 2004 arXiv:0409360
Basic facts and discovery
• Sco-X1 discovered in one of the first Xray observation of the sky (1962)
• ~100 bright (Fx>10-10 cgs) X-ray sources
in the Galaxy, most discovered already
by Uhuru (1971)
• 1034<LX<1038 erg/s
• NS in a binary hypothesis confirmed
soon by discovery of X-ray pulsating
emission and regular eclipses (Cen-X3,
1972)
P=4.84 s
Po=2.087 days
X-ray pulsars
X-ray pulsars
X-ray pulsars
X-ray pulsars
Masses in binaries
Neutron star masses
X-ray binaries
HMXB, LMXB
X-ray binaries
Accretion and B field
Accretion and B field
Accretion and B field
The material coming out from the
companion star (blue arrows) is captured
by the NS. The particle are deviated from
the original trajectory and converge
behind the NS. There they collide,
loosing their energies and then fall
toward the NS. AS they come closer the
grav. Field accelerates them to very high
energies. In the second panel the NS is
surrounded by a strong B fiels, the
incoming matter is very hot and cannot
penetrate the magnetosphere. The
matter move along B lines and continue
to accelerate. B lines converge to poles
and the particles are there focused,
forming an accretion column. The density
is high and the collisions frequent. The
particles loose energy in form of X-rays.
Other particles loose their energy
impacting the NS.
Accretion and B field
• When a strongly magnetic neutron star accretes plasma from a
companion star or the interstellar medium, its magnetic field becomes
dynamically important close to the stellar surface and determines the
properties of the accretion flow. The radius at which the effects of the
magnetic field dominate all others is called the Alfven radius.
• For thin-disk accretion onto a neutron star, the Alfven radius is defined as
the radius at which magnetic stresses remove efficiently the angular
momentum of the accreting material
• For a surface magnetic field strength of 1012 G and a mass accretion rate
~Eddington critical rate, the Alfven radius is 100 neutron-star radii.
• If the stellar spin frequency is smaller than the orbital frequency of matter
at the interaction radius, then the accreting material is forced into
corotation with the star and is channeled along field lines onto the
magnetic poles. An accretion-powered pulsar is produced
• if the stellar spin frequency is larger than the orbital frequency of matter at
the interaction radius, then the material cannot overcome the centrifugal
barrier in order to accrete onto the star. Matter eventually escapes the
neutron star in the form of a wind. “Propeller” regime
High mass X-ray binaries
HMXB
HMXB
• A compact object can accrete matter from a companion star that does not
fill its Roche lobe, if the latter star is losing mass in the form of a stellar
wind. For this process to result in a compact star that is a bright X-ray
source, the companion star has to be massive (≥ 10 M⊙) in order to drive
a strong wind. In this configuration, the optical luminosity of the companion
star dominates the total emission from the system and the rate of mass
transfer is determined by the strength and speed of the wind and the
orbital separation. Such systems are called High-Mass X-ray Binaries.
• ~150 HMXB known, ~30 with good orbital parametes
• because neutron stars in HMXBs accrete for a relatively short period of
time, their magnetic fields do not evolve away from their high birth values,
and hence these neutron stars appear mostly as accretion-powered
pulsars. ~40 pulsating HMXB with P=10-300 sec (0.07s-20min)
• Porb<10days
• The lifetimes of HMXBs are determined by the evolution of the high-mass
companions and are short (105 − 107 yr)
• HMXBs are distributed along the galactic plane, as young stellar
populations do
HMXB X-ray spectra
The accretion is disrupted at
hundreds NS radii and most matter
is funneled into NS poles, on
relatively small areas.
The average spectrum of persistent
HMXB can be approximated by a
broken power law:
With =1.2+/-0.2 c~20 keV F~12
keV
Cold/warm absorption from the star
wind
Iron features
Cyclotron features
• For neutron-star B fielf of 1012 G, the
cyclotron energy on the stellar surface
is11.6 keV and the continuums pectra
are expected to show evidence for
harmonically related cyclotron
resonances cattering features (or
cyclotron lines) in the X-rays.
• Observation of such features was
anticipated from the early days of X-ray
astronomy and expected to lead to
direct measurements B (e.g., Trumper
et al. 1978).
Cyclotron lines
Intermediate mass X-ray binaries
Low Mass X-ray Binary provides
Observational Evidence of NS
Structure
Neutron star
primary
Accretion
disk
Roche
point
Evolved
red dwarf
secondary
LMXB: properties
• 150 known LMXB (2001):
– 130 in the Galaxy,
– 13 in globular clusters,
– 2 in LMC
•
•
•
•
•
•
•
63 are X-ray bursters
75 transient (not always observable)
11 with a black hole (& 8 possible candidates)
Typical luminosity 1036-1038 erg/s
Soft X-ray spectra
Accretion process: Roche-lobe overflow
Orbital periods: from 11 minutes to 17 days
Formation of LMXB
• Direct: Birth as binary system
– More massive star ⇒compact object
• Less massive star fills Roche radius ⇒mass-transfer
⇒LMXB
• Capture:
– Birth of more massive star alone ⇒ compact object
– Close encounter ⇒capture of second star
– High star density ⇒happens almost only in globular
clusters
• Fraction of transients among the BH
systems is > than the fraction of
transients among NS systems and
their outbursts are typically longer
and rarer.
