# LAXPC / AstroSat Study of ~ 1 and ~ 2 mHz Quasi-periodic Oscillations in the Be/X-ray Binary 4U 0115+63 During its 2015 Outburst

## TL;DR

This study reports the detection and analysis of ~1 and ~2 mHz quasi-periodic oscillations in the Be/X-ray binary 4U 0115+63 during its 2015 outburst, using data from AstroSat, NuSTAR, and RXTE, exploring their origins and implications.

## Contribution

First detailed multi-instrument analysis of mHz QPOs in 4U 0115+63, linking observations across different outbursts and discussing potential models for their origin.

## Key findings

- Detection of ~1 and ~2 mHz QPOs during the 2015 outburst.
- Presence of similar mHz QPOs in archival data from 2001-2011.
- Discussion of models explaining the origin of these low-frequency oscillations.

## Abstract

The Be X-ray Binary 4U 0115+63 was observed by Large Area X-ray Proportional Counter (LAXPC) instrument on AstroSat on 2015 October 24 during the peak of a giant Type II outburst. Prominent intensity oscillations at ~ 1 and ~ 2 mHz frequency were detected during the outburst. Nuclear Spectroscopic Telescope Array (NuSTAR) observations made during the same outburst also show mHz quasi periodic oscillations (QPOs). Details of the oscillations and their characteristics deduced from LAXPC/AstroSat and NuSTAR observations are reported in this paper. Analysis of the archival Rossi X-ray Timing Explorer (RXTE)/Proportional Counter Array (PCA) data during 2001-11 also show presence of mHz QPOs during some of the outbursts and details of these QPOs are also reported. Possible models to explain the origin of the mHz oscillations are examined. Similar QPOs, albeit at higher frequencies, have been reported from other neutron star and black hole sources and both may have a common origin. Current models to explain the instability in the inner accretion disk causing the intense oscillations are discussed.