# Discovery and Identification of MAXI J1621-501 as a Type I X-ray Burster with a Super-Orbital Period

## TL;DR

MAXI J1621-501 was identified as a neutron star Low Mass X-ray Binary through detection of 24 Type I X-ray bursts and exhibits a super-orbital period of approximately 78 days, aligning with theoretical predictions.

## Contribution

This study reports the discovery of MAXI J1621-501 as a Type I X-ray burster with a super-orbital period, providing new insights into its accretion dynamics and precession behavior.

## Key findings

- Detected 24 Type I X-ray bursts over 15 months.
- Identified a super-orbital period of ~78 days.
- Spectral analysis revealed a three-component model.

## Abstract

MAXI J1621-501 is the first Swift/XRT Deep Galactic Plane Survey transient that was followed up with a multitude of space missions (NuSTAR, Swift, Chandra, NICER, INTEGRAL, and MAXI) and ground-based observatories (Gemini, IRSF, and ATCA). The source was discovered with MAXI on 2017 October 19 as a new, unidentified transient. Further observations with NuSTAR revealed 2 Type I X-ray bursts, identifying MAXI J1621-501 as a Low Mass X-ray Binary (LMXB) with a neutron star primary. Overall, 24 Type I bursts were detected from the source during a 15 month period. At energies below 10 keV, the source spectrum was best fit with three components: an absorbed blackbody with kT = 2.3 keV, a cutoff power law with index $\Gamma{}$ = 0.7, and an emission line centered on 6.3 keV. Timing analysis of the X-ray persistent emission and burst data has not revealed coherent pulsations from the source or an orbital period. We identified, however, a super-orbital period $\sim{}$78 days in the source X-ray light curve. This period agrees very well with the theoretically predicted radiative precession period of $\sim{}$82 days. Thus, MAXI J1621-501 joins a small group of sources characterized with super-orbital periods.