# Study of the accretion torque during the 2014 outburst of the X-ray pulsar GRO J1744-28

## TL;DR

This study analyzes the 2014 outburst of the X-ray pulsar GRO J1744-28, focusing on its spectral and timing properties, especially the accretion torque's effect on its spin-up behavior, using multi-satellite data.

## Contribution

It provides an updated ephemeris, quantifies the accretion torque's impact on spin-up, and estimates the source distance, offering new insights into the pulsar's behavior during outburst.

## Key findings

- Spin-up rate proportional to mass accretion rate with β≈0.96
- Estimated source distance between 3.4 and 4.1 kpc
- No definitive evidence for a spin-down torque component

## Abstract

We present the spectral and timing analysis of the X-ray pulsar GRO J1744-28 during its 2014 outburst using data collected with the X-ray satellites Swift, INTEGRAL, Chandra, and XMM-Newton. We derived, by phase-connected timing analysis of the observed pulses, an updated set of the source ephemeris. We were also able to investigate the spin-up of the X-ray pulsar as a consequence of the accretion torque during the outburst. Relating the spin-up rate and the mass accretion rate as $\dot{\nu}\propto\dot{M}^{\beta}$, we fitted the pulse phase delays obtaining a value of $\beta=0.96(3)$. Combining the results from the source spin-up frequency derivative and the flux estimation, we constrained the source distance to be between 3.4-4.1 kpc, assuming a disc viscous parameter $\alpha$ to be in the range 0.1-1. Finally, we investigated the presence of a possible spin-down torque by adding a quadratic component to the pulse phase delay model. The marginal statistical improvement of the updated model does not allow us to firmly confirm the presence of this component.