Abstract
We report on the long-term monitoring of X-ray dips from the ultracompact low-mass X-ray binary (LMXB) XB 1916-053. Roughly one-month interval observations were carried out with the Rossi X-ray Timing Explorer (RXTE) during 1996, during which the source varied between dim, hard states and more luminous, soft states. The dip spectra and dip light curves were compared against both the broadband luminosity and the derived mass accretion rate (). The dips spectra could be fitted by an absorbed blackbody plus cutoff power-law nondip spectral model, with additional absorption ranging from 0 to > 100 × 1022 cm-2. The amount of additional blackbody absorption was found to vary with the source luminosity. Our results are consistent with an obscuration of the inner disk region by a partially ionized outer disk. The size of the corona, derived from the dip ingress times, was found to be ~109 cm. The corona size did not correlate with the coronal temperature, but seemed to increase when also increased. We discuss our findings in the context of an evaporated accretion disk corona model and an ADAF-type model.
Highlights
The low-mass X-ray binary (LMXB) XB 1916053 is the most compact X-ray binary which exhibit intensity reductions, or dips
We present results of a long term RXTE monitoring of x-ray dips in XB 1916-053
The persistent emission is best fitted by blackbody and cutoff power law components
Summary
The low-mass X-ray binary (LMXB) XB 1916053 is the most compact X-ray binary which exhibit intensity reductions, or dips. More recent observations with ROSAT (Morley et al 1999), ASCA (Church et al 1997), and Beppo-SAX (Church et al 1998) found the deep dip spectra could be fit with a partially covered blackbody plus power-law model. The partial covering model implies an emission geometry where a compact blackbody is surrounded by an extended corona This picture is supported by RXTE observations of the non-dip emission from XB 1916-053 (Bloser et al 2000a). It is puzzling that effects of the outer disk obscuration of the inner disk region can vary greatly, even in a relatively small range of inclination angles The purpose of this investigation is to study the relationship between mass accretion rate and spectral and timing behavior of the dips in XB 1916-053 using RXTE data. An earlier analysis of the non-dip spectral shape from XB 1916-053 shows a correlated motion along its CCD which implies variation in M (Bloser et al 2000a)
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