CHINESE JOURNAL OF PHYSICS VOL. 39, NO. 4 AUGUST 2001

Determining the Magnetic Inclination of PSR B0656+14 from
its Thermal X-ray Emission

Hsiang-Kuang Chang

Department of Physics, National Tsing Hua University,

Hsinchu, Taiwan 300, R.O.C.
(Received May 18, 2001)

Heated polar caps of a neutron star may betray themselves in thermal X-ray emissions from pulsars as a second, higher-temperature thermal component. Observations of PSR B0656+14 show such a signature. Taking into account the effect of gravity, we look for the allowed ranges of its magnetic inclination angle and the observer's viewing angle constrained by the area ratio of the polar caps to the stellar surface as deduced from spectral fitting, and by the pulsed fraction and pulse width in its X-ray lightcurve. For surface gravity comparable to or smaller than the conventional value of $M/R\approx \text{0.2}$, our results suggest that, for PSR B0656+14, these two angles are within about 15$^\circ$ - 20$^\circ$ and 30$^\circ$ - 40$^\circ$, respectively. The above constraints do not distinguish between the inclination and viewing angles. For M/R = 0.3, no allowed range exists. Because of the large uncertainty in current spectral fitting, our results are only suggestive. However, we demonstrate that the exploitation of constraints from spectral fitting can help further in pinning down the geometry, whereas the earlier work used only the constraints from lightcurves. Geometry determined in this manner can serve as an initial solution for more detailed investigations.

PACS. 97.60.Gb - Pulsars.

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