X-ray Observations of Solitary Neutron Stars an adventure to understand the structure and evolution of 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 sources) some associated with SNRs some truly solitary

o o o o o o o o o o o o o o o o O  -ray pulsar O other RQNS o

* 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, Z m 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 J at 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 approximation is not good enough.

high-energy excess and limb darkening (Wu 2003, master thesis, NTHU) BeppoSAX spectrum of 1RXS J (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 1E PN (top) & MOS (bottom) spectra fitted with two blackbodies (kT = 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 (10 34 – 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 L sd SNR associations CX J (in SMC) 1E (AX J ) G E G U RX J E Kes 73 AXPs return

Soft Gamma Repeaters SGRs ( ) SNR N49 (in LMC) G G G Distance 55 kpc 17 kpc 5 kpc 11 kpc -- (in bursts) L erg s erg s erg s erg s P 8 sec sec (giant flare) ( ) ( ) L erg s erg s (in quiescence) L erg s erg s erg s erg s P sec 5.16 sec (6.4 sec) -- L s-d erg s erg s SGRs (The associations with SNRs are not secure: Gaensler et al. 2001, ApJ 559, 963) return