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“New Views on Accreting X-ray Pulsars,
a brief review of recent results”
Andrea Santangelo
(…and the Magnet Collaboration)
Institut für Astronomie und Astrophysik
Kepler Center for Astro and Particle Physics
Karls-Eberhard-Universität Tübingen
Tokyo 01.12.2010
MAXI International Symposium
Andrea Santangelo,
IAAT KC-Tü
Magnet Collaboration …
A. Santangelo, D. Klochkov, V. Doroshenko, D. Müller, R.
Doroshenko, R. Staubert, M. Sasaki (IAAT), J. Wilms, L.
Barragán, T. Dauser, I. Kreykenbohm, J. Schmid, F.
Schwarm (ECAP), G. Schönherr (AIP), I. Caballero
(Saclay), C. Ferrigno, N. Mowlawi (ISDC), P. Kretschmar
(ESAC), V. McBride (Soton), U. Kraus (Uni Hildesheim),
O. Nishimura (Nagano), K. Postnov, N. Shakura (SAI), K.
Pottschmidt (CRESST/UMBC/GSFC), R. Rothschild, S.
Suchy (UCSD), L. Sidoli (INAF Milano) …
Partial list …
Andrea Santangelo,
IAAT KC-Tü
Liu et al. A&A, 2000, 2005
X-ray Binaries with
a NS: Pulsars?
B  10 G
12
Most of them are
in HMXRB
A few LMXRB
(Her X-1, 4U1626-67,
GX1+4)
Mass Transfer from
the Normal to the
compact star
Wako 10.12.2009
Courtesy of Scientific American
Seminar at RIKEN
Andrea Santangelo,
IAAT KC-Tü
Accretion
Roche Lobe Overflow
Wind Accretion

 ( r )  const

GM1 GM 2 1   2
 ( r )           r 
r  r1 r  r2 2
Lubov & Shu, 1975
Ý 1057 MSun yr1
M
v  103 km s1
Davidson
 & Ostriker, 1973
Andrea Santangelo,
IAAT KC-Tü
SG/X-ray Binaries Wind Accretion

GMm 1
2
2
 m v w  v NS
r
2
racc
2GM
 2
2
v w  v NS



Bozzo+, 2008 SFXT
Interaction between the
inflow wind matter and
the NS magnetosphere:
different regimes at
different luminosities
Kreykenbhom, 2004
Andrea Santangelo,
IAAT KC-Tü
Interaction disk/wind magnetosphere
Details of matter transfer are not understood
Andrea Santangelo,
IAAT KC-Tü
Ghosh & Lamb, 1978, 79
In the Boundary Layer disk is not longer
keplerian
 GM 
k ( r )   3  
 r 
PMagn
B2
2


8 8r 6
At the Boundary Layer disk is disrupted.
The plasma is forced to follow the field
lines and matter is funnelled in accretion
column onto the Neutron Star Magnetic
Poles
Kuster, 2003
Andrea Santangelo,
IAAT KC-Tü
1
2
What is the Corotation Radius ?
At the Corotation Radius the angular
velocity of the magnetosphere equals the
keplerian velocity of the Disk.
rH  rco
1
7


M
4 7
Ý162 730
rHDisk  5.2 10 8 M

 cm
M Sun 
rH  3 10 Km
3
Andrea Santangelo,
IAAT KC-Tü
Not so simple: Open Field lines
Lovelace et al. 1995
Andrea Santangelo,
IAAT KC-Tü
Outflows and conical winds
MHD simulations
Romanova et al., 2009
Andrea Santangelo,
IAAT KC-Tü
Do we have evidence of these outflows?
Andrea Santangelo,
IAAT KC-Tü
Continuous Monitoring of Pulse Period, Pdot, Lx
P
Based on Swift/BAT data
Klochkov +, 2009
Time
.
P
Her X-1
Andrea Santangelo,
IAAT KC-Tü
Strong Spin-down episodes
Outflow episodes!
Andrea Santangelo,
IAAT KC-Tü
Other evidence: decay of Porb
Klochkov +, 2008;
Staubert+, 2009
A decay of the Orbital Period has been
measured
11
1
Ý
POrb  4.85 10 dd
Lx  2 10 37 erg s1

It cannot be reconciled with a
conservative scenario
Matter is ejected!
In strong spin-down episodes the spin-down power is used to expel
matter from the inner disk radius
Ji et al., 2009 (Chandra)
Andrea Santangelo,
IAAT KC-Tü
Surprises from wind/magnetosphere
interactions?
Andrea Santangelo,
IAAT KC-Tü
The strange case of an old friend
GX 301-2
Doroshenko+, 2010
Why is GX 3012 so slow?
Wako 10.12.2009
Seminar at RIKEN
Andrea Santangelo,
IAAT KC-Tü
Torque Balance
d
I
 K   K
dt
Accelerating Torque depends on accretion
rate, wind velocity through kw
The Spin frequency of the source
is determined by accelerating and
braking torques
Davies +, 1979
2
Ý
K   Mkw RA orb
Decelerating Torques
Davidson & Ostriker, 1973
Turbolent Viscosity
Ilarionov & Kompaneets, 1990
Compton Heated Outflows

