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Cyclotron absorp.on features in neutron star systems observed by INTEGRAL
Wei Wang
Na.onal Astronomical Observatories, Beijing, China Cosmic magnetism, Kiama June 7 2010


Contents
· Magne.c fields in neutron stars · History of observing cyclotron line features · Cyclotron absorp.on lines detected by INTEGRAL confirm previous known: Vela X 1; Her X 1; V0332+53; A0535+26 new candidates: 4U 2206+54; 2S 0114+65; IGR J01583+6713


Magne.c fields in Neutron stars
· Magne.c fields of pulsars indirect ways: Assuming a magne.c dipole with meas

urements of spin period P and deriva.ve of P, surface magne.c field given by

then we find three classes of pulsars: normal pulsars: P=30ms ­ 9 s , Bs = 1012 1013 G ; millisecond pulsars: P=1.3 ­ 20 ms, Bs=108 109 G ; magnetars : P=5 12 s, Bs =1014 1015 G


· Magne.c flux conserva.on assump.on neutron star forma.on from massive star collapse
R2NS BNS = R2i Bi some magne.c stars with Bstar= 103 G then from flux conserva.on BNS = 103 (600000/10)2= 3.6x1012 G many magne.c white dwarfs discovered, Bwd=106 8 G neutron stars may form from accre.on induced collapse WD BNS = 106 8 (6000/10)2 = 3.6 x10 11 13 G


· A direct way: cyclotron resonance

scahering features (CRSF)

photons being resonantly scahered by electrons/protons out of the observers line of sight. electron/proton energy are quan.zed into Landau levels, so energy of cyclotron line is a direct measure of magne.c field strength in the scahering region.

on surface of neutron star, observed energy has gravity redshil (1+z) =1.3


Electron cyclotron line measurements:

If B =10

12 13

G , observed E1 = 8 80 keV

High energy space based telescope required ! Proton line or electron line ?
10keV if electron case B = 1.2x1012 G if proton case B = 2.4x1015 G For finding Magnetars B = 1014 15 G electron case E1= 0.8 8 MeV (difficult to detect) proton case E1 = 0.4 ­ 4 keV (contamina.ons with atomic lines)


History
The first cyclotron line in neutron star system observed by a balloon flight (10 hours in 1976): Her X 1 (Truemper et al. 1977; 1978) Originally they thought lines were emission lines at 58 keV and 110 keV Now the absorp.on line features at 42 keV and 80 keV were accepted.


History
· Pair lines in 4U 0115+63 (12/24 keV) by HEAO 1 (White et al. 1983) · Ginga era (1990s): more new cyclotron line discoveries Cep X 1 (30 keV), 4U 1538 52 (20 keV), 4U 1907+09 (21 keV), V0332+53 (27 keV), Vela X 1 (25/50 keV , GX 301 2 (35 keV) · A 0535+26 (1995) : 50 keV by MIR/TMM and HEXE ; 110 keV by CGRO/OSSE


History
· 1996, both RXTE and BeppoSAX were launched. new discoveries of cyclotron lines: 4U 1626 67 (40 keV), Cen X 3 (30 keV), XTE J1946+274 (36 keV), X Per (30 keV), LMC X 4 (100 keV), OAO 1657 415 (36 keV), 4U 1700 37 (37 keV), 1M 0656 072 (36 keV) higher harmonics in known sources: 4U 0115+63 (5 harmonics), Her X 1, 4U1907+09, 4U 1538 52, V 0332+53 (3 harmonics)


4U 0115+63 The spectrum in one pulse phase (Heindl et al. 2000) 5 lines fitted: 13, 24, 36, 48, 62 keV

V 0332+53 The spectrum in an outburst during 2004 Nov 3 lines: 27 keV, 54 keV, 80 keV (Coburn et al. 2006)


INTEGRAL observa.ons
· The INTErna.onal Gamma Ray Astrophysics Laboratory (INTEGRAL) is an European (ESA) Gamma Ray Observatory Satellite Mission for the study of cosmic gamma ray sources in the keV to MeV energy range.
Launch Date: October 17th 2002 Launch Mass: 4000 kg Orbit Period: 72 hours Apogee: 153 000 km Inclina.on: 52.6 degrees

