NICER / ISS Science Nugget
for June 18, 2020

Short-duration milliHertz QPO seen in 4U 1636-53

Publicly available NICER data have been used to produce several science papers by investigators not on the NICER Science Team. One that has just been accepted for publication in the Astrophysical Journal is by Lyu et al., on the low-mass binary system 4U 1636-53.

4U 1636-53 contains a neutron star in orbit with a star comparable in mass to our Sun. Lyu et al. describe measurements of a milliHertz quasiperiodic oscillation (QPO), with a period of 90-110 seconds (see figure). As observed in some other neutron-star X-ray binaries that exhibit mHz QPOs, the periodicity comes and goes with time. It is likely that the detected mHz QPO is due to marginally stable nuclear burning of helium on the neutron star surface. Such marginally stable burning usually precedes sudden, explosive thermonuclear burning, resulting in a so-called Type I X-ray burst, which NICER also detected in this instance.

X-ray light curve and hardness ratio showing a short-duration QPO

Figure: X-ray intensity (top panel) of 4U1636-536 seen by NICER, and its spectral hardness (lower panel), as a function of time. The vertical red lines represent gaps in the data. The interval from roughly 3000-4500 seconds clearly shows the periodicity of the fleeting mHz QPO. Later in the observation, a Type 1 thermonuclear X-ray burst is seen (around time=5000 seconds).

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