Andrea Santangelo,
IAAT KC-Tü
Assuming equilibrium…
kw~0.25-1
Andrea Santangelo,
IAAT KC-Tü
Wako 10.12.2009
Doroshenko et al., 2009
Seminar at RIKEN
CGRO-BATSE
Andrea Santangelo,
IAAT KC-Tü
Wako 10.12.2009
Seminar at RIKEN
Andrea Santangelo,
IAAT KC-Tü
GX 301-2 a hidden “magnetar”?
Not an unique case…
Andrea Santangelo,
IAAT KC-Tü
Vela X-1 (Ps ~283.5 s)_
Doroshenko 2010a, (submitted)
Kreykenbohm+, 2008, Inoue+ 1984
B 101314 G
Other evidence
from: QPOs, Noise
Power Spectrum
Flux drops dramatically

OFF states (Suzaku obs.)
Propeller regime
due to wind density
fluctuations of
clumpy winds?
Gogus+, 2010 in GX 301-2
Andrea Santangelo,
IAAT KC-Tü
Peculiar hard emission at lower luminosity
(Doroshenko +, 2010b, in prep.)
20  40 keV
The source
still pulsates

OFF state
Accretion though at much lower luminosity is there!
We possibly observe the inhibition of the column at
lower luminosities: radiation comes from the polar cap
In “the gated accretion” scenario of Bozzo+ (2008),
we might see here the transition to KH Instab.
Pulse profile
changes at
hard energies
B  2 10 G
13
Andrea Santangelo,
IAAT KC-Tü
Emission from the accretion column
Andrea Santangelo,
IAAT KC-Tü
Basko & Sunyaev, 1976
Accretion Columns
Kuster, 2003
9...11
1
Ý
M 10
M Sun yr
B  10 12 13 G v  0.6  0.8 c
 
Solid Column
L  LEdd
Hallow
Cylinder
Andrea
Santangelo,
IAAT KC-Tü
Transition between two accretion regime depending on the luminosity:
Eddington vs. Sub-Eddington
L 10 erg/sec
37
Right) High accretion rate:
shock is formed, plasma is
decelerated to subsonic speed
and heated. The Plasma then
sinks to the NS surface. Emitted
photons can only escape
perpendicularly to the column
forming a wide Fan beam.

Left) Lower accretion rate No
shock is formed, plasma is
decelerated onto the neutron star
surface by Coulomb collisons;
photons are generated by
Bremsstrahlung and Compton
Cooling. They can escape along
the accretion column, generating
a pencil beam
Wako 10.12.2009
Kretschmar, 1996 Harding, 1994
Seminar at RIKEN
Andrea Santangelo,
IAAT KC-Tü
Cyclotron Lines
Electrons in the magnetosphere plasma move helicodally along the B
field lines: their motion perpendicularly to the B field is quantized in the
Landau levels
mc 2  2 n B
ωn  mc
2
For B<< Bcr
eB
ωc 
γmc
Bcrit
sin 2 θ
sin 2 θ  1
1
1 z
m 2c 3
Bcr 
 4.14  10 13 G
e
ωn  nωc
E e ,c  11.6 B12 keV
Equispaced
12
B in units of 10 Gauss
Andrea Santangelo,
IAAT KC-Tü
Cyclotron lines as absorption lines
Electron is excited
to the n Landau
level
Wako 10.12.2009
Lifetime
is short
De-excitation to
ground state via
single or multiple
photon emission
Seminar at RIKEN
Mean free path is
short, quasi
instantaneous recapturing
of the
Andrea Santangelo,
photon IAAT KC-Tü
Can we probe these two regimes
Using Cyclotron lines…
Wilms+, 2010
+ GX 304-1 ~54 keV P13
Andrea Santangelo,
IAAT KC-Tü
Do cyclotron lines trace the B field of
the Neutron Star?
• Do cyclotron lines trace the B field of the NS?
• How the energy of the line is related to the
luminosity ? Do we observe a change of regime
around 1037 ergs/sec ?
Anticorrelation ?