Sol gamma ray imager IBIS

Perigee: 9000 km


The imager on board INTEGRAL (IBIS) : Coded mask imaging
Performance Energy range: 15 keV 10 MeV Detector area: 3000 cm2 Spectral resolu.on: 9% @ 100 keV Field of view: 9 x 9 deg Angular resolu.on: 12 arcmin Source Loca.on: < 1 arcmin Narrow line sensi.vity (106 s): 2.0 x 10 5 ph s 1 cm 2 Con.nuum sensi.vity (106s): 5 x 10 7 ph s 1 cm 2 keV 1 Timing accuracy: 0.92 ms Mass: 677 kg
Good angular resolu1on, high sensi1vity, spectral resolu1on in 16 ­ 100 keV

ability to search for cyclotron line features


Present exposure map of INTEGRAL all sky surveys (early 2010)
Deep surveys along the Galactic plane in soft gamma-rays Good news for us to study neutron star systems


Vela X 1
Presence of two cyclotron lines is debated for a long time, specially for the 27 keV line (Kreykenbohm et al. 2002; La Barbera et al. 2003). INTEGRAL/IBIS observations confirmed the two lines (Schanne et al. 2007). 27 keV 54 keV


Her X 1
The first cyclotron line (Truemper et al. 1978) The variations of the centroid line energy with the luminosity of the source

39 keV

Klochkov et al. 2007


V0332+53
V 0333+53 is also a Be transient X-ray pulsar P=4.375 s The spectrum of the outburst during 2004 November 3 2 4 7 cyclotron lines: 6 keV 9 keV 4 keV


A0535+26
A Be transient X-ray pulsar, detected in the recurrent outburst states The spectrum of the outburst in 2009 August (Caballero et al. 2010) One line at 45 keV Second at 102 keV 45 keV 102 keV


New Candidates
2S 0114+65
One of the slowest pulsation neutron star P=2.7 hr (Hall et al. 2000; Farrell et al. 2008) And the pulsar is spinning up! (Wang 2010) Bonning & Falanga (2005) found a feature around 40 -50 keV in the residuals. Two-harmonic cyclotron line fit found lines at 22 keV and 44 keV. Need further check!

1990

2005

1x10-6 s s

-1


4U 2206+54
A slow pulsa.on pulsar P=1.54 hr (Reig et al. 2009, Wang 2009)

2004 June

32 keV

only permanent wind fed HMXB with a main sequence donor 2005 Dec Low X ray luminosity The spectra when it became brighter 2 lines were found 29.6±2.8 keV 59.5 ±2.1 keV (Blay et al. 2005, Wang 2009)

30 keV 60 keV


IGR J01583+6713
A hard X-ray transient source discovered by INTEGRAL/IBIS (Steiner et al. 2005). On Dec 6 2005, IBIS detected the new source. The flux decayed; never detected after 2005 Dec 13 (Wang 2010). Optical observations suggested it may be a Be/X-ray star (Halpern et al. 2005). The spectrum of the transient source during the outburst Two cyclotron lines : 35.3 ±1.6 keV 67.9 ±4.8 keV Possible nature: Magnetic neutron star in a Be transient binary? Search for the period in next outburst if possible 35 keV 68 keV


Summary
· At present in about 20 neutron star systems (HMXBs), cyclotron lines are detected; · INTEGRAL discovered 3 new ones; expected to find more (2003 2012); · With each genera.on of new satellites we are getng a wealth of observa.ons on line features in neutron stars, especially with the large number of higher harmonics, now no doubt that these features are cyclotron lines, which are used to measure the magne.c field of neutron stars. · Theore.cal work on cyclotron line is s.ll currently lagging behind observa.ons.


Magnetic field strengths of neutron stars determined by cyclotron line observations

B

(1 ­ 6) x10

12

G