AstroSat detection of a mHz quasi-periodic oscillation and cyclotron line in IGR J19294+1816 during the 2019 outburst

Abstract

We present results from timing and spectral analysis of the high-mass X-ray binary X-ray pulsar IGR J19294+1816 observed using AstroSat during its recent Type-I outburst in 2019 October. AstroSat observations sampled the outburst at the decline phase right after the outburst peak. We carried out timing analysis on the light curves obtained using the Large Area X-ray Proportional Counter (LAXPC) instrument on board AstroSat and measured a spin period of 12.485 065 ± 0.000 015 s. The peak in the power-density spectrum (PDS) corresponding to the spin period of 12.48 s also shows a broadened base. We also detected a quasi-periodic oscillation (QPO) feature at 0.032 ± 0.002 Hz with an RMS fractional amplitude of ∼18 per cent in the PDS. We further carried out a joint spectral analysis using both the Soft X-ray Telescope and the LAXPC instruments and detected a cyclotron resonant scattering feature (CRSF) at 42.7 ± 0.9 keV and an Fe emission line at 6.4 ± 0.1 keV. IGR J19294+1816, being an intermediate spin pulsar, has exhibited a plethora of spectral and timing features during its most recent 2019 outburst, adding it to the list of transients that exhibit both a QPO as well as a CRSF.