• BH transients in quiescence are
significantly fainter than NS
transients.
• These differences are caused by the
different mass ratios of the members
of the binary systems between the
two populations as well as by the
presence of an event horizon in BH
systems.
Transients LMXB
The prevailing model of transient sources is based on the disk instability model
of illuminated accretion disks (van Paradijs 1996; King+ 1996): accretion flows
that extend to large radii ( > 109 − 1010 cm) from the compact object have T<
104 K, at which the anomalous opacity related to the ionization of H renders
them susceptible to a thermal instability. At the off-cycle of the instability,
material piles up at the outer edges of the accretion disk with very little mass
accreted by the central object: quiescent phase. When the disk becomes
unstable, the accretion flow evolves towards the central object at the viscous
timescale, and the system becomes a bright X-ray source in outburst.
Bursts from LMXB
EXO0748-676
origin of X-ray bursts
circumstellar material
Gravitationally Redshifted Neutron Star Absorption
Lines
• XMM-Newton found red-shifted X-ray absorption features
• Cottam et al. (2002, Nature, 420, 51):
- observed 28 X-ray bursts from EXO 0748-676
• Fe XXVI & Fe XXV
z = 0.35
(n = 2 – 3) and O VIII
(n = 1 – 2) transitions
with z = 0.35
ISM
z = 0.35
z = 0.35
ISM
• Red plot shows:
- source continuum
- absorption features
from circumstellar gas
• Note: z = (l-lo)/lo and l/lo = (1 – 2GM/c2r)-1/2
X-ray absorption lines
quiescence
low-ionization
circumstellar
absorber
Low T bursts
High T busts
Fe XXV & O VIII Fe XXVI
(T < 1.2 keV)
(T > 1.2 keV)
redshifted, highly
ionized gas
z = 0.35 due to NS
gravity suggests:
M = 1.4 – 1.8 M
R = 9 – 12 km
Bursts from LMXB
• Two Types of bursts:
• Type I: thermonuclear explosion of He on the neutron star The
material that is accreted on the surface of a weakly-magnetic
neutron star may be compressed to densities and temperatures
for which the thermonuclear burning of helium is unstable. The
ignition of helium results in a rapid (1 s) increase in the X-ray
luminosity of the neutron star, followed by a slower (tens of
seconds) decay that reflects the cooling of the surface layers
that ignited. During bursts coherent oscillations of the observed
X-ray fluxes are often detected. In bursts from two ultracompact millisecond pulsars, in which the spin frequencies of
the stars are known, the asymptotic values of the burst
oscillation frequencies are nearly equal to the spin frequencies
of the NS
• Type II: instabilities of accretion flow onto the neutron star
Spectral and timing properties
X-ray timing properties are correlated with X-ray spectral states. Source states
are qualitatively different, recurring patterns of spectral and timing
characteristics. They arise from qualitatively different inner flow configurations.
Spectral and timing properties:
QPOs
Spectral and timing properties
• Z sources on time scales of hours to a day or so trace out roughly Z shaped
tracks (Fig. 2.4c) in CD/HIDs consisting of three branches connected end-toend and called horizontal branch, normal branch and flaring branch (HB, NB,
FB). kHz QPOs and a15-60Hz QPO called HBO occur on the HB and upper
NB, an 6Hz QPO called NBO on the lower NB, and mostly power-law noise
<1Hz on the FB
• At high Lx atoll sources trace out a well-defined, curved banana branch in
the CD/HIDs
LMXB spectra
• For weak (<109 G) B fields the
accretion disk may touch or come
close to the NS surface and the
accreting matter is distributed over
large areas.
• No pulsations
• Partially Comptonized spectrum
• millisecond radio pulsars were most
often found in binaries with evolved,
low-mass white dwarf companions
(Bhattacharya & van den Heuvel
1991), which were thought to be the
descendents of LMXBs.
• The discovery, with RXTE, of highly
coherent pulsations in the X-ray
fluxes of LMXBs during
thermonuclear X-ray bursts
(Strohmayer et al. 1996) provided
the then strongest evidence for the
presence of neutron stars with
millisecond spin periods in LMXBs.
• However, the first bona fide
millisecond, accretion powered
pulsar was discovered only in 1998,
in a transient ultracompact binary
SAX J1808.4−3658
mmsec pulsars
Black hole binaries
BH binaries
BH binaries
• Found in HMXB, LMXB.
– 3 persistent (Cyg X-1, LMC X-3, LMC X-1)
– many LMXB X-ray Novae (A0620-00, from 50
Crabs to 1uCrab!).
