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T. Narita, J.E. Grindlay (CfA), D. Barret (CESR)
We present analysis of 10 kilosecond ASCA observation of type I X-ray burster 4U1728-34 while it was in the low hard state. The source luminosity in the 0.5-10 keV band was 1\times1037 ergs s-1 at an assumed distance of 4.3 kpc. The ASCA spectrum is best fit by combined power-law and a soft thermal components. The thermal component can be either a kT=0.4 keV, R=18 km blackbody or a R\sqrt{cos\theta}=11.3 km multi-color disk blackbody. No emission lines were detected. The column density to the source was well constrained at 3.20±.05\times1022 cm-2. One moderate sized X-ray burst was also observed in the course of our pointing. Once the persistent emission was subtracted, the burst spectrum can be fit by a blackbody model. We analyzed the time dependent change in the spectrum and found no evidence of photospheric expansion, however the blackbody radius increased asymptotically to 7.5 km as the flux and temperature decreased. If the neutron star radius is interpreted to be 7.5 km, the soft thermal component seen in the persistent emission is likely due to the disk.
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