An indication of the Eddington luminosity during X-ray bursts and constraints on the mass and radius of neutron stars in 4U 1608-52 and MXB 1728-34

Details An indication of the Eddington luminosity during X-ray bursts and constraints on the mass and radius of neutron stars in 4U 1608-52 and MXB 1728-34 An indication of the Eddington luminosity during X-ray bursts and constraints on the mass and radius of neutron stars in 4U 1608-52 and MXB 1728-34

Feb 1989

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1989mnras.236..545f&link_type=abstract

Monthly Notices of the Royal Astronomical Society (ISSN 0035-8711), vol. 236, Feb. 1, 1989, p. 545-557.

Physics

11

Neutron Stars, Stellar Luminosity, Stellar Physics, Stellar Radiation, X Ray Stars, Bolometers, Equations Of State, Light Curve, Red Shift, Stellar Envelopes, Stellar Mass

Scientific paper

Some type-1 X-ray bursts display an intensity dip around the peak, visible only in soft X-ray flux but not in hard X-rays. We show that this dip can be explained in terms of the emergent spectra from the atmosphere of low emissivity and occurs only when the luminosity is very close to the Eddington limit. Hence the peak of the bursts gives a good measure to identify the flux level corresponding to the Eddington limit. Based on the result, constraints on the mass and radius of the neutron stars in 4U 1608-52 and MXB 1728-34 are derived from the variations in the flux and spectrum during the cooling phase observed by Gottwald et al. and Hoffman et al. respectively. Similar mass-radius relations result for both sources, since the accreted material is considered to be hydrogen-rich in 1608-52 and hydrogen-poor in 1728-34. These rule out the very stiff equations of state, and also the very soft equations of state unless the mass is very small. Within current uncertainties, the mass-radius relations for 1608-52 and 1728-34 are consistent with those found for all other sources studied to date, i.e. 4U/MXB 1636-53, 4U 1746-37 and 4U/MXB 1820-30. Further, if the redshift of an absorption line detected from 1608-52 (observed by Nakamura, Inoue & Tanaka 1988) is interpreted in terms of the combination of the gravitational redshift and the transverse Doppler effect due to the rotation of accreted gas, the mass and radius are estimated in the range 1.3-1.6Msun and 9.6-11.0 km, respectively.

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