Measuring accretion disc properties in the transitional millisecond pulsar PSR J1023+0038 using XMM-Newton, NuSTAR, NICER and Chandra

TL;DR

This study uses multi-observatory X-ray data to measure the inner radius of the accretion disc in PSR J1023+0038, confirming the disc extends close to the neutron star surface during high-mode, with implications for gravitational wave emission.

Contribution

It provides the first measurement of the inner disc radius in a transitional millisecond pulsar during high-mode using combined spectral analysis from multiple X-ray observatories.

Findings

Inner disc radius is approximately 16.8 km with 3σ significance.

Detection of Fe emission line at 6.45 keV supports disc extension into magnetosphere.

Results are consistent with models predicting gravitational wave emission from the pulsar.

Abstract

Whether the accretion disc in the X-ray high-mode of transitional millisecond pulsars (tMSP) reaches near the neutron star surface by penetrating the magnetosphere is a crucial question with many implications, including for continuous gravitational wave emission from the pulsar. We attempt to answer this question for the tMSP PSR J1023+0038 by segregating high-mode data and performing detailed spectral analysis using the XMM-Newton EPIC-PN+MOS1+MOS2 joint observations, XMM-Newton+NuSTAR joint observations, NICER and Chandra individual observations during different epochs. With the sum of longest exposures ($\sim$202 ksec of high mode data from $\sim$364 ksec of total exposure), we performed a self-consistent spectral analysis and constrain the inner disc radius 16.8 $\pm$ 3.8 km with at least 3$\sigma$ significance. Such a measurement is found consistent with best-fit spectral values of inner disc radius from other observatory like NICER and a joint observations with XMM-Newton and NuSTAR within 3$\sigma$ limits. We also detect a Fe emission line at 6.45 keV, for the first time from a tMSP, in the Chandra spectrum with 99% significance with an upper limit of the inner disc radius of 21 R$_g$, supporting independently the fact that inner disc extends into neutron stars's magnetosphere during high mode. All results from our analysis imply that the accretion disc is significantly present and extended within the corotation radius of the neutron star in PSR J1023+0038 during the X-ray high-mode of the tMSP PSR J1023+0038. The measured range of inner disc radius is fully consistent with an independent analysis by Bhattacharyya (2020), which suggests continuous gravitational wave emission from this neutron star, and the standard model of X-ray pulsations in accreting MSPs.