# On the curious pulsation properties of the accreting millisecond pulsar IGR J17379-3747

## TL;DR

This study presents NICER observations of the 468 Hz accreting millisecond pulsar IGR J17379-3747, revealing energy-dependent pulsation delays, large pulse fractions, and pulsations at very low luminosities, offering insights into pulsar geometry and transitional states.

## Contribution

It provides detailed spectral and timing analysis of IGR J17379-3747, highlighting extreme pulsation properties and their potential explanation through viewing geometry and magnetic misalignment.

## Key findings

- Energy-dependent pulsation delays (~640 microseconds)
- Pulse fractions exceeding 20%, fluctuations up to 70%
- Detection of pulsations at low luminosity (~5 x 10^33 erg s^-1)

## Abstract

We report on the Neutron Star Interior Composition Explorer (NICER) monitoring campaign of the 468 Hz accreting millisecond X-ray pulsar IGR J17379-3747. From a detailed spectral and timing analysis of the coherent pulsations we find that they show a strong energy dependence, with soft thermal emission lagging about 640 microseconds behind the hard, Comptonized emission. Additionally, we observe uncommonly large pulse fractions, with measured amplitudes in excess of 20% sinusoidal fractional amplitude across the NICER passband and fluctuations of up to ~70%. Based on a phase-resolved spectral analysis, we suggest that these extreme properties might be explained if the source has an unusually favorable viewing geometry with a large magnetic misalignment angle. Due to these large pulse fractions, we were able to detect pulsations down to quiescent luminosities (~5 x 10^33 erg s^-1). We discuss these low-luminosity pulsations in the context of transitional millisecond pulsars.