Millihertz quasi-periodic oscillations in 4U 1636−53 associated with bursts with positive convexity only

Abstract

We investigated the convexity of all type I X-ray bursts with millihertz quasi-periodic oscillations (mHz QPOs) in 4U 1636-53 using archival observations with the Rossi X-ray Timing Explorer. We found that, at a 3.5σ confidence level, in all 39 cases in which the mHz QPOs disappeared at the time of an X-ray burst, the convexity of the burst is positive. The convexity measures the shape of the rising part of the burst light curve and, according to recent models, it is related to the ignition site of bursts on the neutronstar surface. This finding suggests that in 4U 1636−53 these 39 bursts and the marginally-stable nuclear burning process responsible for the mHz QPOs take place at the neutron-star equator. This scenario could explain the inconsistency between the high accretion rate required for triggering mHz QPOs in theoretical models and the relatively low accretion rate derived from observations.

Discovery of an Accretion-rate Independent Absolute RMS Amplitude of Millihertz Quasi-periodic Oscillations in 4U 1636-53

“…We analysed the observations with mHz QPOs reported in Lyu et al (2016) in 4U 1636−53 using the Proportional Counter Array (PCA; Jahoda et al 2006) on board of RXTE. An RXTE observation typically covers 1 to 5 consecutive 90-minute satellite orbits, which usually contains between 1 and 5 ks of useful data separated by 1-4 ks data gaps.…”

“…Furthermore, Lyu et al (2015) found that seven X-ray bursts associated with mHz QPOs in this source were bright, energetic and short, indicating a potential connection between the mHz QPOs and He-rich X-ray bursts. Lyu et al (2016) investigated the convexity of 39 type I X-ray bursts associated with mHz QPOs in 4U 1636−53 and found that all the bursts show positive convexity. This finding suggests that these mHz QPOs and the associated bursts may originate at the equator of the neutron star surface.…”

NICER Discovers mHz Oscillations in the “Clocked” Burster GS 1826−238

“…Similarly to Altamirano et al (2008), they found frequency drift of the mHz oscillations, and suggested the drift time-scale may be set by cooling of the deeper layers, as was previously argued by Keek et al (2009) based on hydrodynamic calculations of helium burning with rotational mixing. Lyu et al (2016) found that the bursts that occurred immediately after an episode of mHz oscillations preferentially showed a rising light curve shape with so-called positive "convexity." Such bursts show fast rise times, and this has been linked to a burst ignition location at or near the star's rotational equator (Maurer & Watts 2008;Cooper & Narayan 2007).…”

“…Such bursts show fast rise times, and this has been linked to a burst ignition location at or near the star's rotational equator (Maurer & Watts 2008;Cooper & Narayan 2007). Thus, Lyu et al (2016) argued that these bursts, and the burning responsible for the mHz QPOs, may occur at the neutron star equator. Stiele et al (2016) reported results from phase-resolved spectroscopy of the mHz oscillations in 4U 1636−536 obtained with XMM-Newton.…”

Testing the Equation of State with Electromagnetic Observations

“…Quasi-Periodic Oscillations (QPOs) at mHz frequencies are detected for a handful of LMXBs and are thought to be associated with quasi-stable burning on the neutron star surface (e.g. Revnivtsev et al 2001;Yu & van der Klis 2002;Heger et al 2007b;Altamirano et al 2008;Linares et al 2012;Keek et al 2014;Lyu et al 2014Lyu et al , 2016. As recently argued by Stiele et al (2016), the maximum black-body emitting radius measured during a QPO cycle provides a lower limit on the radius of the neutron star (it is uncertain whether the entire surface should be emitting).…”