Discovery and Identification of MAXI J1621–501 as a Type I X-Ray Burster with a Super-orbital Period

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 Γ = 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 ∼ 78 days in the source X-ray light curve. This period agrees very well with the theoretically predicted radiative precession period of ∼ 82 days. Thus, MAXI J1621-501 joins a small group of sources characterized with super-orbital periods.

The Swift Deep Galactic Plane Survey (DGPS) Phase I Catalog

“…An example of variable sources uncovered in DGPS observations is displayed in Figure 10. The majority of sources displaying obvious variable behavior were already classified (typically HMXBs, LMXBs, or magnetars; Figure 10), but we were also able to classify a number of variable sources (e.g., Gorgone et al 2019Gorgone et al , 2021O'Connor et al 2022O'Connor et al , 2023aO'Connor et al , 2023b through our follow-up programs, with more classifications in progress.…”

“…We additionally checked archival flux values from available X-ray catalogs. We selected previously unknown or unclassified variable sources with an unabsorbed X-ray flux brighter than F X > 1.0 × 10 −12 erg cm −2 s −1 (0.3-10 keV) for Target of Opportunity (ToO) follow-up with a variety of X-ray satellites, such as XMM-Newton, Chandra, NuSTAR, and NICER, through our approved programs; see, e.g., Gorgone et al (2019Gorgone et al ( , 2021, O'Connor et al (2022O'Connor et al ( , 2023aO'Connor et al ( , 2023b.…”

“…In addition, we made use of multiwavelength observations from the Lowell Discovery Telescope, the South African Astronomical Observatory 1.0 m telescope, and the Southern African Large Telescope. The results of these campaigns were reported in Gorgone et al (2019, 2021), O'Connor et al (2022, 2023a, 2023b.…”

Low-Mass X-ray Binaries

“…Survey and monitoring coverage of the Galactic plane and halo in the X-rays is substantially shallower and less complete than the Galactic center and bulge. Neverthe-less, surveys such as the XMM-Newton or Swift Galactic Plane surveys [348,360] have discovered a handful of LMXBs and LMXB candidates. However, the overwhelming majority of LMXBs identified in the Galactic plane and halo are discovered while exhibiting bright outbursts detected by all-sky monitors such as MAXI or Swift/BAT.…”