Gravitationally Redshifted Absorption Lines in the X-Ray Burst Spectra

NTHUNTHU

2008.01.032008.01.03

Lu, Ting-NiLu, Ting-Ni

X-ray burst X-ray burst

a neutron star and an accreting a neutron star and an accreting companioncompanion

Type I X-ray burstType I X-ray burst has a sharp rise has a sharp rise followed by a slow and gradual followed by a slow and gradual decline of the luminosity profiledecline of the luminosity profile

Type II X-ray burstType II X-ray burst exhibits a quick exhibits a quick pulse shape and may have many fast pulse shape and may have many fast burst separated by minutes burst separated by minutes

Determine properties of neutron Determine properties of neutron starsstars

measuring the gravitational redshift of spectral lines produced in the surface of neutron stars

Previous Observation - Previous Observation - EXO 0748-676

Cottam et al. (2002) reported features in the burst spectra of EXO 0748-676 which they interpreted as gravitationally redshifted absorption lines of Fe xxvi during the early phase of the bursts, and Fe xxv and perhaps O viii during the late phase

The lines would then all have a gravitational redshift of z = 0.35, which would correspond to a neutron star in the mass range of 1.4–1.8 M๏ and in the radius range of 9–12 km.

Obs. Results of GS 1826-24

Light curve of 16 X-ray bursts from GS 1826-24

Obs. Results of GS 1826-24

Representative burst profile (top) and temperature profile (bottom)from GS 182624 as observed with EPIC-pn.

Obs. Results of GS 1826-24

Correlation between the blackbody temperatures and count rates of all bursts detected with EPIC-pn.

Obs. Results of GS 1826-24XMM-Newton RGS average background-subtracted spectra for 16 X-ray bursts from GS 1826-24. The three temperature phases are defined according to the spectral fits with the EPIC-pn data. An absorbed blackbody model is superimposed in red. The Ne x line at 12.1 (with 3.4 σ confidence) Å is included in the low-temperature spectrum.

Discussion The follow-up observations of EXO 0748-676 have fa

iled to again reveal absorption lines from the stellar surface (Cottam et al. 2007)

Discussion

Absorption features in burst spectra are very sensitive to the accretion rate, temperatures, density, and rotation frequency of the neutron star (Bildsten et al. 2003; Chang et al. 2005, 2006).

Temperature of the burst spectra

Discussion

Rotational broadening:

1) 1) Chang et al. (2005) calculated that for an edge-on neutron star with a rotation rate >200 Hz, the absorption features would become undetectable.

2) Chang et al. (2006) also showed that if a rapidly rotating neutron star is seen face-on, the spectral lines will be narrower.

Discussion

Chemical composition of the atmosphere.

The possibility that the lines seen in EXO 0748-676 are not gravitational redshifted iron lines from the surface.

ReferenceReference

Albert K. H. Kong, Jon M. Miller, Mariano Me´ndez, Jean Cottam, Walter H. G. Lewin, Frederik Paerels, Erik Kuulkers, Rudy Wijnands, and Michiel van der Klis 2007, ApJ, 670, L17–L20

Cottam, J., Paerels, F., & Mendez, M. 2002, Nature, 420, 51

Cottam, J., Paerels, F., Mendez, M, Boirin, L., Lewin, W. H. G., Kuulkers, E., & Miller, J. M. 2007, ApJ, in press (arXiv:0709.4062)

THANK YOU !THANK YOU !