[Paper Review] Binary evolution with LOFT

Thomas J. Maccarone, R. A. M. Wijnands+11 more|arXiv (Cornell University)|Jan 12, 2015

Astrophysical Phenomena and Observations6 references3 citations

TL;DR

LOFT will revolutionize the study of binary stellar evolution by enabling deep, all-sky monitoring of X-ray binaries through its Wide Field Monitor (WFM) and high-time-resolution spectroscopy via the Large Area Detector (LAD). It will uncover very faint X-ray binaries, resolve the orbital period distribution of black hole X-ray binaries, and probe neutron star spin-up during transitions to millisecond pulsars, significantly advancing understanding of compact binary evolution and the 'mass gap' between neutron stars and black holes.

ABSTRACT

This is a White Paper in support of the mission concept of the Large Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We discuss the potential of LOFT for the study of very faint X-ray binaries, orbital period distribution of black hole X-ray binaries and neutron star spin up. For a summary, we refer to the paper.

Motivation & Objective

• To overcome severe selection biases in black hole X-ray binary surveys by enabling deep, all-sky monitoring with unprecedented sensitivity.
• To resolve the 'mass gap' between neutron stars and black holes by expanding the sample of systems with accurate mass measurements.
• To study the spin-up and spin-down processes of neutron stars in binaries by detecting and characterizing transitional systems between low-mass X-ray binaries and millisecond radio pulsars.
• To establish a comprehensive census of very faint X-ray binaries (VFXBs) across the Galaxy using the WFM’s sensitivity and wide field of view.
• To enable phase-coherent timing of pulsations in transitional systems by combining rapid response with high temporal resolution, allowing testing of accretion and spin evolution models.

Proposed method

• Utilizing the LOFT Wide Field Monitor (WFM) to conduct continuous, all-sky monitoring in the 2–50 keV energy range with sensitivity down to mCrab fluxes.
• Leveraging the Large Area Detector (LAD) for high-time-resolution X-ray spectroscopy and precise timing of X-ray pulsations in accreting systems.
• Combining WFM alerts with LAD follow-up observations to detect and characterize transient X-ray states, particularly during transitions between LMXB and MSP states.
• Applying phase-coherent timing techniques to pulsations in transitional systems using rapid LAD observations (as short as 300 seconds) to measure spin evolution and torque mechanisms.
• Using radio-derived ephemerides to improve pulse phase tracking and test models of accretion-induced spin-up and ejection-induced spin-down.
• Conducting coordinated multiwavelength follow-up of WFM-triggered outbursts to identify and classify VFXBs and transitional systems.

Experimental results

Research questions

• RQ1 What is the true orbital period distribution of black hole X-ray binaries, and does it match predictions from binary evolution models?
• RQ2 Do the observed 'mass gap' systems between neutron stars and black holes represent a real physical phenomenon or a selection bias?
• RQ3 How do neutron stars achieve millisecond spin periods during accretion, and what are the dominant torques during spin-up and spin-down?
• RQ4 What fraction of very faint X-ray binaries (VFXBs) are actually transitional systems between LMXBs and millisecond pulsars?
• RQ5 Can phase-coherent timing of pulsations in transitional systems constrain the physics of accretion, ejection, and angular momentum transfer?

Key findings

• LOFT’s WFM will detect VFXBs with luminosities as low as ~10^34 erg s⁻¹ across a large fraction of the Galaxy, enabling optical and radio follow-up to determine their nature.
• LOFT is expected to detect pulsations from transitional systems like PSR J1023+0038 in as little as 300 seconds, enabling phase-coherent timing campaigns.
• The mission will significantly expand the sample of neutron stars with measured spin periods, particularly in the millisecond regime, by detecting weak, fast pulsations.
• LOFT’s sensitivity will allow detection of X-ray flares from transitional systems in the 'flaring' state, even at distances up to several kpc, with peak luminosities exceeding 10^37 erg s⁻¹.
• The combination of WFM and LAD will enable the discovery and monitoring of multiple transitional systems during state transitions, providing rare insights into accretion and spin evolution.
• LOFT’s rapid response and high time resolution will allow testing of Applegate-like mechanisms in transitional systems by measuring orbital period variations linked to Roche lobe overflow.