A deep look at the neutron star 4U 1705-44 in a low-hard state

Both neutron star and black hole binaries display a distinct range of spectral states that have been known of for decades. But, a key unanswered question remains: what drives the transition between states? It is still not clear how the geometry of the accretion flow changes through these state transitions, though one key parameter is the inner extent of the accretion disk. In black hole X-ray binaries there is building observational evidence that the inner disk does not truncate down to luminosities of ~1.5E-3 L/L_edd. However, there is yet to be a high quality spectrum of a neutron star LMXB at such low luminosities to test the nature of the accretion disk in these objects. Here, we propose a deep 120 ksec ToO observation of the neutron star LMXB 4U 1705-44 when it transitions into a low/hard state.