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IAUS 291 Neutron stars and pulsars: Challenges and opportunities after 80 Years
The Super-slow Pulsation X-ray
Pulsars in High Mass X-ray Binaries
Wei Wang
National Astronomical Observatories,
Beijing China
The IAU XXVII General Assembly, Beijing August 20-31 2012
Outline
• Super-slow pulsation neutron star systems:
how to define them - Pspin> 1000 sec ?
• Super-slow X-ray pulsars in HMXBs :
Temporal and spectral properties by different
observations
• Possible physical origin/nature and evolutional track
of super-slow X-ray pulsars in HMXBs – a new
population of neutron stars?
Spin period ranges of normal neutron star systems
• Radio pulsars: 1.3ms – 10 sec
• Magnetars: 2 – 12 sec
• Millisecond pulsars in LMXBs : 1.5 -30 ms
Spin period ranges of normal neutron star systems
• Neutron star high-mass X-ray binaries:
Be X-ray transient
(main-sequence companion star)
Supergiant binaries
(supergiant companion)
Corbet diagram
Recently some very slowly pulsation neutron stars are discovered
in some binaries and even in supernova remnants.
The Pspin- Porbit diagram for Super-slow X-ray pulsars in HMXBs
Other candidates of superslow pulsation pulsars
• Two super-slow pulsation X-ray pulsars are also
discovered in the symbiotic low-mass X-ray
binaries: 4U 1954+31 (5 hr); IGR J16358-4724
(5850 s). No orbital modulations are found in two
sources.
• 1E 1613.48−5055 in a young supernova remnant
RCW 103
P = 6.67 hours
(single or binary?)
Why special for superslow pulsation X-ray pulsars
Origin of spin period in X-ray pulsars
Standard evolution of neutron star binaries:
a) ejector state: spin-down like radio pulsars;
b) propeller state: spin-down by interaction between magnetosphere
and stellar winds;
c) accretor state: Pspin reaches a critical value; switch on as X-ray
pulsars as observed.
(Pringle & Rees 1972 Ghosh & Lamb 1978)
The maximum spin period which can be reached in different observed
conditions (magnetic field; accretion rate) :
from several seconds up to near 1000 s.
Then what channels produce the long spin period higher 1000 s ? It is a
key question here.
Temporal and Spectral Properties of the
superslow pulsation pulsars in HMXBs
• Long spin period searches
INTEGRAL (3-day orbit) has advantage for long-duration onsource observations (>100 ks), helpful to determined the spin
period of super-slow pulsation (>1 hour);
Probe of possible evolution of spin period by long-term
monitoring
• Spectral properties and variation in hard X-rays
combined with IBIS and JEM-X, the 3 – 200 keV spectra for
two objects can be extracted – variations; cyclotron
absorption features; hard X-ray tails etc
2S 0114+65
• B1 type supergiant companion
• Orbital period of 11.59 days (Crampton et al. 1985; Corbet et
al. 1999)
• The spin-up trend of the neutron star in 2S 0114+65 was
found in last 20 years:
• 1996 by Hall et al.: Pspin =2.73 hr
• 2004 by Bonning & Falanga: Pspin =2.67 hr (IBIS)
• 2006 by Farrell et al. : Pspin =2.65 hr (RXTE/PCA)
• 2003 Dec – 2008 May by Wang 2011: Pspin =2.67 hr - 2.63 hr
(IBIS )
Spin-up trend of 2S 0114+65 from 1986 - 2008
6.2x10-7 s s-1
8.9x 10-7 s s-1
Spin-up trend seems accelerating
in last 20 years
1.06x 10-6 s s-1
Wang 2011
IBIS 18 -200 keV
Spectral properties of 2S 0114+65
RXTE/PCA 2-9 keV
• The spectrum generally can be described by a (absorbed)
power-law model plus high energy cutoff
• The orbital phase-resolved spectra show the hard spectrum
and low hydrogen column density at the maximum X-ray
luminosity
Wang 2011
Farrell et al. 2008
Hard X-ray tail in the spectrum of 2S 0114+65
The origin of hard X-ray tails is unknown: related to jets? hot corona?
4U 2206+54
• The only persistent X-ray emitting source with a mainsequence star companion O9.5V;
• Orbital period : 9.6 day before 2006, disappears now;
19.12 day modulation was found by Swift and ASM data,
thought to be the real orbital period (Corbet et al. 2007; Wang
2009);
• A slow pulsation X-ray pulsar was recently indentified:
Pspin = 5560 sec (Reig et al. 2009 ; Wang 2009) by both
INTEGRAL and RXTE.