BH binaries light curves
BH binaries transients
•
•
•
•
•
•
6 X-ray novae detected by RossiXTE ASM
U 1543-47: clean example of a
classic light curve with an e-folding
decay time of ≈ 14 days.
XTE J1859+226: another classic light
curve that does show a secondary
maximum (at about 75 days after
discovery). Note the intense
variability near the primary maximum.
XTE J1118+480: One of five X-ray
novae that remained in a hard state
throughout the outburst and failed to
reach the HS state. Note the
prominent precursor peak.
GRO J1655-40:double peaked profile
During the first maximum strong
flaring and intense non-thermal
emission (VH state).
XTE J1550-564: The complex profile
includes two dominant peaks
BH binaries high/soft state
• High accretion rates.
• Geometrically thin, optically
thick disk, Tmax~107K, 1 keV Xrays
• Multicolor disk model, estimate
rin from normalization, T,
inclination and distance
• Weak variability, f-1, no or weak
QPO
BH binaries low/hard state
• Lower accretion rates, a few% of
Eddington
• Hard, non-thermal power law
component ( 1.7)
• steep cut- off near 100 keV
• Comptonization of soft photons by
a hot optically thin plasma. Disk is
faint or undetected.
• presence of a compact and quasisteady radio jet (first in GRS1915,
then Cyg X-1 and others). Flat radio
spectral index
• Strong variability
BHB quiescent state
• BHB spends most of its life in this
state, L-1030.5 - 1033.5 ergs/s, 10-8
outburst L!!
• L/Ledd ~10-8
• Hard spectrum, =1.5-2.1
• Quiescent state may be just an
extremely low state
• In the quiescent state the disk is
truncated at some larger radius and
the interior volume is filled with a
hot (Te 100 keV) advection
dominated accretion flow or ADAF.
Most of the energy released via
viscous dissipation remains in the
accreting gas rather than being
radiated away (as in a thin disk).
The bulk of the energy is advected
with the flow and it is lost in the BH.
Radiative efficiency <0.1-1%.
BH binaries very high state
•
•
•
•
•
•
Both disk and power law component
present, both with a luminosity >0.1 LEdd
Steep power law component, =2.5 up to
1MeV: Compton scattering in a non-thermal
corona
QPOs in both disk and power law
component in the range 0.1-30Hz, both
LFQPO and HFQPO. Persistent. Organized
emission region.
LFQPO<<Keplerian f. BH 10 M⊙, an
orbital frequency near 3 Hz coincides with a
disk radius near 100 Rg , while the
expected radius for maximum X-ray
emission 1-10 Rg. Disk oscillations, spiral
waves.
HFQPO: often commensurate frequencies.
Resonance phenomenon of GR
oscillations.
Explosive formation of radio jets: the
instability that causes impulsive jets is
somehow associated with the VHS state
HFQPOs
BH binaries spectral states
1.
2.
3.
4.
5.
the high/soft (HS) state, a
high intensity state dominated
by thermal emission from an
accretion disk;
the low/hard (LH) state, a low
intensity state dominated by
power law emission and rapid
variability;
the quiescent state an
extraordinarily faint state also
dominated by power law
emission;
the very high (VH) state;
the intermediate state
Jets and radio emission in BHB
• Relativistic, superluminal jets.
• Non-thermal, polarized radio spectra,
indicating shock-accelerated e- emitting
synchrotron
• Very clear correlation between the
presence of jets and the X-ray spectral
state of the accretion flows. Jets appear
when the X-ray spectra of the sources
indicate emission from hot electrons (
100 keV)
• The mechanism responsible for the
heating of electrons in the accretion flow
may be related to the formation of an
outflow, as is the case both for
magnetically active accretion disks
Jets, disks and spectral states
Jets, disks and spectral states
• i low state steady jet Ljet ∝ LX0.5
• ii motion nearly vertical. After a peak motion
nearly horizontal to the left, Source move in the
VHS/IS. Jet persist.
• iii source approaches the jet line between
Jet producing and jet free states. Velocity increases.
Propagation of an internal shock.
• iv source is in the soft state and no jet is produced. Refill of disk.
• The thin disk extend close to the BH. Following phase iv sources drop in
intensity to reach the canonical LS.
• Inner disk is ejected resulting in a disappearence of the inner disk, transition
to LS, jet launch.
Relativistic iron lines
• The first broad Fe Kα line observed for either a BHB or an AGN was
reported in the spectrum of Cyg X-1 based on EXOSAT data. This result
that inspired Fabian et al. (1989) to investigate the production of such a line
in the near vicinity of a Schwarzschild BH, a result that was later
generalized by Laor (1991) to include the Kerr metric.
• Beppo-SAX discovered relativistic lines in several BHB: SAXJ1711+3808,
XTEJ1909+094,GRS1915+105, V4641Sgr
• XMM and Chandra: CCD and gratings
In many cases ISCO consistent
with non-spinning BH
Detection of “smeared edges”