Broadband Observations of the Be/X-ray Binary Pulsar RX J0440.9+4431: Discovery of a Cyclotron Absorption Line

TL;DR

This study analyzes multi-observatory data of the X-ray pulsar RX J0440.9+4431, discovering a cyclotron absorption line at 32 keV that reveals the neutron star's magnetic field and providing insights into its spectral and temporal properties.

Contribution

First comprehensive spectral and temporal analysis of RX J0440.9+4431 across a wide energy band, including the discovery of a cyclotron line and estimation of the magnetic field strength.

Findings

Discovered a ~32 keV cyclotron absorption line.

Estimated the neutron star's magnetic field as ~3.2 x 10^12 G.

Estimated the orbital period as ~155 days.

Abstract

We report the results of an analysis of data obtained with the INTEGRAL, Swift and RXTE observatories during the 2010 April and September outbursts of the X-ray pulsar RX J0440.9+4431. The temporal and spectral properties of the pulsar in a wide energy band (0.6-120 keV) were studied for the first time. We discovered a ~32 keV cyclotron resonant scattering feature in the source spectrum, that allowed us to estimate the magnetic field strength of the neutron star as B~3.2 x 10^12 G. The estimate of the magnetic field strength was confirmed by a comprehensive analysis of the noise power spectrum of the source. Based on the recurrence time between Type I outbursts the orbital period of the binary system can be estimated as ~155 days. We have shown that the pulse profile has a sinusoidal-like single-peaked shape and has practically no dependence on the source luminosity or energy band.