Discovery and Identification of MAXI J1621-501 as a Type I X-Ray Burster with a Super-orbital Period
Göğüş, Ersin; Bahramian, Arash; Chakrabarty, Deepto; Kouveliotou, Chryssa; Stern, Daniel; Kara, Erin; Baring, Matthew G.; Harrison, Fiona A.; Tomsick, John A.; Iwakiri, Wataru; Kaper, Lex; Bozzo, Enrico; Negoro, Hitoshi; Bult, Peter; Hartmann, Dieter H.; Miller-Jones, James; van der Horst, Alexander J.; Huppenkothen, Daniela; Hailey, Charles; Kennea, Jamie; Wijers, Ralph A. M. J.; Beniamini, Paz; Linford, Justin D.; Murata, Katsuhiro; Granot, Jonathan; Gorgone, Nicholas M.; Guiriec, Sylvain; Mazzola, Simona; Younes, George A.
United States, Japan, Netherlands, Switzerland, Turkey, Australia, Israel, Italy
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 two Type I X-ray bursts, identifying MAXI J1621-501 as a low mass x-ray binary 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.