Spin-down trend of 4U 2206+54
Wang 2012; Reig et al. 2012
XMM-Newton
INTEGRAL/IBIS
The spin down trend
is long-term
behaviour ? Any
torque changes ?
Need more
observations
BeppoSAX archive
Average Spin–down
rate of 4.9x10-7 s s-1
Orbital phase-resolved spectra of 4U 2206+54
• The similar behaviour to supergiant X-ray binaries:
• Low column density and hard spectrum around maximum flux
ASM flux peak
(1.5-12 keV)
20 -80 keV
Porbit =19.12 day
Hard X-ray peak
(about 0.1 orbital phase difference)
Cyclotron resonance absorption features in
4U 2206+54
• The features were reported by INTEGRAL/IBIS:
•
fundamental line around 30 keV
•
second harmonic at ~ 60 keV
Wang 2009
Blay et al. 2005
E1 = 29.6± 2.8 keV
E2 = 59.5± 2.1 keV
Bs=3.3x1012 G
Be X transient SXP 1062
• Located in the Small Magellanic Cloud
• Pspin=1032 s, associated with a SNR (age (2-4)x104 yr)
• A large spin-down rate of 3x10-6 s/s.
Haberl et al. 2012
Returning to the question:
what is physical origin for long spin period?
• Li & van den Heuvel (1999): born as a magnetar with B>1014 G,
allow for the neutron star to spin down slower than 1000 s in
Myrs, and field decays to 1012 G at present (super-slow
pulsars as magnetar descendants)
• Ikhsanov (2007): a phase “subsonic propeller” between the
transition from known supersonic propeller state to accretor
state would allow for the long spin period :
Applying the above formulae to the case of 4U 2206+54/2S 0114+65, one
derives the magnetic field of >= 1014 G!
Alternative approaches:
• Spin-down rate in accreting state in standard model
(Lipunov 1992)
for SXP 1062 , B= 3x1014 G
for 4U 2206+54 , B= 5x1013 G
• Recently , a new theory of quasi-spherical accretion for X-ray
pulsars is developed (Shakura et al. 2012):
the magnetic field in wind-fed neutron star systems is given by
However, we still find the derived magnetic field of
2x1014 G for 4U 2206+54
>1014 G for SXP 1062
These super-slow pulsation pulsars could be accreting
magnetars?
Some arguments for magnetar assumption
• Hard X-ray spectrum for superslow X-ray pulsars : power-law with cutoff ,
Γ~1.8 -2.5
magnetars above 20 keV: very hard, single power-law, Γ~1.0 -1.5
• Typical X-ray luminosity of magnetars : 1035 -1036 erg/s ,
4U 2206+54/2S 0114+65: in the range of 1034 -1036 erg/s ;
the source must be powered by accretion not by magnetar activity,
requiring magnetar luminosity lower than 1034 erg/s
• 30 keV lines interpreted as electron lines similar to other neutron star
binaries : normal neutron star or proton lines: magnetar
• Uncertainties and difficulty in explaining the long spin period exist
• Maybe the standard evolution channels is not suitable for these class of
super-slow pulsation X-ray pulsars
Discussion and Summary
• Possible evolution link in slow pulsation X-ray pulsars –
supergiant X-ray pulsars
• If 4U 2206+54/SXP 1062 continue to spin down, spin period is
longer than 10000 s within few hundred years;
• 4U 2206+54 may undergo transition to spin-up channel in
future like 2S 0114+65;
• The possible progenitor of B1 supergiant in 2S 0114+65 is
O9.5V star (main-sequence star type in 4U 2206+54) in stellar
evolutional track (Meynet et al. 1994);
• The spin-up process of 2S 0114+65 will reach the equilibrium
period of less than ~ 1000 s through wind accretion;
• Supergiant X-ray binaries should be the older systems in the
equilibrium spin period range after rapid evolution phase like
2S 0114+65 and 4U 2206+54;
• The lifetime of super-slow pulsation phase (>>1000 s) would
be short, so the objects are rare but now fortunately we
detect them.
A Brief Summary
• Super-slow pulsation X-ray pulsars are
younger X-ray binary systems (age <1 Myr,
even 104 yr);
• They could be a new class of neutron star
system – accreting magnetars.
Thank you for your attention!