[Paper Review] On Low Mass X-ray Binaries and Millisecond Pulsar

L. Burderi, T. Di Salvo|arXiv (Cornell University)|Oct 4, 2013

Pulsars and Gravitational Waves Research2 references3 citations

TL;DR

This paper reviews the evolution and observational properties of accreting millisecond pulsars (AMPs), confirming the Recycling Scenario through the discovery of 14 AMPs since 1998. It presents four independent methods to estimate neutron star magnetic fields, finding consistency within 3σ for SAX J1808.4-3658, resolving long-standing discrepancies in the spin-down and accretion phase models.

ABSTRACT

The detection, in 1998, of the first Accreting Millisecond Pulsar, started an exciting season of continuing discoveries in the fashinating field of compact binary systems harbouring a neutron star. Indeed, in these last three lustres, thanks to the extraordinary performances of astronomical detectors, on ground as well as on board of satellites, mainly in the Radio, Optical, X-ray, and Gamma-ray bands, astrophysicists had the opportunity to thoroughly investigate the so-called Recycling Scenario: the evolutionary path leading to the formation of a Millisecond Radio Pulsar. The most intriguing phase is certainly the spin-up stage during which, because of the accretion of matter and angular momentum, the neutron star accumulates an extraordinary amount of mechanical rotational energy, up to one percent of its whole rest-mass energy. These millisecond spinning neutron stars are truly extreme physical objects: General and Special Relativity are fully in action, since their surfaces, attaining speeds close to one fifth of the speed of light, are located just beyond their Schwartzscild Radius, and electrodynamical forces, caused by the presence of huge surface magnetic fields of several hundred million Gauss, display their spectacular properties accelerating electrons up to such energies to promote pair creation in a cascade process responsible for the emission in Radio and Gamma-ray. The rotational energy is swiftly converted and released into electromagnetic power which, in some cases, causes the neutron star to outshine with a luminosity of one hundred Suns. In this paper I will review some of the most recent discoveries on (accreting) millisecond pulsars.

Motivation & Objective

• To reconcile the Recycling Scenario with observational anomalies in millisecond pulsar populations.
• To resolve the lack of detected millisecond X-ray pulsations in LMXBs prior to 1998.
• To explain the origin of isolated millisecond pulsars through ablation of low-mass companions.
• To estimate neutron star magnetic fields using multiple independent observational methods.
• To validate the role of rotational energy loss via spin-down luminosity in shaping post-accretion evolution.

Proposed method

• Utilized X-ray timing data from RXTE and other satellites to detect coherent millisecond X-ray pulsations in transient LMXBs.
• Applied the Larmor formula for spin-down luminosity: $ L_{\rm SD} = \frac{2}{3c^3} \mu^2 (2\pi/P)^4 $, linking rotational energy loss to magnetic dipole radiation.
• Estimated magnetic fields via X-ray residual luminosity in quiescence (method a), optical reprocessing of dipole radiation (method b), pulse phase delay fitting (method c), and secular spin-down rate analysis (method d).
• Combined multi-wavelength data (X-ray, radio, optical, gamma-ray) to cross-validate magnetic field estimates and evolutionary models.
• Used orbital and spin period measurements to infer companion masses and test evolutionary models involving magnetic braking and gravitational radiation.
• Applied statistical analysis to spin period derivatives $ \dot{P} $, with $ \dot{P}_{-20} $ in units of $ 10^{-20} $, to constrain magnetic field strengths.

Experimental results

Research questions

• RQ1 How do the observed properties of accreting millisecond pulsars support the Recycling Scenario for MSP formation?
• RQ2 Why were millisecond X-ray pulsations not detected in LMXBs before 1998, and what resolved this issue?
• RQ3 What mechanisms explain the existence of isolated millisecond pulsars, and how does spin-down energy loss contribute to their evolution?
• RQ4 Can multiple independent methods yield consistent estimates of neutron star magnetic fields in AMPs?
• RQ5 What is the role of rotational energy release via spin-down in altering the long-term evolution of compact binary systems?

Key findings

• The discovery of coherent 2.5 ms X-ray pulsations in SAX J1808.4-3658 in 1998 confirmed that LMXBs are the progenitors of MSPs, resolving the long-standing lack of detected millisecond X-ray pulsations.
• Four independent methods for estimating magnetic field strength in AMPs yielded consistent results within 3σ for SAX J1808.4-3658, with $ B_8 = 1.62 \pm 0.10 $, validating the robustness of the measurements.
• Magnetic field estimates from secular spin-down ($ \dot{P}_{-18} = -2.75 \pm 0.51 $) yielded $ B_8 = 1.62 \pm 0.10 $, consistent with other methods, confirming the reliability of timing-based spin-down analysis.
• For IGR J17498-2921, the spin period derivative $ \dot{P}_{-18} = +0.39 \pm 0.12 $ indicates spin-up, suggesting ongoing accretion and a low magnetic field strength.
• The spin-down luminosity $ L_{\rm SD} \sim 3.85 \times 10^{35} B_8^2 R_6^6 P_{-3}^{-4} \, \text{erg/s} $ was found to be a dominant energy source in post-accretion phases.
• The estimated magnetic field for XTE J0929-314 from spin-down data is $ \sim 10 $ in $ B_8 $ units, indicating a strong field, consistent with its high $ \dot{P}_{-18} = +2.68 \pm 0.12 $.