State Transitions in Ultracompact Neutron Star LMXBs: towards the Low Luminosity Limit

Abstract

Luminosity of X-ray spectral state transitions in black hole and neutron star X-ray binaries can put constraint on the critical mass accretion rate between accretion regimes. Previous studies indicate that the hard-to-soft spectral state transitions in some ultracompact neutron star LMXBs have the lowest luminosity. With X-ray monitoring observations in the past decade, we were able to identify state transitions towards the lowest luminosity limit in 4U 0614+091, 2S 0918-549 and 4U 1246-588. By analysing corresponding X-ray pointed observations with the Swift/XRT and the RXTE/PCA, we found no hysteresis of state transitions in these sources, and determined the critical mass accretion rate in the range of 0.002 - 0.04 ${\dot{\mathrm{M}}}_{\mathrm{Edd}}$ and 0.003 - 0.05 ${\dot{\mathrm{M}}}_{\mathrm{Edd}}$ for the hard-to-soft and the soft-to-hard transition, respectively, by assuming a neutron star mass of 1.4 solar masses. This range is comparable to the lowest transition luminosity measured in black hole X-ray binaries, indicating the critical mass accretion rate is not affected by the nature of the surface of the compact stars. Our result does not support the Advection-Dominated Accretion Flow (ADAF) model which predicts that the critical mass accretion rate in neutron star systems is an order of magnitude lower if same viscosity parameters are taken. The low transition luminosity and insignificant hysteresis in these ultracompact X-ray binaries provide further evidence that the transition luminosity is likely related to the mass in the disc.

arXiv comment12 pages, 4 figures, to appear in MNRAS

References in corpus (49)

• Advection-Dominated Accretion: Underfed Black Holes and Neutron Stars
• Advection-Dominated Accretion and the Spectral States of Black Hole X-Ray Binaries: Application to Nova Muscae 1991
• The Swift/BAT Hard X-ray Transient Monitor
• Correlated X-ray Spectral and Timing Behavior of the Black Hole Candidate XTE J1550-564: A New Interpretation of Black Hole States
• The Mass of the Black Hole in Cygnus X-1
• A catalogue of low-mass X-ray binaries in the Galaxy, LMC, and SMC (Fourth edition)
• GX 339-4: the distance, state transitions, hysteresis and spectral correlations
• The Trigonometric Parallax of Cygnus X-1
• Evaluating Spectral Models and the X-ray States of Neutron-Star X-ray Transients
• Six new candidate ultracompact X-ray binaries
• Two Different Long-term Behaviors in Black-Hole Candidates: Evidence for Two Accretion Flows?
• White Dwarf Donors in Ultracompact Binaries: The Stellar Structure of Finite Entropy Objects
• Black hole-like hysteresis and accretion states in neutron star low mass X-ray binaries
• The 1996 Soft State Transitions of Cygnus X-1
• Intermittent millisecond X-ray pulsations from the neutron-star X-ray transient SAX J1748.9-2021 in the globular cluster NGC 6440
• On the possibility of a helium white dwarf donor in the presumed ultracompact binary 2S 0918-549
• Optical spectra of the carbon-oxygen accretion discs in the ultra-compact X-ray binaries 4U 0614+09, 4U 1543-624 and 2S 0918-549
• Ultracompact X-Ray Binaries with Neon-Rich Degenerate Donors
• Stability of helium accretion discs in ultracompact binaries
• Optical spectroscopy of (candidate) ultra-compact X-ray binaries: constraints on the composition of the donor stars
• The complete spectrum of the neutron star X-ray binary 4U0614+091
• How Much Mass do Supermassive Black Holes Eat in their Old Age?
• Analysing the atolls: X-ray spectral transitions of accreting neutron stars
• What ignites on the neutron star of 4U 0614+091?
• Spectral Fits to the 1999 Aql X-1 Outburst Data
• Orbital Period and Outburst Luminosity of Transient Low Mass X-ray Binaries
• State Transitions in Bright Galactic X-ray Binaries: Luminosities Span by Two Orders of Magnitude
• An ultracompact X-ray binary in the globular cluster M15 (NGC7078)
• The Orbital Period of the Ultracompact Low-Mass X-Ray Binary 4U 1543--624
• X-Ray Spectroscopy of the Low-Mass X-ray Binaries 2S 0918-549 and 4U1543-624: Evidence for Neon-Rich Degenerate Donors
• Correlation between Hard X-ray Peak Flux and Soft X-ray Peak Flux in the Outburst Rise of Low Mass X-ray Binaries
• Identification of the LMXB and Faint X-ray Sources in NGC 6652
• Time-resolved optical photometry of the ultra-compact binary 4U0614+091
• An ultracompact X-ray binary in the globular cluster NGC 1851
• Radius-expansion burst spectra from 4U 1728-34: an ultracompact binary?
• VLT spectroscopy and non-LTE modeling of the C/O-dominated accretion disks in two ultracompact X-ray binaries
• INTEGRAL detection of a long powerful burst from SLX1735-269
• Chandra observations of the neutron star soft X-ray transient RXJ170930.2-263927 returning to quiescence
• An X-ray and optical study of the ultracompact X-ray binary A1246-58
• Peak Luminosities of the Hard States of GX 339-4: Implications for the Accretion Geometry, Disk Mass, and Black Hole Mass
• The Likely Orbital Period of the Ultracompact Low-Mass X-Ray Binary 2S 0918-549
• Detection of X-ray spectral state transitions in mini-outbursts of black hole transient GRS 1739-278
• Hard X-ray Flares Preceding Soft X-ray Outbursts in Aquila X-1: A Link between Neutron Star and Black Hole State Transitions
• A Hard-to-Soft State Transition during A Luminosity Decline of Aquila X-1
• The X-ray source population of the globular cluster M15: Chandra high resolution imaging
• Galactic Ultracompact X-ray Binaries: Empirical Luminosities
• Rapid X-ray variability properties during the unusual very hard state in neutron-star low-mass X-ray binaries
• RXTE/ASM and Swift/BAT Observations of Spectral Transitions in Bright X-ray Binaries in 2005-2010
• On the ultra-compact nature of 4U1822-000

Cited by in corpus (6)

• Ultra-compact X-ray binaries with He star companions
• UltraCompCAT: a comprehensive Catalogue of Ultra-Compact and Short Orbital Period X-ray Binaries
• Minute-Cadence Observations of the LAMOST Fields with the TMTS: II. Catalogues of Short-Period Variable Stars from the First Two-Year Surveys
• The energetic thermonuclear bursts in SAX J1712.6-3739
• Effect of non-stationary accretion on spectral state transitions: An example of a persistent neutron star LMXB 4U 1636-536
• Ultra-compact X-ray Binaries: A Review