The Super-slow Pulsation X-ray Pulsars in High Mass X-ray Binaries

Wei WangNational Astronomical Observatories,

Beijing China

The IAU XXVII General Assembly, Beijing August 20-31 2012

IAUS 291 Neutron stars and pulsars: Challenges and opportunities after 80 Years

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

The Pspin- Porbit diagram for Super-slow X-ray pulsars in HMXBs

Recently some very slowly pulsation neutron stars are discovered in some binaries and even in supernova remnants.

• 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?)

Other candidates of superslow pulsation 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.

Why special for superslow pulsation X-ray pulsars

Temporal and Spectral Properties of the superslow pulsation pulsars in HMXBs

• Long spin period searches INTEGRAL (3-day orbit) has advantage for long-duration on-

source 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

1.06x 10-6 s s-1

Spin-up trend seems accelerating in last 20 years

Wang 2011

Spectral properties of 2S 0114+65• 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

RXTE/PCA 2-9 keV

IBIS 18 -200 keV

Farrell et al. 2008

Wang 2011

The origin of hard X-ray tails is unknown: related to jets? hot corona?

Hard X-ray tail in the spectrum of 2S 0114+65

4U 2206+54• The only persistent X-ray emitting source with a main-

sequence 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

Average Spin–down rate of 4.9x10-7 s s-1

The spin down trend is long-term behaviour ? Any torque changes ?Need more observations

BeppoSAX archive

INTEGRAL/IBIS

XMM

-New

ton

Wang 2012; Reig et al. 2012

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

Porbit =19.12 day

20 -80 keV

ASM flux peak(1.5-12 keV)

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

Blay et al. 2005

Wang 2009

E1 = 29.6± 2.8 keVE2 = 59.5± 2.1 keVBs=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

• Possible evolution link in slow pulsation X-ray pulsars – supergiant X-ray pulsars

Discussion and Summary

• 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!