Superexpansion as a possible probe of accretion in 4U 1820-30

¹ SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
e-mail: jeanz@sron.nl
² Dept. Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
³ National Superconducting Cyclotron Laboratory, Dept. of Physics & Astronomy, and Joint Institute for Nuclear Astrophysics, Michigan State University, East Lansing, MI 48824, USA
⁴ Los Alamos National Laboratory, B244, Los Alamos, NM 87545, USA

Received: 31 July 2012
Accepted: 2 October 2012

Abstract

The ultracompact X-ray binary 4U 1820-30 is well known for its  ≈170-d superorbital modulation in X-ray flux and spectrum, and the exclusiveness of bursting behavior to the low hard “island” state. In May−June 2009, there was an exceptionally long 51-d low state. This state was well covered by X-ray observations and 12 bursts were detected, 9 with the high-throughput RXTE. We investigate the character of these X-ray bursts and find an interesting change in their photospheric expansion behavior. At the lowest inferred mass accretion rates, this expansion becomes very large in 4 bursts and reaches the so-called superexpansion regime. We speculate that this is due to the geometry of the inner accretion flow being spherical and a decreasing accretion rate: when the flow geometry nearest to the neutron star is spherical and the accretion rate is low, the ram pressure of the accretion disk may become too low to counteract that of the photospheric expansion. In effect, this may provide a novel means to probe the accretion flow. Additionally, we observe a peculiar effect: the well-known cessation of X-ray bursts in the high state is too quick to be consistent with a transition to stable helium burning. We suggest an alternative explanation, that the cessation is due to the introduction of a non-nuclear heat source in the neutron star ocean.