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Many techniques for measuring neutron star radii rely on absolute fluxmeasurements in the X-rays. As a result, one of the fundamental uncertaintiesin these spectroscopic measurements arises from the absolute flux calibrationsof the detectors being used. Using the stable X-ray burster, GS 1826-238, andits simultaneous observations by Chandra HETG/ACIS-S and RXTE/PCA as well as byXMM-Newton EPIC-pn and RXTE/PCA, we quantify the degree of uncertainty in theflux calibration by assessing the differences between the measured fluxesduring bursts. We find that the RXTE/PCA and the Chandra gratings measurementsagree with each other within their formal uncertainties, increasing ourconfidence in these flux measurements. In contrast, XMM-Newton EPIC-pn measures14.0$\pm$0.3 % less flux than the RXTE/PCA. This is consistent with thepreviously reported discrepancy with the flux measurements of EPIC-pn, comparedto EPIC-MOS1, MOS2 and ACIS-S detectors. We also show that any intrinsic timedependent systematic uncertainty that may exist in the calibration of thesatellites has already been implicity taken into account in the neutron starradius measurements.

astrophysical journal/the astrophysical journal

Systematic Uncertainties in the Spectroscopic Measurements of  Neutron-Star Masses and Radii from Thermonuclear X-ray Bursts. III. Absolute  Flux Calibration