Discovery of a 7.8 Hz QPO from the High-Intensity Outburst of Dipping NS LMXB 1A 1744-361

We report on the discovery of a ~7.8 Hz quasi-periodic oscillation (QPO) in the persistent emission immediately following an absorption dip by the dipping but non-eclipsing neutron star low-mass X-ray binary (NS LMXB) 1A 1744-361 with the Neutron star Interior Composition Explorer (NICER). Fitting the QPO with a Lorentzian above a power law continuum, we found the central frequency and full-width half maximum to be 7.8 ± 1.5 Hz and 3.4 ± 1.0 Hz, respectively, which implies a quality factor of 2.3 ± 0.8. The fractional rms amplitude of the QPO was 5.7 ± 1.9% over 0.3-12.0 keV. The QPO was discovered within the three-month long high-intensity outburst that started in late May 2022 (which NICER monitored throughout), one of the longest and brightest outbursts from the source to date. The discovery of the QPO is noteworthy because the source has shown a 2-4 Hz QPO in the low-hard state of the source in the 2004 outburst (seen by RXTE), and it is thought that most QPOs in dipping NS LMXBs are found in this state. However, we have discovered the first QPO in the soft atoll state in the high intensity outburst, contrary to other dipping NS LMXB sources. We will discuss the phenomenology around this QPO. We will also report on the spectral evolution of the source over the entire outburst, as well as compare the spectral properties between the absorption dip and the persistent emission immediately following the absorption dip.