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Title:
Is The "Slow Burster" An Ultracompact Binary?
Authors:
Galloway, Duncan; Yao, Y.; Marshall, H.
Affiliation:
AA(Monash University, Australia), AB(MIT), AC(MIT)
Publication:
American Astronomical Society, HEAD meeting #10, #10.23
Publication Date:
03/2008
Origin:
AAS
Bibliographic Code:
2008HEAD...10.1023G

Abstract

4U 1728-34 is a well-known low-mass X-ray binary and thermonuclear burst source for which the orbital period is presently unknown. We made a 250 ks Chandra HETGS observation in order to carry out high-resolution spectroscopy of the photospheric radius-expansion bursts which are normally observed from the source. We found the source in an unexpected (and rare) high-flux state, exhibiting weak bursts every 2--4 hours with no evidence for radius-expansion. The co-added spectra from 25 bursts detected during the observation exhibited no discrete features; the persistent spectrum was similarly featureless.

The burst profiles and α-values indicate H-poor fuel, but the recurrence times were too short to exhaust the accreted hydrogen via steady burning, for solar composition in the accreted fuel. The burst properties suggest that the source accretes material with significantly sub-solar H-fraction, as from an evolved donor in an ultracompact binary. We also detected at marginal significance a 10.77 min periodic signal in the persistent low-energy X-ray lightcurve which can be attributed to an orbital modulation. These observations strengthen the case for an ultracompact system remarkably similar to 3A 1820-30.

Support for this work was provided by the National Aeronautics and Space Administration through the Smithsonian Astrophysical Observatory contract SV3-73016 to MIT for Support of the Chandra X-Ray Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of the National Aeronautics Space Administration under contract NAS8-03060.

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