# Pulse Phase Variation of Cyclotron Line in HMXB 4U 1907+09 with ASTROSAT LAXPC

## TL;DR

This study analyzes the timing and spectral properties of the HMXB pulsar 4U 1907+09 using AstroSat's LAXPC, confirming cyclotron line features and revealing phase-dependent magnetic field characteristics.

## Contribution

It provides the first detailed phase-resolved spectral analysis of 4U 1907+09 with AstroSat, highlighting the phase dependence of the cyclotron line and magnetic field geometry.

## Key findings

- Confirmed cyclotron line at ~18.5 keV with high significance.
- Detected phase-dependent variation in the cyclotron line parameters.
- Observed energy-dependent pulse profile changes up to 40 keV.

## Abstract

We present timing and spectral analysis of data from an observation of the High Mass X-ray Binary pulsar 4U 1907+09 with the LAXPC instrument onboard AstroSat. The light curve consisted of a flare at the beginning of the observation, followed by persistent emission. The pulsar continues to spin down, and the pulse profile is found to be double-peaked up to 16 keV with the peaks separated by a phase of $\sim0.45$. Significant energy dependence of the pulse profile is seen with diminishing amplitude of the secondary peak above 16 keV, and increasing amplitude of the main peak upto 40 keV and a sharp decline after that. We confirm earlier detections of the Cyclotron Resonance Scattering Feature (CRSF) in 4U 1907+09 at $\sim 18.5\pm 0.2$ keV in the phase-averaged spectrum with a high detection significance. An intensity resolved spectral analysis of the initial flare in the light curve shows that the CRSF parameters do not change with a change in luminosity by a factor of 2.6. We also performed pulse phase-resolved spectral analysis with ten independent phase bins. The energy and the strength of the cyclotron line show pulse phase dependence that is in agreement with previous measurements. Two features from the current observation: different energy dependence of the two pulse peaks and a strong CRSF only around the secondary peak, both indicate a deviation from a dipole geometry of the magnetic field of the neutron star, or complex beaming pattern from the two poles.