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Are there Magnetars
in HMXBs?
The case of
SFXTs
Enrico Bozzo1,2, Maurizio Falanga3, Luigi Stella2
1 INAF - Osservatorio Astronomico di Roma
2 Dipartimento di Fisica – Universita' di Roma “Tor Vergata”
3 CEA Saclay, DSM/DAPNIA/Service d’Astrophysique, France
(ApJ submitted)
Are there Magnetars
in HMXBs?
The case of SFXTs
MAGNETA
RS
HMXBS
SFXTs
magnetic stars
high mass X-ray binaries
supergiant fast X-ray transients
X-Ray Binaries
Cen X-3 the first pulsating X-ray source
to be discovered. (Chodil et al. 1967)
Accretion Process
4.8 s
L acc ~
36
GM NS Ṁ
R NS
38
10 − 10 erg s
−1
X-Ray Binaries
LMXB (low mass X-ray binaries)
HMXB (high mass X-ray binaries)
Accrescimento da disco
Disk Accretion --> short orbital period (<3 d)
(Zhang et al., 2004)
Cent X-3 17 Msun O type star
Porb ~2 d
Wind accretion --> longer spin period (>3 d)
HMXB (high mass X-ray
binaries)
L acc ~
Companion
GM NS Ṁ
R NS
NS
NS
Persistent -->
constant
luminosity
1035-1038 erg s-1
Transients -->
variable
luminosity
~1032-1034 erg s-1 (quiescence)
~1036-1038 erg s-1 (week-to-months long outbursts)
.
Variations of the luminosity <--> Variations of MW along the orbit
HMXB with OB companions
L~102 (OB)
L~103-104 (Be)
(stella et al. 1986)
Modulation of the mass flow rate along the orbit
NOT ENOUGH !!--> INTERACTION PROCESSES
WIND – NS MAGNETOSPHERE
Interaction processes in HMXB with magnetized Neutron Stars
Vw = wind velocity (~1000 Km s-1)
Vx = velocity of compact object (~300 Km s-1)
Ra = capture radius (~4x1010 cm ~ 2-3 R*)
OB
STAR
NS
Ra
WIND
BOW SHOCK
R a=
2 GM
V 2X
X
V 2W
~
2 GM
V 2W
X
10
~ 4 × 10 cm
Ṁ c apt / Ṁ w ~ 10
−5
(Bondi, 1952)
Closer to the neutron star: detailed interaction processes
Ra
Rco
Inflowing
matter
RM
RM~3x10
9
.
B121/3 M-6-1/6
cm
Rco~4x109 Pspin100-2/3 cm
Ra~4x1010 v8-2 cm
B
These radii depend on:


Neutron Star Spin Period

Neutron star magnetic field


Mass loss rate from the
companion star
Velocity of the Wind
Different Position of these radii --> Different interaction regimes
DIRECT ACCRETION REGIME
.
1/3
RM~3x109 B12
Ra
M-6-1/6 cm
Rco~4x109 Pspin1002/3 cm
<
Ra~4x1010 v8-2 cm
Rco
RM
L acc ~
GM NS Ṁ capt
R NS
Lacc~1036 erg s-1
.M ~1016 g s-1
capt
.M ~10-6 M yr-1
w
sun
.
> R ~4x10
-1/6
M-7 cm< R ~4x10
THE CENTRIFUGAL INHIBITION OF ACCRETION: The Centrifugal barrier (“Propeller”)
RM~4.4x10
9
B121/3
co
10
a
Ra
RM
Rco
9
L pro~
=L
Pspin1002/3 cm
v8-2 cm
GM NS Ṁ capt
RM
R NS
acc
RM
~ 10
−3
L acc
Lpro~1033 erg s-1
.M ~1015 g s-1
capt
. ~10-7 M yr-1
M
w
sun
(Stella et al. 1986)
=
HMXB with OB or Be companions
.
Transition from DIRECT ACCRETION --> PROPELLER due to variation in MW
larger eccentricity <--> larger variation in MW --> larger L
.
Everything seems to be well understood ...but...
(stella et al. 1986)
A new class of HMXB: Supergiant Fast X-ray Transients
(Sakano et al. 2002)
SAX J1818.6-1703
AX1845.0-0433
(Bozzo et al, in prep.)
(Sguera et al. 2005)
SFXT: new class, new problems
OB supergiant companion stars (~circular orbits?)
36-1037 erg s-1
 X-ray luminosities ~10

L~104-105
 few hours-long non periodic outbursts
 evidences of P
spin~1000-2000 s
Similar to other
TRANSIENTS

Ra
R
M
R
co
How Can we apply
the wind accretion
theory to SFXT?
Not Similar to other
TRANSIENTS
DIRECT ACCRETION REGIME
.
1/3
RM~3x109 B12
Rco~1.5x1010 Pspin10002/3
M-6-1/6 cm <
cm
R ~4x1010 v -2 cm
a
Ra
8
Rco
RM
L acc ~
GM NS Ṁ capt
R NS
Lacc~1036 erg s-1
.M ~1016 g s-1
capt
.M ~10-6 M yr-1
w
sun
Apparently it's working fine....
Propeller Regime: CENTRIFUGAL BARRIER
B14
RM~2x1010
.
1/3
10
2/3
>
R
~1.5x10
P
co
spin1000
-1/6
M-7 cm<cmR ~4x1010 v -2 cm
a
8
RM
Ra
Rco
L pro~
=L
.
1) Variation of M
GM NS Ṁ capt
RM
R NS
acc
RM
−4
~ 10
Lacc
Open problems:
W
in few hours? few hours orbit? Impossible (OB star radius) !!
2) SFXT with SPIN PERIODS >1000 s REQUIRES superstrong
=
magnetic field
1) Variation of MW in few hours? few hours orbit? Impossible (OB star radius)
!!
.
MW changes on few hours time-scale
CLUMPY WINDS:
Ṁ w ∝

>>
clump
w

w
(Prinjia et al. 2005; Walter et al. 2006)
Bow Shock
NS
2) SFXT with SPIN PERIODS >1000 s REQUIRES superstrong
magnetic
field
Magnetars:
Neutron Stars powered by magnetic energy
MAGNEtic sTARS (B>1014G)
Theoretically ok!
Fast spin (few ms) and differential
rotation generate internal toroidal
field B > 1015 G --> external field
up to 1016 G
(Thompson & Duncan 1992)
Observational evidences?
Evidences in isolated neutron stars (e.g. SGR)
Energy of ~ 1044 ergs caused by large scale rearrangement of core B field
(rotation and accretion power ruled out)
Are There Magnetars in HMXBs? The case of Supergiant Fast X-ray Transients
XMM-NEWTON proposal ACCEPTED !!!