HM ?
Correlation ?
M col
H
?
Ý
M
Andrea Santangelo,
IAAT KC-Tü
Mihara+ 2007, Nakajima 2008
Andrea Santangelo,
IAAT KC-Tü
Correlation or Anti-correlation?
 Caballero et al., 2007, 2009
 A0535+26
 Tsygankov et al., 2006; 2010
Mowlavi et al., 2006
 V0332+53, Outburst 2005
 Anti-correlation is clearly
Wako 10.12.2009
observed
 Staubert et al., 2007
 Her X-1, 5 years of data
 Correlation is clearly
observed; sub-Eddington!
Andrea Santangelo,
IAAT KC-Tü
New Studies using pulse to pulse
variability…
Andrea Santangelo,
IAAT KC-Tü
Pulse to pulse variability…(Klochkov+, P23)
light curve
repeated pulse profile
PCA cts/s/PCU (~3-30 keV)
V0332+53
Time in days
Andrea Santangelo,
IAAT KC-Tü
pulse flux bins
PCA cts/s/PCU (~3-30 keV)
Strong spectral variability
Time in days
Andrea Santangelo,
IAAT KC-Tü
Example: X0115+63
Klochkov+ 2010 (in prep.)
Spectral variability with the single pulse amplitude
With increasing pulse amplitude, the power-law becomes
steeper, the cyclotron line shifts towards lower energies
Andrea Santangelo,
IAAT KC-Tü
Dependence of spectral parameters on pulse height: summary
“negative pulsars”
“positive pulsars”
V0332+53 X0115+63 Her X-1 A0535+26
- flux
?
- flux
-flux
?
?
?
We found two types of spectral dependencies on the single
pulse flux. We interpret them as an indication of two distinct
Santangelo,
accretion regimes (poster by D. Klochkov et al.) Andrea
IAAT KC-Tü
Again instabilities…
Andrea Santangelo,
IAAT KC-Tü
A0535+26 (2005 Outburst)
Caballero
+ 2007, 2008
Wako 10.12.2009
Seminar at RIKEN
Andrea Santangelo,
IAAT KC-Tü
Wako 10.12.2009
Seminar at RIKEN
Andrea Santangelo,
IAAT KC-Tü
Wako 10.12.2009
Seminar at RIKEN
Andrea Santangelo,
IAAT KC-Tü
A question remains… is the cyclotron
line tracing the B field of the NS (in
systems like GX 301-2 and Vela X-1)?
Andrea Santangelo,
IAAT KC-Tü
Doroshenko et al., 2009
Solution of the contradiction?
The Cyclotron lines traces
the field of the production
site and this could be
located at the top of the
column
L ~ 2 T
4
SB EDD
2Rsin H  H ~ 10  30 km
Basko & Sunyaev,1976  high columns can be predicted
Andrea Santangelo,
IAAT KC-Tü
Formation of the spectra is very
complex…
Andrea Santangelo,
IAAT KC-Tü
The continuum?
White+, 1983
POHEI (E) =
Ea
E<Ecut
Eaexp(-(E-Ecut)/Ef))
E>Ecut
1
FDCO( E ) 
E

E
cut
Tanaka, 1986
1  exp(
)
E
NPEX  ( A1 E
Mihara+, 1995
a 1
 A2 E
a 2
)e
E
kT
 ( E  o( E ))e
2
2
E
kT
Approximates thermal comptonization
 Sunyaev and Titarchuk, 1980
Segreto, 2001
Andrea Santangelo,
IAAT KC-Tü
Becker & Wolff, 2005a,b and 2007
Physical Models (high luminosity)
• Accretion mound produces soft
X-rays via bremsstrahlung
• X-rays are up-scattered via
bulk motion comptonization
and diffuse through the walls
of the columns
• Cyclotron emission occurs
together with bbody emission
from a thermal mound
A Radiative shock dominates the
formation of the emitted continuum
Limited to
COLUMN!
Andrea Santangelo,
IAAT KC-Tü
Ferrigno et al., 2009: Model in XSPEC!
A quantitative attempt: 4U 0115+634
Thermal and bulk
Comptonization of
Cyclotron emission.
Thermal
Comptonization
of 0.5 keV BB
Gaussian to
correct the rough
modelling
Cyclotron emission is
concentrated around
the peak. Thermal
comptonization is
almost
constant
Andrea Santangelo,
IAAT KC-Tü
Ferrigno et al., 2009
4U 0115+634 Emission Geometry
Beamed High Energy
emission from the column
(Fan beam)
Low Energy Diffused Halo
producing a fan component
Analysis is being extended to
other sources like 4U1907+67,
Cen X-3, …(Ferrigno et al.
2010)
Magnetic field of Cyclotron emission and of
absorption line forming region are different!
Andrea Santangelo,
IAAT KC-Tü
Light Bending and Geometry
Kraus, 2003, 2010; Sasaki 2010; Caballero 2010
Andrea Santangelo,
IAAT KC-Tü
Kraus et al., 2010
Beam Pattern: three components
Reprocessed radiation
in the accretion
stream
Column component
Low Energy Diffuse
Halo Component
Emission components?
• Isotropic column emission
• Beamed emission
Andrea Santangelo,
IAAT KC-Tü
Conclusions
• Progresses on Accreting Pulsars study have been dramatic in the last
few years
• Discovery of outflows (in Her X-1)
• Pre-outburst flares and magnetospheric instabilities in A0535+26
• Evidence of magnetar-like sources in Binary systems (?)
• A “new” technique: Pulse to pulse variability
• Modelling of the physics of the emission of continuum and of cyclotron
line profiles (Not discussed here!)
• Modelling of Pulse Profiles (Not discussed here!)
• Eventually all this greatly improved our understanding on:
 Emission processes of  Geometry of the
pulsed components  Plasma Parameters
spectral formation
 …of the line
 …B, T,Andrea
tau,
etc…
 …spectral
Wako 10.12.2009 reprocessing
Seminar at RIKEN
Santangelo,
forming region
IAAT KC-Tü
Thanks for listening
Andrea Santangelo,
IAAT KC-Tü