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Light curves of photospheric radius expansion (PRE) bursts, a subset of typeI X-ray bursts, have been used as standard candles to estimate the "nominal PREdistances" for 63% of PRE bursters (bursters), assuming PRE burst emission isspherically symmetric. Model-independent geometric parallaxes of burstersprovide a valuable chance to test models of PRE bursts (PRE models), and can beprovided in some cases by Gaia astrometry of the donor stars in bursters. Wesearched for counterparts to 115 known bursters in the Gaia Early Data Release3, and confirmed 4 bursters with Gaia counterparts that have detected (>3sigma,prior to zero-point correction) parallaxes. We describe a generic approach tothe Gaia parallax zero point as well as its uncertainty using an ensemble ofGaia quasars individually determined for each target. Assuming the sphericallysymmetric PRE model is correct, we refined the resultant nominal PRE distancesof three bursters (i.e. Cen X-4, Cyg X-2 and 4U 0919-54), and put constraintson their compositions of the nuclear fuel powering the bursts. Finally, wedescribe a method for testing the correctness of the spherically symmetric PREmodel using parallax measurements, and provide preliminary results.

publications of the astronomical society of australia/publications astronomical society of australia

Gaia EDR3 parallaxes of type I X-ray bursters and their implications on  the models of type I X-ray bursts: a generic approach to the Gaia parallax  zero-point and its uncertainty