X-ray Observations of Solitary Neutron Stars

an adventure to understand the structure and evolutionof neutron stars

國立清華大學物理系與天文所張祥光

* The concept of neutron stars

* The many faces of neutron stars

* An evolution story?

* Looking closer to a neutron star

* Coming next…

(Baym & Pethick 1979, ARA&A 17, 415)

(Scientific American, Feb. 2003)

The many faces of neutron stars

* radio pulsars .X-ray and –ray emission from pulsars .

* Anomalous X-ray Pulsars (AXPs) .

* Soft Gamma Repeaters (SGRs) .

* other radio-quiet neutron stars . some with possible –ray emission (unidentified EGRET sourc

es)some associated with SNRssome truly solitary

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oO -ray pulsarO other RQNS

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* For radio-quiet neutron stars, to find periodicity and its time derivatives in X-ray data is a crucial issue in understanding their various properties.

* There are different methods to perform periodicity search, e.g., FFT, epoch folding, Zm

2-test, H-test, etc. * The many faces of neutron stars may represent different evolution stages of a neutron star. The scenario is made more complicated by initial conditions and geometrical factors.

Radio image of CTA 1 with ROSAT/PSPC contours

(Slane et al., 2004, ApJ 601, 1045)

Using ASCA and XMM data, a probable period of RX J0007.0+7302 at 127.5 ms was found.(Lin & Chang, 2004, Ap&SS, in press)

(Chang, Lin, Chiu & Liang, 2004, Ap&SS, submitted)

X-ray pulsations from a compact clump in RCW 89

ROSAT/HRI image of RCW 89

(Brazier & Becker, 1997, MNRAS 284, 335)

(Possenti et al., 1996, A&A 313, 565)

(Finley et al., 1992, ApJ 394, L21)

Looking closer to a neutron star…For X-ray thermal emission from neutron stars, the blackbody approximationis not good enough.

high-energy excess and limb darkening(Wu 2003, master thesis, NTHU)

BeppoSAX spectrum of 1RXS J170849.0-400910(Chang, 2004, CJP 42, 135)

Spectral analysis of surface thermal emission from neutron stars may help to determine various properties at the surface, such as

temperature,magnetic field strength,composition, mass-to-radius ratio,viewing geometry, etc.

Looking closer to a neutron star…

The Vela pulsar’s Chandra spectrum

(Pavlov et al., 2001, ApJ 552, L129)

XMM spectrum of

1E1207.4-5209

PN (top) & MOS (bottom) spectra fitted with two blackbodies(kT = 0.211 keV, 0.40 keV)

(Bignami et al., 2003, Nature 423, 725)

Modelling the spectrum of surface thermal emission from neutron stars

2

Part I: the emergent spectrum from a local spot at the surface

the thermal bremsstrahlung cross section in a magnetized plasma

Modelling the spectrum of surface thermal emission from neutron stars

Part II: the composite spectrum from the whole stellar surface

* photon path bending

* temperature distribution over the surface

* features may be smeared out

We are constructing a set of codes, which in particular allows arbitrary magnetic field directions, contribution of thermal conduction, and a more proper treatment of line-broadening and polarization propagation.

* Modelling realistic spectra

Coming next…

Thank You!

* Measuring the polarization

* Are ‘neutron stars’ really neutron stars?

* More observations: temporal and spectral analysis

* Understanding emission mechanisms in neutron star magnetospheres and possible evolutionary scenarios

* Measuring neutron star oscillations (!!!)

(Thompson et al. 1999, ApJ 516, 297)

return

AXPs

Properties of AXPs

• spin periods in a narrow range (~ 6 – 12 s)

• relatively low X-ray luminosity (1034 – 1036 erg s-1), but higher than their spin-down luminosity

• no signature of a binary system

• very soft X-ray spectra

• some of them are associated with supernova remnants

Anomalous X-ray Pulsars

AXPs P (s) log Lsd SNR associations

CX J0110-7211 5.44 33.57 (in SMC) 1E 1048.1-5937 6.45 33.75(AX J1845-0300) 6.97 -- G29.6+0.11E 2259+586 6.98 31.75 G109.1-0.14U 0142+61 8.69 32.07RX J1708-4009 11.0 31.16 1E 1841-045 11.8 33.00 Kes 73

AXPsreturn

Soft Gamma Repeaters

SGRs 0526-66 1806-20 1900+14 1627-41 (1801-23)

SNR N49 (in LMC) G10.0-0.3 G42.8+0.6 G337.0-0.1 -- Distance 55 kpc 17 kpc 5 kpc 11 kpc --

(in bursts)

L 1042 erg s-1 1041 erg s-1 1041 erg s-1 1043 erg s-1 --P 8 sec -- 5.16 sec -- --

(giant flare) (1979.03.05) (1998.08.27)

L 1045 erg s-1 -- 1043 erg s-1 -- --

(in quiescence)

L 1036-37 erg s-1 1035.3 erg s-1 1034.5 erg s-1 1035 erg s-1 --P -- 7.47 sec 5.16 sec (6.4 sec) --

L s-d -- 1033.4 erg s-1 1034.2 erg s-1 -- --

SGRs(The associations with SNRs are not secure: Gaensler et al. 2001, ApJ 559, 963)return