Spectral and timing analysis of the mHz QPOs in the neutron-star low-mass X-ray binary 4U 1636–53

Abstract

We investigate the spectral and timing properties of the millihertz quasiperiodic oscillations (mHz QPOs) in neutron-star low-mass X-ray binary 4U 1636-53 using XMM-Newton and Rossi X-ray Timing Explorer (RXTE) observations. The mHz QPOs in the XMM-Newton/RXTE observations show significant frequency variation and disappear right before type I X-ray bursts.We find no significant correlation between the mHz QPO frequency and the temperature of the neutron-star surface, which is different from theoretical predictions. For the first time we observed the full lifetime of a mHz QPO lasting 19 ks. Besides, we also measure a frequency drift timescale ∼15 ks, we speculate that this is the cooling timescale of a layer deeper than the burning depth, possibly heated by the previous burst. Moreover, the analysis of all X-ray bursts in this source shows that all type I X-ray bursts associated with the mHz QPOs are short, bright and energetic, suggesting a potential connection between mHz QPOs and He-rich X-ray bursts.

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

“…The best-studied object in this regard is 4U 1636−536. The rate of decrease we estimate here for GS 1826 does not appear to be inconsistent with previous reports (Altamirano et al 2008), that is, one can find time intervals reported in the literature for 4U 1636−536 which have approximately similar drift rates (see, for example, Figure 4 in Lyu et al 2015).…”

“…More recently, Lyu et al (2015) also studied episodes of mHz QPOs in 4U 1636−536 using XMM-Newton and RXTE data. 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.…”

Millihertz Quasi-Periodic Oscillations in 4u 1636–536: Putting Possible Constraints on the Neutron Star Size

“…Based on the narrow luminosity range in which mHz QPOs are observed and assuming a He enriched environment (Lyu et al 2015) we assume a hardening factor f col Notes: † : derived from normalization assuming a distance of 6.0 kpc (Galloway et al 2006) and a mass of 1.4 M . Nath et al (2002) based on RXTE observations of the bolometric flux oscillations that occur during the rise of X-ray bursts.…”

“…Including the uncertainty on the distance estimation, the lower limit on the NS radius remains above 10 km. Observations of one life cycle of the mHz QPO in 4U 1636−53 (19 ks; Lyu et al 2015) with NICER will allow to reduce the statistical uncertainties in the radius to about 0.3 km. With the Low en-ergy Focusing Array (LFA) onboard the enhanced X-ray Timing and Polarisation mission (eXTP) it will be possible to reduce the uncertainties even further to 0.15 km, and to constrain the NS radius on ±1 km using current estimates of the source distance and hardening factor.…”

“…The connection between mHz QPOs and nuclear burning has been fostered by the frequency decrease of the mHz QPO in 4U 1636−53 observed prior to the occurrence of a type-I X-ray burst (Altamirano et al 2008b;Lyu et al 2014Lyu et al , 2015. This frequency drift can be reproduced by models of nuclear burning that take mixing processes and crustal heating into account, and could be caused by the cooling of deeper layers of the NS atmosphere (Keek et al 2009